Age-related decline in rate of torque development is accompanied by lower maximal motor unit discharge frequency during fast contractions

Abstract

The aim of this study was to investigate the association between the rate of torque development and maximal motor unit discharge frequency in young and elderly adults as they performed rapid submaximal contractions with the ankle dorsiflexors. Recordings were obtained of the torque exerted by the dorsiflexors during the isometric contractions and the surface and intramuscular electromyograms (EMGs) from the tibialis anterior. The maximal rate of torque development and integrated EMG (percentage of total EMG burst) at peak rate of torque development during fast contractions were lower in elderly than young adults by 48% (P < 0.05) and 16.5% (P < 0.05), respectively. The young adults, but not the elderly adults, exhibited a positive association (r2 = 0.33; P < 0.01) between the integrated EMG computed up to the peak rate of torque development and the maximal rate of torque development achieved during the fast contractions. These age-related changes during fast voluntary contractions were accompanied by a decline (P < 0.001) in motor unit discharge frequency (19, 28, and 34% for first 3 interspike intervals, respectively) and in the percentage of units (45%; P < 0.05) that exhibited double discharges (doublets) at brief intervals (<5 ms). Because aging decreased the maximal rate of torque development of fast voluntary contractions to a greater extent ( approximately 10%) than that of an electrically evoked twitch, collectively the results indicate that the age-related decline in maximal motor unit discharge frequency likely limit, in addition to the slowing of muscle contractile properties, the performance of fast voluntary contractions.

Age-related changes in the rate of muscle activation and rapid force characteristics

“…The present finding that the relative capacity for rapid force production was lower in the old compared to the young men is in agreement with previous studies examining the dorsiflexors (Klass et al 2008) and the leg extensors (Korhonen et al 2006). However, other previous studies have reported that relative RTD or RFD is preserved in older men for the plantar flexors (Thelen et al 1996), dorsiflexors (Thelen et al 1996), leg extensors , and leg flexors .…”

“…Previous studies have examined the age-related reductions in maximal and rapid force capabilities between young and old adults for the leg extensors (Izquierdo et al 1999;Ditroilo et al 2010;Thompson et al 2013), leg flexors , dorsiflexors (Thelen et al 1996;Klass et al 2008), and plantar flexor muscles (Thelen et al 1996;Bemben et al 1991). Collectively, these studies have reported 14-46 % decreases in maximal isometric strength (PF or peak torque) and 25-64 % decreases in the peak rate of force/torque development.…”

“…A potential factor influencing the age-related reduction in relative RFD may be attributed to changes in motor unit recruitment and/or activation patterns (Vandervoort and McComas 1986;Klass et al 2008). For example, Klass et al (2008) have reported that the age-related reduction in RTD was accompanied by a decrease in motor unit discharge rates and the percentage of motor units that discharged with doublets in the old men. However, the 5 The ratio of rate of force development (RFD) variables at 50 and 100-200 ms to peak force (PF) for the young, middle-aged and old men.…”

“…Previous studies have reported age-related reductions in maximal voluntary isometric strength for the muscles of the lower limb such as the leg extensors (Izquierdo et al 1999; Thompson et al 2013), leg flexors , dorsiflexors (Klass et al 2008;Thelen et al 1996), and plantar flexors (Bemben et al 1991;Thelen et al 1996). However, it is often demonstrated that rapid force characteristics [i.e., rate of force or torque development (RFD or RTD)] may decrease during aging to a greater magnitude (39-64 %) than maximal isometric strength (29-46 %; Ditroilo et al 2010;Izquierdo et al 1999; Thompson et al 2013).…”

“…Although several previous authors (see review by Klass et al 2007) have examined the effects of aging on voluntary muscle activation during maximal strength assessments, fewer studies have evaluated the effects of aging on the rate of muscle activation and its influence on RFD. For example, Klass et al (2008) reported that the age associated decline in peak RTD was accompanied by lower motor unit discharge frequencies in the old men. However, contrasting findings have been reported showing a lack of age-related impairments in the rate of muscle activation between middle-aged and healthy old men (Clark and Fielding 2012;Clark et al 2011).…”