The Effect Of Ischemic Preconditioning And Hypoxia On Neuromuscular Function During Intense Exercise

Abstract

Ischemic preconditioning (IPC) has been proposed to preserve neural drive during fatiguing exercise, however the underlying mechanism of this response remains unclear. Previous research has shown exercises impairing local tissue oxygenation to be more favourable in eliciting the humoral effects of IPC. PURPOSE: To determine whether IPC mediated effects on neuromuscular function are dependent on tissue oxygenation. METHODS: Eleven resistance-trained males completed four exercise trials (6 sets of 11 repetitions of maximal effort dynamic single-leg extensions) in either normoxic (fraction of inspired oxygen (FiO2): 21%) or hypoxic (FiO2: 14%) conditions, preceded by treatments of either IPC (3 x 5 min bilateral leg occlusions at 220 mmHg) or sham (3 x 5 min at 20 mmHg). Femoral nerve stimulation was utilized to assess voluntary activation and potentiated twitch characteristics during maximal voluntary contractions (MVCs) performed at baseline, prior to the exercise task and after each set of the exercise task. Tissue oxygenation (via near-infrared spectroscopy), blood oxygenation (via pulse oximetry) and surface electromyography activity was measured throughout the exercise task. RESULTS: MVC and twitch torque declined 62% and 54%, respectively (MVC: 96 ± 24 Nm, 95% CI = 73 to 119 Nm, Cohen’s d = 2.9, p < 0.001; twitch torque: 37 ± 11 Nm, 95% CI = 26 to 48 Nm, d = 1.6, p < 0.001), between pre- and post-exercise measurements without reductions in voluntary activation (mean decrease 0.2 ± 6.2%, 95% CI = -5.7 to 6.1%, d = 0.05, p > 0.21); there were no differences between conditions. Hypoxia reduced both blood and tissue oxygenation by 5% and 6%, respectively, compared to normoxic conditions (blood oxygenation: 4.8 ± 0.3%, 95% CI = 4.7 to 5.0%, d = 1.9, p < 0.001; tissue oxygenation: 3.5 ± 1.5%, 95% CI 2.6 to 4.4%, d = 2.4, p < 0.001), with a further 3% reduction in tissue saturation evident in the hypoxic IPC compared to hypoxic sham trial (mean decrease 1.8 ± 0.7%, 95% CI = 0.5 to 3.5%, d = 1.0, p < 0.05). CONCLUSION: IPC did not affect any measure of neuromuscular function regardless of tissue oxygenation. A reduction in FiO2 did invoke a humoral response and improved muscle O2 extraction during exercise, however it did not manifest into any performance benefit.