Muscle fluid shift does not alter EMG global variables during sustained isometric actions.

Area of Science:

• Space physiology
• Human physiology
• Biomedical engineering

Background:

• Astronauts experience body fluid redistribution in space, altering interstitial water and impedance.
• Electromyographic (EMG) signal features may be affected by these fluid shifts, impacting muscle function assessments.
• Understanding fluid shift effects on EMG is crucial for comparing in-flight and terrestrial muscle performance.

Purpose of the Study:

• To investigate the influence of body fluid shifts on EMG spectral variables during muscle contractile activity.
• To determine if simulated microgravity-induced fluid shifts impact myoelectrical manifestations of muscle fatigue.

Main Methods:

• Ten men performed sustained isometric contractions at 20% and 60% MVC after resting in vertical or supine positions.
• EMG signals were recorded from soleus (SOL), medial (MG), and lateral (LG) gastrocnemius muscles.
• Mean power frequency (MNF) and average rectified value (ARV) initial values and rates of change were analyzed.

Main Results:

• MNF initial values depended on the muscle but were not affected by body position.
• MNF rate of change increased with force and varied by muscle, but remained unaffected by body tilt.
• Fluid shifts from vertical to supine tilt did not impact myoelectrical fatigue indicators.

Conclusions:

• Simulated fluid shifts do not alter myoelectrical manifestations of muscle fatigue.
• The findings suggest that comparing EMG fatigue indices in microgravity and on Earth may not be methodologically limited by fluid distribution changes.