PURPOSE: Diving operations may require personnel to be immersed for extended periods while breathing compressed air or 100% O2. The main objective of this study was to investigate the physiological effects of single and repetitive diving-induced hyperoxic conditions on skeletal muscle performance at 1.35 atmospheres absolute (ATA) on skeletal muscle performance. We hypothesized that following five days of consecutive, resting, long-duration hyperoxic water immersion (WI)s 1) neuromuscular performance would be reduced with a longer recovery time in comparison to air WIs, and 2) an increase in the production of free radicals with augmented inflammatory responses following consecutive hyperbaric, normoxic WIs. METHODS: Twenty-eight healthy, active male divers [30 ± 1 (24-43) yrs, mean ± SEM] completed five consecutive 6-hour resting WIs with 18-hour surface intervals while breathing compressed air (n=15) or 100% O2 (n=13) at 1.35 ATA. Using mixed repeated measures analysis of variance (RANOVA), skeletal muscle performance was assessed immediately before and after each WI, and 24 and 72 hours after the final WI. Assessments included maximum voluntary isometric contractions (MVIC), maximal isokinetic (IK) contractions, 50-repetition maximal IK knee extension (50-rep), maximum handgrip (MHG), and 40% maximal handgrip endurance (MHE). Surface electromyography (sEMG) of the vastus lateralis, rectus femoris, vastus medialis, biceps brachii, and brachioradialis, and local tissue oxygenation via near-infrared spectroscopy (NIRS) on the vastus lateralis, biceps brachii, and forearm flexor muscles were measured during the exercise protocols. All exercise tests were performed on the subject’s right side regardless of limb dominance. Muscle oxidative capacity (MOC) was measured on the vastus lateralis for the Air group only. Venous serum samples were analyzed for superoxide dismutase (SOD), hydrogen peroxide, total antioxidant capacity (TAC), nitrates (NO3), and heat shock protein 90 (HSP90) in the Air group only. RESULTS: Significant decreases were seen in maximal strength on day (D)3 (MVIC knee extension: Air: p<0.001, 2.5%; O2: p=0.016, 4.3%; MVIC elbow flexion: p=0.002, 3.5%, combined group result). The O2 group neuromuscular activation decreased throughout the dive week (DW) and remained reduced through the DW recovery period. The Air group neuromuscular activation increased throughout the DW but returned to baseline by 72-hr post-dive. The O2 group presented with greater post-dive performance measurements (MVIC knee extension: p<0.001, 4.6%) than the Air group. A combined group day main effect demonstrated a decrease in time-to-fatigue for MHE on D5 (p=0.015, 10.2%). The Air group fully recovered by 24-hr post-dive whereas the O2 group time-to-fatigue performance did not recover by 72-hr post-dive (p=0.019, 12.2%). A 24.9% increase (p=0.04) in the MOC rate constant (k) occurred on D4 with a return to baseline by D5. The Air group showed a greater change in deoxygenated hemoglobin formation during MHE than the O2 group (p=0.04, 46.7% difference). In the Air group, the hyperbaric normoxic conditions increased TBARS formation by 11% within 15 minutes post-dive (p=0.018) with SOD formation increased by 8.5% post-dive (p=0.029) and 5% overall by D5 (p=0.04). CONCLUSION: Consecutive, resting, long-duration normoxic and hyperoxic WIs caused small, but noticeable decrements to neuromuscular activation and performance after three days of WI with an adaptation towards recovery by the end of the WI 5. However, subsequent neuromuscular activation is highly affected by the hyperoxic conditions. From the two-group statistical comparison, these changes are due to the hyperoxic exposures, appear to last beyond the 72-hr post-dive recovery period, and seem to delay the onset of muscular fatigue through increased neuromuscular hyperexcitability. Hyperbaric normoxic exposures cause statistically significant increases in oxidative stress and anti-oxidant concentrations. The effects of hyperbaric hyperoxic exposures on free radical production are not tested.
