**Short answer**  
Muscles are controlled by motor neurons in the spinal cord. The number of motor neurons that fire, as well as their individual firing rates govern the control of muscle force.

**Background**  
Muscles consist of contractile elements: the **[muscle fibers][1]**. These muscle fibers are under direct control of the **[motor neurons][2]** in the spinal cord [(Purves *et al.*, 2001)][3], as shown in Fig. 1. The motor neurons in the spinal cord are under direct control of the **[motor cortex][4]** in the brain. When the motor neurons fire, they release [acetylcholine][5], which in turn makes the muscle fibers **[contract][6]**. The force in which a muscle contracts is basically governed by how **strong a single muscle contracts**, and **the number of muscle fibers** that are recruited. 

Firstly, there is the **[rate code][4]** that allow motor neurons to regulate muscle force. An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the associated motor unit (*i.e.*, one or more muscle fibers) generates. When the motor neuron fires a single action potential, the muscle may only slightly twitch. If the motor neuron fires at high rates, however, the second action potential may arrive at the muscle unit before the muscle has had time to recover from the first twitch, and that second action potential will produce a greater amount of force than the first. This is due to an increase in strength of muscle contraction through **action potential summation**. This process has a limit. When the successive action potentials no longer produce a summation of muscle contraction (because the muscle is at its maximum state of contraction), the muscle is in a state called tetanus and is pushed to its limit (source: [Neuroscience Online][4]).

Secondly, there the **[recruitment][4]** of motor neurons (size principle). When a signal is sent to the motor neurons to execute a movement, motor neurons are not all recruited at the same time, or at random. The motor neuron size principle states that, with increasing strength of input onto the motor neurons from the higher brain centers, smaller motor neurons are recruited first, while the larger motor neurons are recruited only when the motor signal increases. Why does this orderly recruitment occur? Because of Ohm’s Law, a small amount of synaptic current will be sufficient to cause the membrane potential of a small motor neuron to reach firing threshold, while the large motor neuron stays below threshold. As the amount of current increases, the membrane potential of the larger motor neuron also increases, until it also reaches firing threshold. This process is called **recruitment** and results in more motor neurons to be activated when the brain signals for the need for a high contractile force. More motor neurons will recruit more muscle fibers (source: [Neuroscience Online][4]).

[![Motorneuron][7]][7]  
<sup>Fig. 1. Motor neurons from the spinal cord innervate the muscles. source: [SE Veterinary Neurology][8]</sup> 

<sub>**References**  
**-** [Purves *et al*. (eds), *Neuroscience*, 2<sup>nd</sup> ed. Sunderland (MA): Sinauer Associates. Motor Neuron-Muscle Relationships][3]  
**-** [University of Texas, Houston Health Science Center. Neuroscience Online][4]</sub>


  [1]: http://classes.midlandstech.edu/carterp/Courses/bio210/chap09/lecture1.html
  [2]: https://en.wikipedia.org/wiki/Motor_neuron
  [3]: http://www.ncbi.nlm.nih.gov/books/NBK10816/
  [4]: http://neuroscience.uth.tmc.edu/s3/chapter01.html
  [5]: http://www.ncbi.nlm.nih.gov/books/NBK11143/
  [6]: http://meat.tamu.edu/ansc-307-honors/muscle-contraction/
  [7]: https://i.sstatic.net/tEW22.jpg
  [8]: http://sevneurology.com/pbvetsociety-spinal-cord-lecture/