Question

By Henneman's size principle , when you are recruiting stronger and stronger motor units because you're doing more strenuous activities , are we activating these stronger motor units while keeping the weeker ones active?, Or are we replacing the weaker motor units when we recruit stronger ones? At the maximal effort of the muscle, are all the motor units recruited or just the strongest ones?

Answer 1

Motor unit recruitment may be regarded as ‘the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction.’

• The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction.
• The motor units are recruited in a precise order in accordance with the magnitude of their force output, in which the small units are recruited first, thus exhibiting task-appropriate recruitment
• The prime mechanism at lower levels of muscle contraction strength is the addition of more motor units, but the firing rate of the initially recruited motor units also enhances. When nearly all motor units are recruited, increase in firing frequency becomes the leading mechanism which increase motor strength. At this level and beyond, motor units may be driven to fire in their secondary range.