Velocity Dependence of Age-Related Muscle Fatigue Resistance

Abstract

The effect of old age on skeletal muscle fatigue resistance is debated due to variations in study findings. While multiple factors likely contribute to disparate results in the literature, the velocity of contractions used to induce fatigue may have a significant influence on age-related fatigue characteristics. Both central motor and intrinsic muscle characteristics change in old age; the extent to which these changes explain the apparent velocity dependence of fatigue outcomes is not known. PURPOSE: To test the hypothesis that an age-related shift in the torque-velocity curve mediates age-related fatigue resistance during rapid, dynamic contractions. METHODS: Knee extension strength and fatigue were tested on 4 separate days in healthy, untrained younger (YW, 23.5 ± 0.9 years [mean ± se], n=10) and older (OW, 68.9 ± 1.4 years, n=10) women. First, the torque-velocity relationship was determined for each participant, using a range of angular velocities (0-400 deg·s-1). Fatigue resistance (maintenance of torque or power expressed as % initial) was assessed in 3 protocols consisting of 4 minutes of intermittent maximal voluntary contractions. Fatigue was compared between YW and OW during isometric contractions (IM), dynamic contractions performed at high velocity (270 deg·s-1, HV), and at an intermediate velocity (IV) corresponding to the velocity eliciting 75% peak torque from the torque-velocity analysis. RESULTS: OW had relatively lower torques with increasing angular velocity compared with YW, indicating a downward shift in the torque-velocity relationship in the old (p = 0.05). OW fatigued less than YW during IM (71.1±3.7% initial vs. 59.7±2.5%, respectively; p = 0.02), but to a greater extent during HV (28.8% vs. 54.4%, p = 0.01). During the IV protocol, fatigue was not different between OW (52.6%) and YW (52.7%, p = 0.98). CONCLUSIONS: Age-related fatigue resistance is highly dependent on contractile velocity. These results should clarify some of the discrepancies in the literature about age-related changes in fatigue resistance. Further, greater fatigue at higher velocities has implications for physical function in the elderly. Support: NIH R01 AG21094