Back squats involve holding a barbell on the back of your shoulders.
Squats require movement at your hips, knees and ankles, forming a compound exercise that requires involvement from numerous muscles. Although the hamstrings are commonly associated with knee flexion, they assist with hip extension during the squat. While they're not responsible for handling a large percentage of the load at the hips, hamstring weakness can limit your squat performance and put you at risk for injury at the knee joints.
Your hamstrings are active throughout a squat, controlling your speed on the way down and helping to bring you back up.
Look at the Squat
The squat exercise can be completed with your own body weight or an array of weighted implements to accommodate all training levels. Despite whether you incorporate weights, the movement of the exercise is the same. With your feet set hip-width apart and pointed forward, you initiate the movement by pushing your hips back in a slightly flexed position. Your knees then bend to lower your hips to the floor. Once your thighs have moved just beyond parallel with the ground, you extend your knees and hips to return to a standing position.
The hamstrings are a collection of three muscles, including the biceps femoris, semitendinosus and semimembranosus, which run down the backs of your upper thighs. The hamstrings originate at the backside of your pelvis and travel down the leg where they then insert at the top of your tibia and fibula, located in the lower leg. Because the hamstrings cross both the hips and knees, they're able to contribute to movements at both joints. As you lower into the squat, your hamstrings assist your gluteal muscles by controlling flexion at the hips. As you rise up out of the squat, your hamstrings contract and work against resistance to extend your hips.
Know the Concentric and Eccentric
During the squat, your hamstrings have to work both concentrically and eccentrically. An eccentric contraction is when your muscles produce a force that's lesser than the load, and thus lengthening as they contract. When you're lowering into the squat, for example, your hips flex, but it's your gluteals and hamstrings that control the speed of the movement. They're eccentrically contracting to control your hips and prevent you from flopping forward at the waist. As you extend the hips and rise up into a standing position, your hamstrings are contracting concentrically, which means they're producing a force that's greater than the load and thus shortening.
Protect the Knees
Not only are the hamstrings assisting with hip extension, they're also responsible for working against the quadriceps to protect the knees during the squat. According to ExRx.net, when your hips and knees are simultaneously extending, such as when you're rising up out of the squat, the hamstrings are responsible for countering the forces of the quadriceps that are pushing the knees forward. If your hamstrings are weak and unable to adequately counter the force produced by the quadriceps, your knees would slide forward during the squat. If the knees move beyond the vertical line of the toes, excess stress is placed on your knee joints.