Electrical Nerve Stimulation Elicits Intensity-dependent Changes in Force Steadiness in Young and Older Adults

Abstract

When individuals attempt to match a submaximal target force by performing a steady isometric contraction, the fluctuations in force are often quantified as force steadiness (coefficient of variation [CV] for force) to provide an index of the neural drive to muscle. PURPOSE: To compare force steadiness of the wrist extensors during evoked and voluntary submaximal contractions in young and older adults. METHODS: Thirteen young (5 men, 25 ± 4 yrs) and 12 older (7 men, 78 ± 5 yrs) adults participated in a 1-hr protocol that involved maximal voluntary contractions (MVCs) and voluntary and evoked isometric contractions to match a 10% MVC target force. Force steadiness during the voluntary contraction was compared with that evoked by wide, high-frequency (1 ms pulses at 100 Hz) and narrow, low-frequency (0.2 ms pulses at 50 Hz) neuromuscular electrical stimulation (NMES), and a voluntary contraction with superimposed submotor transcutaneous electrical stimulation (TENS). CV for force was compared between age groups with unpaired t-tests and within age groups by paired t-tests. RESULTS: CV for force was less for young adults (1.82 ± 0.43%) than older adults (2.80 ± 1.08%) during the voluntary contraction with the wrist extensors (p<0.03). Force steadiness did not differ between age groups during the evoked contractions. However, older adults were steadier during both types of NMES (wide: 2.01 ± 0.67%, p<0.04; narrow: 1.69 ± 0.62%, p<0.02) than during the voluntary contractions (2.80 ± 1.08%). Concurrent TENS did not influence force steadiness for older adults, but young adults were less steady during TENS (2.41 ± 1.02%, p<0.03) than during the voluntary contraction by itself (1.82 ± 0.43%). CONCLUSION: The improvement in force steadiness for older adults during the NMES-evoked contractions indicates that the age-associated decline in force steadiness is attributable to changes in the neural drive to muscle, rather than the mechanical properties of muscle. In contrast, the decline in force steadiness for young adults during concurrent TENS suggests that heightened sensory feedback compromised the neural drive to muscle during the steady contraction.