(04) SUSTAINED MUSCLE OXYGENATION FOLLOWING ARTERIAL OCCLUSION PREDICTS SPECIFIC FORCE IN RESISTANCE TRAINED MEN AND WOMEN

Background: Strength and conditioning professionals routinely focus on a variety of metrics to describe and capture indices of skeletal muscle function. Examining the ratio of muscular strength to size, termed specific force (SF), has grown in popularity due to its ability to quantify muscle quality. In addition, measures of the microvasculature have been associated with skeletal muscle characteristics. However, the precise factors contributing to differences in SF remain unknown.

Purpose: Therefore, our purpose was to determine the relative contributions of skeletal muscle variables such as resting oxygen consumption (mVO₂), muscle oxidative capacity, echo intensity (EI), and microvascular measures to SF.

Methods: 16 college-aged (21±2yr) resistance trained men and women performed unilateral leg extension one-repetition maximum (1RM) trials to define muscular strength. An ultrasound device measured muscle cross-sectional area (mCSA). We quantified SF as the ratio of 1RM to mCSA. Ultrasonography also provided EI of the vastus lateralis (VL), which was corrected for adipose tissue thickness. A near-infrared spectroscopy (NIRS) device was fixed to the VL, and used to assess relative changes in microvascular oxygenated hemoglobin+myoglobin (oxy[heme]) during a vascular occlusion test (VOT). The NIRS-VOT included a 3min baseline, 5min transient ischemia, and 3min re-saturation period. The first 30s of ischemia resulted in a linear decline in oxy[heme], which was defined as resting mVO₂. Immediately following the 5min of ischemia, the first 10s of VL re-saturation was defined as upslope. The maximum value and total area under the curve (AUC) of oxy[heme] following ischemia were quantified. Muscle oxidative capacity was determined after 5min of cycle ergometry at 50% of peak power observed during a prior graded exercise test. Following this submaximal cycle task, a series of brief (e.g., 5s) arterial occlusions (250mmHg) of the femoral artery was performed. During each arterial occlusion, a 3s slope of the oxy[heme] signal was calculated and plotted as a mono-exponential decay curve to yield the rate constant, K. Bivariate associations were examined among the skeletal muscle variables and SF. Significant variables were entered into a stepwise linear regression model to determine meaningful predictors and relative contributions to SF. P-values ≤ 0.05 were considered significant.

Results: The mean±SD of SF was 10.6 ± 2.3 lbs×mCSA ⁻¹ and AUC was 958.7 ± 355.1%×s⁻¹. Specific force was significantly associated with mVO₂ (r=-0.66), upslope (r=0.51), resaturation maximum (r=0.59), AUC (r=0.82), K (r=-0.45), and EI (r=-0.73). The stepwise linear regression model revealed only AUC was a significant predictor of SF (r²=0.68, p< 0.001, β=0.824).

Conclusion: The ability to sustain a state of hyper-saturation (i.e., muscle oxygenation levels above normal resting values) significantly predicted SF to the greatest extent compared to other factors associated with SF. Based on this finding, it is likely that individuals presenting with superior skeletal muscle capillaries and oxygen delivery capacity also produce the greatest amount of force relative to muscle mass. PRACTICAL APPLICATIONS: Strength and conditioning coaches should seek to optimize skeletal muscle oxygen delivery if interested in maximizing force production per unit of muscle mass. This may include programming additional cardiovascular-based exercises (e.g., running and cycling).


Acknowledgements: None