THE EFFECT OF OLD AGE ON PERCEIVED EFFORT AND HEART RATE DURING AN ISOMETRIC FATIGUE TASK

Abstract

In addition to age-related changes in the neuromuscular system of aged humans, alterations in the perception of effort can affect the comparative fatigability of old versus young subjects during voluntary fatigue tasks. The elbow flexors of 7 young (24 ± 2y) and 6 old men (84 ± 1y) were fatigued using repeated (3s on, 2s off) isometric contractions at 60% MVC until force was reduced to less than 60%. Time to fatigue was not different between age groups (4–5 min). At rest and at 1 min intervals, measures of neural and contractile function were made. During the fatigue task, ratings of perceived exertion (RPE), using a modified Borg scale (0–10), and heart rate (HR) were recorded. RPE and HR measures were compared between groups at ∼15, 30, 50, 70, 87 and 100% of the fatigue time. Although HR was greater at each time point and demonstrated a larger range of increase in the young subjects, both groups experienced the same relative increase in HR when data were normalized to the age-predicted HRmax. At the end of the fatigue task, HR was ∼62% of the age-predicted HRmax for both groups. Although greater RPE values were reported for the old men at 15% (5.2 vs 2.4) and 30% (6.0 vs 3.7) of fatigue time, there were no age-related differences for the remaining fatigue time. By the end of the fatigue protocol all subjects reported the maximal RPE value (10). Muscie activation, as assessed by twitch interpolation, surface EMG, and the ratio of 50Hz tetanic tension to MVC force at fatigue were not affected by age, suggesting that fatigue was not due to a central limitation. Thus, despite similar relative HR responses in both groups throughout the fatigue protocol, the old men had a greater perception of effort during the submaximal contractions in the first ∼30% of the protocol. However, the differences in perceived effort were not accompanied by age-related impairments in neural and contractile measures at the end of the protocol, suggesting that this initial elevated perceived effort did not influence fatigue during this task.