Muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions

Abstract

Although high-velocity contractions elicit greater muscle fatigue in older than young adults, the cause of this difference is unclear. We examined the potential roles of resting muscle architecture and baseline contractile properties, as well as changes in voluntary activation and low-frequency fatigue in response to high-velocity knee extensor work. Vastus lateralis muscle architecture was determined in quiescent muscle by ultrasonography in 8 young (23.4±1.8 yrs) and 8 older women (69.6±1.1). Maximal voluntary dynamic (MVDC) and isometric (MVIC), and stimulated (80Hz and 10Hz, 500ms) isometric contractions were performed before and immediately after 120 MVDCs (240˚.s-1 , one every 2s). Architecture variables did not differ between groups (p�0.209), but the half-time of torque relaxation (T 1/2) was longer in older than young women at baseline (151.9±6.0 vs. 118.8±4.4 ms, respectively, p = 0.001). Older women fatigued more than young (to 33.6±4.7% vs. 55.2 ±4.2% initial torque, respectively; p = 0.004), with no evidence of voluntary activation failure (ΔMVIC:80Hz torque) in either group (p�0.317). Low-frequency fatigue (Δ10:80Hz torque) occurred in both groups (p<0.001), as did slowing of T 1/2 (p = 0.001), with no differences between groups. Baseline T 1/2 was inversely associated with fatigue in older (r 2 = 0.584, p = 0.045), but not young women (r 2 = 0.147, p = 0.348). These results indicate that differences in muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions. The inverse association between baseline T 1/2 and fatigue in older women suggests that factors related to slower muscle contractile properties may be protective against fatigue during fast, repetitive contractions in aging.

Muscle fatigue, bioenergetic responses and metabolic economy during load‐ and velocity‐based maximal dynamic contractions in young and older adults

“…Previous work has consistently shown greater fatigue in older than young adults in response to high‐velocity isokinetic (i.e., ≥240°s −1 ) or isotonic contractions (i.e., against a low‐load equivalent to ~20% MVIC) of the knee extensor muscles (Callahan & Kent‐Braun, 2011; Dalton et al., 2012, 2015; Fitzgerald et al., 2020; Sundberg, Kuplic, et al., 2018; Sundberg et al., 2019), as well as other muscles groups (Dalton et al., 2010; Wallace et al., 2016). A systematic review and meta‐analysis indicated greater muscle fatigue in older than young adults in studies where dynamic contractions were performed or muscular power was used as the index of fatigue (Christie et al., 2011).…”

“…We did not include a measure of voluntary activation in the current study, which could have provided useful insight as to the ability of participants to fully activate the knee extensor muscles while supine inside the MR scanner. This decision was based upon the weight of evidence in the literature indicating that deficits in voluntary activation do not likely contribute to age‐related differences in muscle fatigue (Dalton et al., 2010; Fitzgerald et al., 2020; Sundberg, Kuplic, et al., 2018). Thus, although we cannot rule out the presence of voluntary activation failure in the current study, this possibility seems unlikely given previous work in this area, as well as the robust associations between H + and fatigue observed in the present study in both groups.…”

“…The question of whether a lack of complete voluntary activation contributes to age‐related differences in fatigue has been investigated. Using stimulated isometric contractions before and after dynamic contractions, we and others have shown that greater fatigue in older than young adults in the knee extensor and plantar flexor muscles was not due to age‐related deficits in voluntary activation (Dalton et al., 2010; Fitzgerald et al., 2020; Sundberg, Kuplic, et al., 2018). Collectively, these results suggest that age‐related differences in fatigue are due to mechanisms localized within the muscle.…”

“…Participants were seated on a Biodex 4 dynamometer (Biodex System 4, Shirley, NY) with hip and knee angles of 135° and 110°, respectively, to mimic the position used for the contraction protocols in the MR scanner at visits 2 and 3. After securing the participants in this position, they completed 2–3 maximal voluntary isometric contractions (MVICs; 3–5 s each with 1 min rest between contractions) to determine their maximal torque (Fitzgerald et al., 2020). Next, participants completed two 40‐s trials of dynamic contractions (1 every 2 s) with their dominant leg to familiarize them with the contraction protocols to be performed at visits 2 and 3.…”

Impact of low‐load resistance exercise with and without blood flow restriction on muscle strength, endurance, and oxidative capacity: A pilot study

“…Briefly, the participants completed a short set of practice trials to familiarize themselves with the testing protocol, followed immediately by three maximal repetitions per test (1 min rest between repetitions). Finally, an isokinetic knee extensor endurance test comprised of 120 maximal voluntary contractions at 240°/s (one every 2 s for 4 min) was performed to measure the total work (i.e., a measure of absolute endurance) as previously described (Fitzgerald et al., 2020 ). In addition, we also determined the percent change in peak torque measured during the first 20 repetitions relative to the last 20 repetitions (i.e., relative endurance).…”