Muscle coactivation

Muscle coactivation occurs when agonist and antagonist (or synergist) muscles surrounding a joint contract simultaneously to provide joint stability.[1][2] It is also known as muscle cocontraction, since two muscle groups are contracting at the same time. It is able to be measured using electromyography (EMG) from the contractions that occur. The general mechanism of it is still widely unknown. It is believed to be important in joint stabilization, as well as general motor control.[1]

Coactivation of biceps and triceps. The biceps’ EMG is on top, while the triceps’ EMG is on the bottom.

Function

Muscle coactivation allows muscle groups surrounding a joint to become more stable. This is due to both muscles (or sets of muscles) contracting at the same time, which produces compression on the joint. The joint is able to become stiffer and more stable due to this action.[3][4] For example, when the bicep and the triceps coactivate, the elbow becomes more stable. This stabilization mechanism is also important for unexpected loads impeded on the joint, allowing the muscles to coactivate and provide stability to the joint in a quick fashion.[1][3] This mechanism is controlled neuromuscularly, which allows the muscle(s) to contract.[1] This occurs through a motor neuron sending a signal (through creating action potentials) to the muscle fiber to contract by releasing Acetylcholine.[5] When signals are sent to all muscle fibers in a muscle group, the muscle group will contract as a whole.

In the upper limbs, the stability of muscle coactivation allows for precise low-level physical tasks.[1] An example of this would be picking up a small object. By protecting the muscles at the end of their range of motion, the direction of the fine movements is able to be changed.[3] In the lower limbs, stability is important in upright standing balance. The coactivation of different muscle groups allows for proper balance and the ability to adjust weight and to stay upright on uneven ground.[6] It is also believed to be important for postural control by stabilizing the spine.[1] Muscle coactivation is absolutely necessary for learning a fine motor skill or for any activity involving stability.[6] In order for muscle coactivation to occur, it must inhibit reciprocal innervation, which occurs when a muscle contracts and the synergist muscle relaxes.[4] For muscle coactivation to occur, both the muscle and synergist muscle need to contract.

Testing

Muscle coactivation is measured using a technique called electromyography(EMG).[1] This is performed by using surface EMG that responds to electrical activity of the muscle through the skin.[7] Electrical activity is only present in the muscle when the muscle voluntarily contracts.[8] When the muscle is contracted, the EMG is able to display the force of the contraction or how the nerves can respond to stimulation.[9] An EMG of coactivation would display the agonist and antagonist muscle contracting simultaneously. Although it is believed many muscles are involved in the mechanism of coactivation, methods to measure coactivation are finite to specific instances or two muscle systems. Because of this, little is understood about the role of coactivation in a multiple muscle system.[1]

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References
  1. Le, P; Best, TM; Khan, SN; Mendel, E; Marras, WS (February 2017). "A review of methods to assess coactivation in the spine". Journal of Electromyography and Kinesiology. 32: 51–60. doi:10.1016/j.jelekin.2016.12.004. PMID 28039769.
  2. "cocontraction". The Free Dictionary. Retrieved 27 April 2017.
  3. Riemann, Bryan L.; Lephart, Scott M. (2002-01-01). "The Sensorimotor System, Part II: The Role of Proprioception in Motor Control and Functional Joint Stability". Journal of Athletic Training. 37 (1): 80–84. ISSN 1062-6050. PMC 164312. PMID 16558671.
  4. Longstaff, Alan (2005-01-01). BIOS Instant Notes in Neuroscience (2nd ed.). Taylor & Francis. ISBN 978-0415351881.
  5. Purves, Dale (2012-01-01). Neuroscience. Sinauer Associates. ISBN 9780878936953.
  6. Lundy-Ekman, Laurie (2013-08-07). Neuroscience - E-Book: Fundamentals for Rehabilitation (4th ed.). Elsevier Health Sciences. pp. 190–220. ISBN 9780323266482.
  7. Dotan, Raffy; Mitchell, Cameron; Cohen, Rotem; Klentrou, Panagiota; Gabriel, David; Falk, Bareket (2017-04-27). "Child—Adult Differences in Muscle Activation — A Review". Pediatric Exercise Science. 24 (1): 2–21. doi:10.1123/pes.24.1.2. ISSN 0899-8493. PMC 3804466. PMID 22433260.
  8. Klass, Malgorzata; Baudry, Stéphane; Duchateau, Jacques (2007-07-01). "Voluntary activation during maximal contraction with advancing age: a brief review". European Journal of Applied Physiology. 100 (5): 543–551. doi:10.1007/s00421-006-0205-x. PMID 16763836.
  9. "Before, During and After Your EMG Test". www.hopkinsmedicine.org. Retrieved 2017-04-10.
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