Author(s): MARCIANO, A., NERI, M., CRECCHI, M., SAZZI, C., BROCCA, L., PELLEGRINO, M.A., BOTTINELLI, R., PORCELLI, S., Institution: UNIVERSITY OF PAVIA, Country: ITALY, Abstract-ID: 2206

INTRODUCTION:
Inactivity negatively affects cardiorespiratory and muscle functions, decreasing exercise tolerance and dramatically increasing disease risks [1]. Previous data showed a time-dependent impairment of muscle oxidative capacity after severe inactivity (bed rest, BR) [2-4]. No changes in mitochondrial function were observed after 7 days of step reduction (SR) [5], a milder inactivity model. Therefore, we investigated the negative impact of 14 days of SR on muscle oxidative capacity estimated by near infrared spectroscopy in young participants. We hypothesized a longer period of SR could negatively affect muscle oxidative function.
METHODS:
Thirty (23±3 yr; 68±10 kg; 170±7 cm) participants followed a 14 days SR protocol. Before (T1) and after (T2) inactivity, whole-body oxygen uptake (VO2max) was measured during incremental cycling exercise. Vastus lateralis muscle oxygen uptake recovery rate constant (k) was calculated from tissue oxygen saturation index (TSI) changes during repeated transient occlusions performed immediately after leg-extension exercise. Time of ischemia-reperfusion for the occlusions was manipulated to maintain TSI in non-limiting O2 availability condition. Muscle biopsies were also collected and high resolution respirometry (HRR) was performed.
RESULTS:
SR resulted in an 82% reduction of participants’ average daily steps. VO2max was significantly reduced after SR (-4.4% from T1; p 0.022). k was significantly lower in T2 (2.1±0.6 min-1) compared to T1 (2.3±0.6; p=0.031). HRR was not different after inactivity (p>0.05).
CONCLUSION:
Our data confirm that, after a mild reduction in daily activity for two weeks, whole body maximal oxygen uptake loss is related to limitations in oxygen flow at skeletal muscle level measured in-vivo. The absence of changes in maximally [ADP] stimulated mitochondrial respiration and other respirometric parameters after SR, which confirm previous data after 10 days bed rest interventions [4], raises interests towards other assessments of mitochondrial function, such as sensitivity of mitochondrial respiration to sub-maximal [ADP], and impairments upstream of mitochondria.
[1] Booth et al. Physiol Rev (2017); [2] Dirks et al. J Physiol (2019); [3] Salvadego et al. J Physiol (2018); [4] Zuccarelli et al. J Physiol (2021); [5] Edward et al. J Appl Physiol (2021)