Fig. 14.

Back muscle spectral power transition from the down to up phase of a single squat movement and response to accumulated fatigue during exercise. Right back muscle [erector spinae (ES)] spectral power transition from the down and up phase during a squat at the beginning and end of exercise 1 in young (A) and old subjects (B). In contrast to the leg muscle (see Fig. 13), the back muscle spectral power for young subjects (A) does not increase with transition from the down to up phase for any frequency band either at the beginning or end of exercise, reflecting the different role back muscle plays during the squat compared with the leg muscle; whereas force generation is the primary function of the leg muscle, the main role of the back muscle is trunk stabilization. The back muscle spectral power profile and its lack of down/up transition is also observed with accumulation of fatigue at the end of exercise, but with an increased total power that is more pronounced in the down than in the up phase (see Fig. 11B, bottom). Accordingly, young subjects exhibit a higher ratio of the down/phase spectral power at the end vs. beginning of exercise compared with the same ratio for the up phase (Wilcoxon matched-pairs test comparing end/beginning ratio for the down squat phase vs. the same ratio for the up squat phase gives P < 0.01; C). As for the young group (A), back muscle spectral power for old subjects (B) does not increase with transition from the down to up squat phase at the beginning of exercise (Wilcoxon matched-pairs test comparing down vs. up phase at the beginning of exercise gives P > 0.3 for all frequency bands). However, with progression of exercise, there is a significant increase in total spectral power only for the up phase (Wilcoxon matched-pairs test comparing the up phase at the beginning vs. end of exercise gives P < 0.02 for the low- and intermediate-frequency bands; also see Fig. 12B, bottom). Similarly to the leg muscle in old subjects (Fig. 13B), the down phase spectral power for the back muscle (B) does not increase from beginning to end of exercise, indicating that reduced back muscle response to accumulated fatigue during the lengthening contractions of the squat movement and the down phase ratio or the old group (C) is close to 1 for all frequency bands. Error bars in plots represent the group average SE. Red arrows indicate statistically significant changes in spectral power from down to up squat phase. Results for the left back (not shown) are consistent with the results for the right back muscle shown.