Central programming of postural movements: adaptation to altered support-surface configurations

Abstract

We studied the extent to which automatic postural actions in standing human subjects are organized by a limited repertoire of central motor programs. Subjects stood on support surfaces of various lengths, which forced them to adopt different postural movement strategies to compensate for the same external perturbations. We assessed whether a continuum or a limited set of muscle activation patterns was used to produce different movement patterns and the extent to which movement patterns were influenced by prior experience. Exposing subjects standing on a normal support surface to brief forward and backward horizontal surface perturbations elicited relatively stereotyped patterns of leg and trunk muscle activation with 73- to 110-ms latencies. Activity began in the ankle joint muscles and then radiated in sequence to thigh and then trunk muscles on the same dorsal or ventral aspect of the body. This activation pattern exerted compensatory torques about the ankle joints, which restored equilibrium by moving the body center of mass forward or backward. This pattern has been termed the ankle strategy because it restores equilibrium by moving the body primarily around the ankle joints. To successfully maintain balance while standing on a support surface short in relation to foot length, subjects activated leg and trunk muscles at similar latencies but organized the activity differently. The trunk and thigh muscles antagonistic to those used in the ankle strategy were activated in the opposite proximal-to-distal sequence, whereas the ankle muscles were generally unresponsive. This activation pattern produced a compensatory horizontal shear force against the support surface but little, if any, ankle torque. This pattern has been termed the hip strategy, because the resulting motion is focused primarily about the hip joints. Exposing subjects to horizontal surface perturbations while standing on support surfaces intermediate in length between the shortest and longest elicited more complex postural movements and associated muscle activation patterns that resembled ankle and hip strategies combined in different temporal relations. These complex postural movements were executed with combinations of torque and horizontal shear forces and motions of ankle and hip joints. During the first 5-20 practice trials immediately following changes from one support surface length to another, response latencies were unchanged. The activation patterns, however, were complex and resembled the patterns observed during well-practiced stance on surfaces of intermediate lengths.(ABSTRACT TRUNCATED AT 400 WORDS)

Multi-muscle synergies in an unusual postural task: quick shear force production

“…Alexandrov et al (2001) showed that, during the same translation of the center of mass (COM; 1 cm in 500 ms), the A-eigenmovement was associated with an almost four times larger COP excursion than the H-eigenmovement (6.5 cm as compared to 1.7 cm). On the other hand, due to the difference in the inertial properties of the body segments (see Alexandrov et al, 2001), the A-eigenmovement is more efficient for slow motions, while the H-eigenmovement is more efficient for fast COM motions (see also Horak and Nashner, 1986;Runge et al, 1999).…”

“…Indeed, shear force at the level of the support surface does not affect the COP location but is mechanically linked to acceleration of the center of mass of the body. A number of studies investigated EMG patterns in response to a sudden surface translation (Horak and Nashner 1986, Henry et al 1998, Torres-Oviedo and Ting 2007. In those studies, shear force changes were produced by the subjects but they were not explicit task components, rather the means to restore balance.…”

Modulation of ankle muscle postural reflexes in stroke: influence of weight-bearing load

“…The common mode rejection of the EMG system was 115 db and input impedance was 10 Gohm. The MG muscle was used for analysis in the backward translations and the TA muscle was used in the forward translations due to their role as primary recovery muscles for these movements (Horak and Nashner, 1986). The MG was selected because previous studies have reported that the reflex responses of this muscle are load-dependent in healthy individuals (Dietz et al 1989(Dietz et al , 1992.…”

Blood alcohol concentration at 0.06 and 0.10% causes a complex multifaceted deterioration of body movement control

“…However, during more challenging situations, this simplified movement strategy may become insufficient and is subsequently replaced by strategies where corrective movements are introduced about other joints such as at the knees or hip. Such movement patterns are indications of a multi-segmented movement pattern (Horak and Nashner, 1986). Body movement can be captured through kinematic analysis, which is sometimes employed to evaluate the severity or rehabilitation status of a disorder (Blackburn et al, 2003).…”