A motor unit consists of a motor nerve cell (motoneurone) and the muscle fibers it innervates. muscles' ability to generate force is dependent upon an interplay between motor unit recruitment and firing frequency.
In homogeneous muscles such as the adductor pollicis (72 to 91% type I fibers), more motor units are activated with greater workloads up to 50% of maximum force. Motor units are fired more frequently (rate coding) with greater workloads greater than 50% of maximum force (Kulkulka & Clamann 1981).
Larger heterogeneous muscles (combination of Type I and II fibers) such as the biceps brachii and deltoid utilize a different pattern of recruitment and firing frequency (Kulkulka & Clamann 1981; Deluca, et al 1982). In these muscles, motor units are recruited in order according to their size as voluntary contraction increases from zero to maximal force (100% maximum contraction). The small slow twitch oxidative (SO) motor unit is recruited at a low force level. It has a low threshold force for being recruited. Beyond 35% maximum contraction, larger higher threshold fast twitch oxidative glycolytic (FOG) motor units are recruited. Beyond 65% maximum contraction, even larger higher threshold fast twitch glycolytic (FG) motor units are recruited. At the highest extreme, the largest, even higher threshold fast twitch glycolytic (FG) are not recruited until the exertion force exceeds 90% of maximum. The motor unit's firing rate increases as the force of contraction exceeds its recruitment threshold. Fast twitch units require higher firing rates to attain maximum force due to their faster contractile response (Hannerz 1974).
Generally, it's not possible to selectively contract fast twitch muscle fibers. Training with explosive exercises which require high contraction speeds can alter motor unit recruitment patters allowing higher threshold motor units to contract before or in concert with low threshold motor units. (Grimby & Hannerz 1977; Desmedt & Godaux 1978; ter Haar Romeny, et al 1982). Another exception is, the muscle that raises the big toe; only fast twitch muscles are activated. Conversely, slow twitch muscle fibers can be trained to contract faster.
Motor Unit Firing Rates
Maximum firing rates seem to be increased by training and decreased by periods of disuse. Weight training involves increased firing rates since most motor units are activated. Motor units are fired faster during eccentric contraction and slower during concentric and isometric contractions.
Average Innervation Ratios of Muscles
one motor neuron innervates:
- 15 muscle fibers (15:1) in the eye muscles
- 340 muscle fibers (340:1) in the first dorsal interosseous muscle (hand)
- 1,900 muscle fibers (1,900:1) in the tibialis anterior and medial gastrocnemius muscles
Muscle fibers that have higher muscle fibers to motor neuron ratio control gross motor movements. Muscle fibers that have lower ratios control fine motor movements.
In first week of knee extension training, there is a significant reduction in the coactivation of the hamstrings with the quadriceps (Carolan & Cafarelli 1992 in Enoka 1997). This factor alone will increase net knee extensor torque. The ability to extend your knee forcefully depends not only on how much force your quadriceps can generate, but also how effectively you can deactivate the antagonist hamstring muscles. This reduction in coactivation must be learned by performing the actual activity. Incidentally, the learning of coordination of a motor skill extends far beyond just the antagonist activation level.
Learning the coordination and muscle control across multiple joints is required to permit stabilization while allowing maximum force output from multiple joints in a single direction. Consider the involvement of the hamstring as a dynamic stabilizer and role as a knee stabilizer during the squat and leg press (closed chain). Although, antagonist co-contraction may impair the ability to fully activate agonists by reciprocal inhibition (Tyler & Hutton, 1986), antagonist co-contraction can assist ligaments in maintaining joint stability under heavy loads (Baratta et al., 1988). During high velocity (ballistic) movements, antagonist co-contraction may also provide stabilization, precision, and a breaking mechanism (Lestienne, 1979).
Age and Reaction Time
The speed of transmission of impulses along nerves slows by approximately 15 percent between 30 and 80 years of age. Older adults who have always been active have total reaction times that are more similar to those of younger people. The reaction times of older adults with a sedentary lifestyle are much slower.
Force development during bilateral movements (2 legs or 2 arms) is typically less than the sum of the forces developed by each limb independently. This difference is referred to as bilateral deficit and may range from 3% to 25% (Fleck & Kreamer, 2004). Training with bilateral contractions, as done for example by rowers, may eliminate or reduce bilateral deficit. Incidentally, cyclists may display bilateral deficit since they train with alternating or reciprocal movements (Howard & Enoka, 1987).