Muscle fibre typology affects whole-body metabolic rate during isolated muscle contractions and human locomotion.

The wide variation in muscle fibre type distribution across individuals, along with the very different energy consumption rates in slow versus fast muscle fibres, suggests that muscle fibre typology contributes to inter-individual differences in metabolic rate during exercise. However, this has been hard to demonstrate due to the gap between a single muscle fibre and full-body exercises. We investigated the isolated effect of triceps surae muscle contraction velocity on whole-body metabolic rate during cyclic contractions in individuals a priori selected for their predominantly slow (n = 11) or fast (n = 10) muscle fibre typology by means of proton magnetic resonance spectroscopy (1H-MRS). Subsequently, we examined their whole-body metabolic rate during walking and running at 2 m/s, exercises with comparable metabolic rates but distinct triceps surae muscle force and velocity demands (walking: low force, high velocity; running: high force, low velocity). Increasing triceps surae contraction velocity during cyclic contractions elevated net whole-body metabolic rate for both typology groups. However, the slow group consumed substantially less net metabolic energy at the slowest contraction velocity, but the metabolic difference between groups diminished at faster velocities. Consistent with the more economic force production during slow contractions, the slow group exhibited lower metabolic rates than the fast group while running, whereas metabolic rates were similar during walking. These findings provide important insights into the influence of muscle fibre typology on whole-body metabolic rate and emphasize the importance of considering muscle mechanical demands to understand muscle fibre typology related differences in whole-body metabolic rates. KEY POINTS: Muscle fibre typology is often suggested to affect whole-body metabolic rate, yet convincing in vivo evidence is lacking. Using isolated plantar flexor muscle contractions in individuals a priori selected for their predominantly slow or fast muscle fibre typology, we demonstrated that having predominantly slow muscle fibres provides a metabolic advantage during slow muscle contractions, but this benefit disappeared at faster contractions. We extended these results to full-body exercises, where we demonstrated that higher proportions of slow fibres associated with better economy during running but not when walking. These findings provide important insights into the influence of muscle fibre typology on whole-body metabolic rate and emphasize the importance of considering muscle mechanical demands to understand muscle fibre typology related differences in whole-body metabolic rate.

Reduced Intratendinous Sliding in Achilles Tendinopathy During Active Plantarflexion Regardless of Horizontal Foot Position.

PURPOSE: The Achilles tendon consists of three subtendons with the ability to slide relative to each other. As optimal intratendinous sliding is thought to reduce the overall stress in the tendon, alterations in sliding behavior could potentially play a role in the development of Achilles tendinopathy. The aims of this study were to investigate the difference in intratendinous sliding within the Achilles tendon during isometric contractions between asymptomatic controls and patients with Achilles tendinopathy and the effect of changing the horizontal foot position on intratendinous sliding in both groups. METHODS: Twenty-nine participants (13 Achilles tendinopathy and 16 controls) performed isometric plantarflexion contractions at 60% of their maximal voluntary contraction (MVC), in toes-neutral, and at 30% MVC in toes-neutral, toes-in, and toes-out positions during which ultrasound images were recorded. Intratendinous sliding was estimated as the superficial-to-middle and middle-to-deep relative displacement. RESULTS: Patients with Achilles tendinopathy present lower intratendinous sliding than asymptomatic controls. Regarding the horizontal foot position in both groups, the toes-out foot position resulted in increased sliding compared with both toes-neutral and toes-out foot position. CONCLUSION: We provided evidence that patients with Achilles tendinopathy show lower intratendinous sliding than asymptomatic controls. Since intratendinous sliding is a physiological feature of the Achilles tendon, the external foot position holds promise to increase sliding in patients with Achilles tendinopathy and promote healthy tendon behavior. Future research should investigate if implementing this external foot position in rehabilitation programs stimulates sliding within the Achilles tendon and improves clinical outcome.

Do the fasciae of the soleus have a role in plantar fasciitis?

Plantar fasciitis is a chronic, self-limiting, and painful disabling condition affecting the inferomedial aspect of the heel, usually extending toward the metatarsophalangeal joints. There is compelling evidence for a strong correlation between Achilles tendon (AT) loading and plantar aponeurosis (PA) tension. In line with this, tightness of the AT is found in almost 80% of patients affected by plantar fasciitis. A positive correlation has also been reported between gastrocnemius-soleus tightness and heel pain severity in this condition. Despite its high prevalence, the exact etiology and pathological mechanisms underlying plantar heel pain remain unclear. Therefore, the aim of the present paper is to discuss the anatomical and biomechanical substrates of plantar fasciitis with special emphasis on the emerging, though largely neglected, fascial system. In particular, the relationship between the fascia, triceps surae muscle, AT, and PA will be analyzed. We then proceed to discuss how structural and biomechanical alterations of the muscle-tendon-fascia complex due to muscle overuse or injury can create the conditions for the onset of PA pathology. A deeper knowledge of the possible molecular mechanisms underpinning changes in the mechanical properties of the fascial system in response to altered loading and/or muscle contraction could help healthcare professionals and clinicians refine nonoperative treatment strategies and rehabilitation protocols for plantar fasciitis.

Is muscle stiffness a determinant for range of motion in the leg muscles?

Previous training studies with comprehensive stretching durations have reported that an increase in range of motion (ROM) can be related to decreases in muscle stiffness. Therefore, the purpose of this study was to analyze the association between the passive muscle stiffness of three muscle groups (triceps surae, quadriceps, hamstrings) to the respective joint ROM. Thirty-six healthy male soccer players volunteered in this study. After a standardized warm-up, the muscle stiffness was tested via shear wave elastography in six muscles (gastrocnemius medialis and lateralis, rectus femoris, semitendinosus, semimembranosus, and biceps femoris long head). The hip extension, hip flexion, and ankle dorsiflexion ROM were also assessed with a modified Thomas test, a sit and reach test, and a standing wall push test, respectively. We found significant moderate to large correlations between hip flexion ROM and muscle stiffness for the semimembranosus (rP = -0.43; P = 0.01), biceps femoris long head (rP = -0.45; P = 0.01), and overall hamstring stiffness (rP = -0.50; P < 0.01). No significant correlations were found for triceps surae (rP = -0.12; P = 0.51 to 0.67) and rectus femoris muscle stiffness (rP = 0.25; P = 0.14) with ankle dorsiflexion and hip extension ROM, respectively. We conclude that muscle stiffness is an important contributor to hip flexion ROM, but less important for hip extension or ankle joint ROM. Additional contributors to ROM might be tendon stiffness or stretch/pain tolerance.

Speed-specific optimal contractile conditions of the human soleus muscle from slow to maximum running speed.

The soleus is the main muscle for propulsion during human running but its operating behavior across the spectrum of physiological running speeds is currently unknown. This study experimentally investigated the soleus muscle activation patterns and contractile conditions for force generation, power production and efficient work production (i.e. force-length potential, force-velocity potential, power-velocity potential and enthalpy efficiency) at seven running speeds (3.0 m s-1 to individual maximum). During submaximal running (3.0-6.0 m s-1), the soleus fascicles shortened close to optimal length and at a velocity close to the efficiency maximum, two contractile conditions for economical work production. At higher running speeds (7.0 m s-1 to maximum), the soleus muscle fascicles still operated near optimum length, yet the fascicle shortening velocity increased and shifted towards the optimum for mechanical power production with a simultaneous increase in muscle activation, providing evidence for three cumulative mechanisms to enhance mechanical power production. Using the experimentally determined force-length-velocity potentials and muscle activation as inputs in a Hill-type muscle model, a reduction in maximum soleus muscle force at speeds ≥7.0 m s-1 and a continuous increase in maximum mechanical power with speed were predicted. The reduction in soleus maximum force was associated with a reduced force-velocity potential. The increase in maximum power was explained by an enhancement of muscle activation and contractile conditions until 7.0 m s-1, but mainly by increased muscle activation at high to maximal running speed.

Human muscle-tendon unit mechanobiological responses to consecutive high strain cyclic loading.

In response to a mechanical stimulus, tendons have a slower tissue renewal rate compared with muscles. This could, over time, lead to a higher mechanical demand (experienced strain) for the tendon, especially when a high strain magnitude exercise is repeated without sufficient recovery. The current study investigated the adaptive responses of the human triceps surae (TS) muscle-tendon unit (MTU) and extracellular matrix turnover-related biomarkers to repetitive high tendon strain cyclic loading. Eleven young adult males performed a progressive resistance exercise over 12 consecutive days, consisting of high Achilles tendon (AT) strain cyclic loading (90% MVC) with one leg once a day (LegT1) and the alternate leg three times a day (LegT3). Exercise-related changes in TS MTU mechanical properties and serum concentrations of extracellular matrix turnover-related biomarkers were analysed over the intervention period. Both legs demonstrated similar increases in maximal AT force (∼10%) over the 12 day period of exercise. A ∼20% increase in maximal AT strain was found for LegT3 (P<0.05) after 8 consecutive exercise days, along with a corresponding decrease in AT stiffness. These effects were maintained even after a 48 h rest period. The AT mechanical properties for LegT1 were unaltered. Biomarker analysis revealed no sign of inflammation but there was altered collagen turnover and a delay in collagen type I synthesis. Accordingly, we suggest that tendon is vulnerable to frequent high magnitude cyclic mechanical loading as accumulation of micro-damage can potentially exceed the rate of biological repair, leading to increased maximal tendon strain.

Can motor imagery balance the acute fatigue induced by neuromuscular electrical stimulation?

PURPOSE: The combination of motor imagery (MI) and neuromuscular electrical stimulation (NMES) can increase the corticospinal excitability suggesting that such association could be efficient in motor performance improvement. However, differential effect has been reported at spinal level after MI and NMES alone. The purpose of this study was to investigate the acute effect on motor performance and spinal excitability following MI, NMES and combining MI and NMES. METHODS: Ten participants were enrolled in three experimental sessions of MI, NMES and MI + NMES targeting plantar flexor muscles. Each session underwent 60 imagined, evoked (20% MVC) or imagined and evoked contractions simultaneously. Before, immediately after and 10 min after each session, maximal M-wave and H-reflex were evoked by electrical nerve stimulation applied at rest and during maximal voluntary contraction (MVC). RESULTS: The MVC decreased significantly between PRE-POST (- 12.14 ± 6.12%) and PRE-POST 10 (- 8.1 ± 6.35%) for NMES session, while this decrease was significant only between PRE-POST 10 (- 7.16 ± 11.25%) for the MI + NMES session. No significant modulation of the MVC was observed after MI session. The ratio Hmax/Mmax was reduced immediately after NMES session only. CONCLUSION: The combination of MI to NMES seems to delay the onset of neuromuscular fatigue compared to NMES alone. This delay onset of neuromuscular fatigue was associated with specific modulation of the spinal excitability. These results suggested that MI could compensate the neuromuscular fatigue induced acutely by NMES until 10 min after the combination of both modalities.

Lower motor unit discharge rates in gastrocnemius lateralis, but not in gastrocnemius medialis or soleus, in runners with Achilles tendinopathy: a pilot study.

OBJECTIVES: Deficits in muscle performance could be a consequence of a reduced ability of a motor neuron to increase the rate in which it discharges. This study aimed to investigate motor unit (MU) discharge properties of each triceps surae muscle (TS) and TS torque steadiness during submaximal intensities in runners with Achilles tendinopathy (AT). METHODS: We recruited runners with (n = 12) and without (n = 13) mid-portion AT. MU discharge rate was analysed for each of the TS muscles, using high-density surface electromyography during 10 and 20% isometric plantar flexor contractions. RESULTS: MU mean discharge rate was lower in the gastrocnemius lateralis (GL) in AT compared to controls. In AT, GL MU mean discharge rate did not increase as torque increased from 10% peak torque, 8.24 pps (95% CI 7.08 to 9.41) to 20%, 8.52 pps (7.41 to 9.63, p = 0.540); however, in controls, MU discharge rate increased as torque increased from 10%, 8.39 pps (7.25-9.53) to 20%, 10.07 pps (8.89-11.25, p < 0.001). There were no between-group difference in gastrocnemius medialis (GM) or soleus (SOL) MU discharge rates. We found no between-group differences in coefficient of variation of MU discharge rate in any of the TS muscles nor in TS torque steadiness. CONCLUSION: Our data demonstrate that runners with AT may have a lower neural drive to GL, failing to increase MU discharge rate to adjust for the increase in torque demand. Further research is needed to understand how interventions focussing on increasing neural drive to GL would affect muscle function in runners with AT.

Protocols Targeting Afferent Pathways via Neuromuscular Electrical Stimulation for the Plantar Flexors: A Systematic Review.

This systematic review documents the protocol characteristics of studies that used neuromuscular electrical stimulation protocols (NMES) on the plantar flexors [through triceps surae (TS) or tibial nerve (TN) stimulation] to stimulate afferent pathways. The review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement, was registered to PROSPERO (ID: CRD42022345194) and was funded by the Greek General Secretariat for Research and Technology (ERA-NET NEURON JTC 2020). Included were original research articles on healthy adults, with NMES interventions applied on TN or TS or both. Four databases (Cochrane Library, PubMed, Scopus, and Web of Science) were systematically searched, in addition to a manual search using the citations of included studies. Quality assessment was conducted on 32 eligible studies by estimating the risk of bias with the checklist of the Effective Public Health Practice Project Quality Assessment Tool. Eighty-seven protocols were analyzed, with descriptive statistics. Compared to TS, TN stimulation has been reported in a wider range of frequencies (5-100, vs. 20-200 Hz) and normalization methods for the contraction intensity. The pulse duration ranged from 0.2 to 1 ms for both TS and TN protocols. It is concluded that with increasing popularity of NMES protocols in intervention and rehabilitation, future studies may use a wider range of stimulation attributes, to stimulate motor neurons via afferent pathways, but, on the other hand, additional studies may explore new protocols, targeting for more optimal effectiveness. Furthermore, future studies should consider methodological issues, such as stimulation efficacy (e.g., positioning over the motor point) and reporting of level of discomfort during the application of NMES protocols to reduce the inherent variability of the results.

The Role of the Plantaris in Intramuscular Gastrocnemius Equinus Correction.

Equinus deformity is a common cause of foot and ankle pathology. The purpose of our study was to evaluate the role of the plantaris in equinus. Secondary aims were to describe the role of the plantaris in intramuscular gastrocnemius recession and to determine the prevalence of the plantaris in our patient population. We measured ankle dorsiflexion during the steps of a Baumann-type intramuscular gastrocnemius recession. Eighty-nine patients were enrolled in our study. Fourteen of 89 (15.7%) patients did not have a plantaris. A mean dorsiflexion of 9 (interquartile range 6-12)° was obtained after transection of the plantaris tendon and an additional mean 8 (interquartile range 5-10)° was obtained after recession of the gastrocnemius aponeurosis. There was a strong positive correlation (rs = 0.842) of dorsiflexion increase after plantaris transection and dorsiflexion increase after gastrocnemius recession (p < .00). Linear regression showed that for every one-degree of dorsiflexion increase with plantaris transection, there was a predicted dorsiflexion increase of 0.69° with gastrocnemius recession. These results indicate that the plantaris is a component of equinus deformity.

Percutaneous minimal invasive Achilles tendon lengthening improves clinical and radiographic outcomes in severe flexible flatfeet with shortened triceps sureae complex in early childhood: A retrospective study.

BACKGROUND: Severe flexible flatfeet with triceps surae complex shortening are prognostically unfavorable in early childhood and may compromise normal foot development. METHODS: This retrospective, IRB-approved study included 20 children (38 feet) under 6 years with severe flexible flatfeet and triceps surae complex shortening. Treatment included minimally invasive percutaneous achilles tendon lengthening followed by a 4-week cast fixation and corrective orthotic therapy under talo-navicular reposition for at least 6-months. Preoperative weightbearing x-rays and at the last available follow-up included anteroposterior talus-first metatarsal angle and lateral talus pitch, Meary's and talocalcaneal angle and were compared to reference values. ROM, surgeon-rated clinical outcomes and complications/re-interventions were evaluated. RESULTS: Age at surgery was 3.7 years (1.3-5.9 y) and follow-up time was 4.3 years (1.1-8.9 y). No complications occurred. Clinical outcome was good (68 %) to very good (26 %). Ratio of normal angles increased significantly for three angles. Dorsiflexion ROM improved from -5.0 ± 6.8° at baseline to 15.7 ± 7.6°. CONCLUSIONS: With significant improvements in clinical and radiographic outcomes, minimal-invasive percutaneous Achilles tendon lengthening followed by orthotic therapy seems to be a valuable treatment option for selected preschool-aged patients with severe, flexible flatfeet with significantly shortened triceps surae. LEVEL OF EVIDENCE: IV.

Typical m. triceps surae morphology and architecture measurement from 0 to 18 years: A narrative review.

The aim of this review was to report on the imaging modalities used to assess morphological and architectural properties of the m. triceps surae muscle in typically developing children, and the available reliability analyses. Scopus and MEDLINE (Pubmed) were searched systematically for all original articles published up to September 2020 measuring morphological and architectural properties of the m. triceps surae in typically developing children (18 years or under). Thirty eligible studies were included in this analysis, measuring fibre bundle length (FBL) (n = 11), pennation angle (PA) (n = 10), muscle volume (MV) (n = 16) and physiological cross-sectional area (PCSA) (n = 4). Three primary imaging modalities were utilised to assess these architectural parameters in vivo: two-dimensional ultrasound (2DUS; n = 12), three-dimensional ultrasound (3DUS; n = 9) and magnetic resonance imaging (MRI; n = 6). The mean age of participants ranged from 1.4 years to 18 years old. There was an apparent increase in m. gastrocnemius medialis MV and pCSA with age; however, no trend was evident with FBL or PA. Analysis of correlations of muscle variables with age was limited by a lack of longitudinal data and methodological variations between studies affecting outcomes. Only five studies evaluated the reliability of the methods. Imaging methodologies such as MRI and US may provide valuable insight into the development of skeletal muscle from childhood to adulthood; however, variations in methodological approaches can significantly influence outcomes. Researchers wishing to develop a model of typical muscle development should carry out longitudinal architectural assessment of all muscles comprising the m. triceps surae utilising a consistent approach that minimises confounding errors.

Postural support requirements preferentially modulate late components of the gastrocnemius response to transcranial magnetic stimulation.

Mounting evidence suggests that motor evoked potentials (MEPs) recorded in upper limb muscles with postural support roles following transcranial magnetic stimulation receive contributions from both corticospinal and non-corticospinal descending pathways. We tested the hypothesis that neural structures responsible for regulating upright balance are involved in transmitting late portions of TMS-induced MEPs in a lower limb muscle. MEPs were recorded in the medial gastrocnemius muscles of each leg, while participants supported their upright posture in five postural conditions that required different levels of support from the target muscles. We observed that early and late portions of the MEP were modulated independently, with early MEP amplitude being reduced when high levels of postural support were required from a target muscle. Independent modulation of early and late MEPs by altered postural demand suggests largely separable transmission of each part of the MEP. The early component of the MEP is likely generated by fast-conducting corticospinal pathways, whereas the later component may be primarily transmitted along a polysynaptic cortico-reticulospinal pathway.

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.

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 (Peaktwitch ), and peak soleus (EMGSOL ), medial (EMGMG ), and lateral gastrocnemius (EMGLG ) 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 Peaktwitch forces decreased (p ≤ 0.001) immediately after all conditions. No changes were observed for VA (p = 0.365). EMGSOL amplitude reduced (p = 0.040) only after NP, yet EMGLG and EMGMG 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.

Validity and reliability of a novel 3D ultrasound approach to assess static lengths and the lengthening behavior of the gastrocnemius medialis muscle and the Achilles tendon in vivo.

PURPOSE: Human muscle-tendon units (MTUs) are highly plastic and undergo changes in response to specific diseases and disorders. To investigate the pathological changes and the effects of therapeutic treatments, the use of valid and reliable examination methods is of crucial importance. Therefore, in this study, a simple 3D ultrasound approach was developed and evaluated with regard to: (1) its validity in comparison to magnetic resonance imaging (MRI) for the assessment of the gastrocnemius medialis (GM) MTU, muscle belly, and Achilles tendon lengths; and (2) its reliability for static and dynamic length measurements. METHODS: Sixteen participants were included in the study. To evaluate the validity and reliability of the novel 3D ultrasound approach, two ultrasound measurement sessions and one MRI assessment were performed. By combining 2D ultrasound and 3D motion capture, the tissue lengths were assessed at a fixed ankle joint position and compared to the MRI measurements using Bland-Altman plots. The intra-rater and inter-rater reliability for the static and dynamic length assessments was determined using the coefficient of variation, standard error of measurement (SEM), minimal detectable change (MDC95), and intraclass correlation coefficient (ICC). RESULTS: The 3D ultrasound approach slightly underestimated the length when compared with MRI by 0.7%, 1.5%, and 1.1% for the GM muscle belly, Achilles tendon, and MTU, respectively. The approach showed excellent intra-rater as well as inter-rater reliability, with high ICC (≥ 0.94), small SEM (≤ 1.3 mm), and good MDC95 (≤ 3.6 mm) values, with even better reliability found for the static length measurements. CONCLUSION: The proposed 3D ultrasound approach was found to be valid and reliable for the assessment of the GM MTU, muscle belly, and Achilles tendon lengths, as well as the tissue lengthening behavior, confirming its potential as a useful tool for investigating the effects of training interventions or therapeutic treatments (e.g., surgery or conservative treatments such as stretching and orthotics). LEVEL OF EVIDENCE: Level II.

The Reliability and Validity of a Novel Ankle Isometric Plantar Flexion Strength Test.

CONTEXT: Strengthening of ankle plantar flexor muscles is one of the important components in the rehabilitation of many lower limb injuries and disorders. However, there are few simple methods to evaluate ankle plantar flexor strength in clinical situations without using equipment. The purpose of this study was to verify the intraexaminer and interexaminer reliability and the concurrent validity of a novel ankle isometric plantar flexion (PF) strength test (PF break test [PFBT]). DESIGN: Case-control study. METHODS: The intraexaminer and interexaminer reliabilities of the PFBT were evaluated in 57 orthopedic patients and 30 healthy adults. The range of motion of the ankle joint and the number of heel raises were compared between positive and negative PFBT cases. Then, isometric ankle PF torque (ankle PF of 0°, 15°, and 30°) was compared in knee extension and in the 50° flexion position between positive and negative PFBT cases in 30 healthy adults. RESULTS: The intraexaminer reliability (κ) of the PFBT was .81 to .94, and the interexaminer reliability (κ) was .37 to .74. The PFBT positive group had a significantly smaller range of motion in the ankle joint (P < .05) and a significantly lower number of heel raises (P < .001) than the PFBT negative group in orthopedic patients, but no significant differences in healthy adults. The PFBT positive group had less ankle PF torque than the PFBT negative group at all ankle PF angles (P < .01). The isometric PF torque in the end range of ankle PF in knee extension could predict the results of the PFBT and correctly classified 90.0% of cases (P < .001). CONCLUSION: The PFBT is considered a simple and valid method to evaluate isometric ankle PF strength in the clinical setting.

Enthalpy efficiency of the soleus muscle contributes to improvements in running economy.

During human running, the soleus, as the main plantar flexor muscle, generates the majority of the mechanical work through active shortening. The fraction of chemical energy that is converted into muscular work (enthalpy efficiency) depends on the muscle shortening velocity. Here, we investigated the soleus muscle fascicle behaviour during running with respect to the enthalpy efficiency as a mechanism that could contribute to improvements in running economy after exercise-induced increases of plantar flexor strength and Achilles tendon (AT) stiffness. Using a controlled longitudinal study design (n = 23) featuring a specific 14-week muscle-tendon training, increases in muscle strength (10%) and tendon stiffness (31%) and reduced metabolic cost of running (4%) were found only in the intervention group (n = 13, p < 0.05). Following training, the soleus fascicles operated at higher enthalpy efficiency during the phase of muscle-tendon unit (MTU) lengthening (15%) and in average over stance (7%, p < 0.05). Thus, improvements in energetic cost following increases in plantar flexor strength and AT stiffness seem attributed to increased enthalpy efficiency of the operating soleus muscle. The results further imply that the soleus energy production in the first part of stance, when the MTU is lengthening, may be crucial for the overall metabolic energy cost of running.

Optogenetic activation of muscle contraction in vivo.

Purpose: Optogenetics is an emerging alternative to traditional electrical stimulation to initiate action potentials in activatable cells both ex vivo and in vivo. Optogenetics has been commonly used in mammalian neurons and more recently, it has been adapted for activation of cardiomyocytes and skeletal muscle. Therefore, the aim of this study was to evaluate the stimulation feasibility and sustain isometric muscle contraction and limit decay for an extended period of time (1s), using non-invasive transdermal light activation of skeletal muscle (triceps surae) in vivo. MATERIALS AND METHODS: We used inducible Cre recombination to target expression of Channelrhodopsin-2 (ChR2(H134R)-EYFP) in skeletal muscle (Acta1-Cre) in mice. Fluorescent imaging confirmed that ChR2 expression is localized in skeletal muscle and does not have specific expression in sciatic nerve branch, therefore, allowing for non-nerve mediated optical stimulation of skeletal muscle. We induced muscle contraction using transdermal exposure to blue light and selected 10 Hz stimulation after controlled optimization experiments to sustain prolonged muscle contraction. RESULTS: Increasing the stimulation frequency from 10 Hz to 40 Hz increased the muscle contraction decay during prolonged 1s stimulation, highlighting frequency dependency and importance of membrane repolarization for effective light activation. Finally, we showed that optimized pulsed optogenetic stimulation of 10 Hz resulted in comparable ankle torque and contractile functionality to that of electrical stimulation. CONCLUSIONS: Our results demonstrate the feasibility and repeatability of non-invasive optogenetic stimulation of muscle in vivo and highlight optogenetic stimulation as a powerful tool for non-invasive in vivo direct activation of skeletal muscle.

Reliability of Trigger Point Evaluation in the Lower Leg Muscles.

BACKGROUND: Myofascial trigger point diagnosis is a clinical palpatory skill dependent on the patient's subjective response. The inter- and intra-rater reliability of trigger point physical evaluation in the lower leg muscles has rarely been reported. Previous reliability studies suffered from the Kappa paradox. OBJECTIVE: To evaluate the inter- and intra-rater reliability of trigger point recognition in the lower leg muscles implying a specific method to overcome the first Kappa paradox. DESIGN: A reliability study with pre-second examiner exclusion to correct prevalence index. SETTING: Physical therapy outpatient clinic, Beer-Sheva, Israel. SUBJECTS: In sum, 86 soldiers aged 18-30 referred for physical therapy with a diagnosis of musculoskeletal pain consented to take part in this study; 26 were excluded for lacking trigger points, leaving 60 subjects for analysis (31 women, 29 men). METHODS: Both legs were evaluated, and the results were analyzed separately for symptomatic (N = 87) and asymptomatic legs (N = 31). Each subject was evaluated three times, twice by one examiner, and once by a second examiner. Dichotomous findings including palpable taut-band, tenderness, referred pain, and relevance of referred pain were recorded. RESULTS: Inter-rater reliability for active trigger points ranged from 0.49 to 0.75 (median: 0.52) and intra-rater reliability ranged from 0.41 to 0.84 (median: 0.65) and. For total trigger points intra-rater reliability ranged from 0.52 to 0.79 (median: 0.67), and inter-rater reliability ranged from 0.44 to 0.77 (median: 0.66). CONCLUSIONS: Physical examination is a reliable method of trigger point evaluation in lower leg muscles, and it can be used as a diagnostic method for trigger point evaluation.