Muscle coactivation occurs when

agonist An agonist is a chemical that activates a Receptor (biochemistry), receptor to produce a biological response. Receptors are Cell (biology), cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an R ...

and

antagonist muscle Anatomical terminology is used to uniquely describe aspects of skeletal muscle, cardiac muscle, and smooth muscle such as their actions, structure, size, and location. Types There are three types of muscle tissue in the body: skeletal, smooth, a ...

s (or synergist muscles) surrounding a joint contract simultaneously to provide joint stability, and is suggested to depend crucially on supraspinal processes involved in the control of movement. It is also known as muscle cocontraction, since two muscle groups are

contracting A contract is an agreement that specifies certain legally enforceable rights and obligations pertaining to two or more parties. A contract typically involves consent to transfer of goods, services, money, or promise to transfer any of those a ...

at the same time. It is able to be measured using

electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...

(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 Motor control is the regulation of movements in organisms that possess a nervous system. Motor control includes conscious voluntary movements, subconscious muscle memory and involuntary reflexes, as well as instinctual taxes. To control ...

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. For example, when the

biceps The biceps or biceps brachii (, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join to form a single muscle bel ...

and the

triceps The triceps, or triceps brachii (Latin for "three-headed muscle of the arm"), is a large muscle on the ventral, back of the upper limb of many vertebrates. It consists of three parts: the medial, lateral, and long head. All three heads cross the ...

coactivate, the elbow becomes more stable. This stabilization mechanism is also important for unexpected loads impeded on the joint, allowing the muscles to quickly coactivate and provide stability to the joint. This mechanism is controlled neuromuscularly, which allows the muscle(s) to contract. This occurs through a

motor neuron A motor neuron (or motoneuron), also known as efferent neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly o ...

sending a signal (through creating

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

s) to the

muscle fiber A muscle cell, also known as a myocyte, is a mature contractile cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal, smooth, and cardiac (cardiomyocytes). A skeletal muscle cell is long and threadl ...

to contract by releasing

acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

. 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. 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. 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. It is also believed to be important for postural control by stabilizing the spine. Muscle coactivation is absolutely necessary for learning a fine motor skill or for any activity involving stability. In order for muscle coactivation to occur, it must inhibit

reciprocal innervation René Descartes (1596–1650) was one of the first to conceive a model of reciprocal innervation (in 1626) as the principle that provides for the control of agonist and antagonist muscles. Reciprocal innervation describes skeletal muscles as ...

, which occurs when a muscle contracts and the

synergist muscle Anatomical terminology is used to uniquely describe aspects of skeletal muscle, cardiac muscle, and smooth muscle such as their actions, structure, size, and location. Types There are three types of muscle tissue in the body: skeletal, smooth, a ...

relaxes. For muscle coactivation to occur, both the muscle and synergist muscle need to contract.

Testing

Muscle coactivation is measured using a technique called

(EMG). This is performed by using surface EMG that responds to electrical activity of the muscle through the skin. Electrical activity is only present in the muscle when the muscle voluntarily contracts. When the muscle is contracted, the EMG is able to display the force of the contraction or how the nerves can respond to stimulation. 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.

References

{{DEFAULTSORT:Muscle Coactivation Muscular system Nervous system Neuroscience