Effect of acclimatization to high altitude (5,050 m) on motor unit activation pattern and muscle performance.

This study evaluated the influence of acute and chronic hypoxia (up to 40 days at 5,050 m) on the muscle strength, fast-to-slow fiber motor unit (MU) ratio, and MU activation strategy. These factors were studied by measuring the maximal voluntary contraction (MVC) of the elbow flexors and analyzing surface electromyogram (EMG) and soundmyogram (SMG) of the isometrically contracting biceps brachii. From the time and frequency domain analysis of EMG and SMG, the root mean square and mean frequency of the spectra were calculated. SMG and EMG were recorded during 4-s exercises in the 20-100% MVC range before departure; after 2, 15, and 40 days at high altitude; and after the return to sea level. MVC or root mean square and mean frequency at all levels of effort were not significantly different among all experimental conditions. In the present study where optimal living conditions (i.e., food, physical activity, and environmental temperature) were ensured, acute and chronic hypoxia (at least up to 40 days) did not affect the maximal force output or fast-to-slow fiber MU ratio and MU activation pattern during isometric exercise.