Muscle Roles and Contraction Types
Concentric, eccentric and isometric? Agonist, antagonist, synergist and fixator? If you want to know what these terms mean in 'plain English' then it is all revealed right here.
When it comes to training your personal training clients then understanding the different types of contractions that a muscle can perform is vital. It will help you ensure your programme designs are specific to your clients ability and goals as well as keeping them safe with good technique.
Muscle contractions are classified according to the movements they cause and in fitness we are primarily concerned with the following three types of contraction:

1. Concentric contraction: Any contraction where the muscle shortens under load or tension is known as a concentric contraction. For example, the quadriceps muscles in the thigh contract concentrically (shorten) during the upward phase of the squat movement (in the direction of the arrow), as can be seen in the adjacent picture.

2. Eccentric contraction: Muscles not only ‘shorten’ but can also lengthen under load or tension. An eccentric contraction refers to any contraction where the muscle lengthens under load or tension. So in the squat exercise, the quadriceps muscles will contract eccentrically (lengthen) in the downward phase of the movement (the opposite direction of the arrow), as can be seen in the adjacent picture.

3. Isometric contraction: Muscles don’t actually need to move (shorten or lengthen) at all to contract or develop tension. An isometric contraction refers to any contraction of muscles where little or no movement occurs. If during the squat the person stopped moving at a certain point (say halfway up) and held that position for 10 seconds, the quadriceps muscle would be contracting isometrically, it would still be under load/tension but no movement would occur.
Many skeletal muscles contract isometrically in order to stabilise and protect active joints during movement. So while the quadriceps muscles are contracting concentrically during the upward phase of the squat, and eccentrically during the downward phase, many of the deeper muscles of the hip contract isometrically to stabilise the hip joint during the movement.
Concentric and eccentric are also terms used to describe the phase of a movement. The concentric phase is the phase of the movement that is overcoming gravity or load, while the eccentric phase is the phase resisting gravity or load. So for push ups the concentric phase is the up phase where gravity is overcome, and the eccentric phase is the downward phase where gravity is resisted.

What roles do skeletal muscles play during movement?
When completing movements such as walking or squatting, there are a lot of different muscles involved in order to complete the movement smoothly and effectively. They achieve this as they each adopted the appropriate type of contraction (concentric, eccentric or isometric) and have their own specific role that they play during the movement.

There are four different roles that a muscle can fulfil during movement, these roles are:

1. Agonist: The agonist in a movement is the muscle(s) that provides the major force to complete the movement. Because of this agonists are known as the ‘prime movers’. In the bicep curl which produces flexion at the elbow, the biceps muscle is the agonist, as seen in the image below.

The agonist is not always the muscle that is shortening (contracting concentrically). In a bicep curl the bicep is the agonist on the way up when it contracts concentrically, and on the way down when it contracts eccentrically. This is because it is the prime mover in both cases.

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2. Antagonist: The antagonist in a movement refers to the muscles that oppose the agonist. During elbow flexion where the bicep is the agonist, the tricep muscle is the antagonist. While the agonist contracts causing the movement to occur, the antagonist typically relaxes so as not to impede the agonist, as seen in the image above.
The antagonist doesn’t always relax though, another function of antagonist muscles can be to slow down or stop a movement. We would see this if the weight involved in the bicep curl was very heavy, when the weight was being lowered from the top position the antagonist tricep muscle would produce a sufficient amount of tension to help control the movement as the weight lowers.

This helps to ensure that gravity doesn’t accelerate the movement causing damage to the elbow joint at the bottom of the movement. The tricep becomes the agonist and the bicep the antagonist when the elbow extends against gravity such as in a push up, a bench press or a tricep pushdown.

3. Synergist: The synergist in a movement is the muscle(s) that stabilises a joint around which movement is occurring, which in turn helps the agonist function effectively. Synergist muscles also help to create the movement. In the bicep curl the synergist muscles are the brachioradialis and brachialis which assist the biceps to create the movement and stabilise the elbow joint.

4. Fixator: The fixator in a movement is the muscle(s) that stabilises the origin of the agonist and the joint that the origin spans (moves over) in order to help the agonist function most effectively. In the bicep curl this would be the rotator cuff muscles, the ‘guardians of the shoulder joint’. The majority of fixator muscles are found working around the hip and shoulder joints.