The hamstring curl is a resistance exercise that isolates the muscles at the back of the thigh, primarily focusing on knee flexion. This movement contrasts with hip-dominant exercises like deadlifts, which also work the hamstrings. For athletes, the utility of this isolated movement is debated, balancing targeted strength against the dynamic demands of sport. Since the hamstrings are a frequent site of injury in sports involving high-speed running, determining the training value of hamstring curls for performance or injury mitigation is crucial.
How Hamstring Curls Influence Athletic Power and Speed
The hamstrings are bi-articular muscles, meaning they cross both the hip and knee joints, acting as both hip extensors and knee flexors. In high-speed activities like sprinting, propulsion force comes mainly from the hamstrings’ role in hip extension. Traditional hamstring curls involve flexing the knee against resistance, primarily strengthening the knee-flexion component, especially at the end range of the movement.
While knee flexion strength is beneficial for overall muscle development, its direct transfer to maximum sprinting speed is limited because top-end speed relies heavily on powerful hip extension. During the swing phase of a sprint, the hamstrings rapidly decelerate the lower leg before foot contact, a highly dynamic and multi-joint action. Isolated knee flexion strength, often developed concentrically in machine curls, does not fully replicate this dynamic requirement. Athletes must integrate strength gains from curls into multi-joint movements like running and jumping for functional improvement.
Developing Eccentric Strength for Hamstring Strain Prevention
The most compelling reason for athletes to incorporate hamstring curls is for the development of eccentric strength, which is the ability of the muscle to lengthen under tension. Hamstring strain injuries most frequently occur during the late swing phase of high-speed running. At this moment of extreme stress, the muscle is near its maximum length and eccentrically contracting to brake the lower leg’s forward momentum.
Exercises that specifically overload this lengthening phase are highly effective in reducing injury risk. The Nordic Hamstring Curl (NHC) is a prime example, forcing the athlete to resist gravity as the body lowers toward the floor. This training leads to beneficial architectural adaptations, such as an increase in hamstring muscle fascicle length. Research demonstrates that programs incorporating eccentric hamstring training, like the NHC, can reduce the incidence of hamstring strains by approximately 51% in compliant athletes. This targeted eccentric stimulus improves the muscle’s capacity to absorb energy at long lengths, making it more resilient to the high forces experienced during sprinting.
Practical Differences Between Hamstring Curl Methods
Hamstring curl variations differ significantly in how they load the muscle, particularly in relation to hip angle and the phase of contraction emphasized.
Seated Leg Curl
The seated leg curl machine places the athlete’s hips in a flexed position, training the hamstrings at a longer muscle length. This lengthened-position training is often linked to greater muscle hypertrophy, or growth, compared to other methods.
Lying Leg Curl
Conversely, the lying or prone leg curl machine places the hips in a neutral or slightly extended position, training the hamstrings at a relatively shorter length. Both machine variations allow for easy, measurable concentric (lifting) and eccentric (lowering) loading. The seated version tends to offer superior growth stimulus.
Nordic Hamstring Curl (NHC)
The Nordic Hamstring Curl is unique because it is an unsupported bodyweight exercise that exclusively focuses on maximizing eccentric overload. This is its main mechanism for injury prevention. This method is highly specific for building resilience, whereas the machine-based curls are better suited for general strength and muscle mass development.