Muscle contractile properties by surface electrodes compared with those by needle electrodes

We measured the contractile properties of the first dorsal interosseous muscle by surface electrodes and needle electrodes during submaximal voluntary contraction. Twenty-seven single motor units were recorded from 5 volunteers by surface electrodes, which were confirmed by subsequent needle electrode recordings. Twitch tension ranged from 0.6 to 10.0 g with a mean of 3.7 g based on an averaging method triggered by surface myoelectric signals, and the tension calculated from the needle myoelectric signals ranged from 0.8 to 11.7 g with a mean of 3.6 g. Rise time ranged from 33.9 to 81.6 msec with a mean of 49.4 msec for the surface electrode recordings, and from 33.0 to 73.2 msec with a mean of 50.8 msec for the needle electrode recordings. The obtained data of surface electrode recordings indicated a high correlation with that of the needle electrode recordings, r = 0.92 (P < 0.001) for twitch tension and r = 0.91 (P < 0.001) for rise time. We also found a positive correlation between threshold force and twitch tension (by surface electrodes: r = 0.71, P < 0.05; by needle electrodes: r = 0.77, P < 0.05). Our studies indicate that it is possible to record single motor units by non-invasive surface electrodes provided that the unit has a threshold force of less than 2 kg.