Random motor unit activation by electrostimulation

Whether the involvement of motor units is different between surface neuromuscular electrostimulation and voluntary activation remains an unresolved issue. The aim of this pilot study was to verify if motor unit activation during electrostimulation is nonselective/random (i.e., without obvious sequencing related to fibre type), as recently suggested by Gregory and Bickel [6]. Sixteen healthy men randomly performed submaximal isometric contractions (10-s duration) of the quadriceps femoris muscle at 20, 40 and 60 % of maximal voluntary torque under both stimulated and voluntary conditions. During the contractions, paired stimuli were delivered to the femoral nerve (twitch interpolation technique) and the characteristics of the superimposed doublet were compared between the two conditions. For each torque level, time-to-peak torque was significantly longer (p range = 0.05 - 0.0002) during electrostimulation compared to voluntary contractions. Moreover, time-to-peak torque during voluntary trials decreased significantly when increasing the torque level from 20 to 60 % of maximal voluntary torque (p range = 0.03 - 0.0001), whereas it was unchanged during electrostimulation. In conclusion, over-the-muscle electrostimulation would neither result in motor unit recruitment according to Henneman's size principle nor would it result in a reversal in voluntary recruitment order. During electrostimulation, muscle fibres are activated without obvious sequencing related to fibre type.