Isometric force production in high Gz: mechanical effects, proprioception, and central motor commands

BACKGROUND: We have shown in the past that human subjects produce exaggerated isometric responses when exposed to high +Gz. The present study investigated the role played by different factors in this phenomenon.

METHODS: There were 12 healthy non-pilot volunteers who were seated in the gondola of a centrifuge and were exposed to a +1, +1.5, and +3 Gz environment. During each exposure, they produced forces of prescribed magnitudes and directions in their frontal plane using an isometric joystick. From the registered data, we determined initial (100 ms after onset), peak, and end (70 ms before trial end) force.

RESULTS: For a given target magnitude, initial, peak, and end force were each smallest in 1 G, higher in 1.5 G, and higher still in 3 G. This G-related exaggeration was most pronounced for initial, and less for peak and end force. We further found that responses in high G were biased downwards for end force, but not for initial and peak force.

CONCLUSIONS: The G-related excesses of produced force are probably due to less appropriate central commands, since it manifests before proprioception becomes effective. The amelioration of this deficit for peak and end force is probably achieved by partial corrections based on proprioceptive feedback, or by direct mechanical effects. Our findings are relevant for the safe operation of high-performance aircraft during high-G maneuvers.