Task Specific and General Patterns of Joint Motion Variability in Upright- and Hand-Standing Postures

 The preservation of static balance in both upright- and hand-stance is maintained by theprojection of center of mass (CM) motion within the region of stability at the respective base ofsupport. This study investigated, from a degrees of freedom (DF) perspective, whether the stabilityof the CM in both upright- and hand-stances was predicted by the respective dispersion and time-dependent regularity of joint (upright stance—ankle, knee, hip, shoulder, neck; hand stance—wrist,elbow, shoulder, neck) angle and position. Full body three-dimensional (3D) kinematic data werecollected on 10 advanced level junior female gymnasts during 30 s floor upright- and hand-stands.For both stances the amount of the dispersion of joint angle and sway motion was higher than that ofthe CM and center of pressure (CP) with an inverse relation to time-dependent irregularity (SampEn).In upright-standing the variability of neck motion in the anterior–posterior direction was significantlygreater than that of most joints consistent with the role of vision in the control of quiet upright posture.The findings support the proposition that there are both task specific and general properties to theglobal CM control strategy in the balance of upright- and hand-standing induced by the differentactive skeletal-muscular organization and the degeneracy revealed in the multiple distributionalvariability patterns of the joint angle and position in 3D.