Activities of motor units of human tibialis anterior muscle were studied when the subject maintained the force of the muscle at a constant level voluntarily. Motor units were classified into two main groups according to their discharge frequencies and their tensions. Those units which were recruited from the beginning of the contraction and discharged tonically throughout yielded low tensions and units which showed discharge only for a limited period produced higher tensions. © 1981.
Kato, M., Murakami, S., Takahashi, K., & Hirayama, H. (1981). Motor unit activities during maintained voluntary muscle contraction at constant levels in man. Neuroscience Letters, 25(2), 149–154. https://doi.org/10.1016/0304-3940(81)90323-2