Each motor unit has a different activation threshold. Therefore, the magnitude of the motoneuron action potential will determine how many motor units are activated. Additionally, the discharge rate of the motoneuron varies within each motor unit. Therefore, contraction intensity can be controlled by the number of motor units activated and/or the discharge rate of the active units. It's debatable whether humans can voluntarily activate all motor units of a given muscle for a maximal voluntary contraction. The initial strength gains with resistance training are attributed to greater recruitment of available motor units for maximum efforts. This effect occurs before any hypertrophy is evident.