Historical update and new developments on the EMG-force relationships of skeletal muscles

The EMG-force relationships of skeletal muscles are of significant interest to the orthopedic-biomechanics researcher and have been a topic of interest and controversy for the last three decades. We present the developments reported in the literature, from the early 1950s, relevant to this topic, outlining the details of the controversy and providing additional new information resulting from recent studies. New data clearly illustrate that the EMG-force relationships are dependent on the firing rate and recruitment control strategy used by a muscle. Muscles that use motor unit recruitment to obtain the initial 50% of their maximal force, and use firing rate increase to complement the remaining 50%, have a nearly linear EMG-force relationship. Muscles that use recruitment to obtain 60% and up to 100% of their maximal force demonstrate progressive increase in non-linearity of their EMG-force curves. A clear warning is issued against the indiscriminate use of EMG as a representative of muscle force. The recruitment pattern of the muscle under consideration should first be obtained in such a way that the appropriate EMG-force curve is selected.