The contribution of the tendon electrode to M-wave characteristics in the biceps brachii, vastus lateralis and tibialis anterior

In some compound muscle action potentials (M waves) recorded using the belly-tendon configuration, the tendon electrode makes a noticeable contribution to the M wave. However, this finding has only been demonstrated in some hand and foot muscles. Here, we assessed the contribution of the tendon potential to the amplitude of the vastus lateralis, biceps brachii and tibialis anterior M waves, and we also examined the role of this tendon potential in the shoulder-like feature appearing in most M waves. M waves were recorded separately at the belly and tendon locations of the vastus lateralis, biceps brachii and tibialis anterior from 38 participants by placing the reference electrode at a distant (contralateral) site. The amplitude of the M waves and the latency of their peaks and shoulders were measured. In the vastus lateralis, the tendon potential was markedly smaller in amplitude (∼75%) compared to the belly M wave (P = 0.001), whereas for the biceps brachii and tibialis anterior, the tendon and belly potentials had comparable amplitudes. In the vastus lateralis, the tendon potential showed a small positive peak coinciding in latency with the shoulder of the belly-tendon M wave, whilst in the biceps brachii and tibialis anterior, the tendon potential showed a clear negative peak which coincided in latency with the shoulder. The tendon potential makes a significant contribution to the belly-tendon M waves of the biceps brachii and tibialis anterior muscles, but little contribution to the vastus lateralis M waves. The shoulder observed in the belly-tendon M wave of the vastus lateralis is caused by the belly potential, the shoulder in the biceps brachii M wave is generated by the tendon potential, whereas the shoulder in the tibialis anterior M wave is caused by both the tendon and belly potentials. NEW FINDINGS: What is the central question of this study? Does a tendon electrode make a noticeable contribution to the belly-tendon M wave in the vastus lateralis, biceps brachii and tibialis anterior muscles? What is the main finding and its importance? Because the patellar tendon potential is small in amplitude, it hardly influences the amplitude and shape of the belly-tendon M wave of the vastus lateralis. However, for the biceps brachii and tibialis anterior muscles, the potentials at the tendon sites show a large amplitude, and thus have a great impact on the corresponding belly-tendon M waves.

Caffeine does not influence persistent inward current contribution to motoneuron firing

The purpose of this study was to investigate whether caffeine consumption would change persistent inward current (PIC) contribution to motoneuron firing at increased contraction intensities and after repetitive sustained maximal contractions. Before and after the consumption of 6 mg·kg-1 of caffeine or placebo, 16 individuals performed isometric triangular-shaped ramp dorsiflexion contractions (to 20% and 40% of peak torque), followed by four maximal contractions sustained until torque production dropped to 60% of maximum, and consecutive 20% triangular-shaped contractions. Tibialis anterior motor unit firing frequencies were analyzed from high-density surface electromyograms. PIC contribution to motor unit firing was estimated by calculating the delta frequency (ΔF) using the paired motor unit technique. Motoneuron peak firing frequencies at 20% and 40% contractions and total torque-time integral during the repetitive sustained maximal contractions were also assessed. ΔF increased 0.69 peaks per second (pps) (95% CI = -0.98, -0.405; d = -0.87) from 20% to 40% contraction intensities and reduced 0.85 pps (95% CI = 0.66, 1.05; d = 0.99) after the repetitive sustained maximal contractions, regardless of caffeine consumption. Participants produced 337 Nm·s (95% CI = 49.9, 624; d = 0.63) more torque integral during the repetitive sustained maximal contractions after caffeine consumption. A strong repeated-measures correlation (r = 0.61; 95% CI = 0.49, 0.69) was observed between reductions of ΔF and peak firing frequencies after the repetitive sustained maximal contractions. PIC contribution to motoneuron firing increases from 20% to 40% contraction intensities, with no effect of caffeine (on rested tibialis anterior). Repetitive sustained maximal contractions reduced PIC contribution to motoneuron firing, regardless of caffeine or placebo consumption, evidencing that changes in intrinsic motoneuron properties contributed to performance loss. Caffeine-attenuated reduction of torque production capacity was unlikely mediated by PICs.NEW & NOTEWORTHY Persistent inward current (PIC) contribution to motoneuron firing increases with contraction intensities and is reduced after repetitive sustained maximal contractions, regardless of caffeine consumption. Reductions of PIC contribution to motoneuron firing and peak firing frequencies were largely associated, evidencing a novel mechanism underpinning decrements in maximal torque production capacity following repetitive sustained maximal contractions. Caffeine consumption attenuated neuromuscular performance reductions-allowing higher time-torque integral production during repetitive sustained maximal contractions. This was unlikely mediated by PIC.

Effects of jaw clenching and mental stress on persistent inward currents estimated by two different methods

Spinal motoneuron firing depends greatly on persistent inward currents (PICs), which in turn are facilitated by the neuromodulators serotonin and noradrenaline. The aim of this study was to determine whether jaw clenching (JC) and mental stress (MS), which may increase neuromodulator release, facilitate PICs in human motoneurons. The paired motor unit (MU) technique was used to estimate PIC contribution to motoneuron firing. Surface electromyograms were collected using a 32-channel matrix on gastrocnemius medialis (GM) during voluntary, ramp, plantar flexor contractions. MU discharges were identified, and delta frequency (ΔF), a measure of recruitment-derecruitment hysteresis, was calculated. Additionally, another technique was used (VibStim) that evokes involuntary contractions that persist after cessation of combined Achilles tendon vibration and triceps surae neuromuscular electrical stimulation. VibStim measures of plantar flexor torque and soleus activity may reflect PIC activation. ΔF was not significantly altered by JC (p = .679, n = 18, 9 females) or MS (p = .147, n = 14, 5 females). However, all VibStim variables quantifying involuntary torque and muscle activity during and after vibration cessation were significantly increased in JC (p < .011, n = 20, 10 females) and some, but not all, increased in MS (p = .017-.05, n = 19, 10 females). JC and MS significantly increased the magnitude of involuntary contractions (VibStim) but had no effect on GM ΔF during voluntary contractions. Effects of increased neuromodulator release on PIC contribution to motoneuron firing might differ between synergists or be context dependent. Based on these data, the background level of voluntary contraction and, hence, both neuromodulation and ionotropic inputs could influence neuromodulatory PIC enhancement.

Longitudinal changes in intrinsic motoneuron excitability in amyotrophic lateral sclerosis are dependent on disease progression

Increased amplitude of persistent inward currents (PICs) is observed in pre-symptomatic genetically modified SOD1 mice models of amyotrophic lateral sclerosis (ALS). However, at the symptomatic stage this reverses and there is a large reduction in PIC amplitude. It remains unclear whether these changes in PICs can be observed in humans, with cross-sectional studies in humans reporting contradictory findings. In people with ALS, we estimated the PIC contribution to self-sustained firing of motoneurons, using the paired-motor unit analysis to calculate the Δfrequency (ΔF), to compare the weaker and stronger muscles during the course of disease. We hypothesised that, with disease progression, ΔFs would relatively increase in the stronger muscles; and decline in the weaker muscles. Forty-three individuals with ALS were assessed in two occasions on average 17 weeks apart. Tibialis anterior high-density electromyograms were recorded during dorsiflexion (40% of maximal capacity) ramped contractions, followed by clinical tests. ∆F increased from 3.14 (2.57, 3.71) peaks per second (pps) to 3.55 (2.94, 4.17) pps on the stronger muscles (0.41 (0.041, 0.781) pps, standardised difference (d) = 0.287 (0.023, 0.552), P = 0.030). ∆F reduced from 3.38 (95% CI 2.92, 3.84) pps to 2.88 (2.40, 3.36) pps on the weaker muscles (-0.50 (-0.80, -0.21) pps, d = 0.353 (0.138, 0.567), P = 0.001). The ALSFRS-R score reduced 3.9 (2.3, 5.5) points. These data indicate that the contribution of PICs to motoneuron self-sustained firing increases over time in early stages of the disease when there is little weakness before decreasing as the disease progresses and muscle weakness exacerbates, in alignment with the findings from studies using SOD1 mice. KEY POINTS: Research on mouse model of amyotrophic lateral sclerosis (ALS) suggests that the amplitude of persistent inward currents (PICs) is increased in early stages before decreasing as the disease progresses. Cross-sectional studies in humans have reported contradictory findings with both higher and lower PIC contributions to motoneuron self-sustained firing. In this longitudinal (∼17 weeks) study we tracked changes in PIC contribution to motoneuron self-sustained firing, using the ΔF calculation (i.e. onset-offset hysteresis of motor unit pairs), in tibialis anterior muscles with normal strength and with clinical signs of weakness in people with ALS. ΔFs decreased over time in muscles with clinical signs of weakness. The PIC contribution to motoneuron self-sustained firing increases before the onset of muscle weakness, and subsequently decreases when muscle weakness progresses.

Ageing reduces persistent inward current contribution to motor neurone firing: Potential mechanisms and the role of exercise

Nervous system deterioration is a primary driver of age-related motor impairment. The motor neurones, which act as the interface between the central nervous system and the muscles, play a crucial role in amplifying excitatory synaptic input to produce the desired motor neuronal firing output. For this, they utilise their ability to generate persistent (long-lasting) depolarising currents that increase cell excitability, and both amplify and prolong the output activity of motor neurones for a given synaptic input. Modulation of these persistent inward currents (PICs) contributes to the motor neurones' capacities to attain the required firing frequencies and rapidly modulate them to competently complete most tasks. Thus, PICs are crucial for adequate movement generation. Impairments in intrinsic motor neurone properties can impact motor unit firing capacity, with convincing evidence indicating that the PIC contribution to motor neurone firing is reduced in older adults. Indeed, this could be an important mechanism underpinning the age-related reductions in strength and physical function. Furthermore, resistance training has emerged as a promising intervention to counteract age-associated PIC impairments, with changes in PICs being correlated with improvements in muscular strength and physical function after training. In this review, we present the current knowledge of the PIC magnitude decline during ageing and discuss whether reduced serotonergic and noradrenergic input onto the motor neurones, voltage-gated calcium channel dysfunction or inhibitory input impairments are candidates that: (i) explain age-related reductions in the PIC contribution to motor neurone firing and (ii) underpin the enhanced PIC contribution to motor neurone firing following resistance training in older adults.

Intrinsic motor neuron excitability is increased after resistance training in older adults

This study investigated the effects of high-intensity resistance training on estimates of the motor neuron persistent inward current (PIC) in older adults. Seventeen participants (68.5 ± 2.8 yr) completed a 2-wk nonexercise control period followed by 6 wk of resistance training. Surface electromyographic signals were collected with two 32-channel electrodes placed over soleus to investigate motor unit discharge rates. Paired motor unit analysis was used to calculate delta frequency (ΔF) as an estimate of PIC amplitudes during 1) triangular-shaped contractions to 20% of maximum torque capacity and 2) trapezoidal- and triangular-shaped contractions to 20% and 40% of maximum torque capacity, respectively, to understand their ability to modulate PICs as contraction intensity increases. Maximal strength and functional capacity tests were also assessed. For the 20% triangular-shaped contractions, ΔF [0.58-0.87 peaks per second (pps); P ≤ 0.015] and peak discharge rates (0.78-0.99 pps; P ≤ 0.005) increased after training, indicating increased PIC amplitude. PIC modulation also improved after training. During the control period, mean ΔF differences between 20% trapezoidal-shaped and 40% triangular-shaped contractions were 0.09-0.18 pps (P = 0.448 and 0.109, respectively), which increased to 0.44 pps (P < 0.001) after training. Also, changes in ΔF showed moderate to very large correlations (r = 0.39-0.82) with changes in peak discharge rates and broad measures of motor function. Our findings indicate that increased motor neuron excitability is a potential mechanism underpinning training-induced improvements in motor neuron discharge rate, strength, and motor function in older adults. This increased excitability is likely mediated by enhanced PIC amplitudes, which are larger at higher contraction intensities.NEW & NOTEWORTHY Resistance training elicited important alterations in soleus intrinsic motor neuronal excitability, likely mediated by enhanced persistent inward current (PIC) amplitude, in older adults. Estimates of PICs increased after the training period, accompanied by an enhanced ability to increase PIC amplitudes at higher contraction intensities. Our data also suggest that changes in PIC contribution to self-sustained discharging may contribute to increases in motor neuron discharge rates, maximal strength, and functional capacity in older adults after resistance training.

The effects of α-lactalbumin supplementation and handgrip contraction on soleus motoneuron excitability

INTRODUCTION

We tested two strategies that hypothetically increase serotonin availability (α-lactalbumin consumption and a remote submaximal handgrip contraction) on estimates of persistent inward currents (PICs) amplitude of soleus muscle in healthy participants.

METHODS

With a randomised, double-blind, and cross-over design, 13 healthy participants performed triangular-shaped ramp contractions with their plantar flexors (20% of maximal torque), followed by a 30-s handgrip sustained contraction (40% of maximal force) and consecutive repeated triangular-shaped contractions. This was performed before and after the consumption of either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (Corn-starch). Soleus motor units discharge rates were analysed from high-density surface electromyography signals. PICs were estimated by calculating the delta frequency (ΔF) of motor unit train spikes using the paired motor unit technique.

RESULTS

ΔF (0.19 pps; p = 0.001; d = 0.30) and peak discharge rate (0.20 pps; p < 0.001; d = 0.37) increased after the handgrip contraction, irrespective of the consumed supplement. No effects of α-lactalbumin were observed.

CONCLUSIONS

Our results indicate that 40 g of α-lactalbumin was unable to modify intrinsic motoneuron excitability. However, performing a submaximal handgrip contraction before the plantar flexion triangular contraction was capable of increasing ΔF and discharge rates on soleus motor units. These findings highlight the diffused effects of serotonergic input, its effects on motoneuron discharge behaviour, and suggest a cross-effector effect within human motoneurons.

Effects of α-lactalbumin on strength, fatigue and psychological parameters: a randomised double-blind cross-over study

PURPOSE

The neurotransmitter serotonin has a strong effect on behaviour and motor control. Regarding motor control, serotonin contributes to the development of fatigue and is also involved in the ability of motor neurones to operate across a large range of forces (gain control). The consumption of tryptophan-rich supplements (such as α-lactalbumin) is of interest because this amino acid is the only precursor for brain serotonin synthesis. Therefore, the purpose of this study was to determine the effects of α-lactalbumin supplementation on neuromuscular performance.

METHODS

Using a randomised double-blind cross-over design, 16 healthy participants performed plantar flexor and handgrip maximal voluntary contractions, a 30-s submaximal handgrip contraction, and a plantar flexor fatigue protocol before and 90 min after consuming either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (corn-starch). Sleepiness, mood, and cognition were assessed to evaluate any psychological effects.

RESULTS

α-Lactalbumin decreased force steadiness by 25% during the sustained submaximal handgrip contraction (p < 0.01) and induced greater fatigue (15% reduction in total torque-time integral, p = 0.01) during the fatigue protocol. These effects were not observed for the other control beverages. No effects were found for maximal or explosive strength, or psychological measurements.

CONCLUSIONS

40 g of α-lactalbumin increased handgrip force variability and reduced performance during fatiguing muscle contractions but did not influence brief maximal contractions or psychological parameters in healthy individuals. These findings support the hypothesis that the consumption of α-lactalbumin can increase motor neurone input-output gain and exacerbate central fatigue during sustained maximal exercise.

Telehealth multicomponent exercise and health education in breast cancer patients undergoing primary treatment: rationale and methodological protocol for a randomized clinical trial (ABRACE: Telehealth)

BACKGROUND

Current guidelines emphasize cancer patients should increase their physical activity levels, encouraging physical exercise practice as a complementary therapy to mitigate adverse effects during treatment. Telehealth can be a feasible method to improve adherence and interventional support for breast cancer patients, of which most do not meet sufficient physical activity levels after diagnosis. The Adaptations to Breast Cancer and Exercise Using Telehealth (ABRACE: Telehealth) study aims to investigate the effects of a 12-week telehealth multicomponent training program plus a health education program (MTHE), compared to a health education program alone (HE), on physical and psychological outcomes in breast cancer patients undergoing treatment.

METHODS

This study is a randomized controlled trial. Women undergoing primary treatment (during or after chemotherapy) for breast cancer (stages I-III) will be randomly assigned to MTHE (twice a week) or HE (once a week). MTHE components are mobility, aerobic, balance, resistance, and flexibility home-based exercises, supervised by video call. The primary study outcome is cancer-related fatigue. The secondary outcomes are quality of life, symptoms of depression and anxiety, physical activity level, cancer-related cognitive impairment, and functional capacity. Other outcomes are adherence to interventions and a follow-up questionnaire evaluating the individual perception in motivation, lifestyle changes, and main barriers to participation. All outcomes will be remotely assessed before and after intervention. Our analysis will follow the intention-to-treat approach and per-protocol criteria, with additional sub-group analysis.

DISCUSSION

To our knowledge, this is the first randomized clinical trial in breast cancer patients using a face-to-face videoconference strategy to supervise physical exercise. Our hypothesis is of superiority for the effects of MTHE on primary and secondary outcomes compared to the effects of only the health education intervention.

TRIAL REGISTRATION

Adaptations to Breast Cancer and Exercise Using Telehealth (ABRACE: Telehealth), NCT04641377. Registered on 23 November 2021, https://clinicaltrials.gov/ct2/show/NCT04641377.

Effects of Plyometric Training on Physical Performance: An Umbrella Review

BACKGROUND

Plyometric training can be performed through many types of exercises involving the stretch-shortening cycle in lower limbs. In the last decades, a high number of studies have investigated the effects of plyometric training on several outcomes in different populations.

OBJECTIVES

To systematically review, summarize the findings, and access the quality of published meta-analyses investigating the effects of plyometric training on physical performance.

DESIGN

Systematic umbrella review of meta-analyses.

DATA SOURCES

Meta-analyses were identified using a systematic literature search in the databases PubMed/MEDLINE, Scopus, SPORTDiscus, Web of Science, Cochrane Library and Scielo.

ELIGIBILITY CRITERIA FOR SELECTING META-ANALYSES

Meta-analyses that examined the effects of plyometric training on physical fitness in different populations, age groups, and sex.

RESULTS

Twenty-nine meta-analyses with moderate-to-high methodological quality were included in this umbrella review. We identified a relevant weakness in the current literature, in which five meta-analyses included control group comparisons, while 24 included pre-to-post-effect sizes. Trivial-to-large effects were found considering the effects of plyometric training on physical performance for healthy individuals, medium-trivial effects for the sports athletes' groups and medium effects for different sports athletes' groups, age groups, and physical performance.

CONCLUSION

The available evidence indicates that plyometric training improves most related physical fitness parameters and sports performance. However, it is important to outline that most meta-analyses included papers lacking a control condition. As such, the results should be interpreted with caution. PROSPERO number: CRD42020217918.

Variability of Biceps Muscle Stiffness Measured Using Shear Wave Elastography at Different Anatomical Locations With Different Ultrasound Machines

Shear wave elastography is an emerging diagnostic tool used to assess for changes in the stiffness of muscle. Each region of the muscle may have a different stiffness; therefore, the anatomical region should be carefully selected. Machine vendors each have unique methods for calculating the returned stiffness values and, consequently, a high level of agreement in measurement between machines (quantified using the intraclass correlation coefficient [ICC] and Bland-Altman analysis) will allow research findings to be translated to the clinic. This study assessed three locations within the biceps muscle (50% and 75% of the distance between the acromioclavicular joint and antecubital fossa, and superior to distal myotendinous junction [MTJ]) of 32 healthy volunteers with two different machines, the Canon Aplio i600 and SuperSonic Imagine Aixplorer (SSI), to compare the reported shear wave velocities and the variability by coefficient of variation (CV) and ICC. There was no difference in the CV between machines, but a significant difference in the CV at muscle regions, with the 75% location having a 40.2% reduction in CV. The 75% location had the highest ICC values with good posterior mean ICCs of 0.84 on the Canon and 0.83 on the SSI. The 50% and MTJ locations had poor ICC values. The 75% location provided the lowest CV and highest ICC and should be used for future stiffness assessments.

Facilitation-inhibition control of motor neuronal persistent inward currents in young and older adults

A well-coordinated facilitation-inhibition control of motor neuronal persistent inward currents (PICs) via diffuse neuromodulation and local inhibition is essential to ensure motor units discharge at required times and frequencies. Present best estimates indicate that PICs are reduced in older adults; however, it is not yet known whether PIC facilitation-inhibition control is also altered with ageing. We investigated the responses of PICs to (i) a remote handgrip contraction, which is believed to diffusely increase serotonergic input onto motor neurones, and (ii) tendon vibration of the antagonist muscle, which elicits reciprocal inhibition, in young and older adults. High-density surface electromyograms were collected from soleus and tibialis anterior of 18 young and 26 older adults during triangular-shaped plantar and dorsiflexion contractions to 20% (handgrip experiments) and 30% (vibration experiments) of maximum torque (rise-decline rate of 2%/s). A paired-motor-unit analysis was used to calculate ∆F, which is assumed to be proportional to PIC strength. ΔF increased in both soleus (0.55 peaks per second (pps), 16.0%) and tibialis anterior (0.42 pps, 11.4%) after the handgrip contraction independent of age. Although antagonist tendon vibration reduced ΔF in soleus (0.28 pps, 12.6%) independent of age, less reduction was observed in older (0.42 pps, 10.7%) than young adults (0.72 pps, 17.8%) in tibialis anterior. Our data indicate a preserved ability of older adults to amplify PICs following a remote handgrip contraction, during which increased serotonergic input onto the motor neurones is expected, in both lower leg muscles. However, PIC deactivation in response to reciprocal inhibition was impaired with ageing in tibialis anterior despite being preserved in soleus. KEY POINTS: Motor neuronal persistent inward currents (PICs) are facilitated via diffuse neuromodulation and deactivated by local inhibition to ensure motor units discharge at required times and frequencies, allowing normal motor behaviour. PIC amplitudes appear to be reduced with ageing; however, it is not known whether PIC facilitation-inhibition control is also altered. Remote handgrip contraction, which should diffusely increase serotonergic input onto motor neurones, facilitated PICs similarly in both soleus and tibialis anterior of young and older adults. Antagonist tendon vibration, which induces reciprocal inhibition, reduced PICs in soleus in both young and older adults but had less effect in tibialis anterior in older adults. Data from lower-threshold motor units during low-force contractions suggest that PIC facilitation is preserved with ageing in soleus and tibialis anterior. However, the effect of reciprocal inhibition on the contribution of PICs to motor neurone discharge seems reduced in tibialis anterior but preserved in soleus.

Effects of three neuromuscular electrical stimulation methods on muscle force production and neuromuscular fatigue

This study compared the acute responses of three neuromuscular electrical stimulation (NMES) methods on muscle torque-time integral (TTI) and neuromuscular fatigue. Narrow-pulse (0.2 ms; NP), wide-pulse (1 ms; WP), and tendon vibration superimposed onto wide-pulse (WP + VIB)-NMES conditions were applied to sixteen healthy individuals (n = 16) in three separate sessions in a randomized order. Stimulation intensity was set to elicit 20% of maximal voluntary contraction (MVC); the stimulus pattern comprised four sets of 20 repetitions (5 s On and 5 s Off) with a one-minute inter-set interval. TTI was measured for each NMES condition and MVC, voluntary activation (VA), peak twitch torque (Peak_twitch ), and peak soleus (EMG_SOL ), medial (EMG_MG ), and lateral gastrocnemius (EMG_LG ) electromyography were measured before and immediately after each NMES condition. TTI was higher during WP + VIB (19.63 ± 6.34 MVC.s, mean difference = 3.66, p < 0.001, Cohen's d = 0.501) than during WP (15.97 ± 4.79 MVC.s) condition. TTI was higher during WP + VIB (mean difference = 3.79, p < 0.001, Cohen's d = 0.626) than during NP (15.84 ± 3.73 MVC.s) condition. MVC and Peak_twitch forces decreased (p ≤ 0.001) immediately after all conditions. No changes were observed for VA (p = 0.365). EMG_SOL amplitude reduced (p = 0.040) only after NP, yet EMG_LG and EMG_MG amplitudes decreased immediately after all conditions (p = 0.003 and p = 0.013, respectively). WP + VIB produced a higher TTI than WP and NP-NMES, with similar amounts of neuromuscular fatigue across protocols. All NMES protocols induced similar amounts of peripheral fatigue and reduced EMG amplitudes.

Potential role of passively increased muscle temperature on contractile function

Declines in muscle force, power, and contractile function can be observed in older adults, clinical populations, inactive individuals, and injured athletes. Passive heating exposure (e.g., hot baths, sauna, or heated garments) has been used for health purposes, including skeletal muscle treatment. An acute increase in muscle temperature by passive heating can increase the voluntary rate of force development and electrically evoked contraction properties (i.e., time to peak twitch torque, half-relation time, and electromechanical delay). The improvements in the rate of force development and evoked contraction assessments with increased muscle temperature after passive heating reveal peripheral mechanisms’ potential role in enhancing muscle contraction. This review aimed to summarise, discuss, and highlight the potential role of an acute passive heating stimulus on skeletal muscle cells to improve contractile function. These mechanisms include increased calcium kinetics (release/reuptake), calcium sensitivity, and increased intramuscular fluid.

Effects of reciprocal inhibition and whole-body relaxation on persistent inward currents estimated by two different methods

Abstract

Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole‐body relaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well‐established paired motor unit (MU) technique. High‐density surface electromyograms were recorded from gastrocnemius medialis during voluntary, isometric 20‐s ramp, plantarflexor contractions and decomposed into MU discharges to calculate delta frequency (ΔF). Moreover, another technique (VibStim), which evokes involuntary contractions proposed to result from PIC activation, was used. Plantarflexion torque and soleus activity were recorded during 33‐s Achilles tendon vibration and simultaneous 20‐Hz bouts of neuromuscular electrical stimulation (NMES) of triceps surae. ΔF was decreased by RI (n = 15, 5 females) and WBR (n = 15, 7 females). In VibStim, torque during vibration at the end of NMES and sustained post‐vibration torque were reduced by WBR (n = 19, 10 females), while other variables remained unchanged. All VibStim variables remained unaltered in RI (n = 20, 10 females). Analysis of multiple human MUs in this study demonstrates the ability of local, focused inhibition to attenuate the effects of PICs on motoneuron output during voluntary motor control. Moreover, it shows the potential to reduce PICs through non‐pharmacological, neuromodulatory interventions such as WBR. The absence of a consistent effect in VibStim might be explained by a floor effect resulting from low‐magnitude involuntary torque combined with the negative effects of the interventions.

Key points

Spinal motoneurons transmit signals to skeletal muscles to regulate their contraction. Motoneuron firing partly depends on their intrinsic properties such as the strength of persistent (long‐lasting) inward currents (PICs) that make motoneurons more responsive to excitatory input.

In this study, we demonstrate that both reciprocal inhibition onto motoneurons and whole‐body relaxation reduce the contribution of PICs to human motoneuron firing. This was observed through analysis of the firing of single motor units during voluntary contractions.

However, an alternative technique that involves tendon vibration and neuromuscular electrical stimulation to evoke involuntary contractions showed less effect. Thus, it remains unclear whether this alternative technique can be used to estimate PICs under all physiological conditions.

These results improve our understanding of the mechanisms of PIC depression in human motoneurons. Potentially, non‐pharmacological interventions such as electrical stimulation or relaxation could attenuate unwanted PIC‐induced muscle contractions in conditions characterised by motoneuron hyperexcitability.

Ultrasound elastography in the assessment of post-stroke muscle stiffness: a systematic review

Background

Post-stroke muscle stiffness is a major challenge in the rehabilitation of stroke survivors, with no gold standard in clinical assessment. Muscle stiffness is typically evaluated by the Modified Ashworth Scale or the Tardieu Scale; however, these can have low reliability and sensitivity. Ultrasound elastography is an advanced imaging technology that can quantitatively measure the stiffness of a tissue and has been shown to have good construct validity when compared to clinically assessed muscle stiffness and functional motor recovery.

Objective

The purpose of this article is to systematically review the literature regarding the change in muscle stiffness as measured by ultrasound elastography in stroke survivors.

Methods

Scopus, PubMed, Embase, CINAHL, MEDLINE and Cochrane Library were searched for relevant studies that assessed the change in stiffness of post-stroke muscle stiffness measured by ultrasound elastography following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines.

Results

In total, 29 articles were identified, using either strain elastography and shear wave elastography to measure the stiffness of muscles in stroke survivors, most frequently in the biceps and medial gastrocnemius muscles. The stiffness was typically higher in the paretic compared to the non-paretic or healthy control. Other variations that increased the stiffness include increasing the joint angle and introducing a passive stretch or muscle activation. The paretic muscle has also been assessed pre- and post-treatment demonstrating a decrease in stiffness.

Conclusion

Ultrasound elastography is a promising imaging technology for determining the muscle stiffness in stroke survivors with need for a standardized imaging protocol.

DO MOTONEURON DISCHARGE RATES SLOW WITH AGING? A SYSTEMATIC REVIEW AND META-ANALYSIS

Nervous system maladaptation is linked to the loss of maximal strength and motor control with aging. Motor unit discharge rates are a critical determinant of force production; thus, lower discharge rates could be a mechanism underpinning maximal strength and motor control losses during aging. This meta-analysis summarized the findings of studies comparing motor unit discharge rates between young and older adults, and examined the effects of the selected muscle and contraction intensity on the magnitude of discharge rates difference between these two groups. Estimates from 29 studies, across a range of muscles and contraction intensities, were combined in a multilevel meta-analysis, to investigate whether discharge rates differed between young and older adults. Motor unit discharge rates were higher in younger than older adults, with a pooled standardized mean difference (SMD) of 0.66 (95%CI= 0.29-1.04). Contraction intensity had a significant effect on the pooled SMD, with a 1% increase in intensity associated with a 0.009 (95%CI= 0.003-0.015) change in the pooled SMD. These findings suggest that reductions in motor unit discharge rates, especially at higher contraction intensities, may be an important mechanism underpinning age-related losses in maximal force production.

Longing for a Longitudinal Proxy: Acutely Measured Surface EMG Amplitude is not a Validated Predictor of Muscle Hypertrophy

Surface electromyography amplitudes are commonly measured in acute sports and exercise science studies to make inferences about muscular strength, performance, and hypertrophic adaptations that may result from different exercises or exercise-related variables. Here, we discuss the presumptive logic and assumptions underlying these inferences, focusing on hypertrophic adaptations for simplicity's sake. We present counter-evidence for each of its premises and discuss evidence both for and against the logical conclusion. Given the limited evidence validating the amplitude of surface electromyograms as a predictor of longitudinal hypertrophic adaptations, coupled with its weak mechanistic foundation, we suggest that acute comparative studies that wish to assess stimulus potency be met with scrutiny.

Residual force enhancement in human skeletal muscles: A systematic review and meta-analysis

OBJECTIVE

We reviewed and appraised the existing evidence of in vivo manifestations of residual force enhancement in human skeletal muscles and assessed, through a meta-analysis, the effect of an immediate history of eccentric contraction on the subsequent torque capacity of voluntary and electrically evoked muscle contractions.

METHODS

Our search was conducted from database inception to May 2020. Descriptive information was extracted from, and quality was assessed for, 45 studies. Meta-analyses and metaregressions were used to analyze residual torque enhancement and its dependence on the angular amplitude of the preceding eccentric contraction.

RESULTS

Procedures varied across studies with regards to muscle group tested, angular stretch amplitude, randomization of contractions, time window analyzed, and verbal command. Torque capacity in isometric (constant muscle tendon unit length and joint angle) contractions preceded by an eccentric contraction was typically greater compared to purely isometric contractions, and this effect was greater for electrically evoked muscle contractions than voluntary contractions. Residual torque enhancement differed across muscle groups for the voluntary contractions, with a significant enhancement in torque observed for the adductor pollicis, ankle dorsiflexors, ankle plantar flexors, and knee extensors, but not for the elbow and knee flexors. Meta-regressions revealed that the angular amplitude of the eccentric contraction (normalized to the respective joint's full range of motion) was not associated with the residual torque enhancement observed.

CONCLUSION

There is evidence of residual torque enhancement for most, but not all, muscle groups, and residual torque enhancement is greater for electrically evoked than for voluntary contractions. Contrary to our hypothesis, and contrary to generally accepted findings on isolated muscle preparations, residual torque enhancement in voluntary and electrically evoked contractions does not seem to depend on the angular amplitude of the preceding eccentric contraction.

Runners with mid-portion Achilles tendinopathy have greater triceps surae intracortical inhibition than healthy controls

OBJECTIVES

This study aimed to investigate short-interval intracortical inhibition (SICI) and muscle function in the triceps surae of runners with mid-portion Achilles tendinopathy (AT).

METHODS

Runners with (n = 11) and without (n = 13) AT were recruited. Plantar flexor isometric peak torque and rate of torque development (RTD) were measured using an isokinetic dynamometer. Triceps surae endurance was measured as single-leg heel raise (SLHR) to failure test. SICI was assessed using paired-pulse transcranial magnetic stimulation during a sustained contraction at 10% of plantar flexor isometric peak torque.

RESULTS

Triceps surae SICI was 14.3% (95% CI: -2.1 to 26.4) higher in AT than in the control group (57.9%, 95% CI: 36.2 to 79.6; and 43.6% 95% CI: 16.2 to 71.1, p = 0.032) irrespective of the tested muscle. AT performed 16 (95% CI: 7.9 to 23.3, p < 0.001) fewer SLHR repetitions on the symptomatic side compared with controls, and 14 (95% CI: 5.8 to 22.0, p = 0.004), fewer SLHR repetitions on the non-symptomatic compared with controls. We found no between-groups differences in isometric peak torque (p = 0.971) or RTD (p = 0.815).

PERSPECTIVE

Our data suggest greater intracortical inhibition for the triceps surae muscles for the AT group accompanied by reduced SLHR endurance, without deficits in isometric peak torque or RTD. The increased SICI observed in the AT group could be negatively influencing triceps surae endurance; thus, rehabilitation aiming to reduce intracortical inhibition should be considered to improve patient outcomes. Furthermore, SLHR is a useful clinical tool to assess plantar flexor function in AT patients.

Intrinsic motoneuron excitability is reduced in soleus and tibialis anterior of older adults

Age-related deterioration within both motoneuron and monoaminergic systems should theoretically reduce neuromodulation by weakening motoneuronal persistent inward current (PIC) amplitude. However, this assumption remains untested. Surface electromyographic signals were collected using two 32-channel electrode matrices placed on soleus and tibialis anterior of 25 older adults (70 ± 4 years) and 17 young adults (29 ± 5 years) to investigate motor unit discharge behaviors. Participants performed triangular-shaped plantar and dorsiflexion contractions to 20% of maximum torque at a rise-decline rate of 2%/s of each participant's maximal torque. Pairwise and composite paired-motor unit analyses were adopted to calculate delta frequency (ΔF), which has been used to differentiate between the effects of synaptic excitation and intrinsic motoneuronal properties and is assumed to be proportional to PIC amplitude. Soleus and tibialis anterior motor units in older adults had lower ΔFs calculated with either the pairwise [-0.99 and -1.46 pps; -35.4 and -33.5%, respectively] or composite (-1.18 and -2.28 pps; -32.1 and -45.2%, respectively) methods. Their motor units also had lower peak discharge rates (-2.14 and -2.03 pps; -19.7 and -13.9%, respectively) and recruitment thresholds (-1.50 and -2.06% of maximum, respectively) than young adults. These results demonstrate reduced intrinsic motoneuron excitability during low-force contractions in older adults, likely mediated by decreases in the amplitude of persistent inward currents. Our findings might be explained by deterioration in the motoneuron or monoaminergic systems and could contribute to the decline in motor function during aging; these assumptions should be explicitly tested in future investigations.

Lower knee flexion and hip extension rate of torque development in athletes with previous hamstring strain injury

Persistent deficits in strength and voluntary activation have been observed in athletes with a history of hamstring strain injury. The mechanisms contributing to these deficits are poorly understood and consequently may not be appropriately addressed during rehabilitation. This study aimed to investigate the impact of intended knee flexor contraction mode (concentric, eccentric or isometric) on the rate of torque development and surface electromyography (sEMG) rise in athletes with and without a history of unilateral hamstring strain injury. The impact of the previous injury on hip extensor rate of torque development was also investigated. Previously injured limbs exhibited a slower rate of torque development (mean difference = -31%, p = 0.02, Cohen's d = 0.62) and biceps femoris rate of sEMG rise (mean difference = -181% · s^-1, p = 0.003, Cohen's d = 1.10) during intended eccentric knee flexor contractions compared with control limbs. Previously injured (mean difference = -29%, p = 0.01, Cohen's d = 0.85) and contralateral uninjured limbs (mean difference = -31%, p = 0.007, Cohen's d = 0.73) exhibited a slower rate of torque development during isometric hip extensor contractions compared with control limbs. These findings may highlight lower levels of descending input to hamstring motoneurons in previously injured athletes.

Static Stretching Reduces Motoneuron Excitability: The Potential Role of Neuromodulation

Prolonged static muscle stretching transiently reduces maximal muscle force, and this force loss has a strong neural component. In this review, we discuss the evidence suggesting that stretching reduces the motoneuron's ability to amplify excitatory drive. We propose a hypothetical model in which stretching causes physiological relaxation, reducing the brainstem-derived neuromodulatory drive necessary to maximize motoneuron discharge rates.

Estimates of persistent inward currents increase with the level of voluntary drive in low-threshold motor units of plantar flexor muscles

This study tested whether estimates of persistent inward currents (PICs) in the human plantar flexors would increase with the level of voluntary drive. High-density surface electromyograms were collected from soleus and gastrocnemius medialis of 21 participants (29.2 ± 2.6 yr) during ramp-shaped isometric contractions to 10%, 20%, and 30% (torque rise and decline of 2%/s and 30-s duration) of each participant's maximal torque. Motor units identified in all the contraction intensities were included in the paired-motor unit analysis to calculate delta frequency (ΔF) and estimate the PICs. ΔF is the difference in discharge rate of the control unit at the time of recruitment and derecruitment of the test unit. Increases in PICs were observed from 10% to 20% [Δ = 0.6 pulse per second (pps); P < 0.001] and from 20% to 30% (Δ = 0.5 pps; P < 0.001) in soleus and from 10% to 20% (Δ = 1.2 pps; P < 0.001) but not from 20% to 30% (Δ = 0.09 pps; P = 0.724) in gastrocnemius medialis. Maximal discharge rate increased for soleus and gastrocnemius medialis from 10% to 20% [Δ = 1.75 pps (P < 0.001) and Δ = 2.43 pps (P < 0.001), respectively] and from 20% to 30% [Δ = 0.80 pps (P < 0.017) and Δ = 0.92 pps (P = 0.002), respectively]. The repeated-measures correlation identified associations between ΔF and increases in maximal discharge rate for soleus (r = 0.64; P < 0.001) and gastrocnemius medialis (r = 0.77; P < 0.001). An increase in voluntary drive tends to increase PIC strength, which has key implications for the control of force but also for comparisons between muscles or studies when relative force levels might be different. Increases in voluntary descending drive amplify PICs in humans and provide an important spinal mechanism for motor unit discharging, and thus force output modulation.NEW & NOTEWORTHY Animal experiments and computational models have shown that motor neurons can amplify the synaptic input they receive via persistent inward currents. Here we show in humans that this amplification varies proportionally to the magnitude of the voluntary drive to the muscle.

Plantar flexor muscle stretching depresses the soleus late response but not tendon tap reflexes

The purpose of this study was to investigate changes in muscle spindle sensitivity with early and late soleus reflex responses via tendon taps and transcranial magnetic stimulation, respectively, after an acute bout of prolonged static plantar flexor muscle stretching. Seventeen healthy males were tested before and after 5 min (5 × 60-s stretches) of passive static stretching of the plantar flexor muscles. Maximal voluntary isometric torque and M wave-normalized triceps surae muscle surface electromyographic activity were recorded. Both soleus tendon reflexes, evoked by percussion of the Achilles tendon during rest and transcranial magnetic stimulation-evoked soleus late responses during submaximal isometric dorsiflexion were also quantified. Significant decreases in maximal voluntary isometric plantar flexion torque (-19.2 ± 13.6%, p = .002) and soleus electromyographic activity (-20.1 ± 11.4%, p < .001) were observed immediately after stretching, and these changes were highly correlated (r = 0.76, p < .001). No changes were observed in tendon reflex amplitude or latency or peak muscle twitch torque (p > .05). Significant reductions in soleus late response amplitudes (-46.9 ± 36.0%, p = .002) were detected, although these changes were not correlated with changes in maximal electromyographic activity, torque or tendon reflex amplitudes. No changes in soleus late response latency were detected. In conclusion, impaired neural drive was implicated in the stretch-induced force loss; however, no evidence was found that this loss was related to changes in muscle spindle sensitivity. We hypothesize that the decrease in soleus late response indicates a stretch-induced reduction in a polysynaptic postural reflex rather than spindle reflex sensitivity.

Passive muscle stretching reduces estimates of persistent inward current strength in soleus motor units

Prolonged (≥60 s) passive muscle stretching acutely reduces maximal force production at least partly through a suppression of efferent neural drive. The origin of this neural suppression has not been determined; however, some evidence suggests that reductions in the amplitude of persistent inward currents (PICs) in the motoneurons may be important. The aim of the present study was to determine whether acute passive (static) muscle stretching affects PIC strength in gastrocnemius medialis (GM) and soleus (SOL) motor units. We calculated the difference in instantaneous discharge rates at recruitment and de-recruitment (ΔF) for pairs of motor units in GM and SOL during triangular isometric plantar flexor contractions (20% maximum) both before and immediately after a 5 min control period and immediately after five 1 min passive plantar flexor stretches. After stretching, there was a significant reduction in SOL ΔF (-25.6%; 95% confidence interval, CI=-45.1% to -9.1%, P=0.002) but not GM ΔF These data suggest passive muscle stretching can reduce the intrinsic excitability, via PICs, of SOL motor units. These findings (1) suggest that PIC strength might be reduced after passive stretching, (2) are consistent with previously established post-stretch decreases in SOL but not GM EMG amplitude during contraction, and (3) indicate that reductions in PIC strength could underpin the stretch-induced force loss.

Rate of torque development and striatal shape in individuals with prodromal Huntington's disease

The aim of the present study was to quantify explosive joint torque or the ability to develop joint torque rapidly, typically measured as the rate of torque development, in individuals with prodromal Huntington’s disease and healthy controls and its associations with measures of disease burden and striatal pathology. Twenty prodromal Huntington’s disease and 19 healthy control individuals volunteered for this study. Plantar flexor isometric rate of torque development values were evaluated using isokinetic dynamometry. Pathological changes in striatal shape were evaluated using magnetic resonance imaging. Disease burden was evaluated using the disease burden score and cytosine-adenine-guanine age product score. No statistical differences in the rate of torque development were observed between individuals with prodromal Huntington’s disease and healthy controls. However, significant associations were observed between the rate of torque development values and measures of disease burden (r = −0.42 to −0.69) and striatal pathology (r = 0.71–0.60) in individuals with prodromal Huntington’s disease. We found significant associations between lower rate of torque development values and greater striatal shape deflation and disease burden and striatal pathology in individuals with prodromal Huntington’s disease. While no significant differences in the rate of torque development were found between prodromal Huntington’s disease and healthy controls, the noted associations suggest that differences may emerge as the disease advances, which should be investigated longitudinally in future studies.

Is power training effective to produce muscle hypertrophy in older adults? A systematic review and meta-analysis

Power training has been suggested to be effective in improving strength, power, and functional capacity in older adults. However, there is still a lack of systematic investigations reporting its effectiveness for muscle hypertrophy. Thus, this study investigated the effect of power training on muscle hypertrophy and compared its magnitude with traditional moderate-velocity resistance training in older adults. A systematic search was conducted to identify clinical trials investigating the effect of power training on muscle hypertrophy (power training vs. control) and/or comparing the effect of power training versus moderate-velocity resistance training for a meta-analytical approach. Ten studies comparing power training to control conditions and 9 studies comparing power training to moderate-velocity resistance training were selected. Three studies were classified as high quality and 2 were preregistered. The meta-analysis showed that power training was superior for muscle hypertrophy compared with control condition (n = 8 studies; standardised mean difference (SMD) = 0.31; 95% confidence interval (CI) = 0.04, 0.58; p = 0.029), and resulted in similar hypertrophy compared with moderate-velocity resistance training (n = 7 studies; SMD = 0.07; 95% CI = –0.18, 0.32; p = 0.50). No significant heterogeneity was observed (p = 0.46 and 0.54, and I2 = 0% and 0%, respectively). Our data suggest that power training is effective for muscle hypertrophy in older adults, with similar effectiveness as moderate-velocity resistance training. (PROSPERO registration no.: CRD42019128951.)

Novelty It is known that power training might be superior to moderate-velocity resistance training for function improvements in older adults, but there was no meta-analysis investigating its effect on muscle hypertrophy. Power training is effective to induce muscle hypertrophy in older adults to a similar extent as moderate-velocity resistance training.

Resistance training in breast cancer patients undergoing primary treatment: a systematic review and meta-regression of exercise dosage

BACKGROUND

Exercise is recognised as an adjunct therapy for breast cancer patients; however, little is known about the resistance training dose-response. We conducted a systematic review and meta-regression to examine the resistance training dose-response (i.e., volume and intensity) in breast cancer patients undergoing primary treatment.

METHODS

Searches in MEDLINE, CINAHL, and SPORTDiscus were conducted for studies published up to November 2019. Experimental studies that evaluated resistance-based exercise interventions in women with breast cancer undergoing primary treatment were included. Information about resistance training components, average change and change per week, as well as standardised mean difference were extracted, and used for meta-regression analysis. Outcome measures were upper and lower body muscle strength and body composition.

RESULTS

10 trials were included in the systematic review and 4 trials in the dose-response analysis. Resistance training weekly prescribed volume was inversely associated with increases in upper and lower body muscle strength (r² = 98.1-100%; p = 0.009), although there was no relationship between resistance training intensity and strength gains. There was insufficient data for the dose-response analysis of body mass index, percent body fat, and lean mass.

CONCLUSION

Low volume resistance training might be a suitable exercise recommendation for breast cancer patients undergoing primary treatment producing superior benefits for muscle strength compared to higher volume training, regardless of the training intensity. Low volume resistance training may provide a conservative and appropriate approach for breast cancer patients, allowing gradual progression and modification throughout the exercise program.

Caffeine increases motor output entropy and performance in 4 km cycling time trial

Caffeine improves cycling time trial performance through enhanced motor output and muscle recruitment. However, it is unknown if caffeine further increases power output entropy. To investigate the effects of caffeine effects on cycling time trial performance and motor output entropy (MOEn), nine cyclists (VO_2MAX of 55 ± 6.1 mL^.kg^.-1min^-1) performed a 4 km cycling time trial (TT_4km) after caffeine and placebo ingestion in a counterbalanced order. Power output data were sampled at a 2 Hz frequency, thereafter entropy was estimated on a sliding-window fashion to generate a power output time series. A number of mixed models compared performance and motor output entropy between caffeine and placebo every 25% of the total TT_4km distance. Caffeine ingestion improved power output by 8% (p = 0.003) and increased MOEn by 7% (p = 0.018). Cyclists adopted a U-shaped pacing strategy after caffeine ingestion. MOEn mirrored power output responses as an inverted U-shape MOEn during the time trial. Accordingly, a strong inverse correlation was observed between MOEn and power output responses over the last 25% of the TT_4km (p < 0.001), regardless of the ingestion, likely reflecting the end spurt during this period (p = 0.016). Caffeine ingestion improved TT_4km performance and motor output responses likely due to a greater power output entropy.

Lack of cortical or Ia-afferent spinal pathway involvement in muscle force loss after passive static stretching

This study investigated whether modulation of corticospinal-motoneuronal excitability and/or synaptic transmission of the Ia afferent spinal reflex contributes to decreases in voluntary activation and muscular force after an acute bout of prolonged static muscle stretching. Fifteen men performed five 60-s constant-torque stretches (15-s rest intervals; total duration 5 min) of the plantar flexors on an isokinetic dynamometer and a nonstretching control condition in random order on 2 separate days. Maximum isometric plantar flexor torque and triceps surae muscle electromyographic activity (normalized to M wave; EMG/M) were simultaneously recorded immediately before and after each condition. Motor-evoked potentials (using transcranial magnetic stimulation) and H-reflexes were recorded from soleus during EMG-controlled submaximal contractions (23.4 ± 6.9% EMG maximum). No changes were detected in the control condition. After stretching, however, peak torque (mean ± SD; -14.3 ± 7.0%) and soleus EMG/M (-17.8 ± 6.2%) decreased, and these changes were highly correlated (r = 0.83). No changes were observed after stretching in soleus MEP or H-reflex amplitudes measured during submaximal contractions, and interindividual variability of changes was not correlated with changes in EMG activity or maximum torque. During EMG-controlled submaximal contractions, torque production was significantly decreased after stretching (-22.7 ± 15.0%), indicating a compromised muscular output. These data provide support that changes in the excitability of the corticospinal-motoneuronal and Ia afferent spinal reflex pathways do not contribute to poststretch neural impairment.NEW & NOTEWORTHY This study is the first to specifically examine potential sites underlying the decreases in neural activation of muscle and force production after a bout of muscle stretching. However, no changes were found in either the H-reflex or motor-evoked potential amplitude during submaximal contractions.

Passive muscle stretching impairs rapid force production and neuromuscular function in human plantar flexors

PURPOSE

We examined the effect of muscle stretching on the ability to produce rapid torque and the mechanisms underpinning the changes.

METHODS

Eighteen men performed three conditions: (1) continuous stretch (1 set of 5 min), (2) intermittent stretch (5 sets of 1 min with 15-s inter-stretch interval), and (3) control. Isometric plantar flexor rate of torque development was measured during explosive maximal voluntary contractions (MVC) in the intervals 0-100 ms (RTD_V100) and 0-200 ms (RTD_V200), and in electrically evoked 0.5-s tetanic contractions (20 Hz, 20 Hz preceded by a doublet and 80 Hz). The rate of EMG rise, electromechanical delay during MVC (EMD_V) and during a single twitch contraction (EMD_twitch) were assessed.

RESULTS

RTD_V200 was decreased (P < 0.05) immediately after continuous (- 15%) and intermittent stretch (- 30%) with no differences between protocols. The rate of torque development during tetanic stimulations was reduced (P < 0.05) immediately after continuous (- 8%) and intermittent stretch (- 10%), when averaged across stimulation frequencies. Lateral gastrocnemius rate of EMG rise was reduced after intermittent stretch (- 27%), and changes in triceps surae rate of EMG rise were correlated with changes in RTD_V200 after both continuous (r = 0.64) and intermittent stretch (r = 0.65). EMD_V increased immediately (31%) and 15 min (17%) after intermittent stretch and was correlated with changes in RTD_V200 (r = - 0.56). EMD_twitch increased immediately after continuous (4%), and immediately (5.4%), 15 min (6.3%), and 30 min after (6.4%) intermittent stretch (P < 0.05).

CONCLUSIONS

Reductions in the rate of torque development immediately after stretching were associated with both neural and mechanical mechanisms.

Effects of resistance training concentric velocity on older adults' functional capacity: A systematic review and meta-analysis of randomised trials

Reduced levels of functional capacity in older adults are related to lower quality of life, frailty, and sarcopenia, and can increase risk of falling, fractures and hospitalisation. Resistance training is an effective method to attenuate age-related functional declines. Based on the findings that muscle power and explosive strength are strongly associated with functional performance in older adults, it has been suggested that fast-intended-velocity resistance training may elicit greater improvements in functional capacity when compared to moderate-velocity resistance training. However, currently, there is no high-quality systematic review and meta-analysis supporting this assertion. The present study compared the magnitude of functional capacity improvements following resistance training performed with fast-intentional velocity versus moderate velocity. Pubmed, Scopus, and Web of Science databases were searched from inception to January 2019. The following eligibility criteria for selecting studies was adopted: Participants aged ≥60 years; resistance training based intervention for lower limbs performed solely with slow to moderate concentric velocity (≥2 s for each concentric phase) or solely with the intention of maximising velocity (i.e., as fast as possible); and at least one functional test for lower limbs, with pre- and post-intervention measurements. When studies employed multiple functional tests, a single (pooled) standardised mean difference was calculated and presented as combined functional capacity. In addition, functional tests were grouped accordingly to their specificity for the sub-groups meta-analyses. Fifteen studies were selected (high quality, n = 3; and pre-registered, n = 2). The results presented heterogeneity and small-studies publication bias, leading to a biased advantage for fast-intended-velocity resistance training (95%CI = 0.18, 0.65; I² = 45%). Short physical performance battery indicated an advantage for fast-intended-velocity resistance training (95%CI = 0.10, 0.94; I² = 0%). There was no difference for timed up and go (95%CI = -0.07, 0.94; I² = 48%), 30-s chair stand (95%CI = -0.24, 1.39; I² = 71%), 5-times chair stand (95%CI = -1.63, 1.27; I² = 57%) stair climb (95%CI = -1.89, 2.81; I² = 0%), short walk (95%CI = -0.99, 0.96; I² = 21%) and long walk (95%CI = -0.59, 1.00; I² = 0%). These results suggest that there is inconclusive evidence to support the superiority of fast-intended-velocity resistance training to improve functional capacity when compared to moderate-velocity resistance training. These results may have been influenced by the lack of high-quality and pre-registered studies, high heterogeneity, and small-studies publication bias. PROSPERO REGISTRATION NUMBER: CRD42019122251.

Neuromuscular Factors Contributing to Reductions in Muscle Force After Repeated, High-Intensity Muscular Efforts

Multiple neuromuscular processes contribute to the loss of force production following repeated, high-intensity muscular efforts; however, the relative contribution of each process is unclear. In Experiment 1, 16 resistance trained men performed six sets of unilateral isometric plantar flexor contractions of the right leg (3 s contraction/2 s rest; 85% maximal voluntary contraction torque; 90-s inter-set rest) until failure with and without caffeine ingestion (3 mg kg^-1) on two separate days. Corticospinal excitability and cortical silent period (cSP) were assessed before and immediately, 10 and 20 min after the exercise. In Experiment 2, electrically evoked tetanic force and persistent inward current (PIC)-mediated facilitation of the motor neuron pool (estimated using neuromuscular electrical stimulation with tendon vibration) were assessed before and after the same exercise intervention in 17 resistance trained men. Results showed decreases in peak plantar flexion torque (Experiment 1: -12.2%, Experiment 2: -16.9%), electrically evoked torque (20 Hz -15.3%, 80 Hz -15.3%, variable-frequency train -17.9%), and cSP (-3.8%; i.e., reduced inhibition) post-exercise which did not recover by 20 min. Electromyographic activity (EMG; -6%), corticospinal excitability (-9%), and PIC facilitation (-24.8%) were also reduced post-exercise but recovered by 10 min. Caffeine ingestion increased torque and EMG but did not notably affect corticospinal excitability, PIC amplification, or electrically evoked torque. The data indicate that a decrease in muscle function largely underpins the loss of force after repeated, high-intensity muscular efforts, but that the loss is exacerbated immediately after the exercise by simultaneous decreases in corticospinal excitability and PIC amplitudes at the motor neurons.

Volume for Muscle Hypertrophy and Health Outcomes: The Most Effective Variable in Resistance Training

Resistance training is the most effective method to increase muscle mass. It has also been shown to promote many health benefits. Although it is deemed safe and of clinical relevance for treating and preventing a vast number of diseases, a time-efficient and minimal dose of exercise has been the focus of a great number of research studies. Similarly, an inverted U-shaped relationship between training dose/volume and physiological response has been hypothesized to exist. However, the majority of available evidence supports a clear dose-response relationship between resistance training volume and physiological responses, such as muscle hypertrophy and health outcomes. Additionally, there is a paucity of data to support the inverted U-shaped response. Although it may indeed exist, it appears to be much more plastic than previously thought. The overarching principle argued herein is that volume is the most easily modifiable variable that has the most evidenced-based response with important repercussions, be these muscle hypertrophy or health-related outcomes.

Effect of tendon vibration during wide-pulse neuromuscular electrical stimulation (NMES) on muscle force production in people with spinal cord injury (SCI)

Background

Neuromuscular electrical stimulation (NMES) is commonly used in skeletal muscles in people with spinal cord injury (SCI) with the aim of increasing muscle recruitment and thus muscle force production. NMES has been conventionally used in clinical practice as functional electrical stimulation (FES), using low levels of evoked force that cannot optimally stimulate muscular strength and mass improvements, and thus trigger musculoskeletal changes in paralysed muscles. The use of high intensity intermittent NMES training using wide-pulse width and moderate-intensity as a strength training tool could be a promising method to increase muscle force production in people with SCI. However, this type of protocol has not been clinically adopted because it may generate rapid muscle fatigue and thus prevent the performance of repeated high-intensity muscular contractions in paralysed muscles. Moreover, superimposing patellar tendon vibration onto the wide-pulse width NMES has been shown to elicit further increases in impulse or, at least, reduce the rate of fatigue in repeated contractions in able-bodied populations, but there is a lack of evidence to support this argument in people with SCI.

Methods

Nine people with SCI received two NMES protocols with and without superimposing patellar tendon vibration on different days (i.e. STIM and STIM+vib), which consisted of repeated 30 Hz trains of 58 wide-pulse width (1000 μs) symmetric biphasic pulses (0.033-s inter-pulse interval; 2 s stimulation train; 2-s inter-train interval) being delivered to the dominant quadriceps femoris. Starting torque was 20% of maximal doublet-twitch torque and stimulations continued until torque declined to 50% of the starting torque. Total knee extensor impulse was calculated as the primary outcome variable.

Results

Total knee extensor impulse increased in four subjects when patellar tendon vibration was imposed (59.2 ± 15.8%) but decreased in five subjects (− 31.3 ± 25.7%). However, there were no statistically significant differences between these sub-groups or between conditions when the data were pooled.

Conclusions

Based on the present results there is insufficient evidence to conclude that patellar tendon vibration provides a clear benefit to muscle force production or delays muscle fatigue during wide-pulse width, moderate-intensity NMES in people with SCI.

Trial registration

ACTRN12618000022268. Date: 11/01/2018. Retrospectively registered.

The effects of flexibility training on exercise-induced muscle damage in young men with limited hamstrings flexibility

Adaptations to 6 weeks of supervised hamstring stretching training and its potential impact on symptoms of eccentric exercise-induced muscle damage (EIMD) were studied in 10 young, untrained men with limited hamstrings flexibility. Participants performed unilateral flexibility training (experimental leg; EL) on an isokinetic dynamometer, while the contralateral limb acted as control (CL). Hip range of motion (ROM), passive, isometric, and concentric torques, active optimum angle, and biceps femoris and semitendinosus muscle thickness and ultrasound echo intensity were assessed both before and after the training. Additionally, muscle soreness was assessed before and after an acute eccentric exercise bout in both legs (EL and CL) at post-training only. Hip ROM increased (P < .001) only in EL after the training (EL = 10.6° vs CL = 1.6°), but no changes (P > .05) in other criterion measurements were observed. After a bout of eccentric exercise at the end of the program, isometric and dynamic peak torques and muscle soreness ratings were significantly altered at all time points equally in EL and CL. Also, active optimum angle was reduced immediately, 48 and 72 hours post-exercise, and hip ROM was reduced at 48 and 72 hours equally in EL and CL. Finally, biceps femoris muscle thickness was significantly increased at all time points, and semitendinosus thickness and echo intensity significantly increased at 72 hours, with no significant differences between legs. The stretching training protocol significantly increased hip ROM; however, it did not induce a protective effect on EIMD in men with tight hamstrings.

Interpreting Signal Amplitudes in Surface Electromyography Studies in Sport and Rehabilitation Sciences

Surface electromyography (sEMG) is a popular research tool in sport and rehabilitation sciences. Common study designs include the comparison of sEMG amplitudes collected from different muscles as participants perform various exercises and techniques under different loads. Based on such comparisons, researchers attempt to draw conclusions concerning the neuro- and electrophysiological underpinning of force production and hypothesize about possible longitudinal adaptations, such as strength and hypertrophy. However, such conclusions are frequently unsubstantiated and unwarranted. Hence, the goal of this review is to discuss what can and cannot be inferred from comparative research designs as it pertains to both the acute and longitudinal outcomes. General methodological recommendations are made, gaps in the literature are identified, and lines for future research to help improve the applicability of sEMG are suggested.

Effect of tendon vibration during wide-pulse neuromuscular electrical stimulation (NMES) on the decline and recovery of muscle force

BACKGROUND

Neuromuscular electrical stimulation (NMES) is commonly used to activate skeletal muscles and reverse muscle atrophy in clinical populations. Clinical recommendations for NMES suggest the use of short pulse widths (100-200 μs) and low-to-moderate pulse frequencies (30-50 Hz). However, this type of NMES causes rapid muscle fatigue due to the (non-physiological) high stimulation intensities and non-orderly recruitment of motor units. The use of both wide pulse widths (1000 μs) and tendon vibration might optimize motor unit activation through spinal reflex pathways and thus delay the onset of muscle fatigue, increasing muscle force and mass. Thus, the objective of this study was to examine the acute effects of patellar tendon vibration superimposed onto wide-pulse width (1000 μs) knee extensor electrical stimulation (NMES, 30 Hz) on peak muscle force, total impulse before "muscle fatigue", and the post-exercise recovery of muscle function.

METHODS

Tendon vibration (Vib), NMES (STIM) or NMES superimposed onto vibration (STIM + Vib) were applied in separate sessions to 16 healthy adults. Total torque-time integral (TTI), maximal voluntary contraction torque (MVIC) and indirect measures of muscle damage were tested before, immediately after, 1 h and 48 h after each stimulus.

RESULTS

TTI increased (145.0 ± 127.7%) in STIM only for "positive responders" to the tendon vibration (8/16 subjects), but decreased in "negative responders" (-43.5 ± 25.7%). MVIC (-8.7%) and rectus femoris electromyography (RF EMG) (-16.7%) decreased after STIM (group effect) for at least 1 h, but not after STIM + Vib. No changes were detected in indirect markers of muscle damage in any condition.

CONCLUSIONS

Tendon vibration superimposed onto wide-pulse width NMES increased TTI only in 8 of 16 subjects, but reduced voluntary force loss (fatigue) ubiquitously. Negative responders to tendon vibration may derive greater benefit from wide-pulse width NMES alone.

Arm posture-dependent changes in corticospinal excitability are largely spinal in origin

Biceps brachii motor evoked potentials (MEPs) from cortical stimulation are influenced by arm posture. We used subcortical stimulation of corticospinal axons to determine whether this postural effect is spinal in origin. While seated at rest, 12 subjects assumed several static arm postures, which varied in upper-arm (shoulder flexed, shoulder abducted, arm hanging to side) and forearm orientation (pronated, neutral, supinated). Transcranial magnetic stimulation over the contralateral motor cortex elicited MEPs in resting biceps and triceps brachii, and electrical stimulation of corticospinal tract axons at the cervicomedullary junction elicited cervicomedullary motor evoked potentials (CMEPs). MEPs and CMEPs were normalized to the maximal compound muscle action potential (Mmax). Responses in biceps were influenced by upper-arm and forearm orientation. For upper-arm orientation, biceps CMEPs were 68% smaller (P= 0.001), and biceps MEPs 31% smaller (P= 0.012), with the arm hanging to the side compared with when the shoulder was flexed. For forearm orientation, both biceps CMEPs and MEPs were 34% smaller (both P< 0.046) in pronation compared with supination. Responses in triceps were influenced by upper-arm, but not forearm, orientation. Triceps CMEPs were 46% smaller (P= 0.007) with the arm hanging to the side compared with when the shoulder was flexed. Triceps MEPs and biceps and triceps MEP/CMEP ratios were unaffected by arm posture. The novel finding is that arm posture-dependent changes in corticospinal excitability in humans are largely spinal in origin. An interplay of multiple reflex inputs to motoneurons likely explains the results.

Relationships between maximal strength, muscle size, and myosin heavy chain isoform composition and postactivation potentiation

The purpose of this study was to examine the relationships between maximal voluntary postactivation potentiation (PAP) and maximal knee extensor torque, quadriceps cross-sectional area (CSA) and volume, and type II myosin heavy chain (MHC) isoform percentage in human skeletal muscle. Thirteen resistance-trained men completed a test protocol consisting of 2 isokinetic knee extensions at 180°·s(-)(1) performed before and 1, 4, 7, and 10 min after the completion of 4 maximal knee extensions at 60°·s(-)(1) (i.e., a conditioning activity (CA)). Magnetic resonance imaging and muscle microbiopsy procedures were completed on separate days to assess quadriceps CSA and volume and MHC isoform content. Maximal voluntary PAP response was assessed as the ratio of the highest knee extensor torques measured before and after the CA. There were large to very large correlations between maximal voluntary PAP response and maximal knee extensor torque (r = 0.62) and quadriceps CSA (r = 0.68) and volume (r = 0.63). Nonetheless, these correlations were not statistically significant after adjusting for the influence of type II MHC percentage using partial correlation analysis. By contrast, the strongest correlation was observed for type II MHC percentage (r = 0.77), and this correlation remained significant after adjusting for the other variables. Maximal voluntary PAP response is strongly correlated with maximal knee extensor torque and quadriceps CSA and volume, but is mostly clearly associated with the type II myosin isoform percentage in human skeletal muscle.

Intermittent stretch reduces force and central drive more than continuous stretch

INTRODUCTION

The relative contributions of central versus peripheral factors to the force loss induced by acute continuous and intermittent plantarflexor stretches were studied.

METHODS

Eighteen healthy young men with no apparent tissue stiffness limitations randomly performed 1) one 5-min stretch (continuous stretch [CS]), 2) five 1-min stretches (intermittent stretch [IS]), and 3) a control condition, on three separate days. The stretches were constant-torque ankle stretches performed on an isokinetic dynamometer. Gastrocnemius medialis oxygenation status was quantified during stretch using near-infrared spectroscopy. Measures of isometric plantarflexor peak torque (Tpeak), voluntary activation (%VA; interpolated twitch technique), EMG amplitude normalized by Mmax (EMG:M), V-wave amplitude, and excitation-contraction (E-C) coupling efficiency (torque ratio between 20- and 80-Hz tetanic stimulations [20:80]) were taken before, immediately, and 15 and 30 min after each condition.

RESULTS

IS caused substantial cyclic variations in tissue oxygenation, but CS resulted in a greater decrease in oxyhemoglobin concentration. Voluntary Tpeak decreased more after IS (-23.8%) than CS (-14.3%) and remained significantly depressed until 30 min after IS only (-5.6%). EMG:M (-27.7%) and %VA (-15.9%) were reduced only after IS. After CS and IS, the magnitude of decrease in Tpeak was correlated with decreases in EMG:M (r = 0.81 and 0.89, respectively), %VA (r = 0.78 and 0.93), and V-wave (r = 0.51, only after IS). Tetanic torque values (20 and 80 Hz) were decreased after IS (-13.1% and -6.4%, respectively) and CS (-10.9% and -6.7%, respectively), but 20:80 was not different from the control group.

CONCLUSION

These results suggest that IS reduced Tpeak more than CS, and these reductions were strongly associated with a depression in central drive.