Contractile properties of human ankle muscles determined by a systems analysis method for the EMG-force relationship

In order to determine painlessly and non-invasively the contractile properties of human skeletal muscles, the systems analysis method was applied to the relationship between the electromyogram (EMG) of agonist muscles and the isometric force produced by the voluntary contraction of the ankle plantarflexors and dorsiflexors. The subject's task was to produce force impulses at six different ankle positions for both muscles. The EMG(input)-force(output) relationship was approximated using the linear second-order system, and the contraction time (CT), half-relaxation time (HRT) and impulse response (IMP) were estimated. CT, HRT and IMP were found to increase as the muscles were stretched. The CT and HRT were longer for the plantarflexors than for the dorsiflexors, except at extremely plantarflexed positions. These results show qualitative agreement with results obtained by supramaximal electrical stimulation. It is suggested that the method of systems analysis is useful in identifying differences in muscle contractile properties under various force-production conditions.