Determining the extent of neural activation during maximal effort.

Purpose: The purpose of this study was to compare the extent of neural activation assessed by the central activation ratio (CAR) versus activation estimated from T2 magnetic resonance imaging (MRI) and neuromuscular electrical stimulation (NMES).

Methods: Seven college-age individuals volunteered for this study. CAR was determined by manually superimposing a train of NMES (50 Hz, 450-mus biphasic pulses) for 1 s during a maximal voluntary effort. The MRI-NMES method assessed activation by stimulating the knee extensors for 3 min in a 2 s on, 2 s off cycle. T2 MR images were taken at rest and after NMES was administered. Theoretical maximal torque (TMT) of the knee extensors was calculated based on the MRI-NMES activation data. The TMT was then divided by the maximal voluntary isometric contraction (MVIC) of each subject to determine the extent of neural activation during a MVIC.

Results: The results for CAR reveal the percent activation (mean +/- SD) of the quadriceps femoris during a MVIC was 92 +/- 7% for the right thigh and 96 +/- 4% for the left thigh. The MRI-NMES method estimated that MVIC could be achieved if 75 +/- 14% of the knee extensors on the right thigh and 74 +/- 14% on the left thigh were activated. These results are similar to findings that showed MVIC could be achieved by stimulating 71% of the knee extensors.

Conclusions: We conclude that CAR overestimates the extent of neural activation during an MVIC because the 3D shape of the thigh is altered. This will change electric current flow to the axonal motor neuron branches and limit the artificially evoked torque, thereby resulting in an overestimation of CAR.