The quantification of muscle efforts can be very helpful in achieving important goals in clinical or sports biomechanics. However, the estimation of muscle moments and force requires to solve the problem of redundancy inherent in the musculoskeletal system. This paper covers the steps involved in developing an original EMG-based optimization methodology for estimating opposing muscle group moments and individual muscle force during static contractions or dynamic movements, even in the presence of fatigue. The combined use of numerical optimization and EMG data greatly improved our confidence in the estimates of agonist and antagonist moments, as well as muscle force. The generalization of the proposed method might be of particular interest in biomechanics of sport and exercise.