Muscle synergies and joint linkages in sit-to-stand under two initial footpositions

Objective. The purpose of this study was to investigate the activation patt ern of six lower limb muscles during standing up from two initial foot posi tions and to examine relationships between the onsets of muscle activity an d the dynamics of the action. In contrast to earlier studies, an algorithm was used to measure onsets and duration of muscle activity. Method. The activity of biceps femoris, rectus femoris, vastus lateralis, t ibialis anterior, lateral gastrocnemius and medial soleus was studied as ni ne male subjects between the ages of 20 and 35 stood up from two initial fo ot positions (foot forward, foot back). Subjects sat on a height-adjustable seat with their right foot on a force platform and were videotaped as they stood up. Results. When the feet were forward there were significant increases in mov ement duration, displacement and velocity of trunk segment flexion at the h ips reflecting the increased distance the body mass must be moved forward. The sequence of onsets and the duration of muscle activity, however, were c onsistent across the two conditions. Onsets of extension at hip, knee and a nkle, however, correlated with different muscle activity depending on the f oot position. Conclusion. The results suggest that muscles working over different joints are temporally linked and that different muscle linkages may be utilised in the lower limb to coordinate the sit-to-stand action under different funct ional demands.