Disturbances in the voluntary recruitment order of anterior tibial motor units in spastic paraparesis upon fatigue.

The recruitment order of motor units in the tibialis anterior muscle upon fatigue of tonic voluntary contraction was studied in 20 patients with severe spastic paraparesis. An electromyographic technique for secure identification of single motor units was used. Before fatigue the recruitment order is stable and low-frequency units are recruited before high-frequency units; this recruitment pattern agrees with that in normal voluntary activity. When fatigue appears, however, the recruitment order becomes indefinite and high-frequency units can be recruited before low-frequency units; this recruitment pattern agrees with that in normal phasic voluntary activity. Finally, all voluntary activation power disappears, even in the case of units which have never been active. Contraction ability and original recruitment order are restored upon rest but also upon tonic reflex support of the voluntary drive. Whether the fatigue reaction may be due to insufficient gamma motoneurone innervation and whether it is related to spasticity are discussed. The practical physiotherapeutic implications are reviewed.