F waves and motor unit size

Abstract

M responses and twitch contractions were evoked in single motor units (MUs) of the first dorsal interosseus muscle by intramuscular microstimulation of motor axons. Two-hundred nine MUs were studied in 21 subjects. Thirty-five MUs (17%) showed F waves in addition to M responses. Twitch force was used to provide an indirect measure of MU size; additionally, twitch contraction time was measured. There was no select group of MUs generating F waves with regard to the above contraction parameters. However, four of five MUs with very high twitch forces, above 70 mN, generated F waves. We conclude that MUs of all sizes produce F waves with similar probability. Only few MUs with very strong twitch forces, i.e., very large MUs, may be more subject to F-wave production and may be involved in the generation of the so-called repeater F waves.

Do F‐wave measurements detect changes in motor neuron excitability?

“…17 F waves are recruited from the entire motor neuron pool without regard to recruitment threshold. 7,22 Some studies have even suggested preferential activation among the larger motor units. 31 Therefore, H reflexes assess the same motor neuron subpopulation as most excitatory conditioning inputs, which also obey the size principle.…”

Effects of sustained stimulation on the excitability of motoneurons innervating paralyzed and control muscles

“…Because F waves occur with the same incidence in both low-and high-threshold motoneurons (9,23,37), the F-wave amplitude is an indication of the number of motoneurons activated. Hence, amplitude can also reflect the excitability of the motoneuron pool (12).…”

Physiological methods to measure motor function in humans and animals with spinal cord injury

“…A higher incidence of F-waves and F-waves of greater magnitude relative to the maximal motor response from a muscle are considered to be a reflection of greater motoneuron excitability [83][84][85]. Although F-wave studies at the single motor unit or muscle fiber level [86][87][88][89] are more difficult to perform, these data can be used to estimate the conduction velocity of the proximal segment of the peripheral axon, a particularly useful feature when ventral root damage is suspected.…”