Short-latency muscle response patterns to multi-directional, unpredictable perturbations to balance applied to the arm are context dependent

Degenerative cervical myelopathy delays responses to lateral balance perturbations regardless of predictability

The aim of this study was to quantify balance impairments in standing in people with degenerative cervical myelopathy (PwDCM) in response to external perturbations. PwDCM have damage to their spinal cord due to degeneration of the cervical vertebral column, but little is known about balance. Balance was quantified by capturing kinetics, kinematic, and electromyographic data during standing in response to lateral waist pulls. Participants received pulls during predictable and unpredictable contexts in three stance widths at two magnitudes. In response to lateral waist pulls, PwDCM had larger center of mass excursion (P < 0.001) and delayed gluteus medius electromyography onset (P < 0.001) and peak (P < 0.001) timing. These main effects of history of myelopathy were consistent across predictability, stance width, and magnitude. A multilinear regression determined that gluteus medius peak timing + tibialis anterior peak timing most strongly predicted center of mass excursion (R² = 0.50, P < 0.001). These data suggest that PwDCM have delays in generating voluntary and reactive motor commands, contributing to balance impairments. Future rehabilitation strategies should focus on generating rapid muscular contractions. Additionally, frontal plane postural control is regulated by the gluteus medius and the tibialis anterior, whereas other muscles (e.g. gluteus minimus, ankle invertors/evertors) not studied here may also contribute.NEW & NOTEWORTHY Frontal plane reactive postural control is impaired in persons with degenerative cervical myelopathy because of delayed muscle responses. Additionally, postural control varies across stance width, predictability, and perturbation magnitude.

Different flooring surfaces affect infants' crawling performance

This study assessed the influence of different types of flooring on infants' crawling motion patterns and performance. Each participating infant (range: 8.7-12.4 months) was encouraged to crawl on a tatami mat made of woven straw as well as other flooring types such as hardwood, carpet, and joint mat. Material tests were conducted to quantify the friction and shock absorption of the flooring. A three-dimensional motion capture system was used to measure spatiotemporal and kinematic variables during hands-and-knees crawling. An increased crawling rate was associated with a faster cadence of cyclic arm movements, but not with crawling stride length. Hardwood flooring had a significantly lower crawling rate and longer duration of hand-floor contact than tatami, while the crawling stride length and range of motion of joint movements were hardly affected by flooring type. The results of this study suggest a drawback of hardwood flooring in terms of infants' effective quadrupedal locomotion.

Linearity and repeatability of postural responses in relation to peak force and impulse of manually delivered perturbations: a preliminary study

PURPOSE

Postural reactions (PR) of standing subjects have been mostly investigated in response to platform displacements or body perturbations of fixed magnitude. The objective of this study was to investigate the relationship between PR and the peak force and impulse of the perturbation.

METHODS

In ten healthy young men, standing balance was challenged by anteriorly directed perturbations (peak force: 20-60 N) delivered to the back, at the lumbar (L) or inter-scapular (IS) level, by means of a manual perturbator equipped with a force sensor. Postural reactions as expressed by the displacement of the center of pressure (CoP) were recorded using a force platform. Two sets of 20 randomly ordered perturbations (10 to each site) were delivered in two separate testing sessions.

RESULTS

The magnitude of CoP response (∆CoP) was better correlated with the impulse (I) than with the peak force of the perturbation. The normalized response, ∆CoP_n = ∆CoP/I, exhibited good reliability (ICCs of 0.93 for IS and 0.82 for L), was higher with IS than with L perturbations (p < 0.01), and was significantly correlated with the latency of CoP response: r = 0.69 and 0.71 for IS and L, respectively.

CONCLUSION

These preliminary findings support the concept that manually delivered perturbations can be used to reliably assess individual PR and that ∆CoP_n may effectively express a relevant aspect of postural control.

Regulation of dynamic postural control to attend manual steadiness constraints

In daily living activities, performance of spatially accurate manual movements in upright stance depends on postural stability. In the present investigation, we aimed to evaluate the effect of the required manual steadiness (task constraint) on the regulation of dynamic postural control. A single group of young participants ( n = 20) were evaluated in the performance of a dual posturo-manual task of balancing on a platform oscillating in sinusoidal translations at 0.4-Hz (low) or 1-Hz (high) frequencies while stabilizing a cylinder on a handheld tray. Manual task constraint was manipulated by comparing the conditions of keeping the cylinder stationary on its flat or round side, corresponding to low and high manual task constraints, respectively. Results showed that in the low oscillation frequency the high manual task constraint led to lower oscillation amplitudes of the head, center of mass, and tray, in addition to higher relative phase values between ankle/hip-shoulder oscillatory rotations and between center of mass/center of pressure-feet oscillations as compared with values observed in the low manual task constraint. Further analyses showed that the high manual task constraint also affected variables related to both postural (increased amplitudes of center of pressure oscillation) and manual (increased amplitude of shoulder rotations) task components in the high oscillation frequency. These results suggest that control of a dynamic posturo-manual task is modulated in distinct parameters to attend the required manual steadiness in a complex and flexible way.

NEW & NOTEWORTHY We evaluated dynamic postural control on a platform oscillating in sinusoidal translations at different frequencies while performing a manual task with low or high steadiness constraints. Results showed that high manual task constraint led to modulation of metric and coordination variables associated with greater postural stability. Our findings suggest that motor control is regulated in an integrative mode at the posturo-manual task level, with reciprocal interplay between the postural and manual components.

Stability of vertical posture explored with unexpected mechanical perturbations: synergy indices and motor equivalence

We explored the relations between indices of mechanical stability of vertical posture and synergy indices under unexpected perturbations. The main hypotheses predicted higher posture-stabilizing synergy indices and higher mechanical indices of center of pressure stability during perturbations perceived by subjects as less challenging. Healthy subjects stood on a force platform and held in fully extended arms a bar attached to two loads acting downward and upward. One of the loads was unexpectedly released by the experimenter causing a postural perturbations. In different series, subjects either knew or did not know which of the two loads would be released. Forward perturbations were perceived as more challenging and accompanied by co-activation patterns among the main agonist-antagonist pairs. Backward perturbation led to reciprocal muscle activation patterns and was accompanied by indices of mechanical stability and of posture-stabilizing synergy which indicated higher stability. Changes in synergy indices were observed as early as 50-100 ms following the perturbation reflecting involuntary mechanisms. In contrast, predictability of perturbation direction had weak or no effect on mechanical and synergy indices of stability. These observations are interpreted within a hierarchical scheme of synergic control of motor tasks and a hypothesis on the control of movements with shifts of referent coordinates. The findings show direct correspondence between stability indices based on mechanics and on the analysis of multi-muscle synergies. They suggest that involuntary posture-stabilizing mechanisms show synergic organization. They also show that predictability of perturbation direction has strong effects on anticipatory postural adjustment but not corrective adjustments. We offer an interpretation of co-activation patterns that questions their contribution to postural stability.

Origin of directionally tuned responses in lower limb muscles to unpredictable upper limb disturbances

Unpredictable forces which perturb balance are frequently applied to the body through interaction between the upper limb and the environment. Lower limb muscles respond rapidly to these postural disturbances in a highly specific manner. We have shown that the muscle activation patterns of lower limb muscles are organized in a direction specific manner which changes with lower limb stability. Ankle muscles change their activity within 80 ms of the onset of a force perturbation applied to the hand which is earlier than the onset of changes in ground reaction force, ankle angle or head motion. The latency of the response is sensitive to the perturbation direction. However, neither the latency nor the magnitude of the response is affected by stiffening the arm even though this alters the magnitude and timing of motion of the body segments. Based on the short latency, insensitivity of the change in ankle muscle activation to motion of the body segments but sensitivity to perturbation direction we reason that changes in ankle muscle activation are most likely triggered by sensory signals originating from cutaneous receptors in the hand. Furthermore, evidence that the latency of changes in ankle muscle activation depends on the number of perturbation directions suggests that the neural pathway is not confined to the spinal cord.

Effects of amplitude and predictability of perturbations to the arm on anticipatory and reactionary muscle responses to maintain balance

Disturbances to balance arising from forces applied to the upper limb have received relatively little attention compared to disturbances arising from support surface perturbations. In this study we applied fast ramp perturbations to the hand in anterior, posterior, medial and lateral directions. The effects of perturbation predictability and amplitude on the postural response of upper limb, trunk and lower limb muscles were investigated. Perturbations were applied either in blocks of constant amplitude and direction (predictable) or with direction and amplitude varying randomly (random) from trial to trial. The spatial-temporal patterns of anticipatory muscle activation under the predictable condition and the reactionary responses following the perturbation under both conditions were similarly organized. The size of the response increased systematically with the perturbation magnitude for both anticipatory and reactionary changes in muscle activation. However, the slope of the relation between perturbation amplitude and the magnitude of the change in muscle activation was greater when perturbations were predictable than when they were randomly selected. The timing of both the anticipatory and reactionary increases in muscle activation was invariant across perturbation amplitudes. The characteristics of the reactionary responses have a similar organization to the long latency muscle responses to support surface perturbations.