Differential effects of voluntary and involuntary activation on contractile characteristics of two human muscles

To compare the maximal rate of rise of torque (MRRT) of quadriceps femoris and adductor pollicis during voluntary and involuntary contractions, subjects performed voluntary isometric contractions as rapidly as possible over the full range of force-producing capacity. Involuntary contractions were evoked with single shocks and with trains of 10 pulses at 100 Hz at increasing voltages applied directly to the femoral and ulnar nerves. There were linear relationships between MRRT and absolute torque in both muscles during involuntary and voluntary contractions. At the same absolute torque, quadriceps femoris had a higher MRRT than adductor pollicis when both were voluntarily activated (P < 0.05). However, there was no difference in MRRT between these muscles during stimulated contractions. Compared with involuntary tetanic contractions, MRRT during voluntary contractions was the same in quadriceps femoris and was less in adductor pollicis (P < 0.05). These observations suggest that in activating some muscles, such as adductor pollicis, the central nervous system may adopt the strategy of a more gradual excitation to make the contractions more task appropriate.