Hamstring curls are a focused resistance exercise designed to isolate the muscles responsible for bending the knee. The movement involves pulling the lower leg toward the torso against resistance, directly targeting the posterior compartment of the thigh. This action is unique because it removes the hip extension component found in most compound leg exercises. Understanding the muscles involved is fundamental for creating a balanced training program and preventing injury.
The Primary Muscle Group
The muscles performing the hamstring curl belong to a group of three distinct muscles located on the back of the thigh. This primary muscle group consists of the Semitendinosus, the Semimembranosus, and the Biceps Femoris. All three muscles originate from the ischial tuberosity on the pelvis and insert below the knee joint. Their collective function during the curl is knee flexion, the act of drawing the heel toward the glutes.
The Biceps Femoris is situated on the lateral (outer) side of the thigh and has both a long and a short head. The Semitendinosus and the Semimembranosus run along the medial (inner) side. The Semitendinosus is long and thin, while the Semimembranosus is a broader, flatter muscle lying deep to it. The long heads of all three muscles cross both the hip and knee joints. The short head of the Biceps Femoris only crosses the knee joint, meaning it is solely dedicated to knee flexion and is less affected by hip position during the curl.
Supporting and Stabilizing Muscles
While the hamstring group is the sole mover of the knee joint, several other muscles contribute to the movement’s execution. The Gastrocnemius, the larger calf muscle, assists in knee flexion because it crosses the knee joint. Its contribution is small but increases as the knee approaches full flexion. Deeper within the knee lies the Popliteus muscle, which plays a role in initiating the movement by “unlocking” the knee joint from a fully extended position.
Accessory muscles maintain stability and prevent unwanted movement at the hips and torso, especially when using resistance machines. The Gluteal muscles, particularly the Gluteus Maximus, stabilize the pelvis to efficiently direct the force generated by the hamstrings. Core muscles, including the abdominal wall and spinal erectors, engage isometrically to keep the torso fixed. This stabilization prevents the hips from lifting or swaying, which maintains the isolation of the hamstring muscles.
How Exercise Variations Change Muscle Activation
The activation profile of the hamstring group changes depending on the angle of the hip during the exercise. This difference is dictated by the muscle length-tension relationship, which describes how a muscle’s force production changes with its length. The seated hamstring curl positions the hips in a flexed position, placing the hamstrings in a lengthened state from the start. This lengthened position results in greater overall muscle activation across the Semitendinosus, Semimembranosus, and the long head of the Biceps Femoris.
In contrast, the lying or prone hamstring curl places the hips in an extended position, resulting in a shorter initial muscle length. This variation may emphasize the Biceps Femoris short head and often allows the user to handle more weight. However, it does not achieve the same growth stimulus across the full muscle group as the seated variation. Research indicates that training the hamstrings in a lengthened state, as occurs in the seated curl, promotes greater muscle growth (hypertrophy) in the upper regions of the muscle. The choice between variations should therefore be based on whether the training goal is to maximize general strength or to target muscle development at longer muscle lengths.