The modulation of force steadiness by electrical nerve stimulation applied to the wrist extensors differs for young...

We compared the modulation of force steadiness by different types of electrical nerve stimulation in young (n = 13, 25 ± 4 years) and older (n = 12, 78 ± 5 years) adults.

The protocol involved four types of isometric contractions with the wrist-extensor muscles at 10% of the maximal force. Three of the contractions involved electrical nerve stimulation that comprised two forms of neuromuscular electrical stimulation (NMES) to evoke muscle contractions and a voluntary contraction with superimposed transcutaneous electrical nerve stimulation (TENS) at an intensity less than motor threshold.

The coefficient of variation (CV) for force during voluntary wrist extension was less (P = 0.03) for young (1.82 ± 0.43%) than older adults (2.80 ± 1.08%). The CV for force did not differ between age groups during the three types of electrical nerve stimulation but was reduced relative to the value observed during voluntary wrist extension for older adults. In contrast, the CV for force increased during the voluntary contraction with superimposed TENS for young adults but not for older adults. Moreover, there were significant negative correlations in older adults between the CV for force during the voluntary contraction and its decrease with electrical nerve stimulation.

Differences in the CV for force between the evoked and voluntary contractions for the two age groups suggest that the variance in common synaptic input to motor neurons during steady voluntary contractions with the wrist extensors is greater for older adults than young adults.