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Lower yourself into the down phase of a squat and feel the synergist muscles at work.
Mike Powell/Digital Vision/Getty Images
The squat is a common exercise performed all over the world by various levels of athletes, from the competitive level all the way to the recreational level. It can be done with free weights, a barbell or no weights. It can be used to build muscle mass or rehabilitate a knee. It requires balance, poise, strength, focus and flexibility. You can practice the squat in the gym or at home.
Synergist and Eccentric Defined
In order for movement to occur, muscles must contract. According to Andrew Biel's "Trail Guide to the Body," a muscle that leads an action as a primary mover is called an agonist. A muscle that supports the agonist is called a synergist. Muscles that work together in achieving the entire movement are called synergists. Concentric movement is when a muscle is contracting and shortening at the same time. Eccentric movement is when a muscle in contracting but lengthening, simultaneously.
The Eccentric Phase
The squat can be broken down into two sections. The down or lowering phase and the up or standing phase. When you squat down, your hips and knees bend into flexion while your foot deepens into dorsi flexion. During the down phase you are lengthening your quadriceps, glutes and hamstring muscles. Thereby classifying the down/lowering phase as the eccentric section of the squat. Therefore, the up/standing phase is the concentric section. Note that dorsi flexion is when the top of your foot moves toward your shin, or in the instance of the squat, your shin moves toward the top of your foot.
As you lower yourself into the eccentric phase of the squat, all four of your quadriceps are elongated. The four quads are the vastus lateralis, vastus intermedius, vastus medialis and rectus femoris. Note that the rectus femoris crosses in front of the hip. Therefore, even though the lower fibers of the rectus femoris are being stretched due to knee flexion, the upper fibers are shortening due to hip flexion. Similarly, the upper fibers of the hamstring muscles -- biceps femoris, semitendinosus and semimembranosus -- are elongated due to hip flexion, but the lower fibers are flexed due to knee flexion.
Synergist Muscles Continued
The large butt muscle -- called the gluteus maximus -- is also stretching eccentrically during the down phase of the squat. As your knee continues to bend and your foot dorsi flexes, you also lengthen your soleus, the deep calf muscles. The lower fibers of the larger calf muscle, the gastrocnemius, is stretched due to dorsi flexion, but the upper fibers are shortened due to knee flexion. Regardless, your quadriceps, glutes, hamstrings and calves are considered to be in an eccentric state on the down/lowering phase of the squat, which is known as the eccentric phase.