Voluntary activation failure contributes more to plantar flexor weakness than antagonist coactivation and muscle atrophy in chronic stroke survivors

EMG-Constrained and Ultrasound-Informed Muscle-Tendon Parameter Estimation in Post-Stroke Hemiparesis

Secondary morphological and mechanical property changes in the muscle-tendon unit at the ankle joint are often observed in post-stroke individuals. These changes may alter the force generation capacity and affect daily activities such as locomotion. This work aimed to estimate subject-specific muscle-tendon parameters in individuals after stroke by solving the muscle redundancy problem using direct collocation optimal control methods based on experimental electromyography (EMG) signals and measured muscle fiber length. Subject-specific muscle-tendon parameters of the gastrocnemius, soleus, and tibialis anterior were estimated in seven post-stroke individuals and seven healthy controls. We found that the maximum isometric force, tendon stiffness and optimal fiber length in the post-stroke group were considerably lower than in the control group. We also computed the root mean square error between estimated and experimental values of muscle excitation and fiber length. The musculoskeletal model with estimated subject-specific muscle tendon parameters (from the muscle redundancy solver), yielded better muscle excitation and fiber length estimations than did scaled generic parameters. Our findings also showed that the muscle redundancy solver can estimate muscle-tendon parameters that produce force behavior in better accordance with the experimentally-measured value. These muscle-tendon parameters in the post-stroke individuals were physiologically meaningful and may shed light on treatment and/or rehabilitation planning.

A Portable, Neurostimulation-Integrated, Force Measurement Platform for the Clinical Assessment of Plantarflexor Central Drive

Plantarflexor central drive is a promising biomarker of neuromotor impairment; however, routine clinical assessment is hindered by the unavailability of force measurement systems with integrated neurostimulation capabilities. In this study, we evaluate the accuracy of a portable, neurostimulation-integrated, plantarflexor force measurement system we developed to facilitate the assessment of plantarflexor neuromotor function in clinical settings. Two experiments were conducted with the Central Drive System (CEDRS). To evaluate accuracy, experiment #1 included 16 neurotypical adults and used intra-class correlation (ICC2,1) to test agreement of plantarflexor strength capacity measured with CEDRS versus a stationary dynamometer. To evaluate validity, experiment #2 added 26 individuals with post-stroke hemiparesis and used one-way ANOVAs to test for between-limb differences in CEDRS' measurements of plantarflexor neuromotor function, comparing neurotypical, non-paretic, and paretic limb measurements. The association between paretic plantarflexor neuromotor function and walking function outcomes derived from the six-minute walk test (6MWT) were also evaluated. CEDRS' measurements of plantarflexor neuromotor function showed high agreement with measurements made by the stationary dynamometer (ICC = 0.83, p < 0.001). CEDRS' measurements also showed the expected between-limb differences (p's < 0.001) in maximum voluntary strength (Neurotypical: 76.21 ± 13.84 ft-lbs., Non-paretic: 56.93 ± 17.75 ft-lbs., and Paretic: 31.51 ± 14.08 ft-lbs.), strength capacity (Neurotypical: 76.47 ± 13.59 ft-lbs., Non-paretic: 64.08 ± 14.50 ft-lbs., and Paretic: 44.55 ± 14.23 ft-lbs.), and central drive (Neurotypical: 88.73 ± 1.71%, Non-paretic: 73.66% ± 17.74%, and Paretic: 52.04% ± 20.22%). CEDRS-measured plantarflexor central drive was moderately correlated with 6MWT total distance (r = 0.69, p < 0.001) and distance-induced changes in speed (r = 0.61, p = 0.002). CEDRS is a clinician-operated, portable, neurostimulation-integrated force measurement platform that produces accurate measurements of plantarflexor neuromotor function that are associated with post-stroke walking ability.

Electrical stimulation for investigating and improving neuromuscular function in vivo: Historical perspective and major advances

This historical review summarizes the major advances - particularly from the last 50 years - in transcutaneous motor-level electrical stimulation, which can be used either as a tool to investigate neuromuscular function and its determinants (electrical stimulation for testing; EST) or as a therapeutic/training modality to improve neuromuscular and physical function (neuromuscular electrical stimulation; NMES). We focus on some of the most important applications of electrical stimulation in research and clinical settings, such as the investigation of acute changes, chronic adaptations and pathological alterations of neuromuscular function with EST, as well as the enhancement, preservation and restoration of muscle strength and mass with NMES treatment programs in various populations. For both EST and NMES, several major advances converge around understanding and optimizing motor unit recruitment during electrically-evoked contractions, also taking into account the influence of stimulation site (e.g., muscle belly vs nerve trunk) and type (e.g., pulse duration, frequency, and intensity). This information is equally important both in the context of mechanistic research of neuromuscular function as well as for clinicians who believe that improvements in neuromuscular function are required to provide health-related benefits to their patients.

Motor performance, motor impairments, and quality of life after eccentric resistance training in neurological populations: A systematic review and meta-analyses

BACKGROUND

Many overlapping factors impair motor performance and quality of life in neurological patients. Eccentric resistance training (ET) has potential benefits for improving motor performance and treating motor impairments better than some traditional rehabilitation approaches.

OBJECTIVE

To estimate the effect of ET in neurological settings.

METHODS

Seven databases were reviewed up to May 2022 according to PRSIMA guidelines to find randomized clinical trials involving adults with a neurological condition, who underwent ET as set by the American College of Sports Medicine. Motor performance (main outcome) was assessed as strength, power and capacities during activity. Secondary outcomes (impairments) were muscle structure, flexibility, muscle activity, tone, tremor, balance and fatigue. Tertiary outcomes were risk of fall, and self-reports of quality of life.

RESULTS

Ten trials were included, assessed using Risk of Bias 2.0 tool, and used to compute meta-analyses. Effective effects in favour of ET were found for strength and power, but not for capacities during activity. Mixed results were found for secondary and tertiary outcomes.

CONCLUSION

ET may be a promising intervention to better improve strength/power in neurological patients. More studies are needed to improve the quality of evidence underlying changes responsible for these results.

Antagonist muscle torque at the ankle interfere with maximal voluntary contraction under isometric and anisometric conditions

While resultant maximal voluntary contraction (MVC) is commonly used to assess muscular performance, the simultaneous activation of antagonist muscles may dramatically underestimate the strength of the agonist muscles. Although quantification of antagonist torque has been performed in isometric conditions, it has yet to be determined in anisometric conditions. The aim of the study was to compare the mechanical impact of antagonist torque between eccentric, isometric and concentric contractions in PF and DF MVCs. The MVCs in dorsiflexion (DF) and plantar-flexion (PF) were measured in isometric, concentric and eccentric conditions (10° s^-1) in nine healthy men (26.1 ± 2.7 years; 1.78 ± 0.05 m; 73.4 ± 6.5 kg) through two sessions. Electromyographic (EMG) activities from the soleus, gastrocnemius medialis and lateralis, and tibialis anterior muscles were simultaneously recorded. The EMG biofeedback method was used to quantify antagonist torque. Resultant torque significantly underestimated agonist torque in DF MVC (30-65%) and to a lesser extent in PF MVC (3%). Triceps surae antagonist torque was significantly modified with muscle contraction type, showing higher antagonist torque in isometric (29 Nm) than in eccentric (23 Nm, p < 0.001) and concentric (14 Nm, p < 0.001) conditions and resulting in modification of the DF MVC torque-velocity shape. Estimation of the antagonist torque in isometric or anisometric conditions provides new relevant insights to improve neuromuscular performance assessment and to better design strength training and rehabilitation programs related to the torque applied by agonist and antagonist muscles.

Estimation of maximal muscle electromyographic activity from the relationship between muscle activity and voluntary activation

Maximal muscle activity recorded with surface electromyography (EMG) is an important neurophysiological measure. It is frequently used to normalize EMG activity recorded during passive or active movement. However, the true maximal muscle activity cannot be determined in people with impaired capacity to voluntarily activate their muscles. Here, we determined whether maximal muscle activity can be estimated from muscle activity produced during submaximal voluntary activation. Twenty-five able-bodied adults (18 males, mean age 29 yr, range 19-64 yr) participated in the study. Participants were seated with the knee flexed 90° and the ankle in 5° of dorsiflexion from neutral. Participants performed isometric voluntary ankle plantarflexion contractions at target torques, in random order: 1, 5, 10, 15, 25, 50, 75, 90, 95, and 100% of maximal voluntary torque. Ankle torque, muscle activity in soleus, medial and lateral gastrocnemius muscles, and voluntary muscle activation determined using twitch interpolation were recorded. There was a strong log_e-linear relationship between measures of muscle activation and muscle activity in all three muscles tested. Linear mixed models were fitted to muscle activation and log_e-transformed EMG data. Each 1% increase in muscle activation increased muscle activity by a mean of 0.027 ln(mV) [95% confidence interval (CI) 0.025 to 0.029 ln(mV)] in soleus, 0.025 ln(mV) [0.022 to 0.028 ln(mV)] in medial gastrocnemius, and 0.028 ln(mV) [0.026 to 0.030 ln(mV)] in lateral gastrocnemius. The relationship between voluntary muscle activation and muscle activity can be described with simple mathematical functions. In future, it should be possible to normalize recorded muscle activity using these types of functions.NEW & NOTEWORTHY Muscle activity is often normalized to maximal muscle activity; however, it is difficult to obtain accurate measures of maximal muscle activity in people with impaired voluntary neural drive. We determined the relationship between voluntary muscle activation and plantarflexor muscle activity across a broad range of muscle activation values in able-bodied people. The relationship between voluntary muscle activation and muscle activity can be described with simple mathematical functions capable of estimating maximal muscle activity.

Lost in Translation: Simple Steps in Experimental Design of Neurorehabilitation-Based Research Interventions to Promote Motor Recovery Post-Stroke

Stroke continues to be a leading cause of disability. Basic neurorehabilitation research is necessary to inform the neuropathophysiology of impaired motor control, and to develop targeted interventions with potential to remediate disability post-stroke. Despite knowledge gained from basic research studies, the effectiveness of research-based interventions for reducing motor impairment has been no greater than standard of practice interventions. In this perspective, we offer suggestions for overcoming translational barriers integral to experimental design, to augment traditional protocols, and re-route the rehabilitation trajectory toward recovery and away from compensation. First, we suggest that researchers consider modifying task practice schedules to focus on key aspects of movement quality, while minimizing the appearance of compensatory behaviors. Second, we suggest that researchers supplement primary outcome measures with secondary measures that capture emerging maladaptive compensations at other segments or joints. Third, we offer suggestions about how to maximize participant engagement, self-direction, and motivation, by embedding the task into a meaningful context, a strategy more likely to enable goal-action coupling, associated with improved neuro-motor control and learning. Finally, we remind the reader that motor impairment post-stroke is a multidimensional problem that involves central and peripheral sensorimotor systems, likely influenced by chronicity of stroke. Thus, stroke chronicity should be given special consideration for both participant recruitment and subsequent data analyses. We hope that future research endeavors will consider these suggestions in the design of the next generation of intervention studies in neurorehabilitation, to improve translation of research advances to improved participation and quality of life for stroke survivors.

Validity and reliability of an electromyography-based upper limb assessment quantifying selective voluntary motor control in children with upper motor neuron lesions

Current clinical assessments evaluating selective voluntary motor control are measured on an ordinal scale. We combined the Selective Control of the Upper Extremity Scale (SCUES) with surface electromyography to develop a more objective and interval-scaled assessment of selective voluntary motor control. The resulting Similarity Index (SI) quantifies the similarity of muscle activation patterns. We aimed to evaluate the validity and reliability of this new assessment named SISCUES (Similarity Index of the SCUES) in children with upper motor neuron lesions. Thirty-three patients (12.2 years [8.8;14.9]) affected by upper motor neuron lesions with mild to moderate impairments and 31 typically developing children (11.6 years [8.5;13.9]) participated. We calculated reference muscle activation patterns for the SISCUES using data of 33 neurologically healthy adults (median [1st; 3rd quantile]: 32.5 [27.9; 38.3]). We calculated Spearman correlations (ρ) between the SISCUES and the SCUES and the Manual Ability Classification System (MACS) to establish concurrent validity. Discriminative validity was tested by comparing scores of patients and healthy peers with a robust ANCOVA. Intraclass correlation coefficients2,1 and minimal detectable changes indicated relative and absolute reliability. The SISCUES correlates strongly with SCUES (ρ = 0.76, p < 0.001) and moderately with the MACS (ρ = -0.58, p < 0.001). The average SISCUES can discriminate between patients and peers. The intraclass correlation coefficient2,1 was 0.90 and the minimal detectable change was 0.07 (8% of patients' median score). Concurrent validity, discriminative validity, and reliability of the SISCUES were established. Further studies are needed to evaluate whether it is responsive enough to detect changes from therapeutic interventions.

Voluntary activation of knee extensor muscles with transcranial magnetic stimulation

We examined if transcranial magnetic stimulation (TMS) is a valid tool for assessment of voluntary activation of the knee extensors in healthy individuals. Maximal M-waves (M_max) of vastus lateralis (VL) were evoked with electrical stimulation of femoral nerve (FNS); M_max of medial hamstrings (HS) was evoked with electrical stimulation of sciatic nerve branches; motor evoked potentials (MEPs) of VL and HS were evoked with TMS; superimposed twitches (SIT) of knee extensors were evoked with FNS and TMS. In study 1, TMS intensity [69% output (SD: 5)] was optimized for MEP sizes, but guidelines for test validity could not be met. Agonist VL MEPs were too small [51.4% M_max (SD: 11.9); guideline ≥70% M_max] and antagonist HS MEPs were too big [16.5% M_max (SD: 10.3); guideline <10% M_max]. Consequently, the TMS estimated resting twitch [99.1 N (SD: 37.2)] and FNS resting twitch [142.4 N (SD: 41.8)] were different. In study 2, SITs at 90% maximal voluntary contraction (MVC) were similar between TMS [16.1 N (SD: 10.3)] and FNS [20.9 N (SD: 16.7)], when TMS intensity was optimized for this purpose, suggesting a procedure that combines TMS SITs with FNS resting twitches could be valid. In study 3, which tested the TMS intensity [56% output (SD: 18)] that evoked the largest SIT at 90% MVC, voluntary activation from TMS [87.3% (SD: 7.1)] and FNS [84.5% (SD: 7.6)] was different. In sum, the contemporary procedure for TMS-based voluntary activation of the knee extensors is invalid. A modified procedure improves validity but only in individuals who meet rigorous inclusion criteria for SITs and MEPs.NEW & NOTEWORTHY We discovered that the contemporary procedure for assessing voluntary activation of the knee extensor muscles with transcranial magnetic stimulation (TMS) is invalid. TMS activates too few agonist quadriceps motoneurons and too many antagonist hamstrings motoneurons to estimate the resting twitch accurately. A modified procedure, in which TMS-evoked superimposed twitches are considered together with the resting twitch from femoral nerve stimulation, is valid but only in select individuals who meet rigorous eligibility criteria.

Central activation deficits contribute to post stroke lingual weakness in a rat model

Lingual weakness frequently occurs after stroke and is associated with deficits in speaking and swallowing. Chronic weakness after stroke has been attributed to both impaired central activation of target muscles and reduced force-generating capacity within muscles. How these factors contribute to lingual weakness is not known. We hypothesized that lingual weakness due to middle cerebral artery occlusion (MCAO) would manifest as reduced muscle force capacity and reduced muscle activation. Rats were randomized into MCAO or sham surgery groups. Maximum volitional tongue forces were quantified 8 wk after surgery. Hypoglossal nerve stimulation was used to assess maximum stimulated force, muscle twitch properties, and force-frequency response. The central activation ratio was determined by maximum volitional/maximum stimulated force. Genioglossus muscle fiber type properties and neuromuscular junction innervation were assessed. Maximum volitional force and the central activation ratio were significantly reduced with MCAO. Maximum stimulated force was not significantly different. No significant differences were found for muscle twitch properties, unilateral contractile properties, muscle fiber type percentages, or fiber size. However, the twitch/tetanus ratio was significantly increased in the MCAO group relative to sham. A small but significant increase in denervated neuromuscular junctions (NMJs) and fiber-type grouping occurred in the contralesional genioglossus. Results suggest that the primary cause of chronic lingual weakness after stroke is impaired muscle activation rather than a deficit of force-generating capacity in lingual muscles. Increased fiber type grouping and denervated NMJs in the contralesional genioglossus suggest that partial reinnervation of muscle fibers may have preserved force-generating capacity, but not optimal activation patterns.NEW & NOTEWORTHY Despite significant reductions in maximum volitional forces, the intrinsic force-generating capacity of the protrusive lingual muscles was not reduced with unilateral cerebral ischemia. Small yet significant increases in denervated NMJs and fiber-type grouping of the contralesional genioglossus suggest that the muscle underwent denervation and reinnervation. Together these results suggest that spontaneous neuromuscular plasticity was sufficient to prevent atrophy, yet central activation deficits remain and contribute to chronic lingual weakness after stroke.

Reliability of Tibialis Anterior Muscle Voluntary Activation Using the Interpolated Twitch Technique and the Central Activation Ratio in People with Stroke

Voluntary activation (VA) is measured by applying supramaximal electrical stimulation to a muscle during a maximal voluntary contraction (MVC). The amplitude of the evoked muscle twitch is used to determine any VA deficit, and indicates incomplete central neural drive to the motor units. People with stroke experience VA deficits and greater levels of central fatigue, which is the decrease in VA that occurs following exercise. This study investigated the between-session reliability of VA and central fatigue of the tibialis anterior muscle (TA) in people with chronic stroke (n = 12), using the interpolated twitch technique (ITT), adjusted-ITT, and central activation ratio (CAR) methods. On two separate sessions, supramaximal electrical stimulation was applied to the TA when it was at rest and maximally activated, at the start and end of a 30-s isometric dorsiflexor MVC. The most reliable measures of VA were obtained using the CAR calculation on transformed data, which produced an ICC of 0.92, and a lower bound confidence interval in the good range (95% CI 0.77 to 0.98). Reliability was lower for the CAR calculation on non-transformed data (ICC 0.82, 95% CI 0.63 to 0.91) and the ITT and adjusted-ITT calculations on transformed data (ICCs 0.82, 95% CIs 0.51 to 0.94), which had lower bound confidence intervals in the moderate range. The two ITT calculations on non-transformed data demonstrated the poorest reliability (ICCs 0.62, 95% CI 0.25 to 0.74). Central fatigue measures demonstrated very poor reliability. Thus, the reliability for VA in people with chronic stroke ranged from good to poor, depending on the calculation method and statistical analysis method, whereas the reliability for central fatigue was very poor.

How to improve the muscle synergy analysis methodology?

Muscle synergy analysis is increasingly used in domains such as neurosciences, robotics, rehabilitation or sport sciences to analyze and better understand motor coordination. The analysis uses dimensionality reduction techniques to identify regularities in spatial, temporal or spatio-temporal patterns of multiple muscle activation. Recent studies have pointed out variability in outcomes associated with the different methodological options available and there was a need to clarify several aspects of the analysis methodology. While synergy analysis appears to be a robust technique, it remain a statistical tool and is, therefore, sensitive to the amount and quality of input data (EMGs). In particular, attention should be paid to EMG amplitude normalization, baseline noise removal or EMG filtering which may diminish or increase the signal-to-noise ratio of the EMG signal and could have major effects on synergy estimates. In order to robustly identify synergies, experiments should be performed so that the groups of muscles that would potentially form a synergy are activated with a sufficient level of activity, ensuring that the synergy subspace is fully explored. The concurrent use of various synergy formulations-spatial, temporal and spatio-temporal synergies- should be encouraged. The number of synergies represents either the dimension of the spatial structure or the number of independent temporal patterns, and we observed that these two aspects are often mixed in the analysis. To select a number, criteria based on noise estimates, reliability of analysis results, or functional outcomes of the synergies provide interesting substitutes to criteria solely based on variance thresholds.

Central Drive to the Paretic Ankle Plantarflexors Affects the Relationship Between Propulsion and Walking Speed After Stroke

BACKGROUND AND PURPOSE

The ankle plantarflexor muscles are the primary generators of propulsion during walking. Impaired paretic plantarflexion is a key contributor to interlimb propulsion asymmetry after stroke. Poststroke muscle weakness may be the result of a reduced force-generating capacity, reduced central drive, or a combination of these impairments. This study sought to elucidate the relationship between the neuromuscular function of the paretic plantarflexor muscles and propulsion deficits across individuals with different walking speeds.

METHODS

For 40 individuals poststroke, we used instrumented gait analysis and dynamometry coupled with supramaximal electrostimulation to study the interplay between limb kinematics, the neuromuscular function of the paretic plantarflexors (ie, strength capacity and central drive), propulsion, and walking speed.

RESULTS

The strength capacity of the paretic plantarflexors was not independently related to paretic propulsion. Reduced central drive to the paretic plantarflexors independently contributed to paretic propulsion deficits. An interaction between walking speed and plantarflexor central drive was observed. Individuals with slower speeds and lower paretic plantarflexor central drive presented with the largest propulsion impairments. Some study participants with low paretic plantarflexor central drive presented with similarly fast speeds as those with near-normal central drive by leveraging a compensatory reliance on nonparetic propulsion. The final model accounted for 86% of the variance in paretic propulsion (R = 0.86, F = 33.10, P < 0.001).

DISCUSSION AND CONCLUSIONS

Individuals poststroke have latent paretic plantarflexion strength that they are not able to voluntarily access. The magnitude of central drive deficit is a strong indicator of propulsion impairment in both slow and fast walkers.Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A298).

Limited fascicle shortening and fascicle rotation may be associated with impaired voluntary force-generating capacity in pennate muscles of chronic stroke survivors

BACKGROUND

Muscle weakness is one of the most common motor impairments after stroke. A variety of progressive muscular changes are reported in chronic stroke survivors, and it is now feasible to consider these changes as an added source of weakness. However, the net contributions of such muscular changes towards muscle weakness have not been fully quantified.

METHODS

Accordingly, this study aims: (1) to compare muscle architecture of the human medial gastrocnemius between paretic and non-paretic sides in seven chronic hemispheric stroke survivors under passive conditions; (2) to characterize fascicle behavior (i.e., fascicle shortening and fascicle rotation) of the muscle during voluntary isometric contractions; and (3) to assess potential associations between muscle architectural parameters and muscle weakness. Muscle architecture of the medial gastrocnemius (including fascicle length, fascicle pennation angle, and muscle thickness) was characterized using B-mode ultrasonography, and fascicle behavior was then quantified as a function of isometric plantarflexion torque normalized to body mass.

FINDINGS

Our experimental results showed that under passive conditions, there was a significant difference in fascicle length and muscle thickness between paretic and non-paretic muscles, but no difference in resting fascicle pennation angle. However, during isometric contraction, both fascicle shortening and fascicle rotation on the paretic side were significantly decreased, compared to the non-paretic side. Moreover, the relative (i.e., paretic/non-paretic) fascicle rotation-shortening ratio (i.e., fascicle rotation per fascicle shortening) was strongly correlated with the relative maximum voluntary isometric plantarflexion torque.

INTERPRETATION

This association implies that such fascicle changes could impair the force-generating capacity of the muscle in chronic stroke survivors.

Compensatory neuromuscular junction adaptations of forelimb muscles in focal cortical ischemia in rats

INTRODUCTION

Upper limb movements are affected frequently by brain ischemia (BI). Mechanisms involved in recovery and compensatory movements have developed several studies. However, less attention is given to skeletal muscles, where neuromuscular junction (NMJ) has an important role on muscle tropism and functional performance.

METHODS

Animals were divided into two groups: control (C) and BI. Then, animals were skilled to perform single-pellet retrieval task, following these procedures: habituation, shaping, and single-pellet retrieval task. BI was induced using stereotaxic surgery in order to apply endothelin-1 in motor cortex, representative of movements of dominant paw. Reaching task performance was evaluated by single-pellet retrieval task 1 day before BI induction, 4 and 15 days after BI induction. After that, biceps, triceps, fingers flexor, and extensor muscles were extracted. NMJ was assessed in morphometric characteristics (total area, total perimeter, and feret). Muscle fiber cross-sectional area and connective tissue percentage were also evaluated for characterization. Student's t test was used for comparisons between C and BI groups. Tau Kendall's correlation was applied among variables from BI group.

RESULTS

An increase in all NMJ morphometric parameters, as well as increase of atrophy and fibrosis in BI group compared with C. There was a high level of direct correlation between mean values of NMJ morphometry with percentage of success in reaching task in BI group.

CONCLUSION

Brain ischemia-induced NMJ compensatory expansion, muscle atrophy, and fibrosis in forelimb muscles that are related to reaching performance.

Equinovarus foot in stroke survivors with spasticity: a narrative review of muscle–tendon morphology and force production adaptation

Background/Aims

Spastic paresis is the most common motor disorder in stroke survivors and can cause various types of muscle overactivity. This can lead to the development of spastic equinovarus foot, producing an inadequate base of support that limits locomotion and weight transfers. Physical therapists require better knowledge of the effects of spastic equinovarus foot in order to administer effective clinical treatment. Therefore, the aim of the present review was to describe changes in the muscle morphology and force production of stroke survivors with spastic equinovarus foot in relation to gait performance.

Methods

A narrative review of research into the effects of spastic paresis and equinovarus foot in stroke survivors was undertaken.

Results

There were a total of 20 identified studies that observed muscle-tendon morphology and force production in stroke survivors. All studies included in the present review reported several changes in muscle and tendon biomechanical properties as results of the spastic muscle overactivity.

Conclusions

Stroke survivors with spastic equinovarus foot experience muscle and tendon morphology that result in decreased force production, muscle power and gait performance.

The Comparison between Isokinetic Knee Muscles Strength in the Ipsilateral and Contralateral Limbs and Correlating with Function of Patients with Stroke

Objective The aim of this study is to compare the isokinetic knee muscles peak torque measurements and proprioception between the affected and intact limbs of patients with stroke, in addition to finding the correlation between knee muscles strength and lower limb function. Methods Twelve patients with stroke (mean age 64.33 ± 6.140 years), with 3 to 7 months poststroke who can walk 25 feet independently without using or using assistive devices and full passive range of motion were included in the study. Biodex isokinetic dynamometer was used for measuring isokinetic strength at 90°/s, 120°/s, and 150°/s and isometric strength at 60°/s in both flexors and extensors of the knee, whereas proprioception was measured at 45°/s knee flexion, all for affected and intact limbs. Functional measurements were assessed using the Fugl-Meyer Assessment for Lower Limb scale and Barthel Index (BI). Results The differences shown were found to be statistically significant between affected and intact limbs in isokinetic 90°/s flexion ( p = 0.005), extension ( p = 0.0013), and isometric at 60°/s flexion ( p < 0.0001) knee muscle strengths and also the proprioception ( p = 0.05). Significant positive correlation was found between isokinetic affected side knee flexion at 90°/s ( r = 0.903) with BI ( r = 0.704). Conclusion There is a significant difference in peak torque measurements between affected and normal lower limbs of poststroke patients, as well as a significant correlation between the knee strength and lower limb functions. Furthermore, it can also be concluded that the differences in knee proprioception between the affected and intact limbs were shown to be significant.

Estimating Voluntary Activation Of The Elbow And Wrist Muscles In Chronic Hemiparetic Stroke Using Twitch Interpolation Methodology

One of the cardinal motor deficits that occurs after stroke is paresis, a decrease in the voluntary activation of muscles. Paresis leads to a decrease in voluntary joint strength, impacting stroke survivors' ability to perform activities of daily living (ADLs). Quantifying this decrease in voluntary activation is important when designing rehabilitation interventions to address movement impairments and restore the ability to perform ADLs. Twitch interpolation is an experimental technique developed to quantify muscle voluntary activation [1]. This method has been used widely across pathologies but often limited to assessment of the voluntary activation of the plantar flexors, given the ease of activating these muscles through stimulation of the tibial nerve [2]. The complex innervation of elbow and wrist musculature imposes practical difficulties when applying the twitch interpolation technique to these joints [1]. Therefore, only a few studies have used this technique to examine the pathological [3]-[5] upper extremity, with little quantitative data documenting the degree of paresis present in the upper limb after stroke. The goal of this study is to evaluate the feasibility of applying twitch interpolation to quantify voluntary activation of the elbow and wrist flexors and extensors in chronic stroke survivors.

Impaired interlimb coordination is related to asymmetries during pedaling after stroke

OBJECTIVE

To understand whether lower limb asymmetry in chronic stroke is related to paretic motor impairment or impaired interlimb coordination.

METHODS

Stroke and control participants performed conventional, unilateral, and bilateral uncoupled pedaling. During uncoupled pedaling, the pedals were mechanically disconnected. Paretic mechanical work was measured during conventional pedaling. Pedaling velocity and muscle activity were compared across conditions and groups. Relative limb phasing was examined during uncoupled pedaling.

RESULTS

During conventional pedaling, EMG and mechanical work were lower in the paretic than the non-paretic limb (asymmetry). During unilateral pedaling with the paretic limb, muscle activity was larger, but velocity was slower and more variable than during conventional pedaling (evidence of paretic motor impairment). During uncoupled pedaling, muscle activity increased further, but velocity was slower and more variable than in other conditions (evidence of impaired interlimb coordination). Relative limb phasing was impaired in stroke participants. Regression analysis suggested that interlimb coordination may be a stronger predictor of asymmetry than paretic motor impairment.

CONCLUSIONS

Paretic motor impairment and impaired interlimb coordination may contribute to asymmetry during pedaling after stroke.

SIGNIFICANCE

Rehabilitation that addresses paretic motor impairment and impaired interlimb coordination may improve symmetry and maximize improvement.

Long-term use of implanted peroneal functional electrical stimulation for stroke-affected gait: the effects on muscle and motor nerve

BACKGROUND

Peripheral changes to muscle and motor nerves occur following stroke, which may further impair functional capacity. We investigated whether a year-long use of an implanted peroneal FES system reverses stroke-related changes in muscles and motor nerves in people with foot drop in the chronic phase after supratentorial stroke.

METHODS

Thirteen persons with a chronic stroke (mean age 56.1 years, median Fugl-Meyer Assessment leg score 71%) were included and received an implanted peroneal FES system (ActiGait®). Quantitative muscle ultrasound (QMUS) images were obtained bilaterally from three leg muscles (i.e. tibialis anterior, rectus femoris, gastrocnemius). Echogenicity (muscle ultrasound gray value) and muscle thickness were assessed over a one-year follow-up and compared to age-, sex-, height- and weight-corrected reference values. Compound motor action potentials (CMAPs) and motor evoked potentials (MEPs) were obtained from the tibialis anterior muscle. Generalized estimated equation modeling was used to assess changes in QMUS, CMAPs and MEPs outcomes over the follow-up period.

RESULTS

Echogenicity of the tibialis anterior decreased significantly during the follow-up on the paretic side. Z-scores changed from 0.88 at baseline to - 0.15 after 52 weeks. This was accompanied by a significant increase in muscle thickness on the paretic side, where z-scores changed from - 0.32 at baseline to 0.48 after 52 weeks. Echogenicity of the rectus femoris normalized on both the paretic and non-paretic side (z-scores changed from - 1.09 and - 1.51 to 0.14 and - 0.49, respectively). Amplitudes of CMAP and MEP (normalized to CMAP) were reduced during follow-up, particularly on the paretic side (ΔCMAP = 20% and ΔMEP = 14%).

CONCLUSIONS

We show that the structural changes to muscles following stroke are reversible with FES and that these changes might not be limited to electrically stimulated muscles. No evidence for improvement of the motor nerves was found.

Innervation zone distribution of the biceps brachii muscle examined using voluntary and electrically-evoked high-density surface EMG

BACKGROUND

High density surface electromyography (EMG) can be used to estimate muscle innervation zones (IZ). The objective of this study was to compare the differences in the distribution of the biceps brachii (BB) IZ derived from voluntary contractions (VC) and electrical stimulation (ES) of the musculocutaneous nerve.

METHODS

Surface EMG signals were recorded from the medial and lateral BB with two 64-channel high density electrode matrices in eight healthy men. The surface EMG was recorded at different percentages of the maximal voluntary contraction (MVC) force (20-100% MVC) and at different percentages of the current needed to elicit a maximal M-wave (20-100% Imax). The IZs of the medial and lateral BB were identified from the EMG signals and expressed as a row number within a given medial-lateral column.

RESULTS

ES current intensity had no significant effect on the group mean IZ location (p > 0.05). However, The IZ during VC was located more proximally with increasing force (p < 0.05), likely due to muscle shortening. The position of the IZ varied slightly (by up to ~ 8 mm) in a medial-lateral direction under both contraction types, but this spatial effect was not significant. The IZ during ES and weak VC (20, 40% MVC) was similar (p > 0.05), but was more proximal in the latter than the former during 60-100% MVC (p < 0.05).

CONCLUSION

ES can be used to detect spatial differences in IZ location free of the confounding effects of muscle shortening and recruitment order of different sized motor units. The method may prove beneficial for locating the IZ in patients who lack voluntary control of their musculature.

The Relationship Between Blood Flow and Motor Unit Firing Rates in Response to Fatiguing Exercise Post-stroke

We quantified the relationship between the change in post-contraction blood flow with motor unit firing rates and metrics of fatigue during intermittent, sub-maximal fatiguing contractions of the knee extensor muscles after stroke. Ten chronic stroke survivors (>1-year post-stroke) and nine controls participated. Throughout fatiguing contractions, the discharge timings of individual motor units were identified by decomposition of high-density surface EMG signals. After five consecutive contractions, a blood flow measurement through the femoral artery was obtained using an ultrasound machine and probe designed for vascular measurements. There was a greater increase of motor unit firing rates from the beginning of the fatigue protocol to the end of the fatigue protocol for the control group compared to the stroke group (14.97 ± 3.78% vs. 1.99 ± 11.90%, p = 0.023). While blood flow increased with fatigue for both groups (p = 0.003), the magnitude of post-contraction blood flow was significantly greater for the control group compared to the stroke group (p = 0.004). We found that despite the lower magnitude of muscle perfusion through the femoral artery in the stroke group, blood flow has a greater impact on peripheral fatigue for the control group; however, we observed a significant correlation between change in blood flow and motor unit firing rate modulation (r² = 0.654, p = 0.004) during fatigue in the stroke group and not the control group (r² = 0.024, p < 0.768). Taken together, this data showed a disruption between motor unit firing rates and post-contraction blood flow in the stroke group, suggesting that there may be a disruption to common inputs to both the reticular system and the corticospinal tract. This study provides novel insights in the relationship between the hyperemic response to exercise and motor unit firing behavior for post-stroke force production and may provide new approaches for recovery by improving both blood flow and muscle activation simultaneously.

Cryotherapy reduces muscle hypertonia, but does not affect lower limb strength or gait kinematics post-stroke: a randomized controlled crossover study

BACKGROUND

Based on the premise that spasticity might affect gait post-stroke, cryotherapy is among the techniques used to temporarily reduce spasticity in neurological patients. This effective technique would enhance muscle performance, and ultimately, functional training, such as walking. However, understanding whether a decrease in spasticity level, if any, would lead to improving muscle performance and gait parameters is not based on evidence and needs to be clarified.

OBJECTIVES

to investigate the immediate effects of cryotherapy, applied to spastic plantarflexor muscles of subjects post-stroke, on tonus level, torque generation capacity of plantarflexors and dorsiflexors, and angular/spatiotemporal gait parameters.

METHODS

Sixteen chronic hemiparetic subjects participated in this randomized controlled crossover study. Cryotherapy (ice pack) or Control (room temperature sand pack) were applied to the calf muscles of the paretic limb. The measurements taken (before and immediately after intervention) were: 1) Tonus according to the Modified Ashworth Scale; 2) Torque assessments were performed using an isokinetic dynamometer; and 3) Spatiotemporal and angular kinematics of the hip, knee, and ankle (flexion/extension), obtained using a tridimensional movement analysis system (Qualisys).

RESULTS

Cryotherapy decreased plantarflexor tonus but did not change muscle torque generation capacity and did not affect spatiotemporal or angular parameters during gait compared to control application. These findings contribute to the evidence-based approach to clinical rehabilitation post-stroke.

CONCLUSIONS

The findings of this study suggest that cryotherapy applied to the calf muscles of subjects with chronic hemiparesis reduces muscle hypertonia but does not improve dorsiflexors and plantarflexors performance and gait parameters.

CORP: Measurement of upper and lower limb muscle strength and voluntary activation

Muscle strength, the maximal force-generating capacity of a muscle or group of muscles, is regularly assessed in physiological experiments and clinical trials. An understanding of the expected variation in strength and the factors that contribute to this variation is important when designing experiments, describing methodologies, interpreting results, and attempting to replicate methods of others and reproduce their findings. In this review (Cores of Reproducibility in Physiology), we report on the intra- and inter-rater reliability of tests of upper and lower limb muscle strength and voluntary activation in humans. Isometric, isokinetic, and isoinertial strength exhibit good intra-rater reliability in most samples (correlation coefficients ≥0.90). However, some tests of isoinertial strength exhibit systematic bias that is not resolved by familiarization. With the exception of grip strength, few attempts have been made to examine inter-rater reliability of tests of muscle strength. The acute factors most likely to affect muscle strength and serve as a source of its variation from trial-to-trial or day-to-day include attentional focus, breathing technique, remote muscle contractions, rest periods, temperature (core, muscle), time of day, visual feedback, body and limb posture, body stabilization, acute caffeine consumption, dehydration, pain, fatigue from preceding exercise, and static stretching >60 s. Voluntary activation, the nervous system’s ability to drive a muscle to create its maximal force, exhibits good intra-rater reliability when examined with twitch interpolation (correlation coefficients >0.80). However, inter-rater reliability has not been formally examined. The methodological factors most likely to influence voluntary activation are myograph compliance and sensitivity; stimulation location, intensity, and inadvertent stimulation of antagonists; joint angle (muscle length); and the resting twitch.

Strength or Motor Control: What Matters in High-Functioning Stroke?

Background: The two primary motor impairments that hinder function after stroke are declines in strength and motor control. The impact of motor impairments on functional capacity may vary with the severity of stroke motor impairments. In this study, we focus on high-functioning stroke individuals who experience mild to moderate motor impairments and often resume prior activities or return to work. These tasks require the ability to move independently, placing high demands on their functional mobility. Therefore, the purpose of this study was to quantify impairments in strength and motor control and their contribution to functional mobility in high-functioning stroke.

Methods:Twenty-one high-functioning stroke individuals (Fugl Meyer Lower Extremity Score = 28.67 ± 4.85; Functional Activity Index = 28.47 ± 7.04) and 21 age-matched healthy controls participated in this study. To examine motor impairments in strength and motor control, participants performed the following tasks with the paretic ankle (1) maximum voluntary contractions (MVC) and (2) visuomotor tracking of a sinusoidal trajectory. Strength was quantified as the maximum force produced during ankle plantarflexion and dorsiflexion. Motor control was quantified as (a) the accuracy and (b) variability of ankle movement during the visuomotor tracking task. For functional mobility, participants performed (1) overground walking for 7 meters and (2) simulated driving task. Functional mobility was determined by walking speed, stride length variability, and braking reaction time.

Results: Compared with the controls, the stroke group showed decreased plantarflexion strength, decreased accuracy, and increased variability of ankle movement. In addition, the stroke group demonstrated decreased walking speed, increased stride length variability, and increased braking reaction time. The multiple-linear regression model revealed that motor accuracy was a significant predictor of the walking speed and braking reaction time. Further, motor variability was a significant predictor of stride length variability. Finally, the dorsiflexion or plantarflexion strength did not predict walking speed, stride length variability or braking reaction time.

Conclusions: The impairments in motor control but not strength predict functional deficits in walking and driving in high-functioning stroke individuals. Therefore, rehabilitation interventions assessing and improving motor control will potentially enhance functional outcomes in high-functioning stroke survivors.

Motor Unit Properties of the First Dorsal Interosseous in Chronic Stroke Subjects: Concentric Needle and Single Fiber EMG Analysis

The purpose of this study was to better understand changes in motor unit electrophysiological properties in people with chronic stroke based on concentric needle electromyography (EMG) and single fiber EMG recordings. The first dorsal interosseous (FDI) muscle was studied bilaterally in eleven hemiparetic stroke subjects. A significant increase in mean fiber density (FD) was found in the paretic muscle compared with the contralateral side based on single fiber EMG (1.6 ± 0.2 vs. 1.3 ± 0.1, respectively, P = 0.003). There was no statistically significant difference between the paretic and contralateral sides in most concentric needle motor unit action potential (MUAP) parameters, such as amplitude (768.7 ± 441.7 vs. 855.0 ± 289.9 μV), duration (8.9 ± 1.8 vs. 8.68 ± 0.9 ms) and size index (1.2 ± 0.5 vs. 1.1 ± 0.3) (P > 0.18), nor was there a significant difference in single fiber EMG recorded jitter (37.0 ± 9.6 vs. 39.9 ± 10.6 μs, P = 0.45). The increase in FD suggests motor units of the paretic FDI have enlarged due to collateral reinnervation. However, sprouting might be insufficient to result in a statistically significant change in the concentric needle MUAP parameters. Single fiber EMG appears more sensitive than concentric needle EMG to reflect electrophysiological changes in motor units after stroke. Both single fiber and concentric needle EMG recordings may be necessary to better understand muscle changes after stroke, which is important for development of appropriate rehabilitation strategies. The results provide further evidence that motor units are remodeled after stroke, possibly in response to a loss of motoneurons.

Impaired regulation post-stroke of motor unit firing behavior during volitional relaxation of knee extensor torque assessed using high density surface EMG decomposition

The purpose of this study was to use high density surface EMG recordings to quantify stroke-related abnormalities in motor unit firing behavior during repeated sub-maximal knee extensor contractions. A high density surface EMG system (sEMG) was used to record and extract single motor unit firing behavior in the vastus lateralis muscle of 6 individuals with chronic stroke and 8 controls during repeated sub-maximal isometric knee extension contractions. Paretic motor unit firing rates were increased with subsequent contractions (6.19±0.35 pps vs 7.89±0.66 pps, P <; 0.05) during task phases of torque decline as compared to controls (6.95±0.40 pps vs 6.68±0.41 pps). In addition, corresponding rates of torque decline were decreased for the paretic leg as compared to the non-paretic leg. These results suggest that regulation of declining forces may be impaired post stroke due to prolonged firing of paretic motor units.

The role of the ipsilesional side in the rehabilitation of post-stroke subjects

Most stroke lesions occur in the middle cerebral artery territory, presenting a high probability of damage of pathways with predominant ipsilesional disposition, mainly related to postural control. Despite the high probability of bilateral postural control dysfunction based on neuroanatomical fundaments, both research and clinical rehabilitation involving stroke subjects have been focused on contralesional side (also named affected side) impairments, while ipsilesional side (also named non-affected side) impairments have been attributed to an adaptive strategy. This paper aims to present a critical understanding about the state-of-the-art that sustains the hypothesis that stroke subjects with middle cerebral artery territory lesion at the subcortical level show an atypical behaviour in the ipsilateral side associated with the lesion itself and the possible implications.

Lower Extremity Motor Impairments in Ambulatory Chronic Hemiparetic Stroke: Evidence for Lower Extremity Weakness and Abnormal Muscle and Joint Torque Coupling Patterns

Although global movement abnormalities in the lower extremity poststroke have been studied, the expression of specific motor impairments such as weakness and abnormal muscle and joint torque coupling patterns have received less attention. We characterized changes in strength, muscle coactivation and associated joint torque couples in the paretic and nonparetic extremity of 15 participants with chronic poststroke hemiparesis (age 59.6 ± 15.2 years) compared with 8 age-matched controls. Participants performed isometric maximum torques in hip abduction, adduction, flexion and extension, knee flexion and extension, ankle dorsi- and plantarflexion and submaximal torques in hip extension and ankle plantarflexion. Surface electromyograms (EMGs) of 10 lower extremity muscles were measured. Relative weakness (paretic extremity compared with the nonparetic extremity) was measured in poststroke participants. Differences in EMGs and joint torques associated with maximum voluntary torques were tested using linear mixed effects models. Results indicate significant poststroke torque weakness in all degrees of freedom except hip extension and adduction, adductor coactivation during extensor tasks, in addition to synergistic muscle coactivation patterns. This was more pronounced in the paretic extremity compared with the nonparetic extremity and with controls. Results also indicated significant interjoint torque couples during maximum and submaximal hip extension in both extremities of poststroke participants and in controls only during maximal hip extension. Additionally, significant interjoint torque couples were identified only in the paretic extremity during ankle plantarflexion. A better understanding of these motor impairments is expected to lead to more effective interventions for poststroke gait and posture.

Comparison of techniques to determine human skeletal muscle voluntary activation

Determining volitional activation (VA) can provide insights on the cause of muscle weakness in orthopedic and neurological populations. Two electrical stimulation techniques are traditionally used to quantify VA: interpolation (IT) and superimposition (CAR). IT allows for a more accurate VA estimation, however it requires individuals to be stimulated twice, compared to once for CAR, and thus increases stimulation associated discomfort. To date, there is no agreement on what is the best practical technique for calculating quadriceps VA. This paper aims to address this problem by determining what reference force (i.e., using either peak force or force at the time of stimulation) and type of stimulation (train of pulses (burst), doublet, and twitch) is the best technique to use. Our findings showed that the IT with the force at the time of stimulation as a reference should be used to determine VA and that when a burst was used, the VA ratio computations were more accurate. Additionally, using a twitch with a 2ms pulse duration produced reliable VA calculations and may be an acceptable alternative for pain-sensitive subjects. Accurate assessment of VA deficits can help clinicians design rehabilitation programs that are based on subject-specific strength impairments and are more effective.

Differences in excitability properties between medial gastrocnemius, tibialis anterior, and abductor pollicis brevis motor axons

INTRODUCTION

Excitability properties of motor nerves to different muscles are different, but the explanation is uncertain. We characterized motor axon excitability properties to the medial gastrocnemius (MG) in 27 adults, and made comparisons with the peroneal nerve to the tibialis anterior (TA) and median nerve to the abductor pollicis brevis (APB) in 10 subjects.

METHODS

Recordings of multiple excitability properties were made using threshold tracking, stimulating the nerves at the wrist or knee.

RESULTS

Threshold electrotonus and superexcitability differed between nerves (APB>MG>TA axons) that may reflect differences in fast K⁺ conductance. APB axons had larger S2 accommodation and undershoot than TA and MG axons, indicating greater slow K⁺ conductance. TA axons demonstrated greater accommodation during hyperpolarizing currents than MG and APB axons, suggestive of greater inwardly rectifying current.

DISCUSSION

Inherent differences in several conductances underlie nerve differences in excitability, presumably related to muscle or motoneuron properties. Muscle Nerve 57: E60-E69, 2018.

Skeletal muscle changes following stroke: a systematic review and comparison to healthy individuals

BACKGROUND

Despite extensive study of the impact of stroke on muscle and functional performance, questions remain regarding the extent to which changes are due to the neurological injury vs. age-related loss of morphology and force production.

OBJECTIVES

To synthesize available evidence describing post-stroke changes in lower extremity muscle size and strength compared to healthy adults.

METHODS

Scientific literature was searched up to April 2016 to identify studies that included lower extremity muscle size and strength measures in individuals with chronic stroke. Lower extremity muscle size and strength data from healthy controls were sought for comparison. Relative differences were calculated between paretic, nonparetic, and control limbs.

RESULTS

Fifteen studies with 375 participants (61% male; age = 62 ± 5 years; time since stroke = 60 ± 42 months) were included. The paretic limb exhibited deficits of ~13% in thigh muscle size, ~5% in lower leg muscle size, and ~8% in lean leg mass compared to the nonparetic limb. Paretic plantarflexor and knee extensor strength were 52 and 36% lower, respectively, compared to the nonparetic limb. When compared to age-matched control data, both paretic and nonparetic limbs showed deficits in muscle size and strength.

CONCLUSIONS

Age-related differences support the impact of stroke-related sarcopenia as a contributor to hemiparetic muscle dysfunction. Understanding these muscular changes is necessary for designing appropriate exercise interventions aimed at restoring muscle function.

Combined Ultrasound Imaging and Biomechanical Modeling to Estimate Triceps Brachii Musculotendon Changes in Stroke Survivors

The aim of this study was to investigate the changes of musculotendon parameters of triceps brachii in persons after stroke based on subject-specific biomechanical modeling technique combined with in vivo ultrasound measurement. Five chronic stroke survivors and five normal control subjects were recruited. B-mode ultrasound was applied to measure muscle pennation angle and the optimal length of three heads of triceps' brachii at different joint angle positions in resting and isometric contraction. Measured ultrasound data were used to reduce the unknown parameters during the modeling optimization process. The results showed that pennation angles varied with joint angles, and the longhead TRI pennation from stroke group was smaller than the literature value. The maximum isometric muscle stress from persons after stroke was significantly smaller than that found in the unimpaired subjects. The prediction of joint torque fits well with the measured data from the control group, whereas the prediction error is larger in results from persons after stroke. In vivo parameters from ultrasound data could help to build a subject-specific biomechanical model of elbow extensor for both unimpaired and hemiplegic subjects, and then the results driven from the model could enhance the understanding of motor function changes for persons after stroke.

Relationship between foot pain, muscle strength and size: a systematic review

BACKGROUND

Foot pain is common and disabling and thought to be associated with muscle weakness. Understanding the relationship between pain and weakness may help identify effective treatment targets.

OBJECTIVES

To conduct a systematic review to evaluate the relationship between foot pain and foot muscle weakness, or muscle size as a proxy for weakness.

DATA SOURCES

Electronic databases and reference lists were searched for all years to April 2015.

ELIGIBILITY CRITERIA

Full-text articles were retrieved based on the question 'Does the study evaluate an association between foot pain and foot muscle weakness or size?'

DATA EXTRACTION AND SYNTHESIS

Two reviewers independently screened eligible studies, extracted data and completed a methodological rating.

RESULTS

Eight studies were identified evaluating the relationship between foot pain and foot muscle strength (n=6) or size (n=2). Four studies reported a significant relationship between pain and toe flexor force. One study reported a significant relationship between heel pain and reduced forefoot muscle size. One study reported an inconsistent association depending on measurement technique. One study reported no association between pain and hindfoot muscle size. One study reported no association between low to moderate pain and toe flexion force.

LIMITATIONS

Due to data heterogeneity, no data were pooled for meta-analysis.

CONCLUSION

There is evidence of a significant association between foot pain and muscle weakness when foot pain is of high intensity and primarily measured by toe flexion force. However there is inconsistent evidence that lower intensity foot pain is associated with other measures of foot muscle weakness or size. Systematic Research Registry ID reviewregistry166.

Bilateral changes in muscle architecture of physically active people with chronic stroke: A quantitative muscle ultrasound study

OBJECTIVE

Changes in muscle architecture after stroke are usually assessed by investigating inter-limb differences. As a result bilateral changes of muscle architecture might be missed. Our aim was to investigate whether bilateral architectural changes in skeletal muscle can be detected in chronic, physically active stroke patients using quantitative muscle ultrasound (QMUS).

METHODS

Twenty-eight patients (mean time since stroke 5.2years, median Brunnström stage 4) were recruited. QMUS images were obtained bilaterally from 2 arm and 4 leg muscles. Corrected echogenicity (muscle ultrasound grayvalue) and muscle thickness were compared to reference values obtained from healthy subjects. Correlations of muscle changes with demographic, clinical and neurophysiological characteristics were explored.

RESULTS

Out of 6 muscles, a significant increase in mean echogenicity was found in 4 paretic and 3 non-paretic side muscles. Significant decreases in mean muscle thickness were found in 2 paretic side muscles and 1 non-paretic side muscle. Echogenicity of the medial gastrocnemius correlated moderately with walking speed (inversely) and time since stroke.

CONCLUSIONS

This study showed that QMUS is a feasible technique to investigate architectural changes in skeletal muscles in the chronic phase of stroke and that abnormalities can be found in muscles on both the hemiparetic and non-paretic side.

SIGNIFICANCE

Intriguing data on bilateral changes in muscles of people with stroke is presented. Directions for future research are provided.

Contributions of voluntary activation deficits to hand weakness after stroke

BACKGROUND

Hemiparetic stroke survivors often exhibit profound weakness in the digits of the paretic hand, but the relative contribution of potential biomechanical and neurological impairment mechanisms is not known. Establishing sources of impairment would help in guiding treatment.

OBJECTIVE

The present study sought to quantify the role of diminished capacity to voluntarily active finger flexor and extensor muscles as one possible neurological mechanism.

METHODS

Two groups of stroke survivors with "severe" (N = 9) or "moderate" (N = 9) hand impairment and one group of neurologically intact individuals (N = 9) participated. Subjects were asked to create isometric flexion force and extension force, respectively, with the tip of the middle finger. The maximum voluntary force (MVF) and the maximum stimulated force (MSF) produced by an applied train of electrical current pulses (MSF) were recorded for flexion and extension. Percent voluntary activation (PVA) was computed from MVF and MSF.

RESULTS

Significant deficits in both MVF and PVA were observed for stroke subjects compared to control subjects. For example, activation deficits were >80% for extensor digitorum communis (EDC) for the "severe" group. Maximum voluntary force and PVA deficits were greater for EDC than for flexor digitorum superficialis (FDS) for stroke subjects with severe impairment. Maximum voluntary force and PVA correlated significantly for stroke subjects but not for control subjects.

CONCLUSIONS

Although extrinsic finger muscles could be successfully recruited electrically, voluntary excitation of these muscles was substantially limited in stroke survivors. Thus, finger weakness after stroke results predominantly from the inability to fully activate the muscle voluntarily.

Lower limb neuromuscular function and blood flow characteristics in AFO-using survivors of stroke

BACKGROUND AND PURPOSE

Ankle-foot orthoses (AFOs) and gait aids are commonly used by survivors of stroke to ambulate, but they have not been used as inclusion or exclusion criteria for physiological studies. The purpose of this study was to examine differences in neuromuscular function and blood flow characteristics between the lower limbs of ambulatory, AFO-using survivors of stroke (n = 9).

METHODS

The subjects were, on average, 14 years poststroke and had used an AFO for about 7 years. We used the following measures to assess both lower limbs: quadriceps muscular strength (manual muscle testing, 1RM), calf muscular endurance (single-leg heel raises), lower limb soft tissue composition and muscle cross-sectional area, calf resting blood flow (strain gauge plethysmography), and central neural drive of the plantar flexors (nerve stimulation). The sound limb was defined as the control.

RESULTS

Quadriceps strength, triceps surae endurance, and calf cross-sectional area all were greater in the sound limb than in the affected limb (relative differences: 76.3% ± 16.6%, 146.4% ± 24.6%, and 25.6% ± 5.7%, respectively). In addition, resting blood flow, peak torque, and central neural drive of the plantar flexors were significantly greater (relative differences: 38.7% ± 5.9%, 94.4% ± 17.9%, and 43.6% ± 12.0%, respectively) in the sound calf.

DISCUSSION

Our findings confirm that significant decrements in muscle performance, size, and blood flow persist in the affected limb many years after stroke despite a resumption of ambulation.

CONCLUSIONS

This cohort of ambulatory AFO users exhibited large variations in functional abilities. We recommend that future studies in stroke survivors consider AFO use in their research designs.

Ankle antagonist coactivation in the double-support phase of walking: Stroke vs. healthy subjects

INTRODUCTION

Lesions in ipsilateral systems related to postural control in the ipsilesional side may justify the lower performance of stroke subjects during walking.

PURPOSE

To analyze bilateral ankle antagonist coactivation during double support in stroke subjects.

METHODS

Sixteen (8 females; 8 males) subjects with a first isquemic stroke and 22 controls (12 females; 10 males) participated in this study. The double-support phase was assessed through ground reaction forces and the electromyography of ankle muscles was assessed in both limbs.

RESULTS

The ipsilesional limb presented statistically significant differences from the control when assuming specific roles during double support. The tibialis anterior and soleus pair was the one in which this atypical behavior was more pronounced.

CONCLUSION

The ipsilesional limb presents a dysfunctional behavior when a higher postural control activity was demanded.

Excitability properties of motor axons in adults with cerebral palsy

Cerebral palsy (CP) is a permanent disorder caused by a lesion to the developing brain that significantly impairs motor function. The neurophysiological mechanisms underlying motor impairment are not well understood. Specifically, few have addressed whether motoneuron or peripheral axon properties are altered in CP, even though disruption of descending inputs to the spinal cord may cause them to change. In the present study, we have compared nerve excitability properties in seven adults with CP and fourteen healthy controls using threshold tracking techniques by stimulating the median nerve at the wrist and recording the compound muscle action potential over the abductor pollicis brevis. The excitability properties in the CP subjects were found to be abnormal. Early and late depolarizing and hyperpolarizing threshold electrotonus was significantly larger (i.e., fanning out), and resting current-threshold (I/V) slope was smaller, in CP compared to control. In addition resting threshold and rheobase tended to be larger in CP. According to a modeling analysis of the data, an increase in leakage current under or through the myelin sheath, i.e., the Barrett-Barrett conductance, combined with a slight hyperpolarization of the resting membrane potential, best explained the group differences in excitability properties. There was a trend for those with greater impairment in gross motor function to have more abnormal axon properties. The findings indicate plasticity of motor axon properties far removed from the site of the lesion. We suspect that this plasticity is caused by disruption of descending inputs to the motoneurons at an early age around the time of their injury.

Activation of brain areas following ankle dorsiflexion versus plantar flexion: Functional magnetic resonance imaging verification

Changes in activated areas of the brain during ankle active dorsiflexion and ankle active plantar flexion were observed in six healthy subjects using functional magnetic resonance imaging. Excited areas of ankle active dorsiflexion involved the bilateral primary motor area and the primary somatosensory area, as well as the bilateral supplementary sensory area, the primary visual area, the right second visual area, and the vermis of cerebellum. Excited areas of ankle active plantar flexion included the ipsilateral supplementary motor area, the limbic system, and the contralateral corpus striatum. Fine movements of the cerebral cortex control the function of the ankle dorsiflexion to a larger extent than ankle plate flexion, and the function of ankle plate flexion is more controlled by the subcortical area.

Voluntary Activation is Reduced in Both the More- and Less-Affected Upper Limbs after Unilateral Stroke

Objective: Measurement of voluntary activation gives an indication of neural drive to the muscle. This study aimed to identify the site of impairment in neural drive during voluntary contractions post-stroke.

Methods: Elbow-flexor voluntary activation was assessed bilaterally for 10 stroke patients (mean 61.2 ± 12.3 years) and 6 age-matched controls (61.3 ± 14.0 years) by stimulating either the peripheral nerve or the motor cortex during maximal voluntary contractions. Any additional evoked force during maximal contractions implies neural drive is incomplete. Peripheral stimulation can detect deficits at or above the stimulation level, while cortical stimulation can identify suboptimal supraspinal output.

Results: Impairments were apparent on the less-affected side in addition to the more-affected side after stroke in voluntary activation, torque, and electromyographic activity (EMG) response. Maximal torque was reduced by 44% on the more-affected and 31% on the less-affected side compared to healthy controls (p < 0.001). Peripheral voluntary activation was reduced to 81% on the more-affected side and 86% on the less-affected side, with healthy subjects at 96% (p < 0.05). Although EMG was bilaterally impaired after stroke, the pattern of response was different between sides. Voluntary activation could not be calculated for cortical stimulation post-stroke due to variability in the evoked force, but EMG results from cortical stimulation showed significant differences in the neural drive to each side.

Conclusion: Voluntary activation is impaired bilaterally in the upper-limb after stroke, with reduced cortical connectivity on the more-affected side.

Significance: Although the muscle itself did not change post-stroke, altered descending drive and connectivity were the critical factors explaining post-stroke paresis.

Assessment of quadriceps muscle inactivation with a new electrical stimulation paradigm

INTRODUCTION

In this study we evaluated the validity of garment-based quadriceps stimulation (GQS) for assessment of muscle inactivation in comparison with femoral nerve stimulation (FNS).

METHODS

Inactivation estimates (superimposed doublet torque), self-reported discomfort, and twitch and doublet contractile properties were compared between GQS and FNS in 15 healthy subjects.

RESULTS

Superimposed doublet torque was significantly lower for GQS than for FNS at 20% and 40% maximum voluntary contraction (MVC) (P < 0.01), but not at 60%, 80%, and 100% MVC. Discomfort scores were systematically lower for GQS than for FNS (P < 0.05). Resting twitch and doublet peak torque were lower for GQS, and time to peak torque was shorter for GQS than for FNS (P < 0.01).

CONCLUSIONS

GQS can be used with confidence for straightforward evaluation of quadriceps muscle inactivation, whereas its validity for assessment of contractile properties remains to be determined.

Anomalous EMG-force relations during low-force isometric tasks in hemiparetic stroke survivors

Hemispheric brain injury resulting from a stroke is often accompanied by muscle weakness in contralateral limbs. In neurologically intact subjects, appropriate motoneuronal recruitment and rate modulation are utilized to optimize muscle force production. In the present study, we sought to determine whether weakness in an affected hand muscle in stroke survivors is partially attributable to alterations in the control of muscle activation. Specifically, our goal was to characterize whether the surface EMG amplitude was systematically larger as a function of (low) force in paretic hand muscles as compared to contralateral muscles in the same subject. We tested a multifunctional muscle, the first dorsal interosseous (FDI), in multiple directions about the second metacarpophalangeal joint in ten hemiparetic and six neurologically intact subjects. In six of the ten stroke subjects, the EMG-force slope was significantly greater on the affected side as compared to the contralateral side, as well as compared to neurologically intact subjects. An unexpected set of results was a nonlinear relation between recorded EMG and generated force commonly observed in the paretic FDI, even at very low-force levels. We discuss possible experimental as well as physiological factors that may contribute to an increased EMG-force slope, concluding that changes in motor unit (MU) control are the most likely reasons for the observed changes.

Poststroke muscle architectural parameters of the tibialis anterior and the potential implications for rehabilitation of foot drop

Poststroke dorsiflexor weakness and paretic limb foot drop increase the risk of stumbling and falling and decrease overall functional mobility. It is of interest whether dorsiflexor muscle weakness is primarily neurological in origin or whether morphological differences also contribute to the impairment. Ten poststroke hemiparetic individuals were imaged bilaterally using noninvasive medical imaging techniques. Magnetic resonance imaging was used to identify changes in tibialis anterior muscle volume and muscle belly length. Ultrasonography was used to measure fascicle length and pennation angle in a neutral position. We found no clinically meaningful bilateral differences in any architectural parameter across all subjects, which indicates that these subjects have the muscular capacity to dorsiflex their foot. Therefore, poststroke dorsiflexor weakness is primarily neural in origin and likely due to muscle activation failure or increased spasticity of the plantar flexors. The current finding suggests that electrical stimulation methods or additional neuromuscular retraining may be more beneficial than targeting muscle strength (i.e., increasing muscle mass).

Differences in Plantar Flexor Fascicle Length and Pennation Angle between Healthy and Poststroke Individuals and Implications for Poststroke Plantar Flexor Force Contributions

Poststroke plantar flexor muscle weakness has been attributed to muscle atrophy and impaired activation, which cannot collectively explain the limitations in force-generating capability of the entire muscle group. It is of interest whether changes in poststroke plantar flexor muscle fascicle length and pennation angle influence the individual force-generating capability and whether plantar flexor weakness is due to uniform changes in individual muscle force contributions. Fascicle lengths and pennation angles for the soleus, medial, and lateral gastrocnemius were measured using ultrasound and compared between ten hemiparetic poststroke subjects and ten healthy controls. Physiological cross-sectional areas and force contributions to poststroke plantar flexor torque were estimated for each muscle. No statistical differences were observed for any muscle fascicle lengths or for the lateral gastrocnemius and soleus pennation angles between paretic, nonparetic, and healthy limbs. There was a significant decrease (P < 0.05) in the paretic medial gastrocnemius pennation angle compared to both nonparetic and healthy limbs. Physiological cross-sectional areas and force contributions were smaller on the paretic side. Additionally, bilateral muscle contributions to plantar flexor torque remained the same. While the architecture of each individual plantar flexor muscle is affected differently after stroke, the relative contribution of each muscle remains the same.

Muscle atrophy, voluntary activation disturbances, and low serum concentrations of IGF-1 and IGFBP-3 are associated with weakness in people with chronic stroke

BACKGROUND

The muscle weakness that is exhibited poststroke is due to a multifactorial etiology involving the central nervous system and skeletal muscle changes. Insulinlike growth factor 1 (IGF-1) and IGF binding protein 3 (IGFBP-3) have been described as biomarkers of neuromuscular performance in many conditions. However, no information about these biomarkers is available for people with chronic hemiparesis.

OBJECTIVE

The purpose of this study was to investigate possible factors involved in muscle weakness, such as IGF-1 and IGFBP-3 serum concentrations, muscle volume, and neuromuscular performance of the knee flexors and extensors, in people with chronic hemiparesis poststroke.

DESIGN

This was a cross-sectional study.

METHODS

A cross-sectional study was performed on 14 individuals poststroke who were paired with healthy controls. Mobility, function, balance, and quality of life were recorded as outcome measures. Knee flexor and extensor muscle volumes and neuromuscular performance were measured using nuclear magnetic resonance imaging, dynamometry, and electromyography. The serum concentrations of IGF-1 and IGFBP-3 were quantified by enzyme-linked immunosorbent assay (ELISA).

RESULTS

The hemiparetic group had low serum concentrations of IGF-1 (25%) and IGFBP-3 (40%); reduced muscle volume in the vastus medialis (32%), vastus intermedius (29%), biceps femoris (16%), and semitendinosus and semimembranosus (12%) muscles; reduced peak torque, power, and work of the knee flexors and extensors; and altered agonist and antagonist muscle activation compared with controls.

CONCLUSIONS

Low serum concentrations of IGF-1 and IGFBP-3, deficits in neuromuscular performance, selective muscle atrophy, and decreased agonist muscle activation were found in the group with chronic hemiparesis poststroke. Both hemorrhagic and ischemic stroke were considered, and the data reflect a chronic poststroke population with good function.