Fig. 9.

Effects of aging on leg muscle spectral power response to accumulated fatigue at intermediate time scales during exercise and to residual fatigue at large time scales across consecutive exercise segments. The results are obtained following the same experimental protocol, data processing, and analysis procedure and statistical tests as shown for the group of young subjects in Fig. 7 (see methods). Error bars indicate SE. Yellow stars at left indicate the frequency bands, with statistically significant differences in spectral power between beginning and end. A: detailed analysis of the right leg muscle [vastus lateralis (VL)] spectral power contribution of different frequency bands in old subjects shows a very similar shape of the S(f) profile as well as a similar profile evolution from beginning to end of the exercise for individual subjects and their group average in both young (Fig. 7A) and old groups. The spectral power profile S(f) exhibits 2 distinct regimes of frequency bands with 1) different starting levels of power at the beginning of exercise, with higher power for regime 1 of low (5–25 Hz) and intermediate (25–50 Hz) frequencies and lower power for regime 2 of high (50–150 Hz) frequencies), and 2) distinct evolution of frequency regimes in response to accumulation of fatigue with progression of exercise, with more pronounced increase in power for regime 1 compared with regime 2. The observed leg muscle spectral power profile and its evolution with fatigue in the old group is consistent with the leg muscle spectral response of young subjects in Fig. 7, indicating universality in myoelectrical activation during exercise. However, in contrast to the young group, the spectral power in regime 1 of low- and intermediate-frequency bands in old subjects starts at significantly higher levels at the beginning of exercise 1 (with ∼30%, Mann-Whitney U test P < 0.05), indicating significantly elevated initial muscle tone after rest 1 as an effect of aging. Note also that the spectral power increase in regime 1 at the end of exercise 1 is less pronounced (80% for 5–25 Hz and 60% for 25–50 Hz in old vs. 240% and 170%, respectively, in young subjects), indicating a reduced response of the leg muscle activation in old subjects with accumulation of fatigue at intermediate time scales within an exercise. B: similar S(f) profile characteristics and evolution to exercise 1 are also observed for exercise 3. Significantly elevated spectral power in regimes 1 and 2 at the beginning of exercise 3 compared with exercise 1 (Wilcoxon matched-pairs test P < 0.04), with the same level of elevation for the young (Fig. 7b, right) and the old group (right), indicates an effect of residual fatigue from exercises 1 and 2 that is the same for both groups. This residual fatigue effect leads to a reduced response to accumulated fatigue during exercise 3 compared with exercise 1: less pronounced increase of 30% in the 5- to 25-Hz and 20% in the 25- to 50-Hz band in exercise 3 compared with 80% and 60% correspondingly in exercise 1 for old subjects (A and B, right) and ∼50% increase for the 5- to 25-Hz and 25- to 50-Hz bands for exercise 3 compared with 240% and 170%, respectively, for exercise 1 in young subjects (Fig. 7, A and B, right). Note also a similar level of power response in the regime 1 frequency bands at the end of exercise 3 and exercise 1 corresponding to the maximum leg muscle capacity (all exercise segments are performed until full exhaustion; see methods), which is significantly lower in old (A and B, right) compared with young subjects (Fig. 7, A and B, right) (Mann-Whitney U test, P < 0.04). In contrast to regime 1, there is no change in the spectral power of regime 2 of high (50–150 Hz) frequencies. Such episodes of increased evolution in regime 2 during exercise 3 are in contrast to regime 1, where there is a pronounced evolution resulting from residual fatigue at the beginning of exercise 3. This effect is not seen for exercise 1, where there is no effect of residual fatigue and where regime 2 spectral power increases with accumulation of fatigue during the exercise. This behavior of regime 2 in response to accumulated fatigue during exercise and in response to residual fatigue (exercise 3) is consistently observed in both the young (Fig. 7B) and old groups.