Effect of old age on human skeletal muscle force-velocity and fatigue properties

Abstract

It is generally accepted that the muscles of aged individuals contract with less force, have slower relaxation rates, and demonstrate a downward shift in their force-velocity relationship. The factors mediating age-related differences in skeletal muscle fatigue are less clear. The present study was designed to test the hypothesis that age-related shifts in the force-velocity relationship impact the fatigue response in a velocity-dependent manner. Three fatigue protocols, consisting of intermittent, maximum voluntary knee extension contractions performed for 4 min, were performed by 11 young (23.5 ± 0.9 yr, mean ± SE) and 10 older (68.9 ± 4.3) women. The older group fatigued less during isometric contractions than the young group (to 71.1 ± 3.7% initial torque and 59.8 ± 2.5%, respectively; P = 0.02), while the opposite was true during contractions performed at a relatively high angular velocity of 270°·s(-1) (old: 28.0 ± 3.9% initial power, young: 52.1 ± 6.9%; P < 0.01). Fatigue was not different (P = 0.74) between groups during contractions at an intermediate velocity, which was selected for each participant based on their force-velocity relationship. There was a significant association between force-velocity properties and fatigue induced by the intermediate-velocity fatigue protocol in the older (r = 0.72; P = 0.02) and young (r = 0.63; P = 0.04) groups. These results indicate that contractile velocity has a profound impact on age-related skeletal muscle fatigue resistance and suggest that changes in the force-velocity relationship partially mediate this effect.

Fig. 1.

Force-velocity relationships in young and older groups. Shown is mean ±SE torque produced across the range of angular velocities for young and older groups, expressed as a percentage of peak isometric torque [maximal voluntary isometric contractions (MVIC)]. A significant age-velocity interaction (P < 0.01) indicates a downward shift in the force-velocity relationship in the older group compared with the young group. Mean data are absent for older women at 400°·s⁻¹ because 3 of 10 individuals failed to achieve this velocity during maximal voluntary dynamic contractions (MVDC). The dashed, vertical lines represent the velocity at which torque production was 50% of maximum voluntary isometric contraction torque (V₅₀) for each group, which was lower in older (200 ± 12°·s⁻¹) compared with young (250 ± 18°·s⁻¹) women (P = 0.03).

Fig. 2.

Effect of velocity on the relationship between muscle torque and size. Associations are shown between torque production at 3 velocities (0, 120, and 270°·s⁻¹) and fat-free muscle cross-sectional area (CSA; cm²). Associations were significant at all velocities in the young (see text for details). In the older participants, the association was strongest for isometric contractions and weakened as the velocity increased.

Fig. 3.

Changes in torque or power during fatigue protocols. Changes are shown in response to MVIC (A), MVDC at the intermediate velocity (MVDC_int; B), and MVDC at the high velocity (MVDC_hi; C) for young and older groups. Older subjects fatigued less than the young during MVIC (P = 0.02; A), but more during MVDC_hi (P < 0.01; C). There was no difference in fatigue between groups during MVDC_int (P = 0.74; B). Values are expressed relative to baseline values.

Fig. 4.

Fatigue for each protocol. Fatigue at the end of each of three 4-min contraction protocols for young (solid bars) and older (open bars) groups is shown. Values are means ± SE. Differences in fatigue were observed between age groups during MVIC (*P = 0.02) and MVDC_hi (*P < 0.01), whereas no differences were found between groups during MVDC_int (P = 0.74). The young group fatigued to a similar extent in each protocol, whereas older experienced more fatigue in the dynamic contractions, with the greatest fatigue induced during MVDC_hi.

Fig. 5.

Associations between force-velocity characteristics and fatigue. Fatigue (torque, %initial) during repeated MVDC_int was plotted against V₅₀. Linear regression analysis revealed an association between these variables for both young [solid line (y = −0.17x + 95.8), r = −0.63; P = 0.04] and older [dashed line (y = −0.37x + 124.2), r = −0.72; P = 0.02] groups, suggesting that force-velocity characteristics may explain a significant portion of knee extensor fatigue in humans.