Somatic and intramuscular distribution of muscle spindles and their relation to muscular angiotypes

Distribution and length of muscle spindles and their 3D visualisation in the medial gastrocnemius of male and female rats

The spatial distribution of the medial gastrocnemius muscle spindles of 10 male and 10 female rats was analysed under a light microscope, and for the first time, visualised using a 3D model of the muscle. Serial cross-sections of the medial gastrocnemius muscles were separated into 10 divisions along with the proximo-distal axis. The muscle spindles of the rat medial gastrocnemius were predominantly distributed on the proximo-medial divisions of the muscle. There were no sex-related differences in the distribution of the studied receptors. A single division contained an average of 2.71 receptors for animals of both sexes. Moreover, the calculated lengths of male and female rat muscle spindles were comparable, and average lengths did not significantly differ (3.30 ± 1.47 mm for male and 3.26 ± 1.32 mm for female rats). Therefore, the present results fill gaps in recent observations concerning similarities in muscle spindle numbers between male and female animals, despite significant differences in muscle mass and size.

Grip force makes wrist joint position sense worse

Background

The purpose of this study was to investigate how grip force affects wrist joint position sense.

Methods

Twenty-two healthy participants (11 men and 11 women) underwent an ipsilateral wrist joint reposition test at 2 distinct grip forces [0 and 15% of maximal voluntary isometric contraction (MVIC)] and 6 different wrist positions (pronation 24°, supination 24°, radial deviation 16°, ulnar deviation 16°, extension 32°, and flexion 32°).

Results

The findings demonstrated significantly elevated absolute error values at 15% MVIC (3.8 ± 0.3°) than at 0% MVIC grip force [3.1 ± 0.2°, t(20) = 2.303, P = 0.032].

Conclusion

These findings demonstrated that there was significantly worse proprioceptive accuracy at 15% MVIC than at 0% MVIC grip force. These results may contribute to a better comprehension of the mechanisms underlying wrist joint injuries, the development of preventative measures to lower the risk of injuries, and the best possible design of engineering or rehabilitation devices.

Alterations in peripheral joint muscle force control in adults with musculoskeletal disease, injury, surgery, or arthroplasty: A systematic review and meta-analysis

PURPOSE

To systematically review and analyse whether musculoskeletal conditions affect peripheral joint muscle force control (i.e. magnitude and/or complexity of force fluctuations).

METHODS

A literature search was conducted using MEDLINE, CINAHL and SPORTDiscus databases (from inception-8th April 2021) for studies involving: 1) participants with musculoskeletal disease, injury, surgery, or arthroplasty in the peripheral joints of the upper/lower limb; 2) comparison with an unaffected control group or unaffected contralateral limb; and 3) measures of the magnitude and/or complexity of force fluctuations during targeted isometric contractions. The methodological quality of studies was evaluated using a modified Downs and Black Quality Index. Studies were combined using the standardized mean difference (SMD) in a random-effects model.

RESULTS

14 studies (investigating 694 participants) were included in the meta-analysis. There was a significant effect of musculoskeletal conditions on peripheral joint muscle force coefficient of variation (CV; SMD = 0.19 [95 % CI 0.06, 0.32]), whereby individuals with musculoskeletal conditions exhibited greater CV than controls. Subgroup analyses revealed that CV was only greater: 1) when comparison was made between symptomatic and asymptomatic individuals (rather than between affected and contralateral limbs; SMD = 0.22 [95 % CI 0.07, 0.38]); 2) for conditions of the knee (SMD = 0.29 [95 % CI 0.14, 0.44]); and 3) for ACL injury post-surgery (SMD = 0.56 [95 % CI 0.36, 0.75]).

CONCLUSION

Musculoskeletal conditions result in an increase in peripheral joint muscle force CV, with this effect dependent on study design, peripheral joint, and surgical status. The greater force CV is indicative of decreased force steadiness and could have implications for long-term tissue health/day-to-day function.

Skeletal muscle function underpins muscle spindle abundance

Muscle spindle abundance is highly variable within and across species, but we currently lack any clear picture of the mechanistic causes or consequences of this variation. Previous use of spindle abundance as a correlate for muscle function implies a mechanical underpinning to this variation, but these ideas have not been tested. Herein, we use integrated medical imaging and subject-specific musculoskeletal models to investigate the relationship between spindle abundance, muscle architecture and in vivo muscle behaviour in the human locomotor system. These analyses indicate that muscle spindle number is tightly correlated with muscle fascicle length, absolute fascicle length change, velocity of fibre lengthening and active muscle forces during walking. Novel correlations between functional indices and spindle abundance are also recovered, where muscles with a high abundance predominantly function as springs, compared to those with a lower abundance mostly functioning as brakes during walking. These data demonstrate that muscle fibre length, lengthening velocity and fibre force are key physiological signals to the central nervous system and its modulation of locomotion, and that muscle spindle abundance may be tightly correlated to how a muscle generates work. These insights may be combined with neuromechanics and robotic studies of motor control to help further tease apart the functional drivers of muscle spindle composition.

Distribution Heterogeneity of Muscle Spindles Across Skeletal Muscles of Lower Extremities in C57BL/6 Mice

Muscle spindles, an important proprioceptor scattered in the skeletal muscle, participate in maintaining muscle tension and the fine regulation of random movement. Although muscle spindles exist in all skeletal muscles, explanations about the distribution and morphology of muscle spindles remain lacking for the indetermination of spindle location across muscles. In this study, traditional time-consuming histochemical technology was utilized to determine the muscle spindle anatomical and morphological characteristics in the lower extremity skeletal muscle in C57BL/6 mice. The relative distance from spindles to nerve-entry points varied from muscles in the ventral-dorsal direction, in which spindles in the lateral of gastrocnemius were not considered to be close to its nerve-entry point. In the longitudinal pattern, the domain with the highest abundance of spindles corresponded to the nerve-entry point, excluding the tibialis anterior. Spindles are mainly concentrated at the middle and rostral domain in all muscles. The results suggest a heterogeneity of the distribution of spindles in different muscles, but the distribution trend generally follows the location pattern of the nerve-entry point. Histochemical staining revealed that the spindle did not have a symmetrical structure along the equator, and this result does not agree with previous findings. Exploring the distribution and structural characteristics of muscle spindles in skeletal muscle can provide some anatomical basis for the study of muscle spindles at the molecular level and treatment of exercise-related diseases and provide a comprehensive understanding of muscle spindle morphology.

Evaluating Sexual Dimorphism of the Muscle Spindles and Intrafusal Muscle Fibers in the Medial Gastrocnemius of Male and Female Rats

This study sought to investigate the sexual dimorphism of muscle spindles in rat medial gastrocnemius muscle. The muscles were cut transversely into 5–10 and 20 μm thick serial sections and the number, density, and morphometric properties of the muscle spindles were determined. There was no significant difference (p > 0.05) in the number of muscle spindles of male (14.45 ± 2.77) and female (15.00 ± 3.13) rats. Muscle mass was 38.89% higher in males (1.08 vs. 0.66 g in females), making the density of these receptors significantly higher (p < 0.01) in females (approximately one spindle per 51.14 mg muscle mass vs. one per 79.91 mg in males). There were no significant differences between the morphometric properties of intrafusal muscle fibers or muscle spindles in male and female rats (p > 0.05): 5.16 ± 2.43 and 5.37 ± 2.27 μm for male and female intrafusal muscle fiber diameter, respectively; 5.57 ± 2.20 and 5.60 ± 2.16 μm for male and female intrafusal muscle fiber number, respectively; 25.85 ± 10.04 and 25.30 ± 9.96 μm for male and female shorter muscle spindle diameter, respectively; and 48.99 ± 20.73 and 43.97 ± 16.96 μm for male and female longer muscle spindle diameter, respectively. These findings suggest that sexual dimorphism in the muscle spindles of rat medial gastrocnemius is limited to density, which contrasts previous findings reporting differences in extrafusal fibers diameter.

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.

Effect of Knee Joint Angle on Regional Hamstrings Activation During Isometric Knee-Flexion Exercise

CONTEXT

Each hamstring muscle is subdivided into several regions by multiple motor nerve branches, which implies each region has different muscle activation properties. However, little is known about the muscle activation of each region with a change in the knee joint angle. Understanding of regional activation of the hamstrings could be helpful for designing rehabilitation and training programs targeted at strengthening a specific region.

OBJECTIVE

To investigate the effect of knee joint angle on the activity level of several regions within the individual hamstring muscles during isometric knee-flexion exercise with maximal effort (MVCKF).

DESIGN

Within-subjects repeated measures.

SETTING

University laboratory.

PARTICIPANTS

Sixteen young males with previous participation in sports competition and resistance training experience.

INTERVENTION

The participants performed 2 MVCKF trials at each knee joint angle of 30°, 60°, and 90°.

OUTCOME MEASURES

Surface electromyography was used to measure muscle activity in the proximal, middle, and distal regions of the biceps femoris long head (BFlh), semitendinosus, and semimembranosus of hamstrings at 30°, 60°, and 90° of knee flexion during MVCKF.

RESULTS

Muscle activity levels in the proximal and middle regions of the BFlh were higher at 30° and 60° of knee flexion than at 90° during MVCKF (all: P < .05). Meanwhile, the activity levels in the distal region of the BFlh were not different among all of the evaluated knee joint angles. In semitendinosus and semimembranosus, the activity levels were higher at 30° and 60° than at 90°, regardless of region (all: P < .05).

CONCLUSION

These findings suggest that the effect of knee joint angle on muscle activity level differs between regions of the BFlh, whereas that is similar among regions of semitendinosus and semimembranosus during MVCKF.

Piezo2 expressed in proprioceptive neurons is essential for skeletal integrity

In humans, mutations in the PIEZO2 gene, which encodes for a mechanosensitive ion channel, were found to result in skeletal abnormalities including scoliosis and hip dysplasia. Here, we show in mice that loss of Piezo2 expression in the proprioceptive system recapitulates several human skeletal abnormalities. While loss of Piezo2 in chondrogenic or osteogenic lineages does not lead to human-like skeletal abnormalities, its loss in proprioceptive neurons leads to spine malalignment and hip dysplasia. To validate the non-autonomous role of proprioception in hip joint morphogenesis, we studied this process in mice mutant for proprioceptive system regulators Runx3 or Egr3. Loss of Runx3 in the peripheral nervous system, but not in skeletal lineages, leads to similar joint abnormalities, as does Egr3 loss of function. These findings expand the range of known regulatory roles of the proprioception system on the skeleton and provide a central component of the underlying molecular mechanism, namely Piezo2.

Effect of Force Level and Gender on Pinch Force Perception in Healthy Adults

This study investigated the effects of both force level and gender on pinch force reproduction errors in normal participants during an ipsilateral force reproduction task. In total, 20 healthy participants were asked to generate a range of levels of reference forces ranging from 5% to 60% maximal voluntary isometric contraction (MVIC) in increments of 5% MVIC using the tip pinch and to reproduce these forces using the same hand. The results showed that the males more accurately and consistently reproduced the forces than did the females, and the most accurate estimation occurred at approximately 20% to 35% MVIC. This finding can help us better understand the reasons for the higher rate of musculoskeletal disorders in females than in males and to develop tools and preventive strategies to decrease the rate of hand injuries in both genders.

Mechanical regulation of musculoskeletal system development

During embryogenesis, the musculoskeletal system develops while containing within itself a force generator in the form of the musculature. This generator becomes functional relatively early in development, exerting an increasing mechanical load on neighboring tissues as development proceeds. A growing body of evidence indicates that such mechanical forces can be translated into signals that combine with the genetic program of organogenesis. This unique situation presents both a major challenge and an opportunity to the other tissues of the musculoskeletal system, namely bones, joints, tendons, ligaments and the tissues connecting them. Here, we summarize the involvement of muscle-induced mechanical forces in the development of various vertebrate musculoskeletal components and their integration into one functional unit.

The Proprioceptive System Regulates Morphologic Restoration of Fractured Bones

Successful fracture repair requires restoration of bone morphology and mechanical integrity. Recent evidence shows that fractured bones of neonatal mice undergo spontaneous realignment, dubbed "natural reduction." Here, we show that natural reduction is regulated by the proprioceptive system and improves with age. Comparison among mice of different ages revealed, surprisingly, that 3-month-old mice exhibited more rapid and effective natural reduction than newborns. Fractured bones of null mutants for transcription factor Runx3, lacking functional proprioceptors, failed to realign properly. Blocking Runx3 expression in the peripheral nervous system, but not in limb mesenchyme, recapitulated the null phenotype, as did inactivation of muscles flanking the fracture site. Egr3 knockout mice, which lack muscle spindles but not Golgi tendon organs, displayed a less severe phenotype, suggesting that both receptor types, as well as muscle contraction, are required for this regulatory mechanism. These findings uncover a physiological role for proprioception in non-autonomous regulation of skeletal integrity.

Phase contrast synchrotron radiation computed tomography of muscle spindles in the mouse soleus muscle

Muscle spindles are skeletal muscle sensory organs involved in the sensation of position and movement of the body. We have explored the capability of phase contrast computed tomography to visualise muscle spindles in murine skeletal muscle. In particular, we have validated the visualisation of nerve fibres through phase contrast computed tomography using light microscopy on stained histological sections. We further present the first three-dimensional visualisation of muscle spindles in mouse soleus skeletal muscle in conjunction with the neurovascular bundle associated with it.

Inspiratory muscle training affects proprioceptive use and low back pain

PURPOSE

We have shown that individuals with recurrent nonspecific low back pain (LBP) and healthy individuals breathing against an inspiratory load decrease their reliance on back proprioceptive signals in upright standing. Because individuals with LBP show greater susceptibility to diaphragm fatigue, it is reasonable to hypothesize that LBP, diaphragm dysfunction, and proprioceptive use may be interrelated. The purpose of this study was to investigate whether inspiratory muscle training (IMT) affects proprioceptive use during postural control in individuals with LBP.

METHODS

Twenty-eight individuals with LBP were assigned randomly into a high-intensity IMT group (high IMT) and low-intensity IMT group (low IMT). The use of proprioception in upright standing was evaluated by measuring center of pressure displacement during local muscle vibration (ankle, back, and ankle-back). Secondary outcomes were inspiratory muscle strength, severity of LBP, and disability.

RESULTS

After high IMT, individuals showed smaller responses to ankle muscle vibration, larger responses to back muscle vibration, higher inspiratory muscle strength, and reduced LBP severity (P < 0.05). These changes were not seen after low IMT (P > 0.05). No changes in disability were observed in either group (P > 0.05).

CONCLUSIONS

After 8 wk of high IMT, individuals with LBP showed an increased reliance on back proprioceptive signals during postural control and improved inspiratory muscle strength and severity of LBP, not seen after low IMT. Hence, IMT may facilitate the proprioceptive involvement of the trunk in postural control in individuals with LBP and thus might be a useful rehabilitation tool for these patients.

Proprioceptive use and sit-to-stand-to-sit after lumbar microdiscectomy: The effect of surgical approach and early physiotherapy

BACKGROUND

Individuals with non-specific low back pain show decreased reliance on lumbosacral proprioceptive signals and slower sit-to-stand-to-sit performance. However, little is known in patients after lumbar microdiscectomy.

METHODS

Patients were randomly assigned into transmuscular (n=12) or paramedian lumbar surgery (n=13). After surgery, the same patients were randomly assigned into individualized active physiotherapy starting 2 weeks after surgery (n=12) or usual care (n=13). Primary outcomes were center of pressure displacement during ankle and back muscles vibration (to evaluate proprioceptive use), and the duration of five sit-to-stand-to-sit movements, evaluated at 2 (baseline), 8 and 24 weeks after surgery.

FINDINGS

Two weeks after surgery, all patients showed smaller responses to back compared to ankle muscles vibration (P<0.05). Patients that underwent a transmuscular surgical procedure and patients that received physiotherapy switched to larger responses to back muscles vibration at 24 weeks, compared to 2 weeks after surgery (P<0.005), although not seen in the paramedian group and usual care group (P>0.05). Already 8 weeks after surgery, the physiotherapy group needed significantly less time to perform five sit-to-stand-to-sit movements compared to the usual care group (P<0.05).

INTERPRETATION

Shortly after lumbar microdiscectomy, patients favor reliance on ankle proprioceptive signals over lumbosacral proprioceptive reliance to maintain posture, which resembles the behavior of patients with non-specific low back pain. However, early active physiotherapy after lumbar microdiscectomy facilitated higher reliance on lumbosacral proprioceptive signals and early improvement of sit-to-stand-to-sit performance. Transmuscular lumbar surgery favoured recovery of lumbosacral proprioception 6 months after surgery.

CLINICAL TRIAL NUMBER

NCT01505595.

Axially evoked postural reflexes: influence of task

Postural reflexes were recorded in healthy subjects (n = 17) using brief axial accelerations and tap stimuli applied at the vertebra prominens (C7) and manubrium sterni. Short latency (SL) responses were recorded from the soleus, hamstrings and tibialis anterior muscles and expressed as a percentage of the background EMG prior to stimulus onset. In the majority of postural conditions tested, subjects were recorded standing erect and leaning forward with their feet together. The SL response was larger for soleus than for the hamstrings during standing (soleus vs hamstrings; 70.4 vs 28.1 %), whereas the opposite occurred during kneeling (25.3 vs 127.3 %). Concordant head and trunk accelerations produced larger SL responses than discordant accelerations for soleus and hamstrings, but the evoked excitatory response was independent of head direction and as expected for the direction of truncal acceleration. Postural reflexes for soleus and tibialis anterior were strongly affected by conditions that posed a significant threat to postural stability; stimulation at C7 was associated with significant SL enhancement for soleus during anterior lean while sternal stimulation showed SL enhancement for tibialis anterior during posterior lean. Cutaneous anaesthesia applied over the C7 stimulation site had no significant effect on EMG responses, nor did vision or surface type (rigid or compliant). This study provides further evidence that postural reflexes produced by brief axial accelerations are independent of cutaneous receptors, vestibular afferents and ankle proprioceptors, and demonstrates that postural tasks and truncal orientation significantly affect the evoked response, consistent with a role in stabilising posture.

Evaluating the differential electrophysiological effects of the focal vibrator on the tendon and muscle belly in healthy people

Objective

To investigate the electrophysiological effects of focal vibration on the tendon and muscle belly in healthy people.

Methods

The miniaturized focal vibrator consisted of an unbalanced mass rotating offset and wireless controller. The parameters of vibratory stimulation were adjusted on a flat rigid surface as 65 µm at 70 Hz. Two consecutive tests on the different vibration sites were conducted in 10 healthy volunteers (test 1, the Achilles tendon; test 2, the muscle belly on the medial head of the gastrocnemius). The Hoffman (H)-reflex was measured 7 times during each test. The minimal H-reflex latency, maximal amplitude of H-reflex (H_max), and maximal amplitude of the M-response (M_max) were acquired. The ratio of H_max and M_max (HMR) and the vibratory inhibition index (VII: the ratio of the H_max after vibration and H_max before vibration) were calculated. The changes in parameters according to the time and site of stimulation were analyzed using the generalized estimating equation methods.

Results

All subjects completed the two tests without serious adverse effects. The minimal H-reflex latency did not show significant changes over time (Wald test: χ²=11.62, p=0.07), and between the two sites (χ²=0.42, p=0.52). The changes in H_max (χ²=53.74, p<0.01), HMR (χ²=20.49, p<0.01), and VII (χ²=13.16, p=0.02) were significant over time with the adjustment of sites. These parameters were reduced at all time points compared to the baseline, but the decrements reverted instantly after the cessation of stimulation. When adjusted over time, a 1.99-mV decrease in the H_max (χ²=4.02, p=0.04) and a 9.02% decrease in the VII (χ²=4.54, p=0.03) were observed when the muscle belly was vibrated compared to the tendon.

Conclusion

The differential electrophysiological effects of focal vibration were verified. The muscle belly may be the more effective site for reducing the H-reflex compared to the tendon. This study provides the neurophysiological basis for a selective and safe rehabilitation program for spasticity management with focal vibration.

Observational study on the occurrence of muscle spindles in human digastric and mylohyoideus muscles

Although the occurrence of muscle spindles (MS) is quite high in most skeletal muscles of humans, few MS, or even absence, have been reported in digastric and mylohyoideus muscles. Even if this condition is generally accepted and quoted in many papers and books, observational studies are scarce and based on histological sections of a low number of specimens. The aim of the present study is to confirm previous data, assessing MS number in a sample of digastric and mylohyoideus muscles. We investigated 11 digastric and 6 mylohyoideus muscles from 13 donors. Muscle samples were embedded in paraffin wax, cross-sectioned in a rostrocaudal direction, and stained using haematoxylin-eosin. A mean of 5.1 ± 1.1 (range 3–7) MS was found in digastric muscles and mean of 0.5 ± 0.8 (range 0–2) in mylohyoideus muscles. A significant difference (P < 0.001) was found with the control sample, confirming the correctness of the histological procedure. Our results support general belief that the absolute number of spindles is sparse in digastric and mylohyoideus muscles. External forces, such as food resistance during chewing or gravity, do not counteract jaw-opening muscles. It is conceivable that this condition gives them a limited proprioceptive importance and a reduced need for having specific receptors as MS.

H-reflexes reduce fatigue of evoked contractions after spinal cord injury

INTRODUCTION

Neuromuscular electrical stimulation (NMES) over a muscle belly (mNMES) generates contractions predominantly through M-waves, while NMES over a nerve trunk (nNMES) can generate contractions through H-reflexes in people who are neurologically intact. We tested whether the differences between mNMES and nNMES are present in people with chronic motor-complete spinal cord injury and, if so, whether they influence contraction fatigue.

METHODS

Plantar-flexion torque and soleus electromyography were recorded from 8 participants. Fatigue protocols were delivered using mNMES and nNMES on separate days.

RESULTS

nNMES generated contractions that fatigued less than mNMES. Torque decreased the least when nNMES generated contractions, at least partly through H-reflexes (n = 4 participants; 39% decrease), and torque decreased the most when contractions were generated through M-waves, regardless of NMES site (nNMES 71% decrease, n = 4; mNMES, 73% decrease, n = 8).

CONCLUSIONS

nNMES generates contractions that fatigue less than mNMES, but only when H-reflexes contribute to the evoked contractions.

Postural strategy and back muscle oxygenation during inspiratory muscle loading

PURPOSE

Most healthy individuals show a multisegmental control strategy during challenging standing conditions, whereas others show a rigid ankle-steered strategy, which is assumed as suboptimal. Respiratory-demanding tasks exert a perturbing effect on balance, although the underlying mechanisms remain poorly understood. The purpose of this study was to investigate whether inspiratory resistive loading (IRL) affects postural strategy, back muscle oxygenation, and blood volume during postural control.

METHODS

We assessed the acute effects of increased respiratory effort by measuring the center of pressure displacement in 12 healthy individuals during upright standing on an unstable support surface while breathing against an IRL. Simultaneous ankle and back muscle vibration was used to evaluate the proprioceptive strategy (multisegmental vs ankle-steered) during postural control. Back muscles oxygenation and blood volume were assessed using near-infrared spectroscopy (tissue oxygenation index, deoxyhemoglobin, oxyhemoglobin, and combined hemoglobin).

RESULTS

An increased proprioceptive gain at the ankles and an decreased gain at the back were observed after approximately 7 min of IRL. Retrospectively, the group was subdivided on the basis of the participants' dominant proprioceptive use during a baseline postural control. During IRL, the ankle-steered group showed an increased reliance on ankle proprioception compared with a multisegmental group (-5.9 ± 3.1 and 1.0 ± 1.9 cm, respectively, P < 0.05). Tissue oxygenation index, deoxyhemoglobin, oxyhemoglobin, and combined hemoglobin declined progressively in the ankle-steered group during the IRL (from baseline (100%) to -1%, -1%, -45%, and -18%, respectively, P < 0.05), whereas no decline was found in the multisegmental group (from baseline (100%) to 134%, 82%, 129%, and 153%, respectively, P > 0.05).

CONCLUSION

Individuals who adopted an ankle-steered strategy during IRL showed a progressive decline in back muscle oxygenation and blood volume. In contrast, IRL did not affect back muscle oxygenation and blood volume in individuals who showed a multisegmental strategy in upright standing.

Age-related declines in the detection of passive wrist movement

Age-related changes in proprioceptive ability and their contributions to postural instability have been well documented. In contrast, and despite the known importance of proprioceptive feedback in the control of coordinated arm and hand movement, studies focusing on upper limb proprioception in older populations are few and equivocal in their findings. This study focused on kinesthetic awareness about the wrist joint in healthy young and older adults. Passive movement detection thresholds (PMDTs) were twice as high in older compared to young participants. In contrast to previous findings demonstrating asymmetries in static position sense, PMDT did not differ between the dominant and non-dominant wrist joints nor did direction of joint displacement affect PMDT as has been reported for the lower limb. Preliminary analysis indicated that PMDT was significantly higher in older adults categorized as sedentary while active older adults were able to detect passive movement as well as young adults. These findings demonstrate that upper limb kinesthesia is impaired in older adults although the degree of impairment may be influenced by one's level of physical activity.

Muscle dependency of corticomuscular coherence in upper and lower limb muscles and training-related alterations in ballet dancers and weightlifters

It has been well documented that the 15- to 35-Hz oscillatory activity of the sensorimotor cortex shows coherence with the muscle activity during weak to moderate steady contraction. To investigate the muscle dependency of the corticomuscular coherence and its training-related alterations, we quantified the coherence between electroencephalogram (EEG) from the sensorimotor cortex and rectified electromyogram (EMG) from five upper limb (first dorsal interosseous, flexor carpi radialis, extensor carpi radialis, biceps brachii, triceps brachii) and four lower limb muscles (soleus, tibialis anterior, biceps femoris, rectus femoris), while maintaining a constant force level at 30% of maximal voluntary contraction of each muscle, in 24 untrained, 12 skill-trained (ballet dancers), and 10 strength-trained (weightlifters) individuals. Data from untrained subjects demonstrated the muscle dependency of corticomuscular coherence. The magnitude of the EEG-EMG coherence was significantly greater in the distally located lower limb muscles, such as the soleus and tibialis anterior, than in the upper or other lower limb muscles in untrained subjects ( P < 0.05). These results imply that oscillatory coupling between the sensorimotor cortex and spinal motoneurons during steady contraction differs among muscles, according to the functional role of each muscle. In addition, the ballet dancers and weightlifters showed smaller EEG-EMG coherences than the untrained subjects, especially in the lower limb muscles ( P < 0.05). These results indicate that oscillatory interaction between the sensorimotor cortex and spinal motoneurons can be changed by long-term specialized use of the muscles and that this neural adaptation may lead to finer control of muscle force during steady contraction.

Retention of Pax3 expression in satellite cells of muscle spindles

Intrafusal fibers within muscle spindles retain features characteristic of immaturity, unlike the larger and more numerous extrafusal fibers constituting the bulk of skeletal muscle. Satellite cells (SCs), myogenic progenitors, are detected on the surfaces of both intrafusal and extrafusal fibers, but little is known of spindle SCs. We have recently demonstrated that, like their extrafusal counterparts, SCs in muscle spindles of posthatch chickens express paired box transcription factor 7 (Pax7) protein. During vertebrate embryogenesis, myogenic progenitors express both Pax7 and Pax3 proteins. In postnatal mice, Pax3 appears in rare SC subsets, whereas Pax7 is expressed by all SCs within extrafusal fibers. Here we test the hypothesis that Pax3 protein maintains localized expression within SCs of muscle spindles. Immunohistochemical techniques were used to identify SCs by their Pax7 expression within anterior latissimus dorsi muscle excised from posthatch chickens of various ages. A greater percentage of SCs express Pax3 within intrafusal than extrafusal fibers at each age, and the proportion of SCs expressing Pax3 declines with aging. This is the first study to localize Pax3 expression in posthatch avian muscle and within SCs of muscle spindles. We suggest that Pax3-positive SCs are involved in fiber maintenance.

Comparative impact of 2 botulinum toxin injection techniques for elbow flexor hypertonia

OBJECTIVE

To compare 2 techniques of botulinum toxin injection for elbow flexor hypertonia.

DESIGN

Parallel-group, randomized, controlled trial with blinded outcome assessment.

SETTING

Laboratory, tertiary rehabilitation hospital.

PARTICIPANTS

Adults (N=31) with acquired brain injury (21 with traumatic brain injury, 8 with stroke, 2 with hypoxic encephalopathy) provided 36 sets of elbow flexors with Ashworth Scale scores equal to 3.

INTERVENTION

Botulinum toxin type A (BTX-A) was injected with a motor point or a multisite injection technique after obtaining 2 baseline evaluations of the main outcome measures. Motor point technique involved decremental electric stimulation with delivery of 60U of BTX-A (Botox) in 2.4mL or 30U BTX-A in 1.2mL of preservative-free saline at single biceps and brachioradialis motor points, respectively. Distributed injection was performed using electromyographic feedback. Fifteen units in 0.6mL were delivered to each of 4 biceps sites and 2 brachioradialis sites. Total dose (90U) and total injection volume (3.6mL) were identical across groups. Only sites and injection techniques varied. The brachialis was not injected in either group.

MAIN OUTCOME MEASURES

Ashworth Scale, Tardieu catch angle, and root mean square surface electromyographic activity of the biceps, brachialis, and brachioradialis.

RESULTS

Postintervention testing at 3 weeks showed no significant differences between groups (P range, .31-.82 across 3 outcome measures). However, within each group, significant treatment effects were observed on all outcome measures (all P<.01). For the uninjected brachialis muscle, electromyographic reduction was greater for the distributed group.

CONCLUSIONS

In 31 adults with acquired brain injury, single motor point and multisite distributed injections of low-dose, high-volume BTX-A had similar impact. Findings suggest that low-dose, high-volume strategies may have a potential role in reducing drug cost and helping clinicians stay within accepted limits for total body dose in patients with upper motoneuron syndrome requiring many injections.

The distribution and abundance of muscle spindles

This commentary suggests that the distribution and abundance of muscle spindles in different muscles is related to their role as signallers of muscle fascicle length. Large muscles comprising many fascicles will therefore have more spindles than smaller muscles.

Intensity- and muscle-specific fascicle behavior during human drop jumps

The present study was designed to examine fascicle-tendon interaction in the synergistic medial gastrocnemius (MG) and soleus (Sol) muscles during drop jumps (DJ) performed from different drop heights (DH). Eight subjects performed unilateral DJ with maximal rebounds on a sledge apparatus from different DH. During the exercises, fascicle lengths (using ultrasonography) and electromyographic activities were recorded. The results showed that the fascicles of the MG and Sol muscles behaved differently during the contact phase, but the whole muscle-tendon unit and its tendinous tissue lengthened before shortening in both muscles. The Sol fascicles also lengthened before shortening during the ground contact in all conditions. During the braking phase, the Sol activation increased with increasing DH. However, the amplitude of Sol fascicle lengthening was not dependent on DH during the same phase. In the MG muscle, the fascicles primarily shortened during the braking phase in the lower DH condition. However, in the higher DH conditions, the MG fascicles either behaved isometrically or were lengthened during the braking phase. These results suggest that the fascicles of synergistic muscles (MG and Sol) can behave differently during DJ and that, with increasing DH, there may be specific length change patterns of the fascicles of MG but not of Sol.

The Effects of Thoracic Manipulation on Heart Rate Variability: A Controlled Crossover Trial

OBJECTIVE

The objective of this study was to measure the effects of thoracic spinal manipulation on heart rate variability (HRV) in a cohort of healthy young adults.

METHODS

A controlled crossover trial that was conducted on 28 healthy young adults (23 men and 5 women; age range, 18-45 years; mean age, 29 +/- 7 years) measured HRV before and after a sham procedure and a thoracic spinal manipulation.

RESULTS

In healthy young adults, thoracic spinal manipulation was associated with changes in HRV that were not duplicated by the sham procedure. The ratio of the powers of the low-frequency and high-frequency components increased from 0.9562 +/- 0.9192 to 1.304 +/- 1.118 (P = .0030, Wilcoxon signed rank test). In subjects undergoing sham spinal manipulation, there was no statistically significant change in the low-frequency or the high-frequency component of the power spectrum; neither was there any in the ratio of the two regardless of whether the comparison was made using the paired t test or the Wilcoxon signed rank test.

CONCLUSION

High-velocity and low-amplitude manipulation of the thoracic spine appears to be able to influence autonomic output to the heart in ways that are not duplicated by a sham procedure or by other forms of somatic/physical therapies.

An allometric analysis of the number of muscle spindles in mammalian skeletal muscles

An allometric analysis of the number of muscle spindles in relation to muscle mass in mammalian (mouse, rat, guinea-pig, cat, human) skeletal muscles is presented. It is shown that the trend to increasing number as muscle mass increases follows an isometric (length) relationship between species, whereas within a species, at least for the only essentially complete sample (human), the number of spindles scales, on average, with the square root rather than the cube root of muscle mass. An attempt is made to reconcile these apparently discrepant relationships. Use of the widely accepted spindle density (number of spindles g(-1) of muscle) as a measure of relative abundance of spindles in different muscles is shown to be grossly misleading. It is replaced with the residuals of the linear regression of ln spindle number against ln muscle mass. Significant differences in relative spindle abundance as measured by residuals were found between regional groups of muscles: the greatest abundance is in axial muscles, including those concerned with head position, whereas the least is in muscles of the shoulder girdle. No differences were found between large and small muscles operating in parallel, or between antigravity and non-antigravity muscles. For proximal vs. distal muscles, spindles were significantly less abundant in the hand than the arm, but there was no difference between the foot and the leg.

Spinal manipulation and spinal mobilization influence different axial sensory beds

Manipulation and mobilization are two forms of manual therapy commonly employed in the management of musculoskeletal disorders. Spinal manipulation and mobilization are often distinguished from one another by reference to certain biomechanical parameters such as peak force, duration and magnitude of translation. However, as of yet, there is relatively little research which distinguishes between them in terms of neurological mechanisms or clinical effectiveness. Theories concerning the mechanisms underlying the therapeutic effects of manipulation and mobilization commonly make reference to mechanical events such as the release of entrapped tissue or the disruption of intra-articular adhesions. Relatively less attention is given to neural effects. In this paper, we hypothesize that, at least in part, spinal manipulation preferentially influences a sensory bed which, in terms of anatomical location and function, is different from the sensory bed influenced by spinal mobilization techniques. More specifically, we hypothesize that manipulation may particularly stimulate receptors within deep intervertebral muscles, while mobilization techniques most likely affect more superficial axial muscles. In part, our rationale for this hypothesis is based on differences in mechanical advantage of the respective manual procedures on multi-segmental versus short intervertebral muscles.