Recruitment of Small and Large Motor Units

When the muscle is activated initially, the first motor units to fire are small in size and weak in the degree of tension they can generate. Starting with the smallest motor units, progressively larger units are recruited with increasing strength of muscle contraction.
Motor unit recruitment refers to the activation of additional motor units to accomplish an increase in contractile strength in a muscle. A motor unit consists of one motor neuron and all of the muscle fibers it stimulates. All muscles consist of a number of motor units and the fibers belonging to a motor unit are dispersed and intermingle amongst fibers of other units. The muscle fibers belonging to one motor unit can be spread throughout part, or most of the entire muscle, depending on the number of fibers and size of the muscle.[1][2] When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and contract. The activation of one motor neuron will result in a weak but distributed muscle contraction. The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction. Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle, and therefore is a measure of how many muscle fibers of that muscle are activated. The higher the recruitment the stronger the muscle contraction will be. Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons, and thus slow to fast twitch) as contraction increases. This is known as Henneman's size principle. Under some circumstances, the normal order of motor unit recruitment may be altered, such that small motor units cease to fire and larger ones may be recruited.[7][8] This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs.
The force produced by a single motor unit is determined in part by the number of muscle fibers in the unit. Another important determinant of force is the frequency with which the muscle fibers are stimulated by their innervating axon. The rate at which the nerve impulses arrive is known as the motor unit firing rate and may vary from frequencies low enough to produce a series of single twitch contractions to frequencies high enough to produce a fused tetanic contraction. Generally, this allows a 2 to 4-fold change in force. In general, the motor unit firing rate of each individual motor unit increases with increasing muscular effort until a maximum rate is reached. This smooths out the incremental force changes which would otherwise occur as each additional unit was recruited.[9]
Muscle contraction is the activation of tension-generating sites within muscle fibers.[1][2] In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding a heavy book or a dumbbell at the same position.[1] The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.[1]
Muscle contractions can be described based on two variables: force and length. Force itself can be differentiated as either tension or load. Muscle tension is the force exerted by the muscle on an object whereas a load is the force exerted by an object on the muscle.[1] When muscle tension changes without any corresponding changes in muscle length, the muscle contraction is described as isometric.[1][3][4][5] If the muscle length changes while muscle tension remains the same, then the muscle contraction is isotonic.[1][3][4][5] In an isotonic contraction, the muscle length can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction.[1][6] In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time-varying manner.[7] Therefore, neither length nor tension is likely to remain constant when the muscle is active during locomotor activity.
An isometric contraction of a muscle generates tension without changing length.[1][3][4][5] An example can be found when the muscles of the hand and forearm grip an object; the joints of the hand do not move, but muscles generate sufficient force to prevent the object from being dropped.
In isotonic contraction, the tension in the muscle remains constant despite a change in muscle length.[1][3][4][5] This occurs when a muscle's force of contraction matches the total load on the muscle.
In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts.[8] This occurs when the force generated by the muscle exceeds the load opposing its contraction.
In eccentric contraction, the tension generated while isometric is insufficient to overcome the

[@vbarber30]

I am a MT student currently studying the muscular system and this certainly helped to clarify some things for me. Thank you

[@Felipekimst]

0:26 I think it is supposed to be " each muscle fiber is only controlled by one muscle neuron"

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[@ScientistCindy]

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