This depends on: The tension developed by each individual fibre, which depends on: Action potential frequency (frequency-tension relationship);
Fibre length (length-tension relationship); £
Fibre diameter; t
The number of:
Fibres per motor unit;
The smallest motor units have the lowest thresholds for stimulation and the largest motor units have the highest, being recruited only for maximal force of contraction (the size principle of the motor neuron). Individual motor units thus fire asynchronously. Spatial summation refers to an increased number of motor units contracting simultaneously, while temporal summation refers to an increased frequency of contraction of individual motor units.
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