Increased Muscle Coactivation Research Articles

Combined translational and rotational perturbations of standing balance reveal contributions of reduced reciprocal inhibition to balance impairments in children with cerebral palsy.

Balance impairments are common in cerebral palsy. When balance is perturbed by backward support surface translations, children with cerebral palsy have increased co-activation of the plantar flexors and tibialis anterior muscle as compared to typically developing children. However, it is unclear whether increased muscle co-activation is a compensation strategy to improve balance control or is a consequence of reduced reciprocal inhibition. During translational perturbations, increased joint stiffness due to co-activation might aid balance control by resisting movement of the body with respect to the feet. In contrast, during rotational perturbations, increased joint stiffness will hinder balance control as it couples body to platform rotation. Therefore, we expect increased muscle co-activation in response to rotational perturbations if co-activation is caused by reduced reciprocal inhibition but not if it is merely a compensation strategy. We perturbed standing balance by combined backward translational and toe-up rotational perturbations in 20 children with cerebral palsy and 20 typically developing children. Perturbations induced forward followed by backward movement of the center of mass. We evaluated reactive muscle activity and the relation between center of mass movement and reactive muscle activity using a linear feedback model based on center of mass kinematics. In typically developing children, perturbations induced plantar flexor balance correcting muscle activity followed by tibialis anterior balance correcting muscle activity, which was driven by center of mass movement. In children with cerebral palsy, the switch from plantar flexor to tibialis anterior activity was less pronounced than in typically developing children due to increased muscle co-activation of the plantar flexors and tibialis anterior throughout the response. Our results thus suggest that a reduction in reciprocal inhibition causes muscle co-activation in reactive standing balance in children with cerebral palsy.

Reduced reciprocal inhibition during passive spasticity assessments is related with increased muscle co-activation during perturbations of standing balance

Reduced reciprocal inhibition during passive spasticity assessments is related with increased muscle co-activation during perturbations of standing balance

Voluntary muscle coactivation in quiet standing elicits reciprocal rather than coactive agonist-antagonist control of reactive balance.

Muscle coactivation increases in challenging balance conditions as well as with advanced age and mobility impairments. Increased muscle coactivation can occur both in anticipation of (feedforward) and in reaction to (feedback) perturbations, however, the causal relationship between feedforward and feedback muscle coactivation remains elusive. Here, we hypothesized that feedforward muscle coactivation would increase both the body's initial mechanical resistance due to muscle intrinsic properties and the later feedback-mediated muscle coactivation in response to postural perturbations. Young adults voluntarily increased leg muscle coactivation using visual biofeedback before support-surface perturbations. In contrast to our hypothesis, feedforward muscle coactivation did not increase the body's initial intrinsic resistance to perturbations, nor did it increase feedback muscle coactivation. Rather, perturbations with feedforward muscle coactivation elicited a medium- to long-latency increase of feedback-mediated agonist activity but a decrease of feedback-mediated antagonist activity. This reciprocal rather than coactivation effect on ankle agonist and antagonist muscles enabled faster reactive ankle torque generation, reduced ankle dorsiflexion, and reduced center of mass (CoM) motion. We conclude that in young adults, voluntary feedforward muscle coactivation can be independently modulated with respect to feedback-mediated muscle coactivation. Furthermore, our findings suggest feedforward muscle coactivation may be useful for enabling quicker joint torque generation through reciprocal, rather than coactivated, agonist-antagonist feedback muscle activity. As such our results suggest that behavioral context is critical to whether muscle coactivation functions to increase agility versus stability.NEW & NOTEWORTHY Feedforward and feedback muscle coactivation are commonly observed in older and mobility impaired adults and are considered strategies to improve stability by increasing body stiffness prior to and in response to perturbations. In young adults, voluntary feedforward coactivation does not necessarily increase feedback coactivation in response to perturbations. Instead, feedforward coactivation enabled faster ankle torques through reciprocal agonist-antagonist muscle activity. As such, coactivation may promote either agility or stability depending on the behavioral context.

Muscle co-activation in the elderly contributes to control of hip and knee joint torque and endpoint force

We investigated the coordinated activity patterns of muscles based on cosine tuning in the elderly during an isometric force exertion task. We also clarified whether these coordinated activity patterns contribute to the control of hip and knee joint torque and endpoint force as co-activation. Preferred direction (PD) of activity for each muscle in 10 young and 8 older males was calculated from the lower limb muscle activity during isometric force exertion task in various directions. The covariance of endpoint force (η) was calculated from the exerted force data using a force sensor. Relationship between PD and η was used to examine the effect of muscle co-activation on the control of endpoint force. Co-activation between rectus femoris and semitendinosus/biceps femoris increased with changes in muscle PD. Additionally, the η values were significantly low, suggesting that co-activation of multiple muscles may contribute to endpoint force exertion. The mechanism for cooperative muscle activity is determined by the cosine tuning of the PD of each muscle, which affects the generation of hip and knee joint torque and endpoint force exertion. Co-activation of each muscle’s PD changes with age, causing increased muscle co-activation to control torque and force. We demonstrated that co-activation in the elderly is a stabilizer of unsteady joints and a muscle control strategy for cooperative muscle activity.

The relation of energy cost of walking with gait deviation, asymmetry, and lower limb muscle co-activation in children with cerebral palsy: a retrospective cross-sectional study

BackgroundCompared to typically developing children, children with cerebral palsy (CP) have increased energy expenditure during walking, limiting activity and participation. Insight into whether the also deviating and more asymmetric gait with increased muscle co-activation contributes to this increased energy expenditure is important for clinical decision making. The aim of this study was to investigate the relation between energy cost of walking with gait deviation, asymmetry, and muscle co-activation in children with CP.MethodsForty ambulant children with CP, with Gross Motor Function Classification System (GMFCS) level I (N = 35) and II (N = 5), aged between 5-17y, were tested at one or two occasions with 24 weeks in between, resulting in 71 observations. Gross energy cost (J/kg/m) was measured during a 5-min walk test at self-selected speed. From a 3-dimensional gait analyses, kinematic variables and electromyography were extracted to calculate the gait deviation index (GDI) and co-activation index. The relation between energy cost and GDI, GDI asymmetry, and co-activation index of the lower limb muscles was evaluated through mixed model analyses. Height was included to control for growth-related variation.ResultsGait deviation and height combined explained about 40% of the variance in gross energy cost. No significant contribution was found for gait asymmetry or co-activation index.ConclusionsThis cross-sectional study indicates that increased gait deviation contributes to increased energy cost of walking in children with GMFCS level I and II.

Consensus Paper: Ataxic Gait.

The aim of this consensus paper is to discuss the roles of the cerebellum in human gait, as well as its assessment and therapy. Cerebellar vermis is critical for postural control. The cerebellum ensures the mapping of sensory information into temporally relevant motor commands. Mental imagery of gait involves intrinsically connected fronto-parietal networks comprising the cerebellum. Muscular activities in cerebellar patients show impaired timing of discharges, affecting the patterning of the synergies subserving locomotion. Ataxia of stance/gait is amongst the first cerebellar deficits in cerebellar disorders such as degenerative ataxias and is a disabling symptom with a high risk of falls. Prolonged discharges and increased muscle coactivation may be related to compensatory mechanisms and enhanced body sway, respectively. Essential tremor is frequently associated with mild gait ataxia. There is growing evidence for an important role of the cerebellar cortex in the pathogenesis of essential tremor. In multiple sclerosis, balance and gait are affected due to cerebellar and spinal cord involvement, as a result of disseminated demyelination and neurodegeneration impairing proprioception. In orthostatic tremor, patients often show mild-to-moderate limb and gait ataxia. The tremor generator is likely located in the posterior fossa. Tandem gait is impaired in the early stages of cerebellar disorders and may be particularly useful in the evaluation of pre-ataxic stages of progressive ataxias. Impaired inter-joint coordination and enhanced variability of gait temporal and kinetic parameters can be grasped by wearable devices such as accelerometers. Kinect is a promising low cost technology to obtain reliable measurements and remote assessments of gait. Deep learning methods are being developed in order to help clinicians in the diagnosis and decision-making process. Locomotor adaptation is impaired in cerebellar patients. Coordinative training aims to improve the coordinative strategy and foot placements across strides, cerebellar patients benefiting from intense rehabilitation therapies. Robotic training is a promising approach to complement conventional rehabilitation and neuromodulation of the cerebellum. Wearable dynamic orthoses represent a potential aid to assist gait. The panel of experts agree that the understanding of the cerebellar contribution to gait control will lead to a better management of cerebellar ataxias in general and will likely contribute to use gait parameters as robust biomarkers of future clinical trials.

A robust model-based neural-machine interface across different loading weights applied at distal forearm

Musculoskeletal models (MMs) have recently been proposed to decode electromyography (EMG) signals for movement intent recognition. Since the robustness is critical to retain the performance of neural-machine interface (NMI) during daily activities and the loading weight change is one of the critical factors that would affect the performance of NMI, this study aimed to further investigate the robustness of a generic MM-based NMI across different loading conditions. Eight able-bodied (AB) individuals and one individual with a transradial amputation were recruited and tested while performing a real-time virtual wrist/hand posture matching task under different loading weights (AB subjects: 0 kg, 0.567 kg, and 1.134 kg; amputee subject: 0 kg and 0.567 kg) applied at the distal forearm. All tasks were achieved by both AB individuals and the individual with the transradial amputation. There was no significant difference among the real-time performance (completion time, the number of overshoots, and path efficiency) of AB individuals under different loading conditions. We calculated the average muscle activations of each muscle during the initial 0.5 s and last 0.5 s respectively for each target across all subjects and trials. The analysis of muscle activations showed that additional weights caused muscle co-contractions. However, the subjects can cope with the increased muscle co-activation level, modifying muscle activation patterns, and still complete tasks successfully. We obtained similar results from the individual with the transradial amputation. These results demonstrated the robustness of MM-based NMI across different loading conditions. The outcomes indicate the potential of the multi-user NMI toward practical applications.

The Danger of Walking with Socks: Evidence from Kinematic Analysis in People with Progressive Multiple Sclerosis.

Multiple sclerosis (MS) is characterized by gait impairments and severely impacts the quality of life. Technological advances in biomechanics offer objective assessments of gait disabilities in clinical settings. Here we employed wearable sensors to measure electromyography (EMG) and body acceleration during walking and to quantify the altered gait pattern between people with progressive MS (PwPMS) and healthy controls (HCs). Forty consecutive patients attending our department as in-patients were examined together with fifteen healthy controls. All subjects performed the timed 10 min walking test (T10MW) using a wearable accelerator and 8 electrodes attached to bilateral thighs and legs so that body acceleration and EMG activity were recorded. The T10MWs were recorded under three conditions: standard (wearing shoes), reduced grip (wearing socks) and increased cognitive load (backward-counting dual-task). PwPMS showed worse kinematics of gait and increased muscle coactivation than controls at both the thigh and leg levels. Both reduced grip and increased cognitive load caused a reduction in the cadence and velocity of the T10MW, which were correlated with one another. A higher coactivation index at the thigh level of the more affected side was positively correlated with the time of the T10MW (r = 0.5, p < 0.01), Expanded Disability Status Scale (EDSS) (r = 0.4, p < 0.05), and negatively correlated with the cadence (r = −0.6, p < 0.001). Our results suggest that excessive coactivation at the thigh level is the major determinant of the gait performance as the disease progresses. Moreover, demanding walking conditions do not influence gait in controls but deteriorate walking performances in PwPMS, thus those conditions should be prevented during hospital examinations as well as in homecare environments.

Physical and perceptual measures of walking surface complexity strongly predict gait and gaze behaviour.

Walking surfaces vary in complexity and are known to affect stability and fall risk whilst walking. However, existing studies define surfaces through descriptions only. This study used a multimethod approach to measure surface complexity in order to try to characterise surfaces with respect to locomotor stability. We assessed how physical measurements of walking surface complexity compared to participant's perceptual ratings of the effect of complexity on stability. Physical measurements included local slope measures from the surfaces themselves and shape complexity measured using generated surface models. Perceptual measurements assessed participants' perceived stability and surface roughness using Likert scales. We then determined whether these measurements were indicative of changes to stability as assessed by behavioural changes including eye angle, head pitch angle, muscle coactivation, walking speed and walking smoothness. Physical and perceptual measures were highly correlated, with more complex surfaces being perceived as more challenging to stability. Furthermore, complex surfaces, as defined from both these measurements, were associated with lowered head pitch, increased muscle coactivation and reduced walking smoothness. Our findings show that walking surfaces defined as complex, based on physical measurements, are perceived as more challenging to our stability. Furthermore, certain behavioural measures relate better to these perceptual and physical measures than others. Crucially, for the first time this study defined walking surfaces objectively rather than just based on subjective descriptions. This approach could enable future researchers to compare results across walking surface studies. Moreover, perceptual measurements, which can be collected easily and efficiently, could be used as a proxy for estimating behavioural responses to different surfaces. This could be particularly valuable when determining risk of instability when walking for individuals with compromised stability.

Biomechanics of Vertical Posture and Control with Referent Joint Configurations

Our study compared the results of two methods of analysis of postural sway during human quiet standing, the rambling-trembling (Rm-Tr) decomposition and the analysis of the point of intersection of the ground reaction forces (zIP analysis). Young, healthy subjects were required to stand naturally and with an increased level of leg/trunk muscle co-activation under visual feedback on the magnitude of a combined index of muscle activation (muscle mode). The main findings included the shift of zIP toward higher frequencies and strong correlations between Tr and zIP when the subjects stood with increased muscle co-activation. We interpret the results within the idea of whole-body control with a set of primitives associated with referent coordinates in the joint configuration space.

Muscle-Tendon Interaction During Human Dolphin-Kick Swimming.

Without high impact forces, it is not clear how humans can utilize tendon elasticity during low-impact activities. The purpose of the present study was to examine the muscle-tendon behavior together with the electromyographic (EMG) activities of the vastus lateralis (VL) muscle during the human dolphin-kicking. In a swimming pool, each subject (n = 11) swam the 25 m dolphin-kicking at two different speeds (NORMAL and FAST). Surface EMGs were recorded from the VL and biceps femoris (BF) muscles. Simultaneous recordings of the knee joint angle by electro-goniometer and of the VL fascicle length by ultrasonography were used to calculate the muscle-tendon unit and tendinous length of VL (LMTU and LTT, respectively). In the dolphin-kicking, the stretching and shortening amplitudes of VL LMTU did not differ significantly between the two kicking speed conditions. However, both stretching and shortening amplitudes of the VL fascicle length were lower at FAST than at NORMAL speed whereas the opposite was found for the VL LTT values. At FAST, the contribution of the VL tendinous length to the entire VLMTU length changes increased. The EMG analysis revealed at FAST higher agonist VL activation from the late up-beat (MTU stretching) to the early down-beat phases as well as increased muscle co-activation of VL and BF muscles from the late down-beat to early up-beat phases of dolphin-kicking. These results suggest that at increasing kicking speeds, the VL fascicles and tendinous tissues during aquatic movements can utilize tendon elasticity in a similar way than in terrestrial forms of locomotion. However, these activation profiles of VL and BF muscles may differ from their activation pattern in terrestrial locomotion.

Impaired corticomuscular coherence during isometric elbow flexion contractions in humans with cervical spinal cord injury

After spinal cord injury (SCI), the reorganization of the neuromuscular system leads to increased antagonist muscles' co-activation-that is, increased antagonist vs. agonist muscles activation ratio-during voluntary contractions. Increased muscle co-activation is supposed to result from reduced cortical influences on spinal mechanisms inhibiting antagonist muscles. The assessment of the residual interactions between cortical and muscles activity with corticomuscular coherence (CMC) in participants with SCI producing different force levels may shed new lights on the regulation of muscle co-activation. To achieve this aim, we compared the net joint torque, the muscle co-activation and the CMC ~10 and ~20Hz with both agonist and antagonist muscles in participants with SCI and healthy participants performing actual isometric elbow flexion contractions at three force levels. For all participants, overall CMC and muscle co-activation decreased with the increase in the net joint torque, but only CMC ~10Hz was correlated with muscle co-activation. Participants with SCI had greater muscle co-activation and lower CMC ~10Hz, at the highest force levels. These results emphasize the importance of CMC as a mechanism that could take part in the modulation of muscle co-activation to maintain a specific force level. Lower CMC ~10Hz in SCI participants may reflect the decreased cortical influence on spinal mechanisms, leading to increased muscle co-activation, although plasticity of the corticomuscular coupling seems to be preserved after SCI to modulate the force level. Clinically, the CMC may efficiently evaluate the residual integrity of the neuromuscular system after SCI and the effects of rehabilitation.

Contribution of the supplementary motor area and the cerebellum to the anticipatory postural adjustments and execution phases of human gait initiation

Several brain structures including the brainstem, the cerebellum and the frontal cortico-basal ganglia network, with the primary and premotor areas have been shown to participate in the functional organization of gait initiation and postural control in humans, but their respective roles remain poorly understood. The aim of this study was to better understand the role of the supplementary motor area (SMA) and posterior cerebellum in the gait initiation process. Gait initiation parameters were recorded in 22 controls both before and after continuous theta burst transcranial stimulation (cTBS) of the SMA and cerebellum, and were compared to sham stimulation, using a randomized double-blind design study. The two phases of gait initiation process were analyzed: anticipatory postural adjustments (APAs) and execution, with recordings of soleus and tibialis anterior muscles. Functional inhibition of the SMA led to a shortened APA phase duration with advanced and increased muscle activity; during execution, it also advanced muscle co-activation and decreased the duration of stance soleus activity. Cerebellar functional inhibition did not influence the APA phase duration and amplitude but increased muscle co-activation, it decreased execution duration and showed a trend to increase velocity, with increased swing soleus muscle duration and activity. The results suggest that the SMA contributes to both the timing and amplitude of the APAs with no influence on step execution and the posterior cerebellum in the coupling between the APAs and execution phases and leg muscle activity pattern during gait initiation.

Reductions in muscle coactivation and metabolic cost during visuomotor adaptation.

We often have to adapt our movements as we interact with a variety of objects in various conditions on a daily basis. Evidence suggests that motor adaptation relies on a process that minimizes error and effort; however, much of this evidence involved adapting to novel dynamics with physical perturbations to counteract. To examine the generality of the process of minimizing error and effort during motor adaptation, we used a visuomotor adaptation task that did not involve dynamic perturbations. We investigated the time courses of muscle activity, coactivation, and metabolic cost as subjects reached to a target with a visuomotor rotation. We wanted to determine whether subjects would modulate muscle activity, coactivation, and metabolic cost during a visuomotor adaptation task. Interestingly, subjects increased muscle coactivation early during visuomotor adaptation when there were large cursor-trajectory errors but no physical perturbations to reject. As adaptation progressed, muscle activity and coactivation decreased. Metabolic cost followed a similar time course. When the perturbation was removed, typical after-effects were observed: trajectory error increased and then was reduced quickly. This was accompanied by increases in muscle activity, coactivation, and metabolic cost, along with subsequent rapid reductions. These results demonstrate that subjects modulate muscle activity, coactivation, and metabolic cost similarly across different forms of motor adaptation. Overall, our findings suggest that minimization of error and effort may be a general process underlying various forms of motor adaptation.

Effect of auditory constraints on motor performance depends on stage of recovery post-stroke.

In order to develop evidence-based rehabilitation protocols post-stroke, one must first reconcile the vast heterogeneity in the post-stroke population and develop protocols to facilitate motor learning in the various subgroups. The main purpose of this study is to show that auditory constraints interact with the stage of recovery post-stroke to influence motor learning. We characterized the stages of upper limb recovery using task-based kinematic measures in 20 subjects with chronic hemiparesis. We used a bimanual wrist extension task, performed with a custom-made wrist trainer, to facilitate learning of wrist extension in the paretic hand under four auditory conditions: (1) without auditory cueing; (2) to non-musical happy sounds; (3) to self-selected music; and (4) to a metronome beat set at a comfortable tempo. Two bimanual trials (15 s each) were followed by one unimanual trial with the paretic hand over six cycles under each condition. Clinical metrics, wrist and arm kinematics, and electromyographic activity were recorded. Hierarchical cluster analysis with the Mahalanobis metric based on baseline speed and extent of wrist movement stratified subjects into three distinct groups, which reflected their stage of recovery: spastic paresis, spastic co-contraction, and minimal paresis. In spastic paresis, the metronome beat increased wrist extension, but also increased muscle co-activation across the wrist. In contrast, in spastic co-contraction, no auditory stimulation increased wrist extension and reduced co-activation. In minimal paresis, wrist extension did not improve under any condition. The results suggest that auditory task constraints interact with stage of recovery during motor learning after stroke, perhaps due to recruitment of distinct neural substrates over the course of recovery. The findings advance our understanding of the mechanisms of progression of motor recovery and lay the foundation for personalized treatment algorithms post-stroke.

Muscle coactivation during obstacle crossing corresponding to increasing obstacle height in patients with hemiplegia after stroke

Introduction and aim: Stroke patients often encounter difficulties when crossing obstacles, which contribute to a high rate of falls [1]. Previous studies indicated that patients after stroke excessively elevate their leading limbwhile crossing an obstacle to minimize their risk of falling [1]. This adaptive behaviour occurred even though it is difficult to maintain balance by prolonging the single stance time. However, previous studies generally focused on the leading limb in obstacle crossing tasks; therefore, there is little information regarding the trailing limb. More recently, muscle coactivation during gait was reported as a compensation strategy to enhance postural stability in individuals after stroke [2]. Thus, a change in muscle coactivation in the trailing limb corresponding to an increase in obstacle height may be an indicator of postural stability during obstacle crossing. The aim of this study was to investigate changes in muscle coactivation patterns in the trailing limb using electromyography (EMG); these changes corresponded to an increasing obstacle height in stroke patients with hemiplegia. Patients/materials and methods: Twelve sub-acute patients who suffered a stroke less than 6 months previously participated in this study. Each patient performed several trials in an obstacle crossing task. First, patients performed two self-paced normal walking trials on a 5-m walkway. Subsequently, an obstacle was placed 2–2.5m ahead of the start position at a height that was randomly selected from three heights (0 cm, 4 cm, or 8 cm). The patients were instructed to walk at a self-selected speed and step over the obstacle without stopping. In each condition, EMG activity was recorded in the trailing limb from the tibialis anterior and lateral gastrocnemius muscles. The coactivation index (CoI) was calculated for both the paretic and non-paretic limbs when the trailing limb was in the stance phase. The stance phase was further divided into the three following sub-phases: the first and second double stance (DS1 and DS2), and the single stance (SS). Using the Friedman and Wilcoxon signed-rank tests (with Bonferroni correction),weevaluated theCoI calculatedduring each stance sub-phase. Results: Patients tripped over the obstacle in 16.6% trials when the paretic limb trailed and in 3.7% when the non-paretic limb trailed. When the paretic limb trailed, the paretic limb CoI significantly increased only during DS2 corresponding to an increasing obstacle height (p<0.01). In contrast, when the non-paretic limb trailed, the non-paretic limb CoI significantly increased during both SS (p<0.01) and DS2 (p<0.01) corresponding to an increasing obstacle height. The results for the multiple comparisons are shown in Fig. 1. Discussion and conclusions:When the paretic limb trailed, the paretic limb CoI significantly increased only during DS2. However, muscle coactivation decreased during DS2 in normal gait because the postural stability was no longer mainly required in the trailing limb as the weight was shifted to the opposite limb during this phase [3]. Thus, increased muscle coactivation occurring only during DS2 in the paretic limb with an increasing obstacle height may beanunnecessary adaptation that caused thepatients to frequently trip in these trials. In contrast, the non-paretic limb CoI also significantly increased during SSwith increasing an obstacle heightwhen the non-paretic limb trailed. Thus, although instability increased in the SS phase during obstacle crossing, muscle coactivation in the non-paretic limb increased postural stability during this phase, which resulted in a lower rate of trip trials.

Older adults learn less, but still reduce metabolic cost, during motor adaptation

The ability to learn new movements and dynamics is important for maintaining independence with advancing age. Age-related sensorimotor changes and increased muscle coactivation likely alter the trial-and-error-based process of adapting to new movement demands (motor adaptation). Here, we asked, to what extent is motor adaptation to novel dynamics maintained in older adults (≥65 yr)? We hypothesized that older adults would adapt to the novel dynamics less well than young adults. Because older adults often use muscle coactivation, we expected older adults to use greater muscle coactivation during motor adaptation than young adults. Nevertheless, we predicted that older adults would reduce muscle activity and metabolic cost with motor adaptation, similar to young adults. Seated older (n = 11, 73.8 ± 5.6 yr) and young (n = 15, 23.8 ± 4.7 yr) adults made targeted reaching movements while grasping a robotic arm. We measured their metabolic rate continuously via expired gas analysis. A force field was used to add novel dynamics. Older adults had greater movement deviations and compensated for just 65% of the novel dynamics compared with 84% in young adults. As expected, older adults used greater muscle coactivation than young adults. Last, older adults reduced muscle activity with motor adaptation and had consistent reductions in metabolic cost later during motor adaptation, similar to young adults. These results suggest that despite increased muscle coactivation, older adults can adapt to the novel dynamics, albeit less accurately. These results also suggest that reductions in metabolic cost may be a fundamental feature of motor adaptation.

Tremor Irregularity, Torque Steadiness and Rate of Force Development in Parkinson’s Disease

We investigated lower-extremity isometric tremor Approximate Entropy (irregularity), torque steadiness and rate of force development (RFD) and their associations to muscle activation strategy during isometric knee extensions in patients with Parkinson's disease (PD). Thirteen male patients with idiopathic PD and 15 neurologically healthy matched controls performed isometric maximal contractions (extension/flexion) as well as steady submaximal and powerful isometric knee extensions. The patients with PD showed decreased isometric tremor irregularity. Torque steadiness was reduced in PD and the patients had increased muscle coactivation. A markedly lower RFD was found in PD and the decreased RFD correlated with reduced agonist muscle activation. Furthermore, patient RFD correlated with the Movement-Disorder-Society-Unified-Parkinson's-Disease-Rating-Scale 3 (motor part) scores. We concluded that both knee isometric tremor Approximate Entropy and torque steadiness clearly differentiate between patients with PD and healthy controls. Furthermore, severely compromised RFD was found in patients with PD and was associated with decreased agonist muscle activation.

Neuromuscular Function during a Forward Lunge in Meniscectomized Patients

This study aimed to investigate differences in knee joint kinematics, ground reaction force kinetics and neuromuscular activity including muscle coactivation, and medial versus lateral muscle activity during a forward lunge between the operated and contralateral legs of meniscectomized patients. Such differences may represent early changes in neuromuscular function potentially contributing to the development of knee osteoarthritis. Twenty-two unilaterally meniscectomized patients (15 men and 7 women; age = 45.4 ± 5.1 yr, height = 174.3 ± 7.1 cm, weight = 77.3 ± 15.4 kg (mean ± SD)) were investigated on their operated and contralateral legs during a forward lunge using synchronous goniometer, force plate, and EMG recording (vastus lateralis, vastus medialis, biceps femoris, and semitendinosus). During impact loading, an increased level of muscle coactivation (38.0% (29.8%-46.3%) vs 30.1% (23.6%-36.6%), P = 0.02; mean (95% confidence interval)), reduced range of motion (59.4° (54.5°-64.2°) vs 64.9 (60.8°-70.0°), P = 0.01), and increased loading rate (985% BW·s(-1) (653-1317% BW·s(-1)) vs 696% BW·s(-1) (525-868% BW·s(-1)), P = 0.01) were observed in the operated leg compared with the contralateral leg. In addition, different patterns of medial versus lateral neuromuscular activity were observed in the operated and contralateral legs, along with minor differences during the other phases of the forward lunge. The findings of increased muscle coactivation, reduced range of motion, and increased loading rate in the operated compared with the contralateral leg support the hypothesis that meniscectomized patients demonstrate modulations in kinetics, kinematics, and neuromuscular activity, which may represent early changes in neuromuscular function that precede and affect development of knee osteoarthritis. The results also indicate that the forward lunge test is a sensitive measure to identify leg-to-leg differences in neuromuscular function after surgical interventions.

Tradeoff between Stability and Maneuverability during Whole-Body Movements

BackgroundUnderstanding how stability and/or maneuverability affects motor control strategies can provide insight on moving about safely in an unpredictable world. Stability in human movement has been well-studied while maneuverability has not. Further, a tradeoff between stability and maneuverability during movement seems apparent, yet has not been quantified. We proposed that greater maneuverability, the ability to rapidly and purposefully change movement direction and speed, is beneficial in uncertain environments. We also hypothesized that gaining maneuverability comes at the expense of stability and perhaps also corresponds with decreased muscle coactivation.Materials and MethodsWe used a goal-directed forward lean movement task that integrated both stability and maneuverability. Subjects (n = 11) used their center of pressure to control a cursor on a computer monitor to reach a target. We added task uncertainty by shifting the target anterior-posterior position mid-movement. We used a balance board with a narrow beam that reduced the base of support in the medio-lateral direction and defined stability as the probability that subjects could keep the balance board level during the task.ResultsDuring the uncertainty condition, subjects were able to change direction of their anterior-posterior center of pressure more rapidly, indicating that subjects were more maneuverable. Furthermore, medio-lateral center of pressure excursions also approached the edges of the beam and reduced stability margins, implying that subjects were less stable (i.e. less able to keep the board level). On the narrow beam board, subjects increased muscle coactivation of lateral muscle pairs and had greater muscle activity in the left leg. However, there were no statistically significant differences in muscle activity amplitudes or coactivation with uncertainty.Conclusions/SignificanceThese results demonstrate that there is a tradeoff between stability and maneuverability during a goal-directed whole-body movement. Tasks with added uncertainty could help individuals learn to be more maneuverable yet sufficiently stable.