Muscle Coactivation and Strength

This article provides some theory about the concept of muscle coactivation and strength. The importance of this theory is that it helps to explain why Weightlifters must engage in training that is highly specific  to the movement patterns of the Olympic Lifts rather than generalist strength training.

It is presumable that there are few muscles of the body that are not involved in the performance of the Olympic Weightlifting movements of the Snatch and the Clean and Jerk. In virtually every part of the body, large and small muscles will make a contribution to the required execution of a lift as agonists, antagonists or fixators. The following table provides some explanation of these terms:

Action	Explanation of role
Agonist	Muscles that contract to create movement of a joint
Antagonist	Muscles that oppose the contraction of agonists  and limit movement of a joint
Fixator	Muscles that increase in contractile tension to stabilise segments of the body while the action of agonists and antagonists cause movement.

When we think of the force required to lift a weight, there is a tendency to think mostly about muscle contraction within the agonist muscle group. For example, while performing a squat exercise we tend to focus on the contraction of the quadriceps muscle group to cause extension of the knee joint. However, contraction also takes place within the opposing antagonist muscle group, in this case the hamstrings. This contraction of agonist and antagonist simultaneously is called co-activation and thus movement in the knee joint will be the net effect of force generated by the opposing muscle groups (Aagaard et al., 2000).

This co-activation of the antagonist is considered to have an important function in stabilising and protecting the joint (Quinzi et al., 2015, p48). Thus, at every angle of joint articulation, the extent of forces developed within agonists and opposing antagonists controls the speed and extent of movement within a joint.

In the context of Olympic Weightlifting, the performance of the classical lifts requires a high degree of precision of co-activation across multiple joints and this poses a significant motor learning problem. The development of precise co-activation is aspect of neural adaptation. An excellent example of this is found in the actions of muscles that cross the knee joint. During any extension of the knee joint as a result of contraction of the quadriceps, the Biceps Femoris (BF) is co-activated. The BF is bi-articular (crosses two joints) and has a hip extension action and a knee flexion action. These dual actions of the BF create an interesting phenomenon during the pull in Weightlifting. As the weightlifter raises the bar from the floor to the knee, the legs straighten at the knees as a result of the agonist action of the quadriceps. However, as the bar passes the knee, it is normal to see the knees re-bend. This knee re-bending is not as a consequence of purposeful learning of technique but as a result of tension developed in the BF which is acting not only as an antagonist to the quadriceps but also as agonist in hip extension.

The problem for the Weightlifter is exacerbated by the action of the Gastrocnemius, another bi-articular muscle, which not only extends the ankle but flexes the knee. This re-bending of the knee is favourable to the athlete if it occurs after the bar passes the knees. However, for many athletes it is common to see the knees thrust too far forward as the bar approaches the knees. The angle of the knee thus created causes either the bar to hit the shins or else the athlete must develop a pull technique that moves the bar around the knees.

The moment-by-moment precision of tension developed in agonists, antagonists and fixators is a wonderfully complex phenomenon controlled by our central nervous system. The achievement of this control provides efficiency of movement and is perhaps another way to understand what is meant by skill.

However the degree of precision achieved in muscle coactivation and strength of contraction achieved across all joints depends on whether the exercise is familiar or unfamiliar (Busse et al., 2005). A common phenomenon observed by Weightlifting coaches occurs when someone with extensive training experience with weights (but not Weightlifting) attempts to learn the classical lifts. What is observed is a tremendous struggle to cope with quite basic movements because the exercise is unfamiliar and there is excessive coactivation that impairs movement (Busse et al., 2005). The learned coactivation that results from the regular performance of non-Weightlifting exercises becomes a major source of interference and annoyance. In such circumstances, there is not an efficiency of movement and the athlete appears to work excessively hard, even harder than someone with no previous weight-training experience. With persistence, however, the central nervous system modifies the previously learned patterns and thus co-activation appears to reduce in response to learning a new skill (Vereijken, Whiting and Newell cited in Busse et al., 2005).

References:

Aagaard, P., Simonsen, E. B., Andersen, J. L., Magnusson, S. P., Bojsen‐Møller, F., & Dyhre‐Poulsen, P. (2000). Antagonist muscle coactivation during isokinetic knee extension. Scandinavian journal of medicine & science in sports, 10(2), 58-67.

Busse, M. E., Wiles, C. M., & Van Deursen, R. W. M. (2005). Muscle co-activation in neurological conditions. Physical therapy reviews, 10(4), 247-253.

Quinzi, F., Camomilla, V., Felici, F., Di Mario, A., & Sbriccoli, P. (2015). Agonist and antagonist muscle activation in elite athletes: influence of age.European journal of applied physiology, 115(1), 47-56.