Lower Extremity Coordination During Static Stance: The Unique Role of the Vastii Muscles

Abstract

Muscle synergies have been examined during a variety of tasks and many theories about muscle coordination have been proposed. Some studies have been have been performed while pushing against load cells and other have been performed while standing and responding to a perturbation. In this study we examined muscles during a standing posture but with the bottoms of the feet loaded, which mimics many sports (e.g. skiing, rollerblading). PURPOSE: To examine neural control of muscle synergies about the knee during a standing posture. METHODS: Electromyographic activity of nine muscles was examined while subjects stood with their feet rigidly affixed to a force plate (a separate force plate was used for each foot). Subjects were given visual feedback of their forces on the force plate and were asked to use this information to produce specific static postures. Subjects were instructed to equally divide their body weight between the force plates and simultaneously produce forces tangential to the plane of the force plate (20 degree increments in the forward-backward-medial-lateral plane) with their lead foot. The foot defined as the lead foot was changed half-way through the experiment and the order of lead foot selection was randomized relative to each subject's dominant foot. RESULTS: The gastrocnemii were very active when subjects tried to push their foot forward. The adductors were active when subjects tried to move their foot medially and the gluteus medius was active when the foot was pushed laterally. These results were not surprising as they correspond to the natural activity of the muscles. However, both vastii did not follow this expected pattern and were very active when subjects tried to push their legs backwards and were inactive when subjects tried to move their legs forward. The same was not true for the rectus femoris, which was active during both forward and backward pushes, but most active during forward. CONCLUSIONS: These results are very surprising. The vastii are not often observed to act synergistically with the hamstrings except during co-contraction. The lack of vastii and hamstrings activity during forward pushes indicates that this was not a typical co-contraction activation pattern. Further investigation is warranted of the use of the vastii muscles during these postural control tasks. NIH Grant R01 AR046386