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.
Highlights
There was no difference between groups in the maximum rate of torque development (RTD), but T1/2 was slower in older compared with young (Table 2)
Values are mean±SE. 95% CI, 95% confidence intervals for differences in group means; MT, muscle thickness; eFL, estimated fascicle length; Maximal voluntary dynamic (MVDC), maximal voluntary dynamic contraction; MVIC, maximal voluntary isometric contraction; RTD, maximum rate of torque development; T1/2, torque half-relaxation time
This study was designed to evaluate whether age-related differences in muscle architecture, voluntary activation, or contractile properties might be potential mechanisms for the greater muscle fatigue observed in older adults in response to high-velocity muscle contractions
Summary
Maximal torque and power production are lower in older compared with younger adults [1,2,3,4,5,6], due to factors such as a loss of muscle mass [5, 7,8,9,10], slower maximal contraction velocities.
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