Stretch Reflex

What is the stretch reflex, and why should you care?

We will cover the production of force in the next topic, but let’s touch on how your body generates movement. Take a simple example, the bicep curl. In simplistic terms (I’m not going to quiz you on deep anatomical knowledge in this course), the biceps connect from the upper part of the humerus (the bone in your upper body) and the shoulder blade to the radius (one of the bones in your forearm). To allow you to extend your elbow joint and lower your forearm and the Ez-Curl bar, the biceps stretch. To flex your elbow joint and bringing the bar back up the biceps contract.

All physical movement your body does is a result of muscular stretching and contraction.

The human body is quite an extraordinary mechanism, when you think about it. It performs multitudes of functions we don’t normally even recognize, even when closely observing. The stretch reflex is one of these functions. As a muscle stretches (when lowering the Ez-Curl bar in this example) your nervous system detects this stretching and in response as a protective measure triggers a response to initiate a contraction of that muscle, while causing the opposing muscle, the triceps in this example, to relax.

Understanding this process can strengthen your lifts. During the eccentric movement of your squat, while you descend your glutes and hamstrings stretch allowing flexion at your hips. At the same time your quads stretch creating flexion at your knee. As they stretch the stretch reflex kicks in causing the glutes and hamstrings to contract, extending the hips, and contracting the quads extending the knee. These contractions, in effect, create rebound off the bottom of your squat. Learning to use this reflex can drastically improve your power from the bottom of the squat as you begin the concentric movement.

This is, of course, a dramatic simplification of a rather complex process and movement. You can, however, see the utility in this reflex. We will dig into how to use the stretch reflex effectively in a later topic.

Note: It is worth noting that the stretch reflex dissipates rather quickly, fading in from one to seven seconds (one reason why pause squats get progressively more difficult the longer you pause).

Summary

The body has a protective mechanism, the stretch reflex, which causes an involuntary contraction as a muscle is stretched. In most lifts, like the squat, during the eccentric movement key muscle groups are stretched (glutes, hamstrings and quads). As the eccentric movement ends and the concentric movement begins, this stretch reflex can be used to add power to the lift.