Investigating the role of foot placement on the muscular forces of knee extensors in horizontal leg press: A static optimization approach

Horizontal leg press is a basic weight training exercise to strengthen extensor muscles of knee joint. Considering the specific purpose of this training, it´s performed with different techniques. One of the major debates about performing this task concerns foot placement on the foot-plate. In this study, in order to investigate the effect of foot placement on the muscle contraction of knee extensors, a 3-DOF musculoskeletal model of lower-extremity was developed. This model was comprised of 40 skeletal muscles of lower-extremity. Each muscle was modeled with a phenomenological Hill-based model in accompanied with a rigid tendon. Considering the position of feet on the foot-plate (high or low), two types of experiments were designed: Leg Press High (LPH) and Leg Press Low (LPL). 6 healthy volunteers (2 women and 4 men) participated in this study. Experiments were performed at 30% and 70% of each person´s MVC (the maximum weight in which a person could perform the exercise correctly) on the LPL and LPH protocols. Amalgamating the mean values of kinematic data and a constrained static optimization (sum of squared activations as the objective function and discritisized state equations of the multibody system as constraints), the required activation level of each muscle was obtained. The results showed a significant difference between the obtained muscle forces for knee extensors (vastus intermedius, vastus lateralis, vastus medialis, and rectus femoris) in each technique (p<;0.05). The muscle forces obtained in LPH technique were significantly smaller from the same ones in LPL. This observation suggests that if strengthening the knee extensors is of desire, LPH technique is not the correct format for performing the exercise. Another important spotlight in the results concerned the activation patterns of knee extensors. Despite the synergic behavior of all the extensors, because of the different operating range of these muscles, dissimilarity between activation pa- terns was observed.