(14) NON-INVASIVELY MEASURING MUSCLE MITOCHONDRIAL OXIDATIVE CAPACITY FOLLOWING DIFFERENT LOADS OF SQUATS

Muscle oxidative capacity (MOC) is the maximum rate at which the muscle can utilize oxygen, which is associated with phosphocreatine recovery kinetics following exercise. Near-infrared spectroscopy (NIRS) measurement of muscle oxygen uptake (mVO₂) during brief arterial occlusions is a valid, reliable indicator of MOC. However, NIRS measurement of MOC has not been measured following loaded resistance exercise and thus its effect on the measurement is unknown.

Purpose: This study examined non-invasive NIRS measures of MOC in skeletal muscle following body weight (BW), low-load (LOW), and heavy-load (HIGH) squat exercises.

Methods: Seven recreationally active adults with resistance training experience (4 males; 3 females; 21.2 ± 0.1 yr) completed three experimental conditions (BW, LOW, HIGH) on separate days. In each condition, MOC was assessed using the same protocol. The only difference was the squats performed before the repeated occlusions. In BW, participants performed 20 s of bodyweight squats as fast as possible. In LOW, 20 s of barbell back squat was performed at 40% 1-repetition maximum (1RM) following a metronome set at 96 bpm (16 repetitions). In HIGH, 20 s of barbell back squat was performed at 80% 1RM following a metronome set at 48 bpm (8 repetitions). In LOW and HIGH, volume-load (VL) was the matched. MOC was determined from a series of 8 arterial occlusions interspersed with short recoveries (5 s on and 10 s off) that were preceded by 20 s of the condition’s exercise. This was repeated three times and mVO₂ from each was averaged.  Additionally, there were three occlusions (10 s on and 60 s) without prior exercise to measure resting mVO₂. Rapid inflation cuffs placed on the distal portion of the thigh were inflated to 300 mmHg during occlusions and deflated during recovery. Deoxyhemoglobin (HHb), collected at 10 Hz by a NIRS device, was measured at the vastus lateralis. To calibrate the signal to individuals, a 5-min arterial occlusion determined maximal deoxygenation (highest HHb), and the hyperemic response after cuff release determined minimum HHb. The slope of change in HHb during the first 3 s of each occlusion was the mVO₂. Each mVO₂ was plotted and a mono-exponential decay curve was fitted to determine the time constant (τ); τ was indicative of MOC (τ_MOC). A one-way repeated measures ANOVA assessed differences between conditions. Significance was established if p ≤ 0.05. Partial eta squared (η²_p) and Cohen’s d were used to assess effect sizes.

Results: Back squat loads were different by design (p < 0.001; 40% 1RM = 165.5 ± 78.2 lb vs 80% 1RM = 82.7 ± 39.1 lb), but VL was 1307.7 ± 625.5 lb in both conditions. There were significant effects of conditions for τ_MOC (p = 0.017; η²_p = 0.48). Specifically, τ_MOC was significantly faster in BW (50.0 ± 18.9 s) compared to LOW (74.1 ± 16.0 s; p = 0.038; d = 0.81) and HIGH (74.7 ± 29.2 s; p = 0.033; d = 0.82). The difference between LOW and HIGH was not significant (p = 1.00).

Conclusion: Loaded resistance exercise slows measures of MOC, but the magnitude of the load had no effect. VL was lower in BW but was matched between HIGH and LOW. Thus, VL, not load, could play a role in slowing MOC. PRACTICAL APPLICATION: MOC, measured by NIRS, can non-invasively give insight into the recovery kinetics of resistance athletes, however performing the measure following loaded resistance exercise compared to no load (i.e. body weight) can slow the measure and must be taken into consideration.

Acknowledgements: This research was supported by the Coastal Carolina University Professional Enhancement Grant and the Coastal Carolina University Faculty Summer Research Award.