Estimation of tissue stiffness, reflex activity, optimal muscle length and slack length in stroke patients using an electromyography driven antagonistic wrist model

The efficacy of different torque profiles for weight compensation of the hand

Orthotic wrist supports will be beneficial for people with muscular weakness to keep their hand in a neutral rest position and prevent potential wrist contractures. Compensating the weight of the hands is complex since the level of support depends on both wrist and forearm orientations. To explore simplified approaches, two different weight compensation strategies (constant and linear) were compared to the theoretical ideal sinusoidal profile and no compensation in eight healthy subjects using a mechanical wrist support system. All three compensation strategies showed a significant reduction of 47-53% surface electromyography activity in the anti-gravity m. extensor carpi radialis. However, for the higher palmar flexion region, a significant increase of 44-61% in the m. flexor carpi radialis was found for all compensation strategies. No significant differences were observed between the various compensation strategies. Two conclusions can be drawn: (1) a simplified torque profile (e.g., constant or linear) for weight compensation can be considered as equally effective as the theoretically ideal sinusoidal profile and (2) even the theoretically ideal profile provides no perfect support as other factors than weight, such as passive joint impedance, most likely influence the required compensation torque for the wrist joint.

Minimal Clinically Important Difference of Scales Reported in Stroke Trials: A Review

There is a growing awareness of the significance of using minimum clinically important differences (MCIDs) in stroke research. An MCID is the smallest change in an outcome measure that is considered clinically meaningful. This review is the first to provide a comprehensive summary of various scales and patient-reported outcome measures (PROMs) used in stroke research and their MCID values reported in the literature, including a concise overview of the concept of and methods for determining MCIDs in stroke research. Despite the controversies and limitations surrounding the estimation of MCIDs, their importance in modern clinical trials cannot be overstated. Anchor-based and distribution-based methods are recommended for estimating MCIDs, with patient self-evaluation being a crucial component in capturing the patient's perspective on their health. A combination of methods can provide a more comprehensive understanding of the clinical relevance of treatment effects, and incorporating the patient's perspective can enhance the care of stroke patients.

Examining the role of intrinsic and reflexive contributions to ankle joint hyper-resistance treated with botulinum toxin-A

BACKGROUND

Spasticity, i.e. stretch hyperreflexia, increases joint resistance similar to symptoms like hypertonia and contractures. Botulinum neurotoxin-A (BoNT-A) injections are a widely used intervention to reduce spasticity. BoNT-A effects on spasticity are poorly understood, because clinical measures, e.g. modified Ashworth scale (MAS), cannot differentiate between the symptoms affecting joint resistance. This paper distinguishes the contributions of the reflexive and intrinsic pathways to ankle joint hyper-resistance for participants treated with BoNT-A injections. We hypothesized that the overall joint resistance and reflexive contribution decrease 6 weeks after injection, while returning close to baseline after 12 weeks.

METHODS

Nine participants with spasticity after spinal cord injury or after stroke were evaluated across three sessions: 0, 6 and 12 weeks after BoNT-A injection in the calf muscles. Evaluation included clinical measures (MAS, Tardieu Scale) and motorized instrumented assessment using the instrumented spasticity test (SPAT) and parallel-cascade (PC) system identification. Assessments included measures for: (1) overall resistance from MAS and fast velocity SPAT; (2) reflexive resistance contribution from Tardieu Scale, difference between fast and slow velocity SPAT and PC reflexive gain; and (3) intrinsic resistance contribution from slow velocity SPAT and PC intrinsic stiffness/damping.

RESULTS

Individually, the hypothesized BoNT-A effect, the combination of a reduced resistance (week 6) and return towards baseline (week 12), was observed in the MAS (5 participants), fast velocity SPAT (2 participants), Tardieu Scale (2 participants), SPAT (1 participant) and reflexive gain (4 participants). On group-level, the hypothesis was only confirmed for the MAS, which showed a significant resistance reduction at week 6. All instrumented measures were strongly correlated when quantifying the same resistance contribution.

CONCLUSION

At group-level, the expected joint resistance reduction due to BoNT-A injections was only observed in the MAS (overall resistance). This observed reduction could not be attributed to an unambiguous group-level reduction of the reflexive resistance contribution, as no instrumented measure confirmed the hypothesis. Validity of the instrumented measures was supported through a strong association between different assessment methods. Therefore, further quantification of the individual contributions to joint resistance changes using instrumented measures across a large sample size are essential to understand the heterogeneous response to BoNT-A injections.

The impact of age-related increase in passive muscle stiffness on simulated upper limb reaching

Ageing changes the musculoskeletal and neural systems, potentially affecting a person's ability to perform daily living activities. One of these changes is increased passive stiffness of muscles, but its contribution to performance is difficult to separate experimentally from other ageing effects such as loss of muscle strength or cognitive function. A computational upper limb model was used to study the effects of increasing passive muscle stiffness on reaching performance across the model's workspace (all points reachable with a given model geometry). The simulations indicated that increased muscle stiffness alone caused deterioration of reaching accuracy, starting from the edges of the workspace. Re-tuning the model's control parameters to match the ageing muscle properties does not fully reverse ageing effects but can improve accuracy in selected regions of the workspace. The results suggest that age-related muscle stiffening, isolated from other ageing effects, impairs reaching performance. The model also exhibited oscillatory instability in a few simulations when the controller was tuned to the presence of passive muscle stiffness. This instability is not observed in humans, implying the presence of natural stabilizing strategies, thus pointing to the adaptive capacity of neural control systems as a potential area of future investigation in age-related muscle stiffening.

Development of a single device to quantify motor impairments of the elbow: proof of concept

Background

For patients with post-stroke upper limb impairments, the currently available clinical measurement instruments are inadequate for reliable quantification of multiple impairments, such as muscle weakness, abnormal synergy, changes in elastic joint properties and spasticity. Robotic devices to date have successfully achieved precise and accurate quantification but are often limited to the measurement of one or two impairments. Our primary aim is to develop a robotic device that can effectively quantify four main motor impairments of the elbow.

Methods

 The robotic device, Shoulder Elbow Perturbator, is a one-degree-of-freedom device that can simultaneously manipulate the elbow joint and support the (partial) weight of the human arm. Upper limb impairments of the elbow were quantified based on four experiments on the paretic arm in ten stroke patients (mean age 65 ± 10 yrs, 9 males, post-stroke) and the non-dominant arm in 20 healthy controls (mean age 65 ± 14 yrs, 6 males). The maximum strength of elbow flexor and elbow extensor muscles was measured isometrically at 90-degree elbow flexion. The maximal active extension angle of the elbow was measured under different arm weight support levels to assess abnormal synergy. Torque resistance was analyzed during a slow (6°/s) passive elbow rotation, where the elbow moved from the maximal flexion to maximal extension angle and back, to assess elastic joint properties. The torque profile was evaluated during fast (100°/s) passive extension rotation of the elbow to estimate spasticity.

Results

The ten chronic stroke patients successfully completed the measurement protocol. The results showed impairment values outside the 10^th and 90^th percentile reference intervals of healthy controls. Individual patient profiles were determined and illustrated in a radar figure, to support clinicians in developing targeted treatment plans.

Conclusion

The Shoulder Elbow Perturbator can effectively quantify the four most important impairments of the elbow in stroke patients and distinguish impairment scores of patients from healthy controls. These results are promising for objective and complete quantification of motor impairments of the elbow and monitoring patient prognosis. Our newly developed Shoulder Elbow Perturbator can therefore in the future be employed to evaluate treatment effects by comparing pre- and post-treatment assessments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12984-022-01050-2.

Quantifying neural and non-neural components of wrist hyper-resistance after stroke: Comparing two instrumented assessment methods

Patients with poor upper limb motor recovery after stroke are likely to develop increased resistance to passive wrist extension, i.e., wrist hyper-resistance. Quantification of the underlying neural and non-neural elastic components is of clinical interest. This cross-sectional study compared two methods: a commercially available device (NeuroFlexor®) with an experimental EMG-based device (Wristalyzer) in 43 patients with chronic stroke. Spearman's rank correlation coefficients (r) between components, modified Ashworth scale (MAS) and range of passive wrist extension (PRoM) were calculated with 95% confidence intervals. Neural as well as elastic components assessed by both devices were associated (r = 0.61, 95%CI: 0.38-0.77 and r = 0.53, 95%CI: 0.28-0.72, respectively). The neural component assessed by the NeuroFlexor® associated significantly with the elastic components of NeuroFlexor® (r = 0.46, 95%CI: 0.18-0.67) and Wristalyzer (r = 0.36, 95%CI: 0.06-0.59). The neural component assessed by the Wristalyzer was not associated with the elastic components of both devices. Neural and elastic components of both devices associated similarly with the MAS (r = 0.58, 95%CI: 0.34-0.75 vs. 0.49, 95%CI: 0.22-0.69 and r = 0.51, 95%CI: 0.25-0.70 vs. 0.30, 95%CI: 0.00-0.55); elastic components associated with PRoM (r = -0.44, 95%CI: -0.65- -0.16 vs. -0.74, 95%CI: -0.85- -0.57 for NeuroFlexor® and Wristalyzer respectively). Results demonstrate that both methods perform similarly regarding the quantification of neural and elastic wrist hyper-resistance components and have an added value when compared to clinical assessment with the MAS alone. The added value of EMG in the discrimination between neural and non-neural components requires further investigation.

Passive Properties of the Wrist and Fingers Following Chronic Hemiparetic Stroke: Interlimb Comparisons in Persons With and Without a Clinical Treatment History That Includes Botulinum Neurotoxin

Background: Neural impairments that follow hemiparetic stroke may negatively affect passive muscle properties, further limiting recovery. However, factors such as hypertonia, spasticity, and botulinum neurotoxin (BoNT), a common clinical intervention, confound our understanding of muscle properties in chronic stroke. Objective: To determine if muscle passive biomechanical properties are different following prolonged, stroke-induced, altered muscle activation and disuse. Methods: Torques about the metacarpophalangeal and wrist joints were measured in different joint postures in both limbs of participants with hemiparetic stroke. First, we evaluated 27 participants with no history of BoNT; hand impairments ranged from mild to severe. Subsequently, seven participants with a history of BoNT injections were evaluated. To mitigate muscle hypertonia, torques were quantified after an extensive stretching protocol and under conditions that encouraged participants to sleep. EMGs were monitored throughout data collection. Results: Among participants who never received BoNT, no significant differences in passive torques between limbs were observed. Among participants who previously received BoNT injections, passive flexion torques about their paretic wrist and finger joints were larger than their non-paretic limb (average interlimb differences = +42.0 ± 7.6SEM Ncm, +26.9 ± 3.9SEM Ncm, respectively), and the range of motion for passive finger extension was significantly smaller (average interlimb difference = -36.3° ± 4.5°SEM; degrees). Conclusion: Our results suggest that neural impairments that follow chronic, hemiparetic stroke do not lead to passive mechanical changes within the wrist and finger muscles. Rather, consistent with animal studies, the data points to potential adverse effects of BoNT on passive muscle properties post-stroke, which warrant further consideration.

The effect of botulinum toxin-A on neural and non-neural components of wrist hyper-resistance in adults with stroke or cerebral palsy

Background

Botulinum toxin‐A (BoNT) is widely used to manage focal upper limb spasticity and is effective in reducing resistance to passive movement, as measured with the modified Ashworth scale. Discrimination and quantification of the underlying neural and non‐neural components of hyper‐resistance may further improve understanding of the effect of BoNT.

Objective

To explore the effects of BoNT on neural (NC), non‐neural elastic (EC), and viscous (VC) components of resistance to passive wrist extension in adults with stroke or cerebral palsy and the association between the effects on wrist hyper‐resistance components and clinical spasticity, pain and motor function scales.

Design

Pre‐experimental study with pre‐ and post‐intervention measurements at 6 and 12 weeks.

Setting

An outpatient clinic of a hospital.

Participants

Adults with chronic stroke or cerebral palsy indicated for BoNT treatment for hyper‐resistance in the wrist (N = 18).

Interventions

BoNT injections in the wrist and/or finger flexor muscles.

Main Outcome Measures

Wrist hyper‐resistance components, using the NeuroFlexor, and clinical scales (modified Ashworth scale, Tardieu scale, passive wrist extension, pain, Fugl‐Meyer motor assessment of the upper extremity, and action research arm test).

Results

NC was significantly reduced 6 and 12 weeks post‐intervention (median −11.96 Newton, P < .001 and median −9.34 Newton, P = .001, respectively); non‐neural EC and VC showed no change. NC reduction 6 weeks post‐intervention correlated significantly with BoNT dose (Pearson correlation coefficient r_p = −0.56). No significant correlations were found between change scores in wrist hyper‐resistance components and clinical scales.

Conclusions

BoNT affected the neural component of resistance to passive wrist extension, while leaving the non‐neural elastic and viscous components unaffected. This instrumented approach to quantify the effects of BoNT in the wrist and finger flexor muscles on the components of wrist hyper‐resistance may have an added value for BoNT treatment evaluation in clinical practice.

Validity and reliability of myotonometry for assessing muscle viscoelastic properties in patients with stroke: a systematic review and meta-analysis

There is a lack of consensus about the measurement of the muscle viscoelastic features in stroke patients. Additionally, the psychometric properties of the most-commonly used clinical tools remain controversial. Our objective is to investigate the validity and reliability of myotonometry to assess viscoelastic muscle features in stroke survivors. Pubmed, PEDro, Scopus and Cinahl were systematically searched to include studies reporting the psychometric properties of myotonometric devices used in people after stroke. The QUADAS-2 and the COSMIN checklists were used to assess the methodological quality of the studies and the psychometric properties of myotonometry. Nine studies were included in the qualitative synthesis and data from five of these were pooled in a meta-analysis. Overall, low to moderate risk of bias and applicability concerns were observed. Pooled data from intra-rater reliability for muscle tone showed a mean coefficient of correlation of 0.915 (95% CI: 0.880-0.940, I 2 = 69.2%) for upper limbs, and a mean coefficient of 0.785 (95%CI: 0.708-0.844, I 2 = 4.02%) for lower limbs. Myotonometry seems to be a valid and reliable complementary tool to assess muscle viscoelastic properties in stroke survivors, although definite conclusions about concurrent validity need further research.

Leg and lower limb dynamic joint stiffness during different walking speeds in healthy adults

BACKGROUND

The differences and relationship between joint stiffness and leg stiffness can be used to characterize the lower limb behavior during different walking speeds.

RESEARCH QUESTION

This study aimed to investigate the differences in whole leg and lower limb joint stiffness at different walking speeds and the interactions between leg and lower limb joint stiffness.

METHODS

Twenty-seven healthy adults, seventeen males (age: 19.6 ± 2.2 years, height: 176.0 ± 6.0 cm, mass: 69.7 ± 8.9 kg), and ten females (age: 19.1 ± 1.9 years, height: 164.0 ± 3.0 cm, mass: 59.6 ± 3.8 kg), were recruited. Dynamic leg and joint stiffness were calculated during eccentric loading from data recorded using 3D infrared motion analysis and force plates at slow, normal, and fast walking speeds. Differences in dynamic stiffness, joint angles and moments were explored between the walking speeds using Repeated Measures ANOVA with Sidak post-hoc tests. Correlations between leg, joint stiffness, and walking speed were also explored.

RESULTS

The results indicated that the leg dynamic stiffness is decreased by walking speed, however, hip and ankle joint stiffness were increased (p < 0.001) and knee stiffness was unaffected. Leg stiffness showed no correlation with hip, knee, or ankle stiffness. A positive significant correlation was seen between hip and ankle stiffness (p < 0.01) and between knee and ankle stiffness (p < 0.001), however, no correlation was seen between hip and knee stiffness.

SIGNIFICANCE

These results suggest leg stiffness is not associated with lower limb joint stiffness during eccentric loading. This provides new information on the responses of ankle, knee and hip joint stiffness to walking speed.

Time Course of Wrist Hyper-Resistance in Relation to Upper Limb Motor Recovery Early Post Stroke

Background. Patients with an upper limb motor impairment are likely to develop wrist hyper-resistance during the first months post stroke. The time course of wrist hyper-resistance in terms of neural and biomechanical components, and their interaction with motor recovery, is poorly understood. Objective. To investigate the time course of neural and biomechanical components of wrist hyper-resistance in relation to upper limb motor recovery in the first 6 months post stroke. Methods. Neural (NC), biomechanical elastic (EC), and viscous (VC) components of wrist hyper-resistance (NeuroFlexor device), and upper limb motor recovery (Fugl-Meyer upper extremity scale [FM-UE]), were assessed in 17 patients within 3 weeks and at 5, 12, and 26 weeks post stroke. Patients were stratified according to the presence of voluntary finger extension (VFE) at baseline. Time course of wrist hyper-resistance components and assumed interaction effects were analyzed using linear mixed models. Results. On average, patients without VFE at baseline (n = 8) showed a significant increase in NC, EC, and VC, and an increase in FM-UE from 13 to 26 points within the first 6 months post stroke. A significant increase in NC within 5 weeks preceded a significant increase in EC between weeks 12 and 26. Patients with VFE at baseline (n = 9) showed, on average, no significant increase in components from baseline to 6 months whereas FM-UE scores improved from 38 to 60 points. Conclusion. Our findings suggest that the development of neural and biomechanical wrist hyper-resistance components in patients with severe baseline motor deficits is determined by lack of spontaneous neurobiological recovery early post stroke.

Guided Self-rehabilitation Contract vs conventional therapy in chronic stroke-induced hemiparesis: NEURORESTORE, a multicenter randomized controlled trial

Background

After discharge from hospital following a stroke, prescriptions of community-based rehabilitation are often downgraded to “maintenance” rehabilitation or discontinued. This classic therapeutic behavior stems from persistent confusion between lesion-induced plasticity, which lasts for the first 6 months essentially, and behavior-induced plasticity, of indefinite duration, through which intense rehabilitation might remain effective. This prospective, randomized, multicenter, single-blind study in subjects with chronic stroke-induced hemiparesis evaluates changes in active function with a Guided Self-rehabilitation Contract vs conventional therapy alone, pursued for a year.

Methods

One hundred and twenty four adult subjects with chronic hemiparesis (> 1 year since first stroke) will be included in six tertiary rehabilitation centers. For each patient, two treatments will be compared over a 1-year period, preceded and followed by an observational 6-month phase of conventional rehabilitation. In the experimental group, the therapist will implement the diary-based and antagonist-targeting Guided Self-rehabilitation Contract method using two monthly home visits. The method involves: i) prescribing a daily antagonist-targeting self-rehabilitation program, ii) teaching the techniques involved in the program, iii) motivating and guiding the patient over time, by requesting a diary of the work achieved to be brought back by the patient at each visit. In the control group, participants will benefit from conventional therapy only, as per their physician’s prescription.

The two co-primary outcome measures are the maximal ambulation speed barefoot over 10 m for the lower limb, and the Modified Frenchay Scale for the upper limb. Secondary outcome measures include total cost of care from the medical insurance point of view, physiological cost index in the 2-min walking test, quality of life (SF 36) and measures of the psychological impact of the two treatment modalities. Participants will be evaluated every 6 months (D1/M6/M12/M18/M24) by a blinded investigator, the experimental period being between M6 and M18. Each patient will be allowed to receive any medications deemed necessary to their attending physician, including botulinum toxin injections.

Discussion

This study will increase the level of knowledge on the effects of Guided Self-rehabilitation Contracts in patients with chronic stroke-induced hemiparesis.

Trial registration

ClinicalTrials.gov: NCT02202954, July 29, 2014.

Measurement Properties of the NeuroFlexor Device for Quantifying Neural and Non-neural Components of Wrist Hyper-Resistance in Chronic Stroke

Introduction: Differentiating between the components of wrist hyper-resistance post stroke, i.e., pathological neuromuscular activation ("spasticity") and non-neural biomechanical changes, is important for treatment decisions. This study aimed to assess the reliability and construct validity of an innovative measurement device that quantifies these neural and non-neural components by biomechanical modeling. Methods: Forty-six patients with chronic stroke and 30 healthy age-matched subjects were assessed with the NeuroFlexor, a motor-driven device that imposes isokinetic wrist extensions at two controlled velocities (5 and 236°/s). Test-retest reliability was evaluated using intraclass correlation coefficients (ICC) and smallest detectable changes (SDC), and construct validity by testing the difference between patients and healthy subjects and between subgroups of patients stratified by modified Ashworth scale (MAS), and the association with clinical scales. Results: Test-retest reliability was excellent for the neural (NC) and non-neural elastic (EC) components (ICC 0.93 and 0.95, respectively), and good for the viscous component (VC) (ICC 0.84), with SDCs of 10.3, 3.1, and 0.5 N, respectively. NC and EC were significantly higher in patients compared to healthy subjects (p < 0.001). Components gradually increased with MAS category. NC and EC were positively associated with the MAS (r _s 0.60 and 0.52, respectively; p < 0.01), and NC with the Tardieu scale (r _s 0.36, p < 0.05). NC and EC were negatively associated with the Fugl-Meyer Assessment of the upper extremity and action research arm test (r _s ≤ -0.38, p < 0.05). Conclusions: The NeuroFlexor reliably quantifies neural and non-neural components of wrist hyper-resistance in chronic stroke, but is less suitable for clinical evaluation at individual level due to high SDC values. Although construct validity has been demonstrated, further investigation at component level is needed.

Early Shortening of Wrist Flexor Muscles Coincides With Poor Recovery After Stroke

Background. The mechanism and time course of increased wrist joint stiffness poststroke and clinically observed wrist flexion deformity is still not well understood. The components contributing to increased joint stiffness are of neural reflexive and peripheral tissue origin and quantified by reflexive torque and muscle slack length and stiffness coefficient parameters. Objective. To investigate the time course of the components contributing to wrist joint stiffness during the first 26 weeks poststroke in a group of patients, stratified by prognosis and functional recovery of the upper extremity. Methods. A total of 36 stroke patients were measured on 8 occasions within the first 26 weeks poststroke using ramp-and-hold rotations applied to the wrist joint by a robot manipulator. Neural reflexive and peripheral tissue components were estimated using an electromyography-driven antagonistic wrist model. Outcome was compared between groups cross-sectionally at 26 weeks poststroke and development over time was analyzed longitudinally. Results. At 26 weeks poststroke, patients with poor recovery (Action Research Arm Test [ARAT] ≤9 points) showed a higher predicted reflexive torque of the flexors (P < .001) and reduced predicted slack length (P < .001) indicating shortened muscles contributing to higher peripheral tissue stiffness (P < .001), compared with patients with good recovery (ARAT ≥10 points). Significant differences in peripheral tissue stiffness between groups could be identified around weeks 4 and 5; for neural reflexive stiffness, this was the case around week 12. Conclusions. We found onset of peripheral tissue stiffness to precede neural reflexive stiffness. Temporal identification of components contributing to joint stiffness after stroke may prompt longitudinal interventional studies to further evaluate and eventually prevent these phenomena.

FUNCTION AND BIOMECHANICS OF UPPER LIMB IN POST-STROKE PATIENTS — A SYSTEMATIC REVIEW

Current clinical services are struggling to provide the most favorable rehabilitation treatment for patients with stroke, which inspired researchers to investigate and explore the use of rehabilitation devices suitable for the patients and rehabilitation therapy. This review paper addresses the importance of biomechanical features in patients who experienced stroke to the upper limb. First and foremost, a review was done on general biomechanical description associated with motor control, shoulder, elbow, wrist and fingers joint. This included the ability of the patients to move their affected arm and the affect on peak joint torque, range of motion, joint forces, grip strength and muscle activities during the activities of daily living. In addition, we also reviewed the material properties and geometrical condition of tissue in stroke patient. The repercussions of post-stroke patient regarding the bone density, stiffness of muscle as well as the thickness of cartilage are described in this review. Based on the findings, the movement of affected stroke hand is associated with the motor control and material properties of tissue. To strengthen the motor control and maintaining tissue properties, early physical training on patients should be conducted in two to four weeks after stroke. In conclusion, this report suggests a new approach for future biomechanical studies in order to enhance the quality of physiotherapy rehabilitation peculiarly for post-stroke patients.

Similar movements are associated with drastically different muscle contraction velocities

We investigated how kinematic redundancy interacts with the neurophysiological control mechanisms required for smooth and accurate, rapid limb movements. Biomechanically speaking, tendon excursions are over-determined because the rotation of few joints determines the lengths and velocities of many muscles. But how different are the muscle velocity profiles induced by various, equally valid hand trajectories? We used an 18-muscle sagittal-plane arm model to calculate 100,000 feasible shoulder, elbow, and wrist joint rotations that produced valid basketball free throws with different hand trajectories, but identical initial and final hand positions and velocities. We found large differences in the eccentric and concentric muscle velocity profiles across many trajectories; even among similar trajectories. These differences have important consequences to their neural control because each trajectory will require unique, time-sensitive reflex modulation strategies. As Sherrington mentioned a century ago, failure to appropriately silence the stretch reflex of any one eccentrically contracting muscle will disrupt movement. Thus, trajectories that produce faster or more variable eccentric contractions will require more precise timing of reflex modulation across motoneuron pools; resulting in higher sensitivity to time delays, muscle mechanics, excitation/contraction dynamics, noise, errors and perturbations. By combining fundamental concepts of biomechanics and neuroscience, we propose that kinematic and muscle redundancy are, in fact, severely limited by the need to regulate reflex mechanisms in a task-specific and time-critical way. This in turn has important consequences to the learning and execution of accurate, smooth and repeatable movements-and to the rehabilitation of everyday limb movements in developmental and neurological conditions, and stroke.