The Volitional Control of Individual Motor Units Is Constrained within Low-Dimensional Neural Manifolds by Common Inputs.

The implementation of low-dimensional movement control by the central nervous system has been debated for decades. In this study, we investigated the dimensionality of the control signals received by spinal motor neurons when controlling either the ankle or knee joint torque. We first identified the low-dimensional latent factors underlying motor unit activity during torque-matched isometric contractions in male participants. Subsequently, we evaluated the extent to which motor units could be independently controlled. To this aim, we used an online control paradigm in which participants received the corresponding motor unit firing rates as visual feedback. We identified two main latent factors, regardless of the muscle group (vastus lateralis-medialis and gastrocnemius lateralis-medialis). The motor units of the gastrocnemius lateralis could be controlled largely independently from those of the gastrocnemius medialis during ankle plantarflexion. This dissociation of motor unit activity imposed similar behavior to the motor units that were not displayed in the feedback. Conversely, it was not possible to dissociate the activity of the motor units between the vastus lateralis and medialis muscles during the knee extension tasks. These results demonstrate that the number of latent factors estimated from linear dimensionality reduction algorithms does not necessarily reflect the dimensionality of volitional control of motor units. Overall, individual motor units were never controlled independently of all others but rather belonged to synergistic groups. Together, these findings provide evidence for a low-dimensional control of motor units constrained by common inputs, with notable differences between muscle groups.

Comparison of a Wearable Accelerometer/Gyroscopic, Portable Gait Analysis System (LEGSYS+TM) to the Laboratory Standard of Static Motion Capture Camera Analysis.

Examination of gait patterns has been used to determine severity, intervention triage and prognostic measures for many health conditions. Methods that generate detailed gait data for clinical use are typically logistically constrained to a formal gait laboratory setting. This has led to an interest in portable analysis systems for near clinical or community-based assessments. The following study assessed with the wearable accelerometer/gyroscopic, gait analysis system (LEGSYS+TM) and the standard of static motion capture camera (MOCAP) analysis during a treadmill walk at three different walking speeds in healthy participants (n = 15). To compare each speed, 20 strides were selected from the MOCAP data and compared with the LEGSYS+ strides at the same time point. Both scatter and bland-Altman plots with accompanying linear regression analysis for each of the parameters. Each stride parameter showed minimal or a consistent difference between the LEGSYS+ and MOCAP, with the phase parameters showing inconsistencies between the systems. Overall, LEGSYS+ stride parameters can be used in the clinical setting, with the utility of phase parameters needing to be taken with caution.

Reproducibility of Hypermobility Assessment Scales for Children When Performed Using Telehealth versus In-Person Modes.

AIMS: Evaluate reproducibility of hypermobility assessments using in-person versus telehealth modes. METHODS: Hypermobility of 20 children (7-12 years) was evaluated using the Beighton Score, Upper Limb Hypermobility Assessment Tool (ULHAT), and Lower Limb Assessment Score (LLAS) via in-person and telehealth modes. Agreement between the two modes was examined using percentage of exact agreement (%EA and %EA ± 2), Limits of Agreement (LoA) and Smallest detectable change (SDC). Reliability was calculated using intra-class correlation coefficients (ICCs). RESULTS: Agreement between modes for total Scores was best for the Beighton (%EA = fair, %EA ± 2 = good), then the ULHAT (%EA = poor, %EA ± 2 = excellent), and LLAS (%EA = poor, %EA ± 2 = fair). Total scores for all scales showed wide LoA, large SDC (25-31%), and fair to good reliability (ICC = 0.54-0.61). Exact agreement for Generalized Joint Hypermobility classification was excellent for the Beighton (≥7/9 threshold) and fair for the ULHAT and LLAS (≥7/12 threshold). Percentage of individual test items with good/excellent agreement was highest for the Beighton (78%, 7/9 items), then the ULHAT (58%, 14/24) and LLAS (42%, 10/24). CONCLUSION: Total Scores of hypermobility scales showed low exact agreement between in-person and telehealth, but fair-excellent agreement within two points. Classification using the Beighton ≥7/9 threshold was excellent. Research is recommended to increase accuracy of online assessments.

Less common synaptic input between muscles from the same group allows for more flexible coordination strategies during a fatiguing task.

This study aimed to determine whether neural drive is redistributed between muscles during a fatiguing isometric contraction, and if so, whether the initial level of common synaptic input between these muscles constrains this redistribution. We studied two muscle groups: triceps surae (14 participants) and quadriceps (15 participants). Participants performed a series of submaximal isometric contractions and a torque-matched contraction maintained until task failure. We used high-density surface electromyography to identify the behavior of 1,874 motor units from the soleus, gastrocnemius medialis (GM), gastrocnemius lateralis (GL), rectus femoris, vastus lateralis (VL), and vastus medialis (VM). We assessed the level of common drive between muscles in the absence of fatigue using a coherence analysis. We also assessed the redistribution of neural drive between muscles during the fatiguing contraction through the correlation between their cumulative spike trains (index of neural drive). The level of common drive between VL and VM was significantly higher than that observed for the other muscle pairs, including GL-GM. The level of common drive increased during the fatiguing contraction, but the differences between muscle pairs persisted. We also observed a strong positive correlation of neural drive between VL and VM during the fatiguing contraction (r = 0.82). This was not observed for the other muscle pairs, including GL-GM, which exhibited differential changes in neural drive. These results suggest that less common synaptic input between muscles allows for more flexible coordination strategies during a fatiguing task, i.e., differential changes in neural drive across muscles. The role of this flexibility on performance remains to be elucidated.NEW & NOTEWORTHY Redundancy of the neuromuscular system theoretically allows for a redistribution of the neural drive across muscles (i.e., between-muscle compensation) during a fatiguing contraction. Our results suggest that a high level of common input between muscles (e.g., vastus lateralis and medialis) represents a neural constraint making it less likely to redistribute the neural drive across these muscles. In this way, redistribution was only observed across muscles that share little common synaptic input (e.g., gastrocnemius lateralis and medialis).

Postural Control Performance on the Functional Reach Test: Validity of the Kids-Balance Evaluation Systems Test (Kids-BESTest) Criteria.

OBJECTIVE: Evaluate the validity of the Kids-Balance Evaluation Systems Test (Kids-BESTest) clinical criteria for the Functional Reach Test (FRT) forward and lateral with laboratory measures of postural control in children with cerebral palsy (CP). DESIGN: Psychometric study of face, concurrent, and content validity. SETTING: Clinical laboratory. PARTICIPANTS: Children (N=58) aged 7-18 years (ambulant CP n=17, typically developing [TD] n=41). INTERVENTION: Not applicable. MAIN OUTCOME MEASURES: Stability limits in standing were assessed using the Kids-BESTest items for FRT forwards (FRTFORWARD), FRT lateral preferred (FRTLATERAL(P)), and FRT lateral nonpreferred (FRTLATERAL(NP)). Force platforms and kinematic markers were used to collect information on center of pressure (CoP) and joint movement during reach. Analyses included face validity (Kids-BESTest scores compared between CP and TD groups), concurrent validity (agreement between Kids-BESTest scores and digitally derived scores), and content validity (relations between Kids-BESTest scores with kinematic and CoP data). RESULTS: Face validity of Kids-BESTest criteria was demonstrated with lower scores for CP compared to TD groups for FRTFORWARD (P<.001) and FRTLATERAL(NP) (P=.03) and equal scores for FRTLATERAL(P) (P=.12). For concurrent validity, agreement between Kids-BESTest scores and digitally derived scores was good to excellent for FRTLATERAL(both P/NP) (88%-100%) and good for FRTFORWARD (86%-88%) for both groups. For content validity, the CP group Kids-BESTest scores were correlated with CoP-RangeFORWARD during FRTFORWARD (ρ=0.68) and CoP-RangeLATERAL during FRTLATERAL(NP) (ρ=0.57). For kinematic data, correlations were moderate-high between Kids-BESTest scores and range of hip flexion (ρ=0.51) and ankle plantar flexion (ρ=0.75) during FRTFORWARD, and trunk lateral flexion (ρ=0.66) during FRTLATERAL(NP). CONCLUSION: The FRTFORWARD demonstrated face, concurrent, and content validity. The FRTLATERAL(P/NP) demonstrated concurrent validity, but partial face and content validity. To improve validity of Kids-BESTest FRT criteria, additional descriptors have been added under the scoring criteria to enable clinicians to quantify observed reach strategies.

Does adding hip exercises to quadriceps exercises result in superior outcomes in pain, function and quality of life for people with knee osteoarthritis? A systematic review and meta-analysis.

OBJECTIVES: To determine, in people with knee osteoarthritis (KOA): i) the effectiveness of adding hip strengthening exercises to quadriceps exercises and ii) the type of hip strengthening exercise with the greatest evidence for improving pain, function and quality of life. DESIGN: Systematic review with meta-analysis. DATA SOURCES: Medline, Embase, Cochrane, CINAHL and SportDiscus databases were searched from inception to January 2018. ELIGIBILITY CRITERIA FOR SELECTING STUDIES: Randomised controlled trials investigating the effect of adding hip exercises to quadriceps exercises in people with KOA on pain, function and/or quality of life were included. Three subgroups of hip exercises were included: resistance, functional neuromuscular or multimodal exercise. RESULTS: Eight studies were included. Pooled data provide evidence that combined hip and quadriceps exercise is significantly more effective than quadriceps exercise alone for improving walking function (standardised mean difference -1.06, 95% CI -2.01 to -0.12), but not for outcomes of pain (-0.09, 95% CI -0.96 to 0.79), patient-reported function (-0.74, 95% CI -1.56 to 0.08) or stair function (-0.7, 95% CI -1.67 to 0.26). Subgroup analyses reveal that hip resistance exercises are more effective than functional neuromuscular exercises for improving pain (p<0.0001) and patient-reported function (p<0.0001). Multimodal exercise is no more effective than quadriceps strengthening alone for pain (0.13, 95% CI -0.31 to 0.56), patient-reported function (-0.15, 95% CI -0.58 to 0.29) or stair function (0.13, 95% CI -0.3 to 0.57). CONCLUSION: Walking improved after the addition of hip strengthening to quadriceps strengthening in people with KOA. The addition of resistance hip exercises to quadriceps resulted in greater improvements in patient-reported pain and function.

The Relationship of Ultrasound Measurements of Muscle Deformation With Torque and Electromyography During Isometric Contractions of the Cervical Extensor Muscles.

OBJECTIVES: Speckle tracking analysis (STA) of ultrasound (US) images quantifies the longitudinal deformation of a region of muscle tissue to provide a mechanical measure of muscle activity. As the validity and reliability of this method has not yet been adequately assessed, the aim of this study was to determine the validity and reliability of STA in the dorsal neck muscles during isometric neck extension contractions. METHODS: Twenty volunteers performed 3 repetitions of isometric neck extension in a dynamometer at 10%, 20%, 40%, 60%, and 80% of maximal voluntary torque while US and surface electromyography (EMG) data were recorded. Speckle tracking analysis was then used to calculate measurements of muscle deformation. The relationship among torque, muscle deformation (separate for each muscle and summed), and EMG was evaluated using linear regressions and Spearman's correlation coefficients. The reliability of EMG and muscle deformation was determined using intraclass correlation coefficients (ICCs). RESULTS: Muscle deformation in 4 of the 5 muscles when examined separately and collectively was significantly related to torque (P < .05); however, the relationship was weak (r2 = 0.03-0.18). In contrast, a strong linear relationship was observed between torque and EMG (P < .001, r2 = 0.83). Poor to moderate reliability of muscle deformation measures (ICC: 0.02-0.69) was found compared with EMG, which was highly reliable (ICC: 0.67-0.90). CONCLUSION: These results suggest that the validity of US STA measurements for isometric contractions of the dorsal neck muscles is questionable. Further investigation into this method is required before it can be used as a tool to measure muscle activity.

Do individual differences in the distribution of activation between synergist muscles reflect individual strategies?

Individual differences in the distribution of activation between synergist muscles have been reported during a wide variety of tasks. Whether these differences represent actual individual strategies is unknown. The aims of this study were to: (i) test the between-day reliability of the distribution of activation between synergist muscles, (ii) to determine the robustness of these strategies between tasks, and to (iii) describe the inter-individual variability of activation strategies in a large sample size. Eighty-five volunteers performed a series of single-joint isometric tasks with their dominant leg [knee extension and plantarflexion at 25% of maximal voluntary contraction (MVC)] and locomotor tasks (pedalling and walking). Of these participants, 62 performed a second experimental session that included the isometric tasks. Myoelectrical activity of six lower limb muscles (the three superficial heads of the quadriceps and the three heads of the triceps surae) was measured using surface electromyography (EMG) and normalized to that measured during MVC. When considering isometric contractions, distribution of normalized EMG amplitude among synergist muscles, considered here as activation strategies, was highly variable between individuals (15.8% < CV < 42.7%) and robust across days (0.57 < ICC < 0.82). In addition, individual strategies observed during simple single-joint tasks were correlated with those observed during locomotor tasks [0.37 < r < 0.76 for quadriceps (n = 83); 0.30 < r < 0.66 for triceps surae (n = 82); all P < 0.001]. Our results provide evidence that people who bias their activation to a particular muscle do so during multiple tasks. Even though inter-individual variability of EMG signals has been well described, it is often considered noise which complicates the interpretation of data. This study provides evidence that variability results from actual differences in activation strategies.

Reproducibility of the Kids-BESTest and the Kids-Mini-BESTest for Children With Cerebral Palsy.

OBJECTIVE: To evaluate the reproducibility, including reliability and agreement, of the Kids Balance Evaluation Systems Test (Kids-BESTest) and the short form of Kids-BESTest (Kids-Mini-BESTest) for measuring postural control in school-aged children with cerebral palsy. DESIGN: Psychometric study of intrarater, interrater, and test-retest reliability and agreement. SETTING: Clinical laboratory and home. PARTICIPANTS: Convenience sample of children (N=18) aged 8 to 17 years with ambulant cerebral palsy (CP) (Gross Motor Function Classification System I-II) with spastic or ataxic motor type. INTERVENTION: Not applicable. MAIN OUTCOME MEASURES: Postural control was assessed using the Kids-BESTest and the Kids-Mini-BESTest. An experienced physiotherapist assessed all children in real time and the testing session was videotaped. The same physiotherapist viewed and scored the video twice, at least 2 weeks apart, to assess intrarater reproducibility. Another experienced physiotherapist scored the same video to determine interrater reproducibility. Thirteen children returned for a repeat assessment with the first physiotherapist within 6 weeks and their test-retest performance was rated in real time and with video. RESULTS: Excellent reliability was observed for both the Kids-BESTest (intraclass correlation coefficient [ICC] 0.96-0.99) and Kids-Mini-BESTest (ICC 0.79-0.98). The smallest detectable change was good to excellent for all Kids-BESTest agreement analyses (5%-9%), but poor to good for Kids-Mini-BESTest analyses (9%-16%). CONCLUSION: The Kids-BESTest shows an excellent ability to discriminate postural control abilities of school-aged children with CP and it has a low smallest detectable change, suitable for use as a preintervention and postintervention outcome measure. Although the Kids-Mini-BESTest is 5 to 10 minutes shorter to administer, it has poorer reproducibility and focuses only on falls-related balance, which excludes 2 domains of postural control.

Effect of experimental muscle pain on the acquisition and retention of locomotor adaptation: different motor strategies for a similar performance.

As individuals with musculoskeletal disorders often experience motor impairments, contemporary rehabilitation relies heavily on the use of motor learning principles. However, motor impairments are often associated with pain. Although there is substantial evidence that muscle pain interferes with motor control, much less is known on its impact on motor learning. The objective of the present study was to assess the effects of muscle pain on locomotor learning. Two groups (Pain and Control) of healthy participants performed a locomotor adaptation task (robotized ankle-foot orthosis perturbing ankle movements during swing) on two consecutive days. On day 1 (acquisition), hypertonic saline was injected in the tibialis anterior (TA) muscle of the Pain group participants, while Control group participants were pain free. All participants were pain free on day 2 (retention). Changes in movement errors caused by the perturbation were assessed as an indicator of motor performance. Detailed analysis of kinematic and electromyographic data provided information about motor strategies. No between-group differences were observed on motor performance measured during the acquisition and retention phases. However, Pain group participants had a residual movement error later in the swing phase and smaller early TA activation than Control group participants, thereby suggesting a reduction in the use of anticipatory motor strategies to overcome the perturbation. Muscle pain did not interfere with global motor performance during locomotor adaptation. The different motor strategies used in the presence of muscle pain may reflect a diminished ability to anticipate the consequences of a perturbation. NEW & NOTEWORTHY This study shows that experimental muscle pain does not influence global motor performance during the acquisition or next-day retention phases of locomotor learning. This contrasts with previous results obtained with cutaneous pain, emphasizing the risk of directly extrapolating from one pain modality to another. Muscle pain affected motor strategies used when performing the task, however: it reduced the ability to use increased feedforward control to overcome the force field.

Neuromechanical coupling within the human triceps surae and its consequence on individual force-sharing strategies.

Little is known about the factors that influence the coordination of synergist muscles that act across the same joint, even during single-joint isometric tasks. The overall aim of this study was to determine the nature of the relationship between the distribution of activation and the distribution of force-generating capacity among the three heads of the triceps surae [soleus (SOL), gastrocnemius medialis (GM) and gastrocnemius lateralis (GL)]. Twenty volunteers performed isometric plantarflexions, during which the activation of GM, GL and SOL was estimated using electromyography (EMG). Functional muscle physiological cross-sectional area (PCSA) was estimated using imaging techniques and was considered as an index of muscle force-generating capacity. The distribution of activation and PCSA among the three muscles varied greatly between participants. A significant positive correlation between the distribution of activation and the distribution of PCSA was observed when considering the two bi-articular muscles at intensities ≤50% of the maximal contraction (0.51

Muscle tone assessments for children aged 0 to 12 years: a systematic review.

AIM: The aim of this study was to identify and examine the psychometric properties of muscle tone assessments for children aged 0 to 12 years. METHOD: Four electronic databases were searched to identify studies that included assessments of resting and/or active muscle tone. Methodological quality and overall psychometric evidence of studies were rated using the COnsensus-based Standards for the selection of health Measurement INstruments checklist. RESULTS: Twenty-one assessments were identified from 97 included studies. All assessments were broad developmental assessments that included muscle tone items or subscales. Most assessments (16/21) were designed for young children (<2y). Four assessments measured resting and active tone and demonstrated at least moderate validity or reliability: the Amiel-Tison Neurological Assessment (ATNA) at term, Neonatal Intensive Care Unit Network Neurobehavioral Scale (NNNS), Premie-Neuro for newborn infants, and the Hammersmith Infant Neurological Examination (HINE) for infants aged 2 months to 2 years. For children over 2 years, the Neurological Sensory Motor Developmental Assessment (NSMDA) assesses resting and active tone but has limited validity. INTERPRETATION: The ATNA at term, NNNS, Premie-Neuro, HINE, and NSMDA can assess resting and active tone in infants and/or children. Further psychometric research is required to extend reliability, validity, and responsiveness data, particularly for older children. WHAT THIS PAPER ADDS: This is the first review of muscle tone assessments for children aged 0 to 12 years. Twenty-one assessments contain muscle tone items and 16 are for children under 2 years. Four assessments are reliable or valid to measure both resting and active tone.

Heterogeneity of passive elastic properties within the quadriceps femoris muscle-tendon unit.

PURPOSE: The purpose of this study was to compare regional elastic properties between anterior and posterior regions of the patellar tendon, and individual quadriceps muscles, over a range of knee flexion angles. METHODS: An isokinetic dynamometer passively positioned the non-dominant knee of 19 young, healthy participants, at 25°, 40°, 55°, 70° and 85° flexion. Shear wave velocity (SWV, an index of tissue elasticity) was measured using ultrasound shear wave elastography in a relaxed (passive) state, confirmed by electromyography. RESULTS: SWV of the patellar tendon and quadriceps muscles increased with knee flexion (longer muscle-tendon unit; P < 0.001). Within the proximal third of the patellar tendon, SWV was lower in the posterior than anterior region at 70° (P = 0.002) and 85° (P < 0.001), but not at 25°, 40° or 55° (region-by-angle interaction, P = 0.007). No differences were found between anterior and posterior regions within the middle third of the patellar tendon (P = 0.332). For the quadriceps muscles, a significant muscle-by-angle (P < 0.001) interaction was also observed. SWV of VL was greater than VM at 55° (P = 0.005), 70° (P = 0.001) and 85° (P < 0.001), but not at 25° or 40°. SWV of RF was lower than VL at all angles (all P < 0.002) and lower than VM at 55°, 70° and 85° (all P < 0.002). CONCLUSIONS: Passive knee flexion at and beyond 70° was associated with non-uniform elastic properties within the proximal patellar tendon and between individual quadriceps muscles. To what extent this heterogeneity of passive elastic properties contributes to injury remains unknown.

Shear-wave velocity of the patellar tendon and quadriceps muscle is increased immediately after maximal eccentric exercise.

PURPOSE: To determine whether stiffness of the patellar tendon and quadriceps muscles is altered immediately after and 48 h after a single bout of maximal eccentric exercise of the knee extensor muscles. METHODS: Thirteen healthy individuals [group mean (SD) age 22.4 (3.5) years; 7 female] performed a single bout of maximal eccentric exercise of the non-dominant knee extensors, using an isokinetic dynamometer. Shear-wave velocity (an index of tissue stiffness) was recorded from the patellar tendon, vastus medialis (VM), rectus femoris (RF) and vastus lateralis (VL), before, following (post0), and 48 h after (post48) exercise. To investigate features of exercise induced muscle damage, maximal voluntary isometric contraction (MVIC) and self-reported pain and stiffness (numerical rating scales 0 = no pain/stiffness to 100 = worst imaginable pain/stiffness) were measured before, post0, and post48 exercise. Serum creatine kinase (CK) was measured before and post48 exercise. RESULTS: Compared to preexercise, MVIC decreased and self-reported pain and stiffness increased at post0 and post48 and CK levels increased at post48 (all p < 0.01). Compared to preexercise, shear-wave velocity was greater at post0 for the patellar tendon [15.9 (24.6)%, p = 0.01] and RF [23.6 (16.7)%, p < 0.001], each returning to baseline by post48. No significant differences were observed for VL or VM post0 or post48 exercise. CONCLUSION: Maximal eccentric exercise produced an immediate increase in the stiffness of the patellar tendon and RF, resolving by 48 h. As this change was not observed in VL and VM, future studies may explore heterogeneity within synergist muscles following eccentric exercise.

Adductor magnus: An EMG investigation into proximal and distal portions and direction specific action.

Cadaveric studies indicate that adductor magnus is structurally partitioned into at least two regions. The aim of this study was to investigate the direction-specific actions of proximal and distal portions of adductor magnus, and in doing so determine if these segments have distinct functional roles. Fine-wire EMG electrodes were inserted into two portions of adductor magnus of 12 healthy young adults. Muscle activity was recorded during maximum voluntary isometric contractions (MVICs) across eight tests (hip flexion/extension, internal/external rotation, abduction, and adduction at 0°, 45°, and 90° hip flexion). Median activity within each action (normalized to peak) was compared between segments using repeated measures nonparametric tests (α = 0.05). An effect size (ES = z-score/√sample size) was calculated to determine the magnitude of difference between muscle segments. The relative contribution of each muscle segment differed significantly during internal rotation (P < 0.001; ES = 0.88) and external rotation (P = 0.003, ES = 0.79). The distal portion was most active during extension [median (interquartile range); 100(0)% MVIC)] and internal rotation [58(34)% MVIC]. The proximal portion was most active during extension [100(49)% MVIC] and adduction [59(64)%MVIC], with low level activity during external rotation [15(41)%MVIC]. This study suggests that adductor magnus has at least two functionally unique regions. Differences were most evident during rotation. The different direction-specific actions may imply that each segment performs separate roles in hip stability and movement. These findings may have implications on injury prevention and rehabilitation for adductor-related groin injuries, hamstring strain injury, and hip pathology. Clin. Anat. 31:535-543, 2018. © 2018 Wiley Periodicals, Inc.

Muscle Coordination and the Development of Musculoskeletal Disorders.

The present article examines the hypothesis that each individual has unique muscle coordination strategies (or signatures) that will have specific mechanical effects on their musculoskeletal system. As such, some strategies would make some people more at risk of developing musculoskeletal disorders than others. Identification of individual coordination strategies might provide insight into the development of musculoskeletal disorders.

Motor adaptations to local muscle pain during a bilateral cyclic task.

The aim of this study was to determine how unilateral pain, induced in two knee extensor muscles, affects muscle coordination during a bilateral pedaling task. Fifteen participants performed a 4-min pedaling task at 130 W in two conditions (Baseline and Pain). Pain was induced by injection of hypertonic saline into the vastus medialis (VM) and vastus lateralis (VL) muscles of one leg. Force applied throughout the pedaling cycle was measured using an instrumented pedal and used to calculate pedal power. Surface electromyography (EMG) was recorded bilaterally from eight muscles to assess changes in muscle activation strategies. Compared to Baseline, during the Pain condition, EMG amplitude of muscles of the painful leg (VL and VM-the painful muscles, and RF-another quadriceps muscle with no pain) was lower during the extension phase [(mean ± SD): VL: -22.5 ± 18.9%; P < 0.001; VM: -28.8 ± 19.9%; P < 0.001, RF: -20.2 ± 13.9%; P < 0.001]. Consistent with this, pedal power applied by the painful leg was also lower during the extension phase (-16.8 ± 14.2 W, P = 0.001) during Pain compared to Baseline. This decrease was compensated for by an 11.3 ± 8.1 W increase in pedal power applied by the non-painful leg during its extension phase (P = 0.04). These results support pain adaptation theories, which suggest that when there is a clear opportunity to compensate, motor adaptations to pain occur to decrease load within the painful tissue. Although the pedaling task offered numerous possibilities for compensation, only between-leg compensations were systematically observed. This finding is discussed in relation to the mechanical and neural constraints of the pedaling task.

Perspectives on Postural Control Dysfunction to Inform Future Research: A Delphi Study for Children With Cerebral Palsy.

OBJECTIVE: To identify whether consensus can be achieved in how clinicians and researchers define, describe, assess, and treat postural control dysfunction in children with cerebral palsy (CP). DESIGN: Delphi study with 3 iterative rounds. SETTING: Electronic survey. PARTICIPANTS: Researchers and/or clinicians (N=43) from 7 countries with a mean ± SD of 20±11 years of experience working with children with CP participated. Participants included authors of published works on postural control in CP (identified from a recent systematic review), members of the Australasian Academy of CP and Developmental Medicine, and 2 major Australian rehabilitation providers. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: The Delphi study consisted of 3 iterative rounds of surveys. In Round 1, respondents answered open-ended questions regarding their views on (1) definition items for postural control, (2) theoretical frameworks, (3) methods for assessment, and (4) interventions for postural control dysfunction in children with CP. Rounds 2 and 3 were made up of items generated by participants in Round 1 and combined with items identified from the literature. Participants indicated their level of agreement for each item on a 7-point Likert scale. Threshold for consensus was ≥85% agreement. RESULTS: Of 306 items generated, 174 reached consensus by Round 3. Most postural control definition items (90%) achieved consensus. Two theoretical frameworks (14%) reached consensus. Less than half (42%) of assessment items reached consensus. More individual assessment items (89%) reached consensus than multi-item tools (4%). Just over half (61%) of the items generated for interventions reached consensus. CONCLUSION: Consensus was achieved for a postural control definition. However, substantial research is needed to establish a comprehensive, postural control-specific framework and suite of assessments. These would provide a foundation to improve intervention selection and dosage.

Nature of the coupling between neural drive and force-generating capacity in the human quadriceps muscle.

The force produced by a muscle depends on both the neural drive it receives and several biomechanical factors. When multiple muscles act on a single joint, the nature of the relationship between the neural drive and force-generating capacity of the synergistic muscles is largely unknown. This study aimed to determine the relationship between the ratio of neural drive and the ratio of muscle force-generating capacity between two synergist muscles (vastus lateralis (VL) and vastus medialis (VM)) in humans. Twenty-one participants performed isometric knee extensions at 20 and 50% of maximal voluntary contractions (MVC). Myoelectric activity (surface electromyography (EMG)) provided an index of neural drive. Physiological cross-sectional area (PCSA) was estimated from measurements of muscle volume (magnetic resonance imaging) and muscle fascicle length (three-dimensional ultrasound imaging) to represent the muscles' force-generating capacities. Neither PCSA nor neural drive was balanced between VL and VM. There was a large (r = 0.68) and moderate (r = 0.43) correlation between the ratio of VL/VM EMG amplitude and the ratio of VL/VM PCSA at 20 and 50% of MVC, respectively. This study provides evidence that neural drive is biased by muscle force-generating capacity, the greater the force-generating capacity of VL compared with VM, the stronger bias of drive to the VL.

Elastography for Muscle Biomechanics: Toward the Estimation of Individual Muscle Force.

Estimation of individual muscle force remains one of the main challenges in biomechanics. This review presents a series of experiments that used ultrasound shear wave elastography to support the hypothesis that muscle stiffness is linearly related to both active and passive muscle forces. Examples of studies that used measurement of muscle stiffness to estimate changes in muscle force are presented.