Adaptations in maximal motor unit discharge rate to strength training in young and older adults

Six young (mean = 23 years) and 6 older (mean = 76 years) adults participat ed in isometric resistance training 5 days/week for 6 weeks. The task invol ved isometric fifth finger abduction. Maximal motor unit discharge rates (M UDRs) were obtained from the abductor digiti minimi of each hand at 0, 2, 1 4, and 42 days of training using a quadrifilar needle electrode and automat ic spike recognition software. In agreement with previous findings, maximal MUDR at baseline was significantly lower in older adults (P < 0.001), aver aging 51.5 (+/-17.13) Hz in young and 43.3 (+/-14.88) Hz in older adults. I n response to resistance training, maximal Voluntary force increased 25% in young and 33% in older subjects (P < 0.001). Maximal MUDR increased signif icantly (11% young, 23% older) on day 2 [F(3,36) = 2.58, P < 0.05], but in older subjects returned to baseline levels thereafter. These adaptations in abductor digiti minimi MUDR suggest a two-part response to strengthening f ifth finger abduction: early disinhibition followed by altered MU activatio n. (C) 2001 John Wiley & Sons, Inc.