Referent configuration of the body: a global factor in the control of multiple skeletal muscles

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.