The effects of foot position and posture on activation of vastus medialis during rehabilitation exercises

Previous research has shown that muscle activation patterns of the anterior thigh influence knee joint mechanics and in particular, patellar tracking. Vastus lateralis and vastus medialis muscles are located on the lateral and medial sides of the quadriceps group respectively. They are the primary muscles responsible for maintaining patellar tracking through the patellofemoral groove in order to prevent knee conditions such as patellofemoral pain syndrome. If either the vastus lateralis or vastus medialis muscle has a predominant level of activation, then the patellar will tend to be drawn to one side of the patellofemoral groove. Rehabilitation protocols for treating patellar maltracking and patellofemoral pain often focus on improving muscle activation of vastus medialis. Vastus medialis is the smallest anterior thigh muscle responsible for leg extension, with lower activation levels often resulting in the patellar being drawn laterally. Although rehabilitation protocols may focus on medial muscle activation, both muscles are innervated by the same nerve, suggesting that improved vastus medialis muscle activation will also result in improved vastus lateralis muscle activation. To combat this problem it has been suggested that turning the foot out to 45 degrees may improve vastus medialis activation due to the interaction of the adductor magnus muscle. Another theory suggests that vastus medialis will be in a relatively higher position than vastus lateralis when seated, and therefore, the muscle needs to work harder to oppose gravity. Three exercises (leg press, squat and terminal leg extension) were assessed in order to determine if commonly employed rehabilitation protocols can improve vastus medialis activation. The exercises were performed with two foot positions (parallel/vertical and turned out to 45 degrees) at 5 0 % of 3 R M and the resultant E M G activity was normalised to a series of isometric M V C trials. Single differential electrodes were placed on the muscle bellies of vastus medialis and vastus lateralis, running parallel to the muscle fibres, with an earth electrode over the patella. Synchronised E M G and video data were collected to allow for the assessment of muscle activation during seated and standing posture exercises, open and closed kinetic chain exercises and for both foot positions to determine if any of these exercise conditions would result in improved vastus medialis muscle activation. The results showed that any improved vastus medialis activation also coincided with improved vastus lateralis activation, with both muscles contracting to perform leg extension motion. Higher levels of muscle activation generally occurred when the foot was in a parallel/vertical foot position, and the squat exercise resulted in higher levels of muscle activation than both of the seated exercises. The two closed kinetic chain exercises also demonstrated higher levels of muscle activation when the leg was in a flexed position, while the open kinetic chain exercise produced higher levels of muscle activation as the leg moved toward an extended position.