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 (T1/2) 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 T1/2 (p = 0.001), with no differences between groups. Baseline T1/2 was inversely associated with fatigue in older (r2 = 0.584, p = 0.045), but not young women (r2 = 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 T1/2 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.

Fig 1. Ultrasound image from a representative young woman showing estimated fascicle length (eFL), muscle thickness (MT), and pennation angle (ϴ).

Fig 2. Representative baseline series of contractions.

Data are from one young woman.

Fig 3. Age-related differences in muscle fatigue and recovery.

Older women experienced greater fatigue (*p = 0.005). There was a group×time interaction (†p = 0.03) for recovery of peak torque over the first 5 min, but both groups fully recovered within 5 min following contractions. Data are mean±SE. The dark and light grey shaded boxes represent the contraction protocol and recovery period, respectively.

Fig 4. Fatigue and recovery of contractile properties in young and older women.

A) 10:80-Hz torque at baseline, fatigue, and throughout recovery. The 10:80-Hz torque ratio decreased with fatigue (main effect of time: #p = 0.001) and increased over the first 5 minutes of recovery in both groups (main effect of time: #p≤0.01). B) T_1/2 at baseline, fatigue, and throughout recovery. Relaxation was slower in older compared with young women at baseline, fatigue, and following 5 minutes of recovery (main effect of group: *p≤0.04). In both groups, T_1/2 slowed with fatigue and recovered within 5 minutes of recovery in both groups (main effect of time: #p = 0.001), with no groupxtime interactions. Data are mean±SE. The dark and light grey shaded boxes represent the contraction protocol and recovery period, respectively.

Fig 5. Recovery of maximal voluntary isometric and stimulated torque measures in young and older women.

A) Maximal voluntary isometric torque throughout recovery. Torque decreased less in young than older adults in response to fatigue, but both groups recovered within 5 min. B) 80-Hz torque throughout recovery. The torque elicited by an 80-Hz stimulation was only different after 30 min recovery, such that it was greater in young than older adults. C) 10-Hz torque throughout recovery. No group differences were observed at any time point.

Fig 6. Associations between half relaxation time and fatigue.

Fatigue (peak torque, % initial) was plotted against half relaxation time (T_1/2) at baseline. Linear regression analysis revealed an association between these variables for older (r² = 0.584, p = 0.045), but not young women (r² = 0.147, p = 0.348), suggesting the age-related slowing of T_1/2 may prevent excessive muscle fatigue in older women.