No abstract

Redundancy is associated with the ability of the nervous system to select different interjoint coordinations and movement trajectories to achieve the same motor goal. The nervous system may coordinate multiple degrees of freedom (DF) by combining them in a task-specific way to control them as a unit or synergy. Some movements may be accomplished using only one synergy, whereas other movements may employ several synergies. To investigate the problem of interjoint coordination, we applied principal component (PC) analysis to eight types of movement in healthy male subjects: forward squats, backward squats, sideways squats, squats on one leg, walking three steps, stepping in place, going up a step, and going down a step. Angular changes in four DF were analyzed: thigh flexion-extension, knee flexion-extension, ankle flexion-extension, thigh abduction-adduction, with the former three DF investigated in all movements. For many movements, two synergies were sufficient to account for more than 95% of DF angular excursions. Squatting on one leg could be described using only one synergy (99%). The angle between the vectors representing PCs for movements produced with the right and left legs could be less than 10 degrees for some movements but could reach 25 degrees for other movements. The nervous system may thus use somewhat different interjoint coordinations while producing movements on the right and the left sides. The angle between the first PCs of different movements could be smaller than 10 degrees. Thus there may be a common but adjustable basic synergy that is used to produce different movements. Additional synergies provide the transition from one movement to another.

In addition to local biomechanical and reflex factors influencing muscle activation, global factors may be used by the nervous system to control all muscles in a coherent and task-specific way. It has been hypothesized that a virtual or referent (R) configuration of the body determined by muscle recruitment thresholds specified by neural control levels is such a factor. Due to the threshold nature of the R configuration, the activity of each muscle depends on the difference between the actual (Q) and the R configuration of the body. The nervous system modifies the R configuration to produce movement. One prediction of this hypothesis is that the Q and R configurations may match each other, most likely in movements with reversals in direction, resulting in a minimum in the electromyographic (EMG) activity level of muscles involved. The depth of the minima is constrained by the degree of coactivation of opposing muscle groups. Another prediction is that EMG minima in the activity of multiple muscles may occur not only when the movement is assisted but also when it is opposed by external forces (e.g., gravity). To verify these predictions, we analyzed EMG patterns of 16-21 functionally diverse muscles of the legs, trunk, and arms during jumping and stepping in place. One EMG minimum in the activity of all muscles regularly occurred near the apex of the jump. A minimum was also observed near the point of transition of the body from flexion to extension leading to a jump. During stepping in place, the activity of muscles of each side of the body was usually minimized near the beginning and near the end of the stance phase as well as during the maximum elevation of the foot. Since EMG minima occurred not only during gravity-assisted but also gravity-opposed movement reversals, it is concluded that neural factors (such as matching between the Q and R) rather than mechanical factors are responsible for minimizing the EMG activity in these movements.