Figure 2. Isometric ramp contraction.

This was performed immediately after each SSF trial to test the sensitivity of the control unit. As torque increased, the firing frequency of the control unit also increased, indicating the sensitivity of the control unit to increased descending drive. The arrow denotes the firing frequency of the control unit (9.93 Hz) during the SSF trial. Since the control unit's firing frequency increases well beyond this point with further voluntary effort, we conclude that the control unit was not firing at a saturated frequency during the SSF trial.