The study of motor unit activation during exercise provides information on the function of the neuromuscular system and changes in muscle oxygenation for metabolic functions during exercise. The aim of this study was to examine the changes in muscle oxygenation and motor unit activation during resistance exercise and the correlation between these changes in normoweight and obese women.
A total of 25 women participated who were divided, based on body fat percentage (BF), into normoweight (BF: 25.4±3.41, age: 20.7±0.89 years, n=13) and obese (BF: 40.47±6.07, age: 20.5±0.97 years, n=12) and performed in the single leg extension exercise 3 sets of 10 repetitions at 60% of 1RM with a 3′ rest between sets. During the performance of the exercise, electromyographic (EMG) activity of the vastus lateralis muscle was recorded and through decomposition of the EMG signal, the amplitude of the motor unit action potential (MUAP), the firing rate (FR) was determined. Also, the change in oxygenation of the vastus lateralis muscle was recorded by near-infrared spectroscopy (NIRS) and the slope of the change in oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin during the performance of the sets was calculated.
There was a significant increase (p<0.05) in EMGrms activity and FR of the motor units during the performance of the sets with no difference (p<0.05) in the change between the two groups. The slope of increase in HHb and decrease in O2Hb was higher (p<0.05) in normoweight women. There was no significant correlation between the rate of change of the EMGrms signal (r = 0.08-0.30) and FR (r = 0.04-0.28) with the rate of change of O2Hb and HHb in each set either in the whole sample or in each group separately.
In conclusion, normoweight and obese women show similar changes in neuromuscular activation during resistance exercise but different metabolic function with greater deoxygenation in normoweight women. The lack of correlation between the change in neuromuscular activation and muscle oxygenation is likely due to the increased use of anaerobic energy production mechanisms during the performance of this exercise program.