Author(s): VIKMOEN, O., TEIEN, H.K.2, TANSØ, R.2, AANDSTAD, A.3, LANDER, E.1, CUMMING, K.T.1, HELKALA, K.4, RAASTAD, T.1, Institution: NORWEGIAN SCHOOL OF SPORTS SCIENCES: NORGES IDRETTSHOGSKOLE, Country: NORWAY, Abstract-ID: 2217

INTRODUCTION:
Strenuous military field exercises (MFE) causes muscle mass loss and long-lasting physical performance impairments. However, their effect on muscle efficiency during submaximal work and associated cellular muscle changes are poorly understood. Furthermore, the effects of MFE in women are not well documented. This study aimed to investigate the impact of strenuous MFE on efficiency during submaximal work, muscle aerobic enzyme content, muscle glycogen content, and to identify potential sex differences in these aspects.
METHODS:
Efficiency was tested on a cycle ergometer (200 W for men and 150 W for women) and CMJ on a force plate in 10 men and in 8 women before (T0), and 1 (T1), 7 (T7), and 14 (T14) days after a 10-days MFE consisting of rigorous physical activity with food and sleep restriction. Muscle biopsies from m. vastus lateralis were collected at T0, T1, and T7.
RESULTS:
Countermovement jump height was decreased with 6.7±6.2% (p≤0.001) at T1 and was still decreased at T14 with 4.1±4.7% (p=0.003) for the sexes combined. There were no sex differences in the changes from T0
The men improved gross efficiency (GE) at T1 (0.7±0.7 pp, p=0.019), whereas it decreased in women (–0.9±1.0 pp, p=0.042). The change was different between sexes (p=0.002), and these group differences persisted at T7 (p=0.022). At T14, GE returned to pre-values in women, while it remained improved in men (0.4±0.6 pp, p=0.038), with no sex differences in the changes from T0.
Heart rate (HR) decreased at T1 in both men (–12.9±3.4%, p≤0.001) and women (–4.6±4.7%, p=0.030), with a larger reduction in men (p≤0.001). HR remained reduced in men throughout recovery but returned to pre-values at T7 in women. Blood lactate concentration did not change significantly in either sex, though women showed a larger (p=0.037) non-significant increase at T1 (29±44%, p=0.198) compared to a non-significant decrease in men (–11±31%, p = 0.109). There were no sex differences in changes in respiratory exchange ratio or perceived exertion during submaximal cycling.
At T7, the sexes combined reduced the content of citrate synthase (–23±28%, p=0.005) and tended to reduce to content of COX4 (–18±43%, p=0.056), with no sex differences.
The changes in muscle glycogen content were similar in Type I and II fibers for both sexes. At T1, glycogen content in the muscle fibers combined was reduced in both men and women with 8±10% (p=0.033) and 8±8% (p=0.02), respectively. At T7, the percentage change from T0 was different between sexes (p=0.009), where the men had increased glycogen content with 10±6% (p≤0.001), while the women returned to T0 values.
CONCLUSION:
A demanding MFE resulted in improved cycling efficiency in men but impaired efficiency in women. The reasons for this sex difference remain unclear, though it is likely unrelated to changes in aerobic enzyme levels or glycogen stores, which were similar between men and women