The antagonist is the tricep brachii and serves primarily to stabilize the elbow joint as the movement is performed. The amount of stability and control needed is dependent upon the type of exercise. In our example, as the elbow flexors contract concentrically, the elbow extensors contract eccentrically. Using this data and neglecting the weight of the forearm and hand, we can calculate the joint torque needed to at least hold the weight steady in this particular position. In any case, the amount of co-contractor torque will subtract from the agonist torque and result in less weight being lifted.
Joints become more stable as muscles generate force to cause joint compression.
Complex Multi-Joint Lifts vs. The squat is a perfect example to show this phenomenon. Figure 2 shows the major muscles involved in the squat exercise. Let's examine some of the multi-joint muscles. During the squat exercise, the hamstrings are very active in keeping the trunk upright.