Electromyographic indices of muscle fatigue during simulated air combat maneuvering.

Pilots exposed to high levels of headward (+Gz) acceleration must perform voluntary muscle contractions in order to maintain head-level arterial pressure. To study the possibility that muscular fatigue can limit human +Gz duration tolerance, electromyographic (EMG) activity and EMG indices of muscular fatigue were measured during a simulated air combat maneuvering (SACM) centrifuge profile. Eight experienced male volunteers were exposed to a +4-7 Gz centrifuge profile until volitional fatigue. Electrical activity (EMG) was recorded from 7 muscles: biceps brachii (BB), latissimus dorsi (LD), pectoralis major (PM), rectus abdominis (RA), vastus lateralis (VL), biceps femoris (BF) and gastrocnemius (GN). EMG and force during isometric contractions of the same muscles were also recorded at 1 G. Root mean square (RMS) and mean power frequency (MPF) were calculated for each second of EMG data. G-tolerance time averaged 256 +/- 33 s (mean +/- SD). RMS activity was expressed relative to activity during a maximal muscle contraction. The mean values (%) for each muscle during the 7 Gz plateaux were: RA, 30.8; BB, 26.4; LD, 44.0; PM, 48.5; VL, 43.4; BF, 31.4; GN, 39.3. The estimated level of contraction relative to a MVC (%) was: RA, 36.6; BB, 30.5; LD, 43.9 and PM, 61.4. There was no significant difference between contraction levels for any of the muscles studied. RMS activity did not increase over time and MPF decreased significantly only in BF and LD, however, these decreases were small. EMG activity and estimated contraction intensities were considered to be low to moderate. These results suggest that it is unlikely that fatigue in the muscles studied would limit G-tolerance time.