Motor Output Variability Research Articles

Contralateral cutaneous reflex modulation during gait in individuals with and without chronic ankle instability

IntroductionChronic ankle instability (CAI), a common seqeula to ankle injury is characterized by a variety of sensorimotor deficits extending beyond the previously injured limb. Cutaneous reflexes have been identified as a potential contributor to these functional limitations with recent studies identifying alterations in reflex patterns following sural nerve stimulation among those with CAI. To date, no studies have measured cutaneous reflexes of the unaffected limb in this population, therefore, the objective of this study was to measure contralateral cutaneous reflexes during gait in individuals with unilateral CAI and healthy controls. MethodsMuscle activity of 6 lower limb muscles was measured in nineteen participants while receiving random, non-noxious sural nerve stimulations during a walking task. ResultsControl reflex patterns were generally well-aligned with previous literature while CAI patterns varied from controls in several muscles throughout the gait cycle. Namely, a lack of lateral gastrocnemius facilitation during late stance and medial gastrocnemius inhibition at midstance. Additionally, a lack of significant BF facilitation throughout contralateral swing was noted. These results indicate reflex alterations extend beyond the affected limb in those with unilateral CAI indicating changes at the spinal level following lateral ankle sprains (LAS). Considering the symptom variability in CAI, the lack of significant reflexes exhibited by the CAI group may be due to increased variability in motor output between subjects or between stimulation trials. ConclusionsThese findings highlight the importance of identifying reflex alterations arising from LAS and subsequently treating these limitations through rehabilitation targeting systemic neural pathways rather than local deficits.

The too many facets of motor output variability. Comment on “From neural noise to co-adaptability: Rethinking the multifaceted architecture of motor variability” by Casartelli, L., Maronati, C., & Cavallo, A.

The too many facets of motor output variability. Comment on “From neural noise to co-adaptability: Rethinking the multifaceted architecture of motor variability” by Casartelli, L., Maronati, C., & Cavallo, A.

Motor Output Variability in Movement Disorders: Insights From Essential Tremor.

Findings on individuals with essential tremor suggest that tremor (within-trial movement unsteadiness) and inconsistency (trial-to-trial movement variance) stem from distinct pathologies and affect function uniquely. Nonetheless, the intricacies of inconsistency in movement disorders remain largely unexplored, as exemplified in ataxia where inconsistency below healthy levels is associated with greater pathology. We advocate for clinical assessments that quantify both tremor and inconsistency.

Combined cervical transcutaneous with lumbosacral epidural stimulation improves voluntary control of stepping movements in spinal cord injured individuals.

Introduction: Lumbosacral spinal cord neuromodulation has shown the ability to restore voluntary control and stepping in individuals with chronic spinal cord injury. Methods: We combined cervical transcutaneous and lumbar epidural stimulation to explore the brain-spinal connectomes and their influence in spinal excitability and interlimb coupling. Four individuals with a prior implanted lumbosacral spinal cord epidural stimulator participated in the study. We assessed lower extremity muscle activity and kinematics during intentional stepping in both non-weight bearing and weight-bearing environments. Results: Our results showed an inhibition of motor evoked potentials generated by spinal cord epidural stimulation when cervical transcutaneous stimulation is applied. In contrast, when intentional stepping is performed in a non-weight bearing setting, range of motion, motor output amplitude, and coordination are improved when cervical transcutaneous and lumbar epidural stimulations are combined. Similarly, with both stimulations applied, coordination is improved and motor output variability is decreased when intentional stepping is performed on a treadmill with body weight support. Discussion: Combined transcutaneous cervical and epidural lumbar stimulation demonstrated an improvement of voluntary control of stepping in individuals with chronic motor complete paralysis. The immediate functional improvement promoted by the combination of cervical and lumbar stimulation adds to the body of evidence for increasing spinal excitability and improvement of function that is possible in individuals with chronic paralysis.

Inter-Individual Variability in Motor Output Is Driven by Recruitment Gain in the Corticospinal Tract Rather Than Motor Threshold.

Variability in the response of individuals to various non-invasive brain stimulation protocols is a major problem that limits their potential for clinical applications. Baseline motor-evoked potential (MEP) amplitude is the key predictor of an individual’s response to transcranial magnetic stimulation protocols. However, the factors that predict MEP amplitude and its variability remain unclear. In this study, we aimed to identify the input–output curve (IOC) parameters that best predict MEP amplitude and its variability. We analysed IOC data from 75 subjects and built a general linear model (GLM) using the IOC parameters as regressors and MEP amplitude at 120% resting motor threshold (RMT) as the response variable. We bootstrapped the data to estimate variability of IOC parameters and included them in a GLM to identify the significant predictors of MEP amplitude variability. Peak slope, motor threshold, and maximum MEP amplitude of the IOC were significant predictors of MEP amplitude at 120% RMT and its variability was primarily driven by the variability of peak slope and maximum MEP amplitude. Recruitment gain and maximum corticospinal excitability are the key predictors of MEP amplitude and its variability. Inter-individual variability in motor output may be reduced by achieving a uniform IOC slope.

TU-127. Inter-individual variability in motor output is driven by recruitment gain rather than motor threshold

TU-127. Inter-individual variability in motor output is driven by recruitment gain rather than motor threshold

Large‐scale optical recording of respiration‐modulated neurons in ventrolateral medulla reveals cycle‐to‐cycle variability compatible with multifunctionality

Respiration-modulated neurons form a distributed network along the ventral respiratory column (VRC). While anatomically constrained locations are hypothesized to mediate respiration and its coordination with swallow, their interactions are poorly understood. These networks can be recorded optically in the sagittally-sectioned rodent hindbrain (SSRH) preparation. We used neonate (P3-P5) mice, genetically encoded with the Ca2+ indicator GCaMP6F, to record the VRC at high sampling rates, over epochs long enough to robustly characterize the intrinsic cycle-to-cycle variability in both motor output and neuronal onset times. We sought to identify where inspiratory bursts initiated by sorting neuronal onset times relative to ventral root onset. Our findings did not support the hypothesis that respiratory rhythm arises out of the activity of an anatomically compact, dedicated central pattern generating network: earliest neurons were distributed along the VRC, and on a cycle-to-cycle basis, earliest neurons differed. In part, variability in onset times could be accounted for by slow system-level variables: significant correlations between expiratory durations both before and after an inspiratory burst, and neuronal onset times and burst duration were found. We propose that these networks are multifunctional, regulating the coordination of orofacial, swallow, and respiratory behaviors, as well as blood-gas homeostasis. Networks at the mediodorsal edge of the SSRH that carry lung afferent feedback, as well as command networks for orofacial behaviors such as swallow and vocalization are at the surface of SSRH preparation. Thus we can test this hypothesis by evoking associated reflexes and behaviors in vitro using electrical stimulation, and observing respiratory network dynamics.

Motor Unit Discharge Variability Is Increased in Mild-To-Moderate Parkinson's Disease.

Individuals with Parkinson's disease (PD) demonstrate deficits in muscle activation such as decreased amplitude and inappropriate bursting. There is evidence that some of these disturbances are more pronounced in extensor vs. flexor muscles. Surface EMG has been used widely to quantify muscle activation deficits in PD, but analysis of discharge of the underlying motor units may provide greater insight and be more sensitive to changes early in the disease. Of the few studies that have examined motor unit discharge in PD, the majority were conducted in the first dorsal interosseous, and no studies have measured motor units from extensor and flexor muscles within the same cohort. The objective of this study was to characterize the firing behavior of single motor units in the elbow flexor and extensor muscles during isometric contractions in people with mild-to-moderate PD. Ten individuals with PD (off-medication) and nine healthy controls were tested. Motor unit spike times were recorded via intramuscular EMG from the biceps and triceps brachii muscles during 30-s isometric contractions at 10% maximum voluntary elbow flexion and elbow extension torque, respectively. We selected variables of mean motor unit discharge rate, discharge variability, and torque variability to evaluate motor abnormalities in the PD group. The effects of group, muscle, and group-by-muscle on each variable were determined using separate linear mixed models. Discharge rate and torque variability were not different between groups, but discharge variability was significantly higher in the PD group for both muscles combined (p < 0.0001). We also evaluated the asymmetry in these motor variables between the triceps and biceps for each individual participant with PD to evaluate whether there was an association with disease severity. The difference in torque variability between elbow flexion and extension was significantly correlated with both the Hoehn and Yahr scale (rho = 0.71) and UPDRS (rho = 0.62). Our findings demonstrate that variability in motor output, rather than decreased discharge rates, may contribute to motor dysfunction in people with mild-to-moderate PD. Our findings provide insight into altered neural control of movement in PD and demonstrate the importance of measuring from multiple muscles within the same cohort.

Does variability in motor output at individual joints predict stride time variability in gait? Influences of age, sex, and plane of motion

Old age is associated with variability in gait motor output, particularly in females, and is linked to fall risk. However, little is known about how older age and sex affect variability in the outputs of individual joints, and how these variabilities contribute to the collective gait output. Healthy adults aged 18–99 years (N = 102, 57 females) completed six trials of straight walking at self-selected speed. Stride time variability (coefficient of variation) and variabilities of lower limb tridimensional joint angles (standard deviations: SD) were calculated. Age * Sex (A * S) mixed models were conducted on all measures and year-by-year rates of change were subsequently estimated. Correlations and stepwise linear regression analyses were computed between joint angular variabilities and stride time variability. Each year of age was associated with 0.022% higher stride time variability (A: p = .002), 0.07° lower variability in peak ankle dorsiflexion (A: p = .004), 0.002–0.098° higher variability in mean ankle inversion/eversion, mean pelvic obliquity, and pelvic rotation range of motion (A: p < .05), and 0.024° higher variability in knee flexion/extension range of motion in males (A * S: p = .003). Higher variability in mean ankle and hip flexion/extension and in mean ankle inversion/eversion correlated with (ρ = 0.211–0.336; ps < 0.05) and independently predicted higher stride time variability (ps < 0.05), together explaining 21.9% of variance. Results suggest that higher stride time variability with older age may be produced by a shift from sagittal plane variability to frontal plane variability at the ankle.

Superficial Shoulder Muscle Synergy Analysis in Facioscapulohumeral Dystrophy During Humeral Elevation Tasks.

Facioscapulohumeral dystrophy (FSHD) is a progressive muscle-wasting disease which leads to a decline in upper extremity functionality. Although the scapulohumeral joint's stability and functionality are affected, evidence on the synergetic control of the shoulder muscles in FSHD individuals is still lacking. The aim of this paper is to understand the neuromuscular changes in shoulder muscle control in people with FSHD. Upper arm kinematics and electromyograms (EMG) of eight upper extremity muscles were recorded during shoulder abduction-adduction and flexion-extension tasks in eleven participants with FSHD and 11 healthy participants. Normalized muscle activities were extracted from EMG signals. Non-negative matrix factorization was used to compute muscle synergies. Maximum muscle activities were compared using non-parametric analysis of variance. Similarities between synergies were also calculated using correlation. The Biceps Brachii was significantly more active in the FSHD group (25±2%) while Trapezius Ascendens and Serratus Anterior were less active (32±7% and 39±4%, respectively). Muscle synergy weights were altered in FSHD individuals and showed greater diversity while controls mostly used one synergy for both tasks. The decreased activity by selected scapula rotator muscles and muscle synergy weight alterations show that neuromuscular control of the scapulohumeral joint is less consistent in people with FSHD compared to healthy participants. Assessments of muscle coordination strategies can be used to evaluate motor output variability and assist in management of the disease.

The Impact of Mental Fatigue on Force and Motor Unit Firing Variability in Young Adults

Mental fatigue leads to declines in performance of tasks such as cycling time trial performance and skill-based outcomes such as soccer shot accuracy. The neuromuscular mechanisms leading to these declines are not well understood. Although force variability has been shown to increase under dual-task conditions, it is not known if these results extend to conditions of mental fatigue. PURPOSE: The purpose of this study was to assess the impact of mental fatigue on variability in motor output in healthy, young individuals. Specifically, we sought to determine if a task that induces mental fatigue has effects on force and motor unit firing variability. METHODS: Nineteen participants (10 female, 9 male) performed 10-s isometric contractions at 20 and 50% maximum voluntary contraction (MVC) before, during, and after completing 20 min of the psychomotor vigilance task (PVT). The PVT is a sustained attention task that induces mental fatigue, as indicated by increases in reaction time (RT) to visual stimuli. Force and indwelling motor unit (MU) firings were measured prior to and immediately following performance of the PVT (single task), and within the first and final minutes of PVT performance (dual task). Subjective ratings of fatigue were also obtained using a 10-point Likert scale before and after the PVT. RESULTS: Reaction time increased by 14% from the beginning of the PVT (276.14 ± 31.54 ms) to the end (314.15 ± 37.74 ms, p<0.001). Subjects also subjectively reported greater levels of fatigue following the PVT (4.95 ± 1.84) compared to before (3.00 ± 1.20, p<0.001), indicating successful induction of mental fatigue. For the coefficient of variation (CV) of force there was no significant main effect of time (p=0.14) or contraction intensity (p=0.33), and no significant interaction (p=0.51). For the CV of the MU interspike interval there was a main effect of contraction intensity with a greater CV of the interspike interval during the 50% MVC (19.51 ± 3.84%) than the 20% MVC (14.87 ± 3.59%, p<0.001) contractions. However, there was no significant main effect of time (p=0.83) and no significant interaction (p=0.23). CONCLUSION: Inducing mental fatigue did not lead to changes in the variability of force production or motor unit firing during isometric contractions at 20 and 50% MVC as a single-task or during a concurrent cognitive task.

Visual load and variability of muscle activation: Effects on reactive driving of older adults

BackgroundThe functional significance of the increase in motor output variability with increased visual information processing in older adults remains unclear. Here, we test the hypothesis that increased visual information processing increases muscle activation variability in older adults and impairs their ability to react as fast and as precisely as young adults during a simulated reactive driving task. MethodsFourteen young and sixteen older adults performed a reactive driving simulation task that required responding to unexpected brake lights of the car ahead during a simple reaction time task (low visual information processing condition) and a choice reaction time task with “no go” trials condition (high visual information processing condition). We quantified the following: 1) reactive driving performance – combination of premotor response time, motor response time, and brake force error; 2) motor output variability – brake impulse variability; 3) muscle activation variability – variability in the tibialis anterior (TA) muscle activity. ResultsThe increase in information processing exacerbated the impaired reactive driving performance in older adults. The best predictor of this impairment was the increase in brake force error. The impaired reactive driving performance was related to brake impulse variability and variability in the TA activity. ConclusionsThis study provides novel evidence that increased information processing increases muscle activation variability in older adults with detrimental consequences to their ability to perform a simulated reactive driving task.

Motor-Evoked Pain Increases Force Variability in Chronic Jaw Pain

Musculoskeletal pain changes how people move. Although experimental pain is associated with increases in the variability of motor output, it is not clear whether motor-evoked pain in clinical conditions is also associated with increases in variability. In the current study, we measured jaw force production during a visually guided force paradigm in which individuals with chronic jaw pain and control subjects produced force at 2% of their maximum voluntary contraction (low target force level) and at 15% of their maximum voluntary contraction (high target force level). State measures of pain were collected before and after each trial. Trait measures of pain intensity and pain interference, self-report measures of jaw function, and measures of depression, anxiety, and fatigue were also collected. We showed that the chronic jaw pain group exhibited greater force variability compared with controls irrespective of the force level, whereas the accuracy of force production did not differ between groups. Furthermore, predictors of force variability shifted from trait measures of pain intensity and pain interference at the low force level to state measures of pain intensity at the high force level. Our observations show that motor-evoked jaw pain is associated with increases in force variability that are predicted by a combination of trait measures and state measures of pain intensity and pain interference. PerspectiveChronic jaw pain is characterized by increases in variability during force production, which can be predicted by pain intensity and pain interference. This report could help clinicians better understand the long-term consequences of chronic jaw pain on the motor system.

Intra-Subject Consistency during Locomotion: Similarity in Shared and Subject-Specific Muscle Synergies.

Human locomotion is a complex motor task. Previous research hypothesized that muscle synergies reflect the modular control of muscle groups operated by the Central Nervous System (CNS). Despite the high stride-to-stride variability characterizing human gait, most studies analyze only a few strides. This may be limiting, because the intra-subject variability of motor output is neglected. This gap could be filled by recording and analyzing many gait cycles during a single walking task. In this way, it can be investigated if CNS recruits the same muscle synergies consistently or if different strategies are adopted during the locomotion task. The aim of this work is to investigate the intra-subject consistency of muscle synergies during overground walking. Twelve young healthy volunteers were instructed to walk for 5 min at their natural pace. On the average, 181 ± 10 gait cycles were analyzed for each subject. Surface electromyography was recorded from 12 muscles of the dominant lower limb and the trunk. Gait cycles were grouped into subgroups containing 10 gait cycles each. The consistency of the muscle synergies extracted during the gait trial was assessed by measuring cosine similarity (CS) of muscle weights vectors, and zero-lag cross-correlation (CC) of activation signals. The average intra-subject CS and CC were 0.94 ± 0.10 and 0.96 ± 0.06, respectively. We found five synergies shared by all the subjects: high consistency values were found for these synergies (CS = 0.96 ± 0.05, CC = 0.97 ± 0.03). In addition, we found 10 subject-specific synergies. These synergies were less consistent (CS = 0.80 ± 0.20, CC = 0.89 ± 0.14). In conclusion, our results demonstrated that shared muscle synergies were highly consistent during walking. Subject-specific muscle synergies were also consistent, although to a lesser extent.

Motor Output Variability Impairs Driving Ability in Older Adults: Reply to Stinchcombe, Dickerson, Weaver, and Bedard.

Motor Output Variability Impairs Driving Ability in Older Adults: Reply to Stinchcombe, Dickerson, Weaver, and Bedard.

Sex differences in spatial accuracy relate to the neural activation of antagonistic muscles in young adults.

Sex is an important physiological variable of behavior, but its effect on motor control remains poorly understood. Some evidence suggests that women exhibit greater variability during constant contractions and poorer accuracy during goal-directed tasks. However, it remains unclear whether motor output variability or altered muscle activation impairs accuracy in women. Here, we examine sex differences in endpoint accuracy during ankle goal-directed movements and the activity of the antagonistic muscles. Ten women (23.1±5.1years) and 10 men (23±3.7years) aimed to match a target (9° in 180ms) with ankle dorsiflexion. Participants performed 50 trials and we recorded the endpoint accuracy and the electromyographic (EMG) activity of the primary agonist (Tibialis Anterior; TA) and antagonist (Soleus; SOL) muscles. Women exhibited greater spatial inaccuracy (Position error: t=-2.65, P=0.016) but not temporal inaccuracy relative to men. The motor output variability was similar for the two sexes (P>0.2). The spatial inaccuracy in women was related to greater variability in the coordination of the antagonistic muscles (R 2 0.19, P=0.03). These findings suggest that women are spatially less accurate than men during fast goal-directed movements likely due to an altered activation of the antagonistic muscles.

Age-associated differences in motor output variability and coordination during the simultaneous dorsiflexion of both feet

Older adults are more variable than young adults on tasks that demand the simultaneous control of more than one effector, and the difference between age groups may be related to their different capacity of coordinating the force output of the involved effectors. The goal of this study was to determine whether age-associated differences in motor output variability during tasks involving the simultaneous dorsiflexion of two feet can be partially explained by differences in coordination and possibly attenuated by physical training. Ten young and 22 old adults (10 trained and 12 untrained old adults) volunteered to participate in the study. Trained older adults had experience in a high-intensity mixed modality training (MMT) regime for a minimum of 1 year. Volunteers performed successive trials of a constant force task and a goal-directed task, with and without visual feedback. Within- and between-trial variability were calculated and coordination was quantified using the uncontrolled manifold (UCM) approach (i.e., co-variation of the force outputs of both feet were used to quantify a motor synergy index). Older adults exhibited greater variability and lower synergy (p < .05), independently of physical training status, than young adults. Removal of visual feedback caused greater variability and lower synergy for all groups (p < .05). Our results suggest that older adults exhibit greater motor output variability in tasks involving the simultaneous dorsiflexion of both feet possibly due to a lack of coordination between the feet.

What Is the Contribution of Ia-Afference for Regulating Motor Output Variability during Standing?

Motor variability is an inherent feature of all human movements, and describes the system‘s stability and rigidity during the performance of functional motor tasks such as balancing. In order to ensure successful task execution, the nervous system is thought to be able to flexibly select the appropriate level of variability. However, it remains unknown which neurophysiological pathways are utilized for the control of motor output variability. In responding to natural variability (in this example sway), it is plausible that the neuro-physiological response to muscular elongation contributes to restoring a balanced upright posture. In this study, the postural sway of 18 healthy subjects was observed while their visual and mechano-sensory system was perturbed. Simultaneously, the contribution of Ia-afferent information for controlling the motor task was assessed by means of H-reflex. There was no association between postural sway and Ia-afference in the eyes open condition, however up to 4% of the effects of eye closure on the magnitude of sway can be compensated by increased reliance on Ia-afference. Increasing the biomechanical demands by adding up to 40% bodyweight around the trunk induced a specific sway response, such that the magnitude of sway remained unchanged but its dynamic structure became more regular and stable (by up to 18%). Such regular sway patterns have been associated with enhanced cognitive involvement in controlling motor tasks. It therefore appears that the nervous system applies different control strategies in response to the perturbations: The loss of visual information is compensated by increased reliance on other receptors; while the specific regular sway pattern associated with additional weight-bearing was independent of Ia-afferent information, suggesting the fundamental involvement of supraspinal centers for the control of motor output variability.

Temporary Nerve Block at Selected Digits Revealed Hand Motor Deficits in Grasping Tasks.

Peripheral sensory feedback plays a crucial role in ensuring correct motor execution throughout hand grasp control. Previous studies utilized local anesthesia to deprive somatosensory feedback in the digits or hand, observations included sensorimotor deficits at both corticospinal and peripheral levels. However, the questions of how the disturbed and intact sensory input integrate and interact with each other to assist the motor program execution, and whether the motor coordination based on motor output variability between affected and non-affected elements (e.g., digits) becomes interfered by the local sensory deficiency, have not been answered. The current study aims to investigate the effect of peripheral deafferentation through digital nerve blocks at selective digits on motor performance and motor coordination in grasp control. Our results suggested that the absence of somatosensory information induced motor deficits in hand grasp control, as evidenced by reduced maximal force production ability in both local and non-local digits, impairment of force and moment control during object lift and hold, and attenuated motor synergies in stabilizing task performance variables, namely the tangential force and moment of force. These findings implied that individual sensory input is shared across all the digits and the disturbed signal from local sensory channel(s) has a more comprehensive impact on the process of the motor output execution in the sensorimotor integration process. Additionally, a feedback control mechanism with a sensation-based component resides in the formation process for the motor covariation structure.

Editorial: A Hand at Work: Effects of Aging.

Editorial: A Hand at Work: Effects of Aging.