The hamstrings are a group of three muscles located along the back of the thigh, extending from the pelvis to the knee joint. This muscle group includes the semimembranosus, the semitendinosus, and the biceps femoris. Their primary actions are the extension of the hip and the flexion of the knee. Whether running effectively “builds” these muscles, meaning increasing their size (hypertrophy), depends heavily on the intensity of the activity.
The Hamstring’s Primary Role in Running
The hamstring muscles perform two distinct functions during the running gait cycle. In the stance phase, when the foot is on the ground, the hamstrings contract concentrically (shortening) to help extend the hip. This action contributes to the powerful force that propels the body forward, shared with the gluteal muscles.
The most demanding role occurs during the swing phase, when the leg moves forward through the air. The hamstrings engage in a powerful eccentric contraction, meaning they lengthen while under tension. This action rapidly decelerates the lower leg, controlling the forward swing and preparing the foot for a controlled landing. During high-speed running, peak muscle forces can reach up to nine times body weight.
How Different Running Styles Affect Engagement
The type of running performed dictates the stimulus placed upon the hamstring muscle fibers. Low-intensity, steady-state running, such as a long jog, primarily recruits slow-twitch muscle fibers suited for endurance. This style increases muscular endurance and efficiency, but it offers a limited stimulus for increasing muscle size. The overall time under tension is high, but the mechanical tension per stride is relatively low.
Conversely, high-intensity running, like sprinting, interval training, or running uphill, increases muscle fiber recruitment and force production. Sprinting demands maximum eccentric strength from the hamstrings to rapidly decelerate the leg during the late swing phase. This high-force, eccentric loading is a stronger stimulus for strength development than steady-state running. Running uphill also increases resistance, forcing greater engagement of the hamstrings and glutes to overcome gravity.
Why Running Alone Limits Hamstring Hypertrophy
While high-speed running develops hamstring strength and resilience, it often fails to provide the necessary stimulus for significant muscle hypertrophy. Building substantial muscle size requires progressive overload, which involves continually increasing the mechanical tension and volume placed on the muscle over time. Typical running, even sprinting, does not always provide the consistent, high mechanical tension needed for maximal growth compared to resistance training.
Muscle growth is maximized when the muscle is trained under high tension while in a lengthened position, which is difficult to replicate consistently in running. Effective hypertrophy programs deliberately incorporate heavy eccentric loading to cause microtrauma and subsequent repair and growth. To maximize hamstring size and strength, runners should incorporate supplementary exercises, including:
- The Romanian Deadlift (RDL), which focuses on hip extension under load.
- The Nordic Hamstring Curl, which emphasizes intense eccentric strength.
- The seated leg curl, which provides direct, high-tension stimulation.
- The lying leg curl, which provides direct, high-tension stimulation.