This study investigates the muscular synergies of the lower limb during human walking, estimated using a constrained canonical polyadic decomposition of muscle lever arms. The results confirm the existence of synergies complementary to synergies previously derived from non-negative matrix factorization of computed muscle activations or measured electromyographic signals. Furthermore, we present a method for computing musculotendon forces based on the use of 'synergistic forces', computed using lever arm synergies. This method is evaluated through the estimation of knee contact forces for one subject equipped with an instrumented knee prosthesis.