Linear and non-linear surface EMG/force relationships in human muscles. An anatomical/functional argument for the existence of both.

The EMG/force relations of various human muscles were examined over the full isometric force range using uniform experimental methods. The muscles chosen reflected varying fiber composition, fiber distribution and force generating patterns (recruitment/frequency coding). Our results generally suggest linear relations for muscles of near uniform fiber composition. Non-linear relations were obtained from muscles of mixed fiber composition. The non-linearity persisted despite variations in recording configuration, electrode placement and limb position. These were suggested by others to account for the lack of linearity in the basic EMG/force relation. The roles of uneven fiber distribution, motor unit potential amplitudes and possible supratetanic motor unit driving are discussed within the context of muscle force generating mechanisms as possible explanations for non-linear EMG/force relations.