Task-level feedback can explain temporal recruitment of spatially fixed muscle synergies throughout postural perturbations

Abstract

Recent evidence suggests that complex spatiotemporal patterns of muscle activity can be explained with a low-dimensional set of muscle synergies or M-modes. While it is clear that both spatial and temporal aspects of muscle coordination may be low dimensional, constraints on spatial versus temporal features of muscle coordination likely involve different neural control mechanisms. We hypothesized that the low-dimensional spatial and temporal features of muscle coordination are independent of each other. We further hypothesized that in reactive feedback tasks, spatially fixed muscle coordination patterns-or muscle synergies-are hierarchically recruited via time-varying neural commands based on delayed task-level feedback. We explicitly compared the ability of spatially fixed (SF) versus temporally fixed (TF) muscle synergies to reconstruct the entire time course of muscle activity during postural responses to anterior-posterior support-surface translations. While both SF and TF muscle synergies could account for EMG variability in a postural task, SF muscle synergies produced more consistent and physiologically interpretable results than TF muscle synergies during postural responses to perturbations. Moreover, a majority of SF muscle synergies were consistent in structure when extracted from epochs throughout postural responses. Temporal patterns of SF muscle synergy recruitment were well-reconstructed by delayed feedback of center of mass (CoM) kinematics and reproduced EMG activity of multiple muscles. Consistent with the idea that independent and hierarchical low-dimensional neural control structures define spatial and temporal patterns of muscle activity, our results suggest that CoM kinematics are a task variable used to recruit SF muscle synergies for feedback control of balance.

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

“…Therefore, we suggest that somatosensory information from cutaneous receptors in the hand is largely responsible for the earliest change in the ankle muscle response rather than the somatosensory information from more distant receptors because of hand-to-legs propagation of the effects of the perturbation and the latency of motion of the thorax and the ankle joints relative to EMG. Although COM acceleration onset preceded the earliest ankle muscle activation and COM feedback has been proposed as a control signal for lower limb muscle activation in a recent model of balance control for support surface perturbations (Safavynia and Ting, 2012), on average the onset of COM motion only preceded the onset of ankle muscle activation by 13 ms. It is unlikely that this lead time would be sufficient for the central nervous system to compute COM motion and transmit an appropriate directionally tuned command to the ankle muscles.…”

Differentiation of gluteus medius and minimus activity in weight bearing and non-weight bearing exercises by M-mode ultrasound imaging

“…The high variability between gluteus minimus and the deep gluteus medius in weight bearing suggests a functional differentiation, e.g. an adaption to slight differences in the position of the center of mass (Safavynia and Ting, 2012) or other mechanical demands (Herrel et al, 2008). The study findings suggest that such differentiation can be exercised in weight shift to one legged stance but not in side-lying abduction.…”

Graded muscle contractions determined by temporal recruitment

“…It has been researched by Safavynia and Ting, who found that measuring the frequency of the peaks rather than the amplitude of S the peaks is a more reliable and accurate depiction of the graded potential [3]. For future work, muscle fatigue should be taken into consideration.…”

“…It has been proven that temporal recruitment is easier to reconstruct [3]. Using this theory, it is more effective to interpret the strength of a muscle contraction by finding the frequency of an EMG signal instead of the amplitude of an EMG signal.…”

“…The strength of a muscle contraction can be increased via two types of recruitment: spatial recruitment and temporal recruitment [3]. Spatial recruitment is the increase in number of motor units in order to increase strength.…”