Comparison of acute responses in spinal excitability between older and young people after neuromuscular electrical stimulation

PURPOSE

This study aims at comparing acute responses in spinal excitability, as measured by H-reflex, between older and young individuals, following a single session of NMES superimposed onto voluntary isometric contractions of the ankle plantar-flexor muscles (NMES+), with respect to passive NMES (pNMES) and voluntary isometric contractions only (ISO).

METHODS

Thirty-two volunteers, 16 older (OLDER) and 16 young (YOUNG), were asked to sustain a constant force at 20% of maximal voluntary isometric contraction (MVIC) of the ankle plantar-flexor muscles in the dominant limb during each of the 3 conditions (NMES+ , pNMES and ISO). Fifteen repetitions of 6 s were performed, with a resting interval of 6 s between repetitions. Before and after each condition, soleus H-reflexes were elicited by percutaneous electrical stimulation of the posterior tibial nerve and H-reflex amplitudes recorded by surface EMG.

RESULTS

In OLDER, H-reflex amplitude did not change following any experimental condition (ISO: p = 0.203; pNMES: p = 0.542; NMES+: p = 0.431) compared to baseline. On the contrary, in YOUNG, H-reflex amplitudes significantly increased (p < 0.000) and decreased (p = 0.001) following NMES+ and pNMES, respectively, while there was no significant change in reflex responses following ISO (p = 0.772).

CONCLUSION

The lack of change in H-reflex responses following either NMES+ or pNMES might reflect a reduced ability of older people in modulating spinal excitability after the conditions. Specifically, an age-related alteration in controlling mechanisms at presynaptic level was suggested.

Effectiveness of instrument assisted soft tissue mobilization versus foam rolling on trigger point release in calf muscles

BACKGROUND

Instrument assisted soft tissue mobilization and foam rolling are two techniques that have been proven effective in treating Myofascial Trigger Points, irrespective of the type of trigger point. However, little is known about the comparative effectiveness of Instrument assisted soft tissue mobilization and foam rolling. This study proposed to evaluate the effectiveness of either technique on plantar flexors trigger points, ankle dorsiflexion, and lower limb power present in the calf muscles in non-symptomatic patients.

METHOD

Forty-two subjects with bilateral calf muscle tightness, at least one trigger point in the calf muscle, and fulfilling the inclusion criteria were randomly assigned to either of the groups. Group A was treated for gastrocnemius and soleus trigger points using Instrument assisted soft tissue mobilization and Group B was treated using the Foam Rolling method. Treatment was given every alternate day, a total of 3 sessions. Subjects were evaluated on 1st and 3rd sessions for pre-post differences of ankle dorsiflexion Range of motion in weight bearing and non-weight bearing position, pressure pain threshold for gastrocnemius trigger point 1(G1), 2(G2), and soleus point 1(S1) on both sides, and lower limb power.

RESULT

Within group analyses, both groups had shown statistically significant results for all parameters except gastrocnemius trigger point 2 of foam rolling. For between group comparison foam rolling had a statistically significant result in non-weight bearing ankle dorsiflexion range of motion.

CONCLUSION

Both Instrument assisted soft tissue mobilization and Foam rolling were equally effective for treating calf trigger points. But foam rolling was more effective in improving ankle dorsiflexion range of motion.

A forefoot strike pattern during 18° uphill walking leads to greater ankle joint and plantar flexor loading

BACKGROUND

The ankle joint is one of the most involved joints in uphill walking. Furthermore, it is well known that toe walking increases the external dorsiflexion moment in the first half of stance during level walking. However, the effects of different foot-strike patterns on plantar flexor muscle forces, ankle joint forces, and other lower limb joint and muscle forces are unknown.

RESEARCH QUESTION

Do foot-strike patterns during 18° uphill walking affect lower limb sagittal joint angles and moments, as well as joint contact and muscle forces?

METHODS

This study was based on a data subset from previous publications, analysing uphill walking on an 18° ramp at a preset speed of 1.1 m/s in 18 male participants (34 limbs analyzed, 27 ± 5 years). Participants were divided into two groups based on their foot-strike pattern at initial contact: heel (HC) and forefoot (FC). Lower limb sagittal joint angles and moments as well as joint contact and muscle forces were assessed. Differences between the groups were assessed using two-sample t-tests.

RESULTS

FC showed increased soleus and gastrocnemius muscle forces as well as ankle joint forces during loading response and mid stance compared to HC. The soleus muscle force impulse was 51.1% higher in the FC group than in the HC group (p < 0.001). On the other hand, FC had a lower absolute centre of mass vertical displacement and reduced knee and hip joint, as well as iliopsoas and hamstring muscle force impulses.

SIGNIFICANCE

In terms of plantar flexor and ankle joint loading, it is advantageous to exhibit a heel strike pattern. The current results can be used to recommend foot-strike patterns for uphill walking, particularly in the presence or prevention of musculoskeletal issues.

The force response of muscles to activation and length perturbations depends on length history

Recent studies have demonstrated that muscle force is not determined solely by activation under dynamic conditions, and that length history has an important role in determining dynamic muscle force. Yet, the mechanisms for how muscle force is produced under dynamic conditions remain unclear. To explore this, we investigated the effects of muscle stiffness, activation and length perturbations on muscle force. First, submaximal isometric contraction was established for whole soleus muscles. Next, the muscles were actively shortened at three velocities. During active shortening, we measured muscle stiffness at optimal muscle length (L0) and the force response to time-varying activation and length perturbations. We found that muscle stiffness increased with activation but decreased as shortening velocity increased. The slope of the relationship between maximum force and activation amplitude differed significantly among shortening velocities. Also, the intercept and slope of the relationship between length perturbation amplitude and maximum force decreased with shortening velocity. As shortening velocities were related to muscle stiffness, the results suggest that length history determines muscle stiffness and the history-dependent muscle stiffness influences the contribution of activation and length perturbations to muscle force. A two-parameter viscoelastic model including a linear spring and a linear damper in parallel with measured stiffness predicted history-dependent muscle force with high accuracy. The results and simulations support the hypothesis that muscle force under dynamic conditions can be accurately predicted as the force response of a history-dependent viscoelastic material to length perturbations.

Swimming exercise and nano-l-arginine supplementation improve oxidative capacity and some autophagy-related genes in the soleus muscle of aging rats

The present research aims to evaluate the effect of swimming exercise and chitosan-coated l-arginine on mitochondrial oxidation, BCL2 Interacting Protein 3 (Bnip3), NIP-like protein × (Nix), B-cell lymphoma-extra-large (Bcl-xL) and autophagy-related protein light chain 3(LC3) expression in soleus muscle of aging rats. In this experimental research, 25 male Wistar rats were assigned into five groups randomly: young, old, old + Nano l-arginine (Nano L-a), old + exercise (Ex), and old + Nano l-arginine (Nano L-a) + exercise (Ex) (n = 5 in each). They performed a swimming exercise program five days a week for six weeks. To determine the relative strength for rats before and after performing these interventions, the 1repetition maximum (1RM) test was done as a pre and post-test. The exercise program started with 20 min and after four sessions, gradually increased to 60 min and this time was maintained until the completion of the training period. l-arginine coated with chitosan nanoparticles was given to the rats in the l-arginine-supplemented group via gavage at a dosage of 500 mg/kg/day, five days a week, for six weeks. Additionally, the rats in all groups were fed a normal diet (2.87 kcal/g and 15 % energy from fat). Upon the completion of the protocol implementation, the rats were sacrificed and the soleus muscle was fixed and frozen to determine hematoxylin and eosin (H&E) staining, immunohistochemistry (IHC), gene expression analysis, levels of reactive oxygen species (ROS), and total antioxidant capacity (TAC). The results from the present research indicated that swimming exercise and Nano l-arginine improve the strength and histology of muscle tissue in old rats (p < 0.05). Aging significantly increased the expression of Nix and Bnip3 (p < 0.05) and reduced the Bcl-xL gene expression (p < 0.05). The expression of LC3 protein also increased with aging (p < 0.05). Therapeutic interventions, such as combined treatment (old + Nano L-a + Ex) for old animals, reduced the amount of this protein in soleus muscle (p < 0.05). The ROS values also showed a significant reduction only in the old + Nano L-a + Ex group compared to the old group. Moreover, TAC values show a significant decrease in the old and old + Ex groups in comparison to the young group. The use of arginine supplement, especially in nano form, along with swimming exercise seems to reduce the oxidative damage to the elderly muscle tissue, which has a positive effect on the structure and function of the soleus muscle. Since these interventions only had a significant effect on LC3 protein, further studies with more diverse measurement methods for autophagy are suggested.

Comparisons in muscle compound action potential parameters measured during standing, walking, and running

The muscle compound action potential (M-wave) has been used as an indicator of peripheral muscle conditions during rest or on isometric muscle contraction. The present study aimed to compare the M-wave parameters during standing, walking, and running. Seventeen young males performed four sets of repeated maximal isometric plantar flexion. Before, after, and between the repeated contraction tasks, M-waves in the soleus muscle were measured during standing, walking, and running. M-waves on walking and running were elicited at the beginning of the swing phase when the soleus muscle was not voluntarily activated. From the detected M-waves, the amplitude, area, and latency for first and second phases were calculated. Amplitudes for first and second phases were not significantly different between standing and walking/running. The area and latency for both phases during walking/running were significantly lower than those during standing (p < 0.05). Significant correlations in amplitude and area were found between standing and walking/running for the first phase (p < 0.05), but not for the second phase (p > 0.05). These results suggest that assessments of the M-wave amplitude for the first phase can be applied to walking and running in the same way as for standing in the soleus muscle.

Force loss induced by inhibiting cross-bridge cycling is mitigated in eccentric contraction

We examined whether the force loss induced by the 2,3-butanedione monoxime affects isometric and eccentric forces differently. Single skinned muscle fibers were activated at an average sarcomere length of 2.4 µm and then stretched to 3.0 µm. This trial was performed with and without 2,3-butanedione monoxime to calculate the magnitude of force loss attained at the several time points: pre-stretch phase at 2.4 µm, eccentric phase, end of eccentric contraction, and post-stretch phase at 3.0 µm. The magnitude of force loss was significantly larger in the pre-stretch phase than the other time points. Further, the mitigated force loss in the eccentric contraction was more prominent in the long condition than in the short condition. We suggest that the eccentric force is relatively preserved compared to the reference isometric force (pre-stretch) when cross-bridge cycling is inhibited, possibly because of the contribution of the elastic force produced by titin.

Soleus muscle and Achilles tendon compressive stiffness is related to knee and ankle positioning

Changes in fascicle length and tension of the soleus (SOL) muscle have been observed in humans using B-mode ultrasound to examine the knee from different angles. An alternative technique of assessing muscle and tendon stiffness is myometry, which is non-invasive, accessible, and easy to use. This study aimed to estimate the compressive stiffness of the distal SOL and Achilles tendon (AT) using myometry in various knee and ankle joint positions. Twenty-six healthy young males were recruited. The Myoton-PRO device was used to measure the compressive stiffness of the distal SOL and AT in the dominant leg. The knee was measured in two positions (90° of flexion and 0° of flexion) and the ankle joint in three positions (10° of dorsiflexion, neutral position, and 30° of plantar flexion) in random order. A three-way repeated-measures ANOVA test was performed. Significant interactions were found for structure × ankle position, structure × knee position, and structure × ankle position × knee position (p < 0.05). The AT and SOL showed significant increases in compressive stiffness with knee extension over knee flexion for all tested ankle positions (p < 0.05). Changes in stiffness relating to knee positioning were larger in the SOL than in the AT (p < 0.05). These results indicate that knee extension increases the compressive stiffness of the distal SOL and AT under various ankle joint positions, with a greater degree of change observed for the SOL. This study highlights the relevance of knee position in passive stiffness of the SOL and AT.

Triceps surae muscle force potential and force demand shift with altering stride frequency in running

While it is well recognized that the preferred stride frequency (PSF) in running closely corresponds to the metabolically optimal frequency, the underlying mechanisms are still unclear. Changes in joint kinematics when altering stride frequency will affect the muscle-tendon unit lengths and potentially the efficiency of muscles crossing these joints. Here, we investigated how fascicle kinematics and forces of the triceps surae muscle, a highly energy consuming muscle, are affected when running at different stride frequencies. Twelve runners ran on a force measuring treadmill, adopting five different frequencies (PSF; PSF±8%; PSF±15%), while we measured joint kinematics, whole-body energy expenditure, triceps surae muscle activity, and soleus (SOL; N = 10) and gastrocnemius medialis (GM; N = 12) fascicle kinematics. In addition, we used dynamic optimization to estimate SOL and GM muscle forces. We found that SOL and GM mean muscle fascicle length during stance followed an inverted U-relationship with the longest fascicle lengths occurring at PSF. Fascicle lengths were shortest at frequencies lower than PSF. In addition, average SOL force was greater at PSF-15% compared to PSF. Overall, our results suggest that reduced SOL and GM muscle fascicle lengths, associated with reduced muscle force potential, together with greater SOL force demand, contribute to the increased whole-body energy expenditure when running at lower than PSF. At higher stride frequencies, triceps surae muscle kinematics and force production were less affected suggesting that increased energy expenditure is rather related to higher cost of leg swing and greater cost of force production.

Residual force enhancement is reduced in permeabilized fiber bundles from mdm muscles

Residual force enhancement (RFE) is the increase in steady-state force after active stretch relative to the force during isometric contraction at the same final length. The mdm mutation in mice, characterized by a small deletion in N2A titin, has been proposed to prevent N2A titin-actin interactions so that active mdm muscles are more compliant than WT. This decrease in active muscle stiffness is associated with reduced RFE. We investigated RFE in permeabilized soleus (SOL) and extensor digitorum longus (EDL) fiber bundles from wild type and mdm mice. On each fiber bundle, we performed active and passive stretches from an average sarcomere length of 2.6 - 3.0 µm at a slow rate of 0.04 µm/s, as well as isometric contractions at the initial and final lengths. One-way ANOVA showed that SOL and EDL fiber bundles from mdm mice exhibited significantly lower RFE than WT (P<0.0001). This result is consistent with previous observations in single myofibrils and intact muscles. However, it contradicts the results from a previous study which appeared to show that compensatory mechanisms could restore titin force enhancement in single fibers from mdm psoas. We suggest that residual force enhancement measured previously in mdm single fibers was an artifact of the high variability in passive tension found in degenerating fibers, which begins after ∼24 days of age. The results are consistent with the hypothesis that RFE is reduced in mdm skeletal muscles due to impaired Ca2+ dependent titin-actin interactions resulting from the small deletion in N2A titin.

Bridging the muscle genome to phenome across multiple biological scales

Muscle is highly hierarchically organized, with functions shaped by genetically controlled expression of protein ensembles with different isoform profiles at the sarcomere scale. However, it remains unclear how isoform profiles shape whole-muscle performance. We compared two mouse hindlimb muscles, the slow, relatively parallel-fibered soleus and the faster, more pennate-fibered tibialis anterior (TA), across scales: from gene regulation, isoform expression and translation speed, to force-length-velocity-power for intact muscles. Expression of myosin heavy-chain (MHC) isoforms directly corresponded with contraction velocity. The fast-twitch TA with fast MHC isoforms had faster unloaded velocities (actin sliding velocity, Vactin; peak fiber velocity, Vmax) than the slow-twitch soleus. For the soleus, Vactin was biased towards Vactin for purely slow MHC I, despite this muscle's even fast and slow MHC isoform composition. Our multi-scale results clearly identified a consistent and significant dampening in fiber shortening velocities for both muscles, underscoring an indirect correlation between Vactin and fiber Vmax that may be influenced by differences in fiber architecture, along with internal loading due to both passive and active effects. These influences correlate with the increased peak force and power in the slightly more pennate TA, leading to a broader length range of near-optimal force production. Conversely, a greater force-velocity curvature in the near-parallel fibered soleus highlights the fine-tuning by molecular-scale influences including myosin heavy and light chain expression along with whole-muscle characteristics. Our results demonstrate that the individual gene, protein and whole-fiber characteristics do not directly reflect overall muscle performance but that intricate fine-tuning across scales shapes specialized muscle function.

Treadmill running prevents atrophy differently in fast- versus slow-twitch muscles in a rat model of rheumatoid arthritis

To investigate the effects of treadmill running on two different types of skeletal muscle, we established a rat model of collagen-induced arthritis (CIA). The skeletal muscles studied were the extensor digitorum longus (EDL), which is rich in fast-twitch muscle fibers, and the soleus, which is rich in slow-twitch muscle fibers. The histological and transcriptional changes in these muscles at 14 and 44 days after immunosensitization were compared between rats that were forced to exercise (CIA ex group) and free-reared CIA rats (CIA no group). Change in protein expression was examined on day 14 after a single bout of treadmill running. Treadmill running had different effects on the relative muscle weight and total and fiber cross-sectional areas in each muscle type. In the soleus, it prevented muscle atrophy. Transcriptional analysis revealed increased eukaryotic translation initiation factor 4E (Eif4e) expression on day 14 and increased Atrogin-1 and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) expression on day 44 in the soleus in the CIA ex group, suggesting an interaction between muscle type and exercise. A single bout of treadmill running increased the level of Eif4e and p70S6K and decreased that of Atrogin-1 in the soleus on day 14. Treadmill running prevented muscle atrophy in the soleus in a rat model of rheumatoid arthritis via activation of mitochondrial function, as evidenced by increased PGC-1α expression.

What did the ankle say to the knee? Estimating knee dynamics during landing - A systematic review and meta-analysis

BACKGROUND

Landing-based measures of the knee are often used to assess risk of anterior cruciate ligament (ACL) injury and inform prevention strategies. There is less understanding of the ankle's influence on knee measures during landing.

OBJECTIVE

1. Examine interactions of dynamic ankle measures alongside various subject and task characteristics on knee dynamics in vertical landing and 2. Determine whether ankle measures alone can estimate dynamic knee measures associated with ACL injury risk.

DESIGN

Systematic review and meta-analysis.

METHODS

Electronic databases Medline, EMBASE, CINAHL, Web of Science and Cochrane were screened for studies that included measurement of initial contact angles and internal joint moments of both the ankle and knee during landing in uninjured individuals.

RESULTS

28 studies were included for analysis. Using 1144 landing trials from 859 individuals, RRelief F algorithm ranked dynamic ankle measures more important than landing task and subject characteristics in estimating knee dynamics. An adaptive boosting model using four dynamic ankle measures accurately estimated knee extension (R² = 0.738, RMSE = 3.65) and knee abduction (R² = 0.999, RMSE = 0.06) at initial contact and peak knee extension moment (R² = 0.988, RMSE = 0.13) and peak knee adduction moment (R² = 1, RMSE = 0.00).

CONCLUSIONS

Dynamic ankle measures can accurately estimate initial contact angles and peak moments of the knee in vertical landing, regardless of landing task or individual subject characteristics. This study provides a theoretical basis for the importance of the ankle in ACL injury prevention.

Does Ankle Exoskeleton Assistance Impair Stability During Walking in Individuals with Cerebral Palsy?

Lower-limb exoskeletons have the potential to improve mobility in individuals with movement disabilities, such as cerebral palsy (CP). The goal of this study was to assess the impact of plantar-flexor assistance from an untethered ankle exoskeleton on dynamic stability during unperturbed and perturbed walking in individuals with CP. Seven participants with CP (Gross Motor Function Classification System levels I-III, ages 6-31 years) completed a treadmill walking protocol under their normal walking condition and while wearing an ankle exoskeleton that provided adaptive plantar-flexor assistance. Pseudo-randomized treadmill perturbations were delivered during stance phase by accelerating one side of a split-belt treadmill. Treadmill perturbations resulted in a significant decrease in anteroposterior minimum margin-of-stability (- 32.1%, p < 0.001), and a significant increase in contralateral limb step length (8.1%, p = 0.005), integrated soleus activity during unassisted walking (23.4%, p = 0.02), and peak biological ankle moment (9.6%, p = 0.03) during stance phase. Plantar-flexor assistance did not significantly alter margin-of-stability, step length, soleus activity, or ankle moments during both unperturbed and perturbed walking. These results indicate that adaptive plantar-flexor assistance from an untethered ankle exoskeleton does not significantly alter dynamic stability maintenance during unperturbed and perturbed walking for individuals with CP, supporting future research in real-world environments.

Highly Selective Partial Neurectomy for Lower-Extremity Spasticity: 2-Dimensional Operative Video

Spasticity is a common debilitating condition after central nervous system injury. The principal therapies-sedating antispasticity medications and focal therapies (eg, botulinum toxin)-may not provide sufficient reduction in tone, have intolerable side effects, and lose efficacy over time. Selective neurectomy is a surgical option for durable reduction in tone.1-6 Although commonly performed internationally, neurectomies are used less often within the USA. We present the surgical case of a 23-yr-old woman with cerebral palsy and severe spastic diplegia. Medications, injections, and tendon surgeries had failed to relieve her spastic lower extremities. She presented with crouch gait, adductor scissoring, and bilateral equinovarus. She had previously benefitted significantly from bilateral hamstring and obturator neurectomies, with improvement in mobility. She desired tibial neurectomies for her bilateral equinovarus, which impacted her gait substantially. To reduce unwanted plantarflexion and internal rotation tone, selective neurectomies of the motor nerves to the medial and lateral gastrocnemius, soleus, and posterior tibialis muscles were performed. The surgical video details a technique for longitudinally opening the epineurium, separating small nerves into their individual fascicles, and subdividing submillimeter fascicles by approximately 50% to 60% of the fascicle cross-sectional area. She reported significant improvement in gait, her modified Ashworth score dropped from 3 to 0, and her motor power remained unchanged, which is equivalent or better than most published results.2,7 Neurectomies are a highly effective procedure for focal extremity spasticity. Additional clinical series or trials would help establish the appropriate indications and durability and quantify the risks and benefits. The patient consented to treatment and publication.

Characterization of neutral sphingomyelinase activity and isoform expression in rodent skeletal muscle mitochondria

Skeletal muscle is composed of fiber types that differ in mitochondrial content, antioxidant capacity, and susceptibility to apoptosis. Ceramides have been linked to oxidative stress-mediated apoptotic intracellular signalling and the enzyme neutral sphingomyelinase (nSMase) is, in part, responsible for generating these ceramides through the hydrolysis of sphingomyelin. Despite the role of ceramides in mediating apoptosis, there is a gap in the literature regarding nSMase in skeletal muscle mitochondria. This study aimed to characterize total nSMase activity and individual isoform expression in isolated subsarcolemmal (SS) mitochondria from soleus, diaphragm, plantaris, and extensor digitorum longus (EDL). Total nSMase activity did not differ between muscle types. nSMase2 content was detectable in all muscles and higher in EDL, soleus, and plantaris compared to diaphragm whereas nSMase3 was undetectable in all muscles. Finally, total nSMase activity positively correlated to nSMase2 protein content in soleus but not the other muscles. These findings suggest that nSMase associated with SS mitochondria may play a role in intracellular signalling processes involving ceramides in skeletal muscle and nSMase2 may be the key isoform, specifically in slow twitch muscle like soleus. Further studies are needed to fully elucidate the specific contribution of nSMase, along with the role of the various isoforms and mitochondrial subpopulation in generating mitochondrial ceramides in skeletal muscle, and its potential effects on mediating apoptosis.

Posture dependent ankle and foot muscle responses evoked by Achilles' tendon vibration

To investigate the link between the triceps surae and the intrinsic muscles of the foot, often underestimated in posture maintenance, we asked how Achilles' tendon vibration modulates the EMG activity of the soleus and flexor digitorum brevis (FDB) muscles during different postural tasks: sitting, standing and forward leaning. Young healthy participants (n = 19, age = 24 ± 7.4 years) stood for 60 s in three visually controlled postures, while vibration (1.5-1.8 mm, 80 Hz) was bilaterally applied over the Achilles' tendon during the middle 20 s. Center of Pressure (CoP) and EMG activity of the soleus and FDB muscle were summarized in 5 s epochs and compared across time (before, during and after vibration) and postural tasks. Achilles' tendon vibration shifted the CoP position forward in sitting and backward in standing and leaning and increased the root mean square of the CoP velocity to a greater extent in standing and leaning compared to sitting. Soleus and FDB EMG amplitude also increased in response to vibration. These responses were posture dependent, being greater in standing (soleus: 57 %, FDB: 67 % relative to pre-vibration) compared to sitting (soleus: 36 %, FDB: 27 % relative to pre-vibration) and leaning (soleus: 26 %, FDB: 8% relative to pre-vibration). After vibration offset, both soleus and FDB showed sustained activation across all three postures. Results highlight the presence of Ia afferent projections from the soleus to the α motor neurons of the FDB muscle triggered by Achilles' tendon vibration. This link is posture dependent serving a functional role in standing and forward leaning in the presence of externally applied perturbations.

Control of rat muscle nitrate levels after perturbation of steady state dietary nitrate intake

The roles of nitrate and nitrite ions as nitric oxide (NO) sources in mammals, complementing NOS enzymes, have recently been the focus of much research. We previously reported that rat skeletal muscle serves as a nitrate reservoir, with the amount of stored nitrate being highly dependent on dietary nitrate availability, as well as its synthesis by NOS1 enzymes and its subsequent utilization. We showed that at conditions of increased NO need, this nitrate reservoir is used in situ to generate nitrite and NO, at least in part via the nitrate reductase activity of xanthine oxidoreductase (XOR). We now further investigate the dynamics of nitrate/nitrite fluxes in rat skeletal muscle after first increasing nitrate levels in drinking water and then returning to the original intake level. Nitrate/nitrite levels were analyzed in liver, blood and several skeletal muscle samples, and expression of proteins involved in nitrate metabolism and transport were also measured. Increased nitrate supply elevated nitrate and nitrite levels in all measured tissues. Surprisingly, after high nitrate diet termination, levels of both ions in liver and all muscle samples first declined to lower levels than the original baseline. During the course of the overall experiment there was a gradual increase of XOR expression in muscle tissue, which likely led to enhanced nitrate to nitrite reduction. We also noted differences in basal levels of nitrate in the different types of muscles. These findings suggest complex control of muscle nitrate levels, perhaps with multiple processes to preserve its intracellular levels.

Relationship between resting medial gastrocnemius stiffness and drop jump performance

Although the influence of the series elastic element of the muscle-tendon unit on jump performance has been investigated, the corresponding effect of the parallel elastic element remains unclear. This study examined the relationship between the resting calf muscle stiffness and drop jump performance. Twenty-four healthy men participated in this study. The shear moduli of the medial gastrocnemius and the soleus were measured at rest as an index of muscle stiffness using ultrasound shear wave elastography. The participants performed drop jumps from a 15 cm high box. The Spearman rank correlation coefficient was used to examine the relationships between shear moduli of the muscles and drop jump performance. The medial gastrocnemius shear modulus showed a significant correlation with the drop jump index (jump height/contact time) (r = 0.414, P = 0.044) and jump height (r = 0.411, P = 0.046), but not with contact time (P > 0.05). The soleus shear modulus did not correlate with these jump parameters (P > 0.05). These results suggest that the resting medial gastrocnemius stiffness can be considered as one of the factors that influence drop jump performance. Therefore, increase in resting muscle stiffness should enhance explosive athletic performance in training regimens.

Comparing cortico-motor hotspot identification methods in the lower extremities post-stroke: MEP amplitude vs. latency

Transcranial magnetic stimulation (TMS) is a technique used to probe and measure cortico-motor responses of the nervous system. However, lower extremity (LE) specific methodology has been slow to develop. In this retrospective analysis, we investigated what motor evoked potential metric, amplitude (MEP_amp) or latency (MEP_lat), best distinguished the motor-cortical target, i.e. hotspot, of the tibialis anterior and soleus post-stroke. Twenty-three participants with stroke were included in this investigation. Neuronavigation was used to map hotspots, derived via MEP_amp and MEP_lat, over a 3cm × 5cm grid. Distances between points with the greatest response within a session and between days were compared. Both criterion, amplitude and latency, provided poor identification of locations between trials within a session, and between multiple visits. Identified hotspots were similar only 15 % and 8% of the time between two assessments within the same session, for amplitude and latency respectively. However, MEP_amp was more consistent in identifying hotspots, evidenced by locations being less spatially distant from each other (Amplitude: 1.4 cm (SD 0.10) Latency: 1.7 (SD 1.04), P = 0.008) within a session and between days (Amplitude: 1.3 cm (SD 0.95), Latency 1.9 cm (SD 1.14), P = 0.004). While more work is needed to develop LE specific methodology for TMS, especially as it applies to investigating gait impairments, MEP_amp appears to be a more consistent criterion for hotspot identification when compared to MEP_lat. It is recommended that future works continue to use MEP_amp when identifying tibialis anterior and soleus hotspots using neuronavigation.

Differences in soleus H-reflex to M-wave ratio between obese and non-obese individuals

BACKGROUND

The aim of this cross-sectional comparative study was to investigate potential differences in soleus H-reflex to M-wave ratio between obese and non-obese individuals under different postural constraints.

METHODS

A total of 23 obese individuals and 23 matched non-obese controls took part in the study. The maximal soleus H-reflex to M-wave ratio (peak-to-peak amplitude) was quantified in three different conditions: seated at rest, seated with a light contraction of the plantar flexors and during bipedal quiet standing.

FINDINGS

Soleus H-reflex to M-wave ratio was significantly higher in obese than in non-obese individuals in all the experimental conditions (p < 0.01).

INTERPRETATION

Obese individuals showed larger normalized soleus H-reflex responses than their non-obese counterparts, probably as a reflect of long-term neurophysiological adaptations to excessive body weight and postural demands.

Soleus strain: an underestimated injury?

The medial gastrocnemius is the most commonly injured muscle in the calf, and while traumatic lesions of the medial gastrocnemius are frequently described in the medical literature and clinical and sonographic diagnoses are easy, those of the soleus are less easily recognisable clinically or by sonography. We present a case of traumatic lesions of the medial gastrocnemius, diagnosed clinically and with ultrasound, while MRI also detected two lesions of the soleus. The case presented and the review of the literature highlight how clinical and ultrasound examinations can lead to misunderstandings about traumatic injury to the soleus.

H-reflex and M-wave responses after voluntary and electrically evoked muscle cramping

PURPOSE

Despite the widespread occurrence of muscle cramps, their underlying neurophysiological mechanisms remain unknown. To better understand the etiology of muscle cramps, this study investigated acute effects of muscle cramping induced by maximal voluntary isometric contractions (MVIC) and neuromuscular electrical stimulation (NMES) on the amplitude of Hoffmann reflexes (H-reflex) and compound muscle action potentials (M-wave).

METHODS

Healthy men (n = 14) and women (n = 3) participated in two identical sessions separated by 7 days. Calf muscle cramping was induced by performing MVIC of the plantar flexors in a prone position followed by 2.5-s NMES over the plantar flexors with increasing frequency and intensity. H-reflexes and M-waves evoked by tibial nerve stimulation in gastrocnemius medialis (GM) and soleus were recorded at baseline, and after MVIC-induced cramps and the NMES protocol.

RESULTS

Six participants cramped after MVIC, and H-reflex amplitude decreased in GM and soleus in Session 1 (- 33 ± 32%, - 34 ± 33%, p = 0.031) with a similar trend in Session 2 (5 cramped, p = 0.063), whereas the maximum M-wave was unchanged. After NMES, 11 (Session 1) and 9 (Session 2) participants cramped. H-reflex and M-wave recruitment curves shifted to the left in both sessions and muscles after NMES independent of cramping (p ≤ 0.001).

CONCLUSION

Changes in H-reflexes after a muscle cramp induced by MVIC and NMES were inconsistent. While MVIC-induced muscle cramps reduced H-reflex amplitude, muscle stretch to end cramping was a potential contributing factor. By contrast, NMES may potentiate H-reflexes and obscure cramp-related changes. Thus, the challenge for future studies is to separate the neural consequences of cramping from methodology-based effects.

The twisted structure of the fetal calcaneal tendon is already visible in the second trimester

INTRODUCTION

The progress in morphological science results from the greater possibilities of intra-pubic diagnosis and treatment of congenital disabilities, including the motor system. However, the structure and macroscopic development of the calcaneal tendon have not been investigated in detail. Studies on the adult calcaneal tendon showed that the calcaneal tendon is composed of twisted subtendons. This study aimed to investigate the internal structure of the fetal calcaneal tendon in the second trimester.

MATERIALS AND METHODS

Thirty-six fetuses fixed in 10% formaldehyde were dissected using the layer-by-layer method and a surgical microscope.

RESULTS

The twisted structure of the calcaneal tendon was revealed in all specimens. The posterior layer of the calcaneal tendon is formed by the subtendon from the medial head of the gastrocnemius muscle. In contrast, the anterior layer is formed by the subtendon from the lateral head of the gastrocnemius muscle. The subtendon from the soleus muscle constitutes the anteromedial outline of the calcaneal tendon. The lateral outline of the calcaneal tendon is formed by the subtendon originating from the medial head of the gastrocnemius muscle. In contrast, the medial outline is formed by the subtendon from the soleus muscle. In most of the examined limbs, the plantaris tendon attached to the tuber calcanei was not directly connected to the calcaneal tendon.

CONCLUSIONS

The twisted structure of the subtendons of the fetal calcaneal tendon is already visible in the second trimester and is similar to that seen in adults.

The stretch-shortening cycle effect is prominent in the inhibited force state

It has been suggested that residual force enhancement (RFE) contributes to the work enhancement observed in stretch-shortening cycles (SSC). Based on recent findings that RFE was preserved in the reduced force state, one may speculate that the SSC effect may be preserved in the reduced force state as well. The purpose of this study was to examine the magnitude of the SSC effect in inhibited skeletal muscle force states. Normal and inhibited force conditions were analyzed using skinned rabbit soleus fibres (N = 18). The inhibited force condition was achieved by adding 2,3-Butanedione monoxime into the activating solution. For both conditions, a SSC test and a pure shortening test were performed. In the SSC tests, fibres were activated at an average sarcomere length of 2.4 μm, and then stretched to 3.0 μm. Immediately after the end of the stretch, fibres were shortened to 2.4 μm. In the pure shortening tests, fibres were activated at an average sarcomere length of 3.0 μm and then shortened to 2.4 μm. The relative increase in mechanical work in the shortening phase of the SSC compared to the pure shortening condition was defined as the SSC effect index, and the magnitude of the SSC effect was compared between the normal and the inhibited force condition. The SSC effect was greater in the inhibited compared to the normal force condition (p < 0.001). We conclude that the SSC effect is at least in part preserved in the reduced force state.

MyoSight-semi-automated image analysis of skeletal muscle cross sections

BACKGROUND

Manual analysis of cross-sectional area, fiber-type distribution, and total and centralized nuclei in skeletal muscle cross sections is tedious and time consuming, necessitating an accurate, automated method of analysis. While several excellent programs are available, our analyses of skeletal muscle disease models suggest the need for additional features and flexibility to adequately describe disease pathology. We introduce a new semi-automated analysis program, MyoSight, which is designed to facilitate image analysis of skeletal muscle cross sections and provide additional flexibility in the analyses.

RESULTS

We describe staining and imaging methods that generate high-quality images of immunofluorescent-labelled cross sections from mouse skeletal muscle. Using these methods, we can analyze up to 5 different fluorophores in a single image, allowing simultaneous analyses of perinuclei, central nuclei, fiber size, and fiber-type distribution. MyoSight displays high reproducibility among users, and the data generated are in close agreement with data obtained from manual analyses of cross-sectional area (CSA), fiber number, fiber-type distribution, and number and localization of myonuclei. Furthermore, MyoSight clearly delineates changes in these parameters in muscle sections from a mouse model of Duchenne muscular dystrophy (mdx).

CONCLUSIONS

MyoSight is a new program based on an algorithm that can be optimized by the user to obtain highly accurate fiber size, fiber-type identification, and perinuclei and central nuclei per fiber measurements. MyoSight combines features available separately in other programs, is user friendly, and provides visual outputs that allow the user to confirm the accuracy of the analyses and correct any inaccuracies. We present MyoSight as a new program to facilitate the analyses of fiber type and CSA changes arising from injury, disease, exercise, and therapeutic interventions.

Differences in stretch-shortening cycle and residual force enhancement between muscles

It has been suggested that cross bridge kinetics and residual force enhancement (RFE) affect force in the stretch-shortening cycle (SSC). Because cross bridge kinetics and titin isoforms, which are thought to be related to RFE, differ between muscles, the SSC effect may be also muscle-dependent. Thus, we compared the SSC effect between psoas and soleus muscles, which have a distinct fiber type distribution and different titin isoforms. Four tests (SSC, SSC control, RFE, RFE control) were conducted using isolated, skinned fibers of psoas and soleus. In the SSC tests, fibers were activated at an average sarcomere length of 2.4 μm, stretched to 3.0 μm, and shortened to 2.4 μm. In the SSC control tests, fibers were activated at an average sarcomere length of 3.0 μm and then shortened to 2.4 μm. The relative increase in mechanical work obtained during shortening between tests was defined as the SSC effect. In the RFE tests, fibers were activated at an average sarcomere length of 2.4 μm and then stretched to 3.0 μm, while the RFE control tests consisted of an isometric contraction at 3.0 μm. The difference in steady-state isometric force between tests was defined as RFE. The SSC effect was greater in soleus than in psoas, while the RFE was the same for both muscles. Since the SSC effect was greater in soleus, while the RFE was the same, the observed greater SSC effect is probably not directly caused by RFE, but may be related to differences in cross bridge kinetics.

[Effects of electroacupuncture combined with passive stretch exercise on disused muscle atrophy and expression of skeletal troponin in mice]

OBJECTIVE

To observe the efficacy of electroacupuncture (EA) plus passive stretch exercise in the treatment of disused atrophy of gastrocnemius and soleus muscles in mice.

METHODS

Fifty C57BL/6 mice were randomly and equally divided into 5 groups: blank control, model, passive stretch exercise (exercise), EA and EA+exercise groups. The muscular atrophy model was established by fixing the gastrocnemius and soleus muscles with plaster immobilization (by putting the right leg into a plastic vial and then twining the vial with medical plaster bandage from the ankle upwards to the thigh and groin to maintain the knee-joint flexion and ankle joint plantar flexion for 7 days). EA (2 Hz/100 Hz, 1 mA)was applied to bilateral "Zusanli"(ST36) for 10 min, once a day for 4 weeks. For mice with the passive exercise, the plastic vial was removed first, followed by pulling out the hindleg to seize the toes to stretch them until the right hindleg is fully extended, then, pushed the leg towards the body. The procedures were repeated once again and again for 10 min. The exercise was conducted once daily, for 4 weeks. The cross-sectional area of fast and slow muscle fibers of the soleus and gastrocnemius was measured under electronic microscope after ATPase histochemical stain and the expression of slow skeletal muscle troponin (TNNI1) and fast skeletal muscle troponin (TNNI2) in the soleus and gastrocnemius was detected by Western blot.

RESULTS

Compared with the blank control group, the cross-sectional areas of the fast and slow muscle fibers of the soleus and gastrocnemius muscles were significantly decreased in the model group (P<0.05, P<0.01). Following the interventions, the cross-sectional areas of the fast and slow muscle fibers of soleus muscle in the EA+exercise group, and those of the fast and slow muscle fibers of the gastrocnemius muscle in the EA and EA+exercise groups, and the expression levels of TNNI1 and TNNI2 proteins in the gastrocnemius muscle of the EA+exercise group were significantly increased in comparison with the model group (P<0.05, P<0.01).

CONCLUSION

EA combined with passive stretch exercise can promote the recovery of the soleus and gastrocnemius muscles in disused muscle atrophy mice, which may be related to its effect in up-regulating the expression of TNNI1 and TNNI2 proteins.

Characterizing the effect of low intensity transcranial magnetic stimulation on the soleus H-reflex at rest

Modulation of a Hoffmann (H)-reflex following transcranial magnetic stimulation (TMS) has been used to assess the nature of signals transmitted from cortical centers to lower motor neurons. Further characterizing the recruitment and time-course of the TMS-induced effect onto the soleus H-reflex adds to the discussion of these pathways and may improve its utility in clinical studies. In 10 healthy controls, TMS was used to condition the soleus H-reflex using TMS intensities from 65 to 110% of the resting motor threshold (RMT). Early facilitation [- 5 to - 3 ms condition-test (C-T) interval] was evident when TMS was 110% of RMT (P < 0.05). By comparison, late facilitation (+ 10 to + 20 ms C-T interval) was several times larger and observed over a wider range of TMS intensities, including 65-110% of RMT. The early inhibition (- 3 to - 1 ms C-T interval) had a low TMS threshold and was elicited over a wide range of intensity from 65% to 95% of RMT (all P < 0.05). A second inhibitory phase was seen ~ 4 ms later (+ 1 to + 4 ms C-T intervals) and was only observed for a TMS intensity of 95% of RMT (P < 0.05). The present findings reaffirm that subthreshold TMS strongly modulates soleus motor neurons and demonstrates that distinct pathways can be selectively probed at discrete C-T intervals when using specific TMS intensities.

Modulation of spinal excitability following neuromuscular electrical stimulation superimposed to voluntary contraction

Purpose

Neuromuscular electrical stimulation (NMES) superimposed on voluntary muscle contraction has been recently shown as an innovative training modality within sport and rehabilitation, but its effects on the neuromuscular system are still unclear. The aim of this study was to investigate acute responses in spinal excitability, as measured by the Hoffmann (H) reflex, and in maximal voluntary contraction (MVIC) following NMES superimposed to voluntary isometric contractions (NMES + ISO) compared to passive NMES only and to voluntary isometric contractions only (ISO).

Method

Fifteen young adults were required to maintain an ankle plantar-flexor torque of 20% MVC for 20 repetitions during each experimental condition (NMES + ISO, NMES and ISO). Surface electromyography was used to record peak-to-peak H-reflex and motor waves following percutaneous stimulation of the posterior tibial nerve in the dominant limb. An isokinetic dynamometer was used to assess maximal voluntary contraction output of the ankle plantar flexor muscles.

Results

H-reflex amplitude was increased by 4.5% after the NMES + ISO condition (p < 0.05), while passive NMES and ISO conditions showed a decrease by 7.8% (p < 0.05) and no change in reflex responses, respectively. There was no change in amplitude of maximal motor wave and in MVIC torque during each experimental condition.

Conclusion

The reported facilitation of spinal excitability following NMES + ISO could be due to a combination of greater motor neuronal and corticospinal excitability, thus suggesting that NMES superimposed onto isometric voluntary contractions may provide a more effective neuromuscular stimulus and, hence, training modality compared to NMES alone.

Modulation of spinal cord excitability following remote limb ischemic preconditioning in healthy young men

Remote limb ischemic preconditioning (RIPC) has shown to improve dynamic postural control in humans. However, studies on the underlying adaptations of spinal cord networks have never been performed. The present work addresses this issue by investigating parameters from the soleus H-reflex recruitment curve (RC), presynaptic mechanisms of reflex modulation (presynaptic inhibition-PSI, and post activation depression-PAD), and the excursion of the center of pressure (CP) recorded during 1 min in upright stance over a compliant surface. A sham ischemic protocol (partial obstruction of blood flow) was applied to the contralateral thigh along four consecutive days. The same procedure was repeated with full obstruction (RIPC) three days after ending the sham protocol. Data were collected before and after both sham and RIPC protocols. The follow-up data were collected five days after the last ischemic intervention. Significant reduction was detected for both the fast oscillations of the CP (higher frequency components) and the parameter estimated from the RC corresponding to the high amplitude H-reflexes (p < 0.05). Even though the magnitude of effects was similar, it was washed out within three days after sham, but persisted for at least five days after RIPC. No significant differences were found for PSI and PAD levels across conditions. These findings indicate that RIPC leads to enduring changes in spinal cord excitability for the latest reflexively recruited motoneurons, along with improvement in balance control. However, these adaptations were not mediated by the presynaptic mechanisms currently assessed.

Adaptive Ankle Resistance from a Wearable Robotic Device to Improve Muscle Recruitment in Cerebral Palsy

Individuals with cerebral palsy can have weak and poorly coordinated ankle plantar flexor muscles that contribute to inefficient walking patterns. Previous studies attempting to improve plantar flexor function have had inconsistent effects on mobility, likely due to a lack of task-specificity. The goal of this study was to develop, validate, and test the feasibility and neuromuscular response of a novel wearable adaptive resistance platform to increase activity of the plantar flexors during the propulsive phase of gait. We recruited eight individuals with spastic cerebral palsy to walk with adaptive plantar flexor resistance provided from an untethered exoskeleton. The resistance system and protocol was safe and feasible for all of our participants. Controller validation demonstrated our ability to provide resistance that proportionally- and instantaneously-adapted to the biological ankle moment (R = 0.92 ± 0.04). Following acclimation to resistance (0.16 ± 0.02 Nm/kg), more-affected limbs exhibited a 45 ± 35% increase in plantar flexor activity (p = 0.02), a 26 ± 24% decrease in dorsiflexor activity (p < 0.05), and a 46 ± 25% decrease in co-contraction (tibialis anterior and soleus) (p = 0.02) during the stance phase. This adaptive resistance system warrants further investigation for use in a longitudinal intervention study.

The impact of a high-fat diet in mice is dependent on duration and age, and differs between muscles

Prolonged high-fat diets (HFDs) can cause intramyocellular lipid (IMCL) accumulation that may negatively affect muscle function. We investigated the duration of a HFD required to instigate these changes, and whether the effects are muscle specific and aggravated in older age. Muscle morphology was determined in the soleus, extensor digitorum longus (EDL) and diaphragm muscles of female CD-1 mice from 5 groups: young fed a HFD for 8 weeks (YS-HFD, n=16), young fed a HFD for 16 weeks (YL-HFD, n=28) and young control (Y-Con, n=28). The young animals were 20 weeks old at the end of the experiment. Old (70 weeks) female CD-1 mice received either a normal diet (O-Con, n=30) or a HFD for 9 weeks (OS-HFD, n=30). Body mass, body mass index and intramyocellular lipid (IMCL) content increased in OS-HFD (P≤0.003). In the young mice, this increase was seen in YL-HFD and not YS-HFD (P≤0.006). The soleus and diaphragm fibre cross-sectional area (FCSA) in YL-HFD was larger than that in Y-Con (P≤0.004) while OS-HFD had a larger soleus FCSA compared with that of O-Con after only 9 weeks on a HFD (P<0.001). The FCSA of the EDL muscle did not differ significantly between groups. The oxidative capacity of fibres increased in young mice only, irrespective of HFD duration (P<0.001). High-fat diet-induced morphological changes occurred earlier in the old animals than in the young, and adaptations to HFD were muscle specific, with the EDL being least responsive.

Novel human models for elucidating mechanisms of rate-sensitive H-reflex depression

BACKGROUND

This study used novel human neurophysiologic models to investigate whether the mechanism of rate-sensitive H-reflex depression lies in the pre-synaptic or post-synaptic locus in humans. We hypothesized that pre-synaptic inhibition would suppress Ia afferents and H-reflexes without suppressing alpha motor neurons or motor evoked potentials (MEPs). In contrast, post-synaptic inhibition would suppress alpha motor neurons, thereby reducing H-reflexes and MEPs.

METHODS

We recruited 23 healthy adults with typical rate-sensitive H-reflex depression, 2 participants with acute sensory-impaired spinal cord injury (SCI) (to rule out influence of sensory stimulation on supra-spinal excitability), and an atypical cohort of 5 healthy adults without rate-sensitive depression. After a single electrical stimulation to the tibial nerve, we administered either a testing H-reflex or a testing MEP at 50-5000 ms intervals.

RESULTS

Testing MEPs were not diminished in healthy subjects with or without typical rate-sensitive H-reflex depression, or in subjects with sensory-impaired SCI. MEP responses were similar in healthy subjects with versus without rate-sensitive H-reflex depression.

CONCLUSIONS

Results from these novel in vivo human models support a pre-synaptic locus of rate-sensitive H-reflex depression for the first time in humans. Spinal reflex excitability can be modulated separately from descending corticospinal influence. Each represents a potential target for neuromodulatory intervention.

Proteomic analysis revealed different responses to hypergravity of soleus and extensor digitorum longus muscles in mice

Investigating protein abundance profiles is important to understand the differences in the slow and fast skeletal muscle characteristics. The profiles in soleus (Sol) and extensor digitorum longus (EDL) muscles in mice exposed to 1 g or 3 g for 28 d were compared. The biological implications of the profiles revealed that hypergravity exposure activated a larger number of pathways involved in protein synthesis in Sol. In contrast, the inactivation of signalling pathways involved in oxidative phosphorylation were conspicuous in EDL. These results suggested that the reactivity of molecular pathways in Sol and EDL differed. Additionally, the levels of spermidine synthase and spermidine, an important polyamine for cell growth, increased in both muscles following hypergravity exposure, whereas the level of spermine oxidase (SMOX) increased in EDL alone. The SMOX level was negatively correlated with spermine content, which is involved in muscle atrophy, and was higher in EDL than Sol, even in the 1 g group. These results indicated that the contribution of SMOX to the regulation of spermidine and spermine contents in Sol and EDL differed. However, contrary to expectations, the difference in the SMOX level did not have a significant impact on the growth of these muscles following hypergravity exposure. SIGNIFICANCE: The skeletal muscle-specific protein abundance profiles result in differences in the characteristics of slow and fast skeletal muscles. We investigated differences in the profiles in mouse slow-twitch Sol and fast-twitch EDL muscles following 28-d of 1 g and 3 g exposure by LC-MS/MS analysis and label-free quantitation. A two-step solubilisation of the skeletal muscle proteins increased the coverage of proteins identified by LC-MS/MS analysis. Additionally, this method reduced the complexity of samples more easily than protein or peptide fractionation by SDS-PAGE and offline HPLC while maintaining the high operability of samples and was reproducible. A larger number of hypergravity-responsive proteins as well as a prominent increase in the wet weights was observed in Sol than EDL muscles. The biological implications of the difference in the protein abundance profiles in 1 g and 3 g groups revealed that the reactivity of each molecular pathway in Sol and EDL muscles to hypergravity exposure differed significantly. In addition, we found that the biosynthetic and interconversion pathway of polyamines, essential factors for cell growth and survival in mammals, was responsive to hypergravity exposure; spermidine and spermine contents in Sol and EDL muscles were regulated by different mechanisms even in the 1 g group. However, our results indicated that the difference in the mechanism regulating polyamine contents is unlikely to have a significant effect on the differences in Sol and EDL muscle growth following hypergravity exposure.

Non-uniformity of displacement and strain within the Achilles tendon is affected by joint angle configuration and differential muscle loading

Although the Achilles tendon (AT) has been studied for more than a century, a complete understanding of the mechanical and functional consequences of AT structural organization is currently lacking. The aim of this study was to assess how joint angle configuration affects subtendon displacement and strain of soleus (SOL) and lateral gastrocnemius (LG) muscles. Knots sutured onto SOL and LG subtendons of 12 Wistar rats, were videotaped to quantify displacements and the ankle torque was assessed for different isometric activation conditions (i.e., individual and simultaneous) of the triceps surae muscles. Changing ankle and knee joint angle affected the magnitude of displacement, relative displacement and strain of both SOL and LG subtendons. SOL subtendon behavior was not only affected by changes in ankle angle, but also by changes in knee angle. Displacement of SOL subtendon decreased (28-49%), but strain increased in response to knee extension. Independent of joint angle configuration, stimulation of any combination of the muscles typically resulted in displacements and strains of LG and SOL subtendons. Typically SOL displaced more but LG displaced more when stimulated at longer muscle lengths. Our results demonstrate that the distinct subtendons of the Achilles tendon can move and deform differently, but are not fully independent. Within the AT, there appears to be a precarious balance between sliding allowance and mechanical connectivity between subtendons.

Data on dysfunctional muscle contraction and genes contractile expression associated with chlorpyrifos exposure in slow twitch skeletal muscle

Chlorpyrifos (CPF) is a toxic organophosphate commonly used worldwide. Its residues are being detected in different environmental matrixes and hence in the food chain. Repeated CPF exposure might pose health risk for the general population on long term. This data article contains the data of contractility impairment further to dietary exposure to CPF on a hind limb skeletal muscle; soleus, a typical slow twitch skeletal muscle. Thirty adult male rats Sprague Dawley are divided into three groups receiving the following daily diet for 6 weeks: Group 1 (vehicle), Group 2: CPF1 (CPF 1mg/kg/day) and Group 3: CPF5 (CPF 5 mg/kg/day). Soleus twitch tension and fatigability index are determined at the end of the treatment. The activity of acteylcholinesterase enzyme is assessed in the tissues homogenate. Additionally, we examined the expression levels of ryanodine type 1 receptor (RyR1), ATPase Sarcoplasmic/Endoplasmic Reticulum Ca²⁺ Transporting 1 (Atp2a1), ATPase Sarcoplasmic/Endoplasmic Reticulum Ca²⁺ Transporting 2 (Atp2a2) and nicotinic acetylcholine receptor (nAChR) in CPF-exposed skeletal muscle tissue using quantitative real time polymerase chain reaction. CPF exposure at two different doses induced an increase in twitch contraction in soleus muscle along with an increase in fatigability index. These increases are accompanied by low level of acetylcholinesterase enzyme activity as well as modification in genes level expression of nAChR, RyR1, Atp2a1 and Atp2a2 involved in contractility.

Habitual foot strike pattern does not affect simulated triceps surae muscle metabolic energy consumption during running

Foot strike pattern affects ankle joint work and triceps surae muscle-tendon dynamics during running. Whether these changes in muscle-tendon dynamics also affect triceps surae muscle energy consumption is still unknown. In addition, as the triceps surae muscle accounts for a substantial amount of the whole-body metabolic energy consumption, changes in triceps surae energy consumption may affect whole-body metabolic energy consumption. However, direct measurements of muscle metabolic energy consumption during dynamic movements is difficult. Model-based approaches can be used to estimate individual muscle and whole-body metabolic energy consumption based on Hill type muscle models. In this study, we use an integrated experimental and dynamic optimization approach to compute muscle states (muscle forces, lengths, velocities, excitations and activations) of 10 habitual midfoot/forefoot striking and nine habitual rearfoot striking runners while running at 10 and 14 km h^-1 The Achilles tendon stiffness of the musculoskeletal model was adapted to fit experimental ultrasound data of the gastrocnemius medialis muscle during ground contact. Next, we calculated triceps surae muscle and whole-body metabolic energy consumption using four different metabolic energy models provided in the literature. Neither triceps surae metabolic energy consumption (P>0.35) nor whole-body metabolic energy consumption (P>0.14) was different between foot strike patterns, regardless of the energy model used or running speed tested. Our results provide new evidence that midfoot/forefoot and rearfoot strike patterns are metabolically equivalent.

Differences in muscle attachment proportion within the most common location of medial tibial stress syndrome in vivo

INTRODUCTION

The medial tibial stress syndrome is one of the most common causes of running-related injuries. The primary study objective was to observe the attachment proportion of flexor digitorum longus and soleus, at the most common location of medial tibial stress syndrome, using ultrasonography, on a large cohort of young males and females to evaluate for gender-based anatomical differences. The secondary objective of this study was to investigate the relationship between the anatomical features and medial tibial stress syndrome.

METHODS

In this study, we observed whether or not flexor digitorum longus and/or soleus attached at the middle and distal thirds of the medial margin of the tibia (most common location of medial tibial stress syndrome) using ultrasonography. History of medial tibial stress syndrome was defined by inquiries.

RESULTS

The Chi² tests showed that the attachment proportion of the soleus in female participants was significantly higher than that observed in male participants. In addition, Chi² testing showed that there were no significant differences between attachment proportion of soleus of legs with history of medial tibial stress syndrome and legs without history of medial tibial stress syndrome, in both male and female participants.

CONCLUSIONS

These results suggested that the anatomical features of flexor digitorum longus might be involved in medial tibial stress syndrome development, whereas the anatomical features of the soleus might not be involved in medial tibial stress syndrome development.

LEVEL OF EVIDENCE

III, cross-sectional study.

Objective assessment of stiffness in the gastrocnemius muscle in patients with symptomatic Achilles tendons

Objectives

To establish quantitative values of stiffness for the gastrocnemius muscle in patients diagnosed with Achilles tendinopathy.

Design

Case–control study with a single (cross sectional) time point of patients diagnosed with unilateral Achilles tendinopathy and an asymptomatic group.

Methods

Sample of 50 participants: 25 participants with symptomatic unilateral Achilles tendinopathy (symptomatic group) with the asymptomatic side of this group used as a control (control group 1). A third group of 25 participants with asymptomatic Achilles tendons (control group 2). Mechanical stiffness parameters, including natural oscillation frequency (F), dynamic stiffness (S), mechanical stress relaxation time (R), logarithmic decrement (D) and creep (C), were assessed using the MyotonPRO. Measurements at the medial and lateral gastrocnemii were taken as non-weight bearing (NWB) and weight bearing (WB).

Results

Significant (p<0.05) differences were observed between the mean symptomatic group and control groups 1 and 2 in the WB and NWB conditions for natural oscillation frequency (F), dynamic stiffness (S), and creep (C). There was a significant reduction in S WB and conversely a significant increase in S of the symptomatic group NWB. Creep (C) and mechanical stress relaxation time (R) were significantly (p<0.05) less in the symptomatic group NWB. Significant differences were observed between the NWB and WB data sets for logarithmic decrement (D). NWB D demonstrated significantly (p<0.05) higher scores than the WB condition.

Conclusion

This study demonstrated that there was a significant reduction in stiffness of the gastrocnemius muscle in participants with Achilles tendinopathy compared with the two control groups. The WB MyotonPRO measures for the symptomatic groups of F and S were significantly lower, indicating a decrease in the tonal properties (F) and dynamic stiffness (S) of the muscle. The application of MyotonPRO technology to measure changes in stiffness in the gastrocnemius muscle may be useful to measure as part of a rehabilitation programme where specific attention could be placed to increase the tonal properties of the muscle. Further studies are required to develop a robust clinical application of this technology, particularly with regard to Achilles tendinopathy.

Contribution of the Achilles tendon to force potentiation in a stretch-shortening cycle

Muscle force during concentric contractions is potentiated by a preceding eccentric contraction: a phenomenon known as the stretch-shortening cycle (SSC) effect. Tendon elongation is often considered to be the primary factor for this force potentiation. However, direct examination of the influence of tendon elongation on the SSC effect has not been made. The aim of this study was to evaluate the contribution of tendon elongation to the SSC effect by comparing the magnitude of the SSC effect in the rat soleus with and without the Achilles tendon. The rat soleus was subjected to concentric contractions without pre-activation (CON) and concentric contractions with an eccentric pre-activation (ECC). For the 'with-tendon' condition, the calcaneus was rigidly fixed to a force transducer, while for the 'without-tendon' condition, the soleus was fixed at the muscle-tendon junction. The SSC effect was calculated as the ratio of the mechanical work done during the concentric phase for the ECC and the CON conditions. Substantial and similar (P=0.167) SSC effects were identified for the with-tendon (318±86%) and the without-tendon conditions (271±70%). The contribution of tendon elongation to the SSC effect was negligible for the rat soleus. Other factors, such as pre-activation and residual force enhancement, may cause the large SSC effects and need to be evaluated.

Influence of stretch magnitude on the stretch-shortening cycle in skinned muscle fibres

The mechanical work attained during muscle fibre shortening is increased by prior stretching. Recently, we suggested that residual force enhancement (RFE) may contribute to this enhanced work. RFE can be changed reliably by changing the stretch magnitude. Therefore, the purpose of this study was to examine the effect of stretch magnitude, and by association RFE, on the mechanics of the stretch-shortening cycle (SSC) in skinned skeletal muscle fibres. Three tests were performed using skinned rabbit soleus fibres (N=18). The first test was a pure shortening contraction in which fibres were activated and then shortened from an average sarcomere length of 3.3 μm to 3.0 μm. The second test was a SSC in which fibres were activated and stretched from 3.0 μm to 3.3 μm, and then shortened to 3.0 μm. The third test was a SSC in which fibres were activated and stretched from 2.4 μm to 3.3 μm, and then shortened to 3.0 μm. The mechanical work during shortening and the force maintained 15 s after the end of shortening were determined. The relative increase in mechanical work with respect to the pure shortening condition was greater for the large than for the small stretch condition (P<0.001). Similarly, the relative increase in force 15 s after the end of shortening was greater for the large than for the small stretch condition (P=0.043). We conclude that increasing the magnitude of stretch results in an increase in mechanical work and increased force at steady state following the stretch, probably because of the greater RFE.

Impacts of prolonged chlorpyrifos exposure on locomotion and slow-and fast- twitch skeletal muscles contractility in rats

AIM

Investigate the effect of dietary exposure to chlorpyrifos on locomotion and contraction of soleus andextensor digitorum longus (edl) involved in locomotion. Methods: Rats were fed diets containing 1 or 5 mg kg-1 of chlorpyrifos for six weeks. Locomotion has been assessed weekly using beam walking and beam balance tests. Soleus and edl were removed to study contractile properties, myofibrillar protein content and myosin heavy chain isoforms.

RESULTS

Animals treated with 5 mg kg-1 chlorpyrifos had a decrease body weight. An increase by 28% and 24% in latency time assessed by beam walking test and a decrease by 9% and 13% in the beam balance time was reported after 6 weeks of 1 and 5 chlorpyrifos exposure respectively. The contractile properties in soleus showed an increase in twitch amplitude by 25% and 63% in 1 and 5 doses respectively, without modification in the contraction time and half relaxation time. edl treated with 1 mg kg-1 showed a decrease by 35%, 42% and 22% in twitch amplitude, contraction time and half relaxation time respectively. edl treated with 5 mg kg-1 showed an increase of 23% in twitch amplitude without modification of the other parameters. These changes were associated with modification of myofibrillar protein content in all treated groups. Myosin heavy chain isoforms were altered in both skeletal muscles treated with 1 mg kg-1.

CONCLUSION

Exposure to chlorpyrifos can alter the locomotion and produce physiological changes in a dose and muscle type related manner.

Carbenoxolone enhances peripheral insulin sensitivity and GLUT4 expression in skeletal muscle of obese rats: Potential participation of UBC9 protein

AIM

This study investigates the insulin sensitizer effect of carbenoxolone (CBX) and potentially involved peripheral mechanisms.

MAIN METHODS

Taking glucose transporter 4 (GLUT4) as a marker of glucose disposal, we investigated the CBX effects on whole-body insulin sensitivity and solute carrier 2a4 (Slc2a4)/GLUT4 expression in visceral (VAT) and subcutaneous (SAT) adipose tissues and soleus muscle of monosodium glutamate (MSG)-induced obese rats. Sterol regulatory element binding protein (SREBP1), an enhancer of Slc2a4 expression was analyzed through mRNA content and SREBP1-binding to Slc2a4 promoter. Finally, the small ubiquitin-modifier conjugating enzyme 9 (UBC9), whose low content indicates accelerated GLUT4 degradation was analyzed in soleus.

KEY FINDINGS

Hypercorticosteronemia, hyperinsulinemia and low glucose decay rate in the insulin tolerance test of obese rats were restored by CBX (P < 0.05). Slc2a4/GLUT4 increased in SAT (P < 0.05) and decreased in VAT (P < 0.01) of obese rats. In soleus, obesity increased Slc2a4 but decreased GLUT4 (P < 0.01), possibly by accelerating GLUT4 degradation, as suggested by decreased UBC9 (P < 0.01). CBX restored both UBC9 and GLUT4 contents. SREBP1 did not participate in the Slc2a4 transcriptional regulation.

SIGNIFICANCE

The insulin sensitizer effect of CBX involves the increase of GLUT4 expression in soleus, indicating an increased glucose disposal in skeletal muscle. This observation reinforces the skeletal muscle as the main site of insulin-induced glucose uptake and sheds new light on the metabolic effects of 11βHSD1 inhibitors, since most of the studies so far have focused on its effects on liver and adipose tissues.

Training-induced increase in Achilles tendon stiffness affects tendon strain pattern during running

Background

During the stance phase of running, the elasticity of the Achilles tendon enables the utilisation of elastic energy and allows beneficial contractile conditions for the triceps surae muscles. However, the effect of changes in tendon mechanical properties induced by chronic loading is still poorly understood. We tested the hypothesis that a training-induced increase in Achilles tendon stiffness would result in reduced tendon strain during the stance phase of running, which would reduce fascicle strains in the triceps surae muscles, particularly in the mono-articular soleus.

Methods

Eleven subjects were assigned to a training group performing isometric single-leg plantarflexion contractions three times per week for ten weeks, and another ten subjects formed a control group. Before and after the training period, Achilles tendon stiffness was estimated, and muscle-tendon mechanics were assessed during running at preferred speed using ultrasonography, kinematics and kinetics.

Results

Achilles tendon stiffness increased by 18% (P < 0.01) in the training group, but the associated reduction in strain seen during isometric contractions was not statistically significant. Tendon elongation during the stance phase of running was similar after training, but tendon recoil was reduced by 30% (P < 0.01), while estimated tendon force remained unchanged. Neither gastrocnemius medialis nor soleus fascicle shortening during stance was affected by training.

Discussion

These results show that a training-induced increase in Achilles tendon stiffness altered tendon behaviour during running. Despite training-induced changes in tendon mechanical properties and recoil behaviour, the data suggest that fascicle shortening patterns were preserved for the running speed that we examined. The asymmetrical changes in tendon strain patterns supports the notion that simple in-series models do not fully explain the mechanical output of the muscle-tendon unit during a complex task like running.

Plantarflexor strength and endurance deficits associated with mid-portion Achilles tendinopathy: The role of soleus

OBJECTIVES

Determine how the strength and endurance of the plantar flexors are affected by Achilles tendinopathy and whether one muscle is more affected than another.

DESIGN

Case control study.

SETTING

University Laboratory.

PARTICIPANTS

39 Runners with mid-portion Achilles tendinopathy and 38 healthy runners participated in this study.

MAIN OUTCOME MEASURES

Isokinetic dynamometry was completed bilaterally in two knee positions on all subjects to assess the torque and endurance capacity of the plantar flexors.

RESULTS

Subjects with Achilles tendinopathy were statistically weaker (by 26.1Nm Concentric 90°/sec, 14,8Nm Concentric 225°/sec and 55.5Nm Eccentric 90°/sec for knee extended testing and 17.3Nm, 10.1Nm and 52.3Nm for the flexed knee respectively) than healthy controls at all isokinetic test speeds and contraction modes irrespective of knee position (p value = <0.001). The endurance capacity of the plantar flexors was significantly reduced (Total work done 613.5Nm less) in subjects with Achilles tendinopathy when compared to the healthy controls (p value = <0.001).

CONCLUSIONS

Achilles tendinopathy is associated with large deficits in plantar flexor torque and endurance. The deficits are bilateral in nature and appear to be explained by a greater loss of the soleus force generating capacity rather than the gastrocnemius.

H-reflex modulation preceding changes in soleus EMG activity during balance perturbation

When balance is compromised, postural strategies are induced to quickly recover from the perturbation. However, neuronal mechanisms underlying these strategies are not fully understood. Here, we assessed the amplitude of the soleus (SOL) H-reflex during forward and backward tilts of the support surface during standing (n = 15 healthy participants). Electrical stimulation of the tibial nerve was applied randomly before platform tilt (control) and 0, 25, 50, 75, 100 or 200 ms after tilt onset. During backward tilt, a significant decrease in H-reflex amplitude was observed at 75, 100 and 200 ms. The onset of the decreased H-reflex amplitude significantly preceded the onset of the SOL EMG decrease (latency: 144 ± 16 ms). During forward tilt, the amplitude of the H-reflex increased at 100 and 200 ms after tilt onset. The onset of H-reflex increase did not occur significantly earlier than the onset of the SOL EMG increase (127 ± 5 ms). An important inter-subject variability was observed for the onset of H-reflex modulation with respect to EMG response for each direction of tilt, but this variability could not be explained by the subject's height. Taken together, the results establish the time course of change in SOL H-reflex excitability and its relation to the increase and decrease in SOL EMG activity during forward and backward tilts. The data presented here also suggest that balance mechanisms may differ between forward and backward tilts.

Menstrual cycle variation and gender difference in muscle stiffness of triceps surae

BACKGROUND

We aimed to investigate the menstrual cycle variation and the gender difference of the triceps surae muscle stiffness and passive stiffness of the ankle joint.

METHODS

The subjects of the study included 12 healthy young women and 12 healthy young men. Shear elastic moduli of the lateral gastrocnemius, medial gastrocnemius, and soleus muscles were measured as an index of muscle stiffness using shear wave elastography. The passive stiffness of the ankle joint was calculated from passive torque when the ankle joint was passively dorsiflexed. Measurements were conducted in the follicular, ovulatory, and luteal phases to examine the menstrual cycle variation.

FINDINGS

There was no difference noted in the passive stiffness or triceps surae muscle stiffness for young women in the menstrual cycle. As for gender differences, passive stiffness in all menstrual phases in women was lower while the soleus muscle stiffness in women was higher, compared to that in men.

INTERPRETATION

Our findings suggest that passive stiffness and muscle stiffness did not differ in the menstrual cycle. In addition, the measured part of the soleus was more tensioned in women than in men.

Skeletal muscle secretion of IL-6 is muscle type specific: Ex vivo evidence

Emerging evidence indicates that skeletal muscle possesses endocrine function to secret myokines. Interleukin 6 (IL-6) is a well-characterized myokine that is involved in regulation of metabolism and muscle function. Metabolism type and contractile dynamics vary in different muscle types. It is not clear, however, if IL-6 secretion differs in different muscle types. In this study, we first established an ex vivo approach to test the inducible muscle secretion. Freshly isolated muscles were incubated in Krebs solution at 37 °C with oxygen supply. Secreted IL-6 in the incubation media was measure using Western blot and ELISA assay. We first confirmed that the IL-6 release was inducible by treating the incubated muscle with a cytokine stimulant. We demonstrated that physiological temperature (37 °C) and O₂ supply were essential for the induction of IL-6 release from the incubated muscle, suggesting it is a controlled secretion rather than a spontaneous leak. Using this approach, we found that IL-6 release was only inducible from soleus muscle but not EDL muscle. We further showed that IL-6 protein level was higher in slow oxidative muscle fibers. Moreover, we showed that EDL, although lacks of IL-6 release, surely has inducible secretory function that had different secretory pattern from soleus.

Characterizing the corticomotor connectivity of the bilateral ankle muscles during rest and isometric contraction in healthy adults

The investigation of the corticomotor connectivity (CMC) to leg muscles is an emerging research area, and establishing reliability of measures is critical. This study examined the measurement reliability and the differences between bilateral soleus (SOL) and tibialis anterior (TA) CMC in 21 neurologically intact adults. Using single pulse transcranial magnetic stimulation (TMS), each muscle's CMC was assessed twice (7 ± 2 days apart) during rest and active conditions. CMC was quantified using a standardized battery of eight measures (4/condition): motor threshold during resting (RMT), motor evoked potential amplitude and latency (raw and normalized to height) in both conditions, contralateral silent period (CSP) during active. Using two reliability metrics (intraclass correlation coefficient and coefficient of variation of method error; good reliability: ≥0.75 and ≤15, respectively) and repeated-measures ANOVA, we investigated the reliability and Muscle X Body Side interaction. For both muscles, RMT, resting raw and normalized latencies, and active raw latency demonstrated good reliability, while CSP had good reliability only for TA. Amplitude did not demonstrate good reliability for both muscles. SOL CMC was significantly different from TA CMC for all measures but CSP; body side had no significant effect. Therefore, only certain measures may reliably quantify SOL and TA CMC while different CMC (except CSP) between SOL and TA suggests dissimilar corticospinal drive to each muscle regardless of the side.