Hamstrings Exercises

Biarticular Hip-Extension and Knee-Flexion Anatomy

The hamstrings cross both the hip and the knee, and that biarticular structure means they perform two mechanically distinct jobs: extending the hip and flexing the knee. Hip-hinge exercises such as the Romanian deadlift and good mornings train the hip-extension side almost exclusively, since the knee stays largely still while the hip does the work. Knee-flexion exercises, curling the heel toward the glutes, train the other half, and the two halves do not fully substitute for each other; a lifter who only hinges builds strong hip-extension hamstring strength while leaving the knee-flexion function comparatively undertrained. The glute-ham raise is unusual in that it loads both functions in the same rep, extending the hip while the knee also flexes, which is why it is often treated as a complete hamstring exercise rather than a half-pattern like a pure hinge.

Short-Head Biceps Femoris Isolation

Most of the hamstring group, the semitendinosus, semimembranosus, and the long head of the biceps femoris, are biarticular and cross both joints. The short head of the biceps femoris is the exception: it originates on the femur itself rather than the pelvis, so it crosses only the knee and contributes to knee flexion alone, with no role in hip extension. Because hinge exercises never load the knee-flexion function, they cannot reach the short head at all; only a knee-flexion exercise, a leg curl variation, trains it. This is the direct mechanical reason a hinge-only hamstring program leaves one head of the muscle group essentially untouched regardless of how much hinge volume is performed.

The Seated Curve

Hip angle during a knee-flexion exercise changes hamstring length at the hip end and therefore how much tension the biarticular fibers can produce at the knee end. A lying leg curl keeps the hip extended, which lengthens the biarticular hamstring fibers across the hip and increases the stretch-tension they’re under during knee flexion. A seated leg curl flexes the hip, shortening those same biarticular fibers at the hip end and shifting more of the relative work toward the hip-independent short head of the biceps femoris. Neither position is strictly superior, they train a genuinely different length-tension relationship, which is why seated vs lying leg curl is worth alternating rather than treating as two interchangeable versions of the same exercise.

Before heavier hinge work, the world’s greatest stretch dynamically mobilizes the hamstrings alongside the hips and thoracic spine as part of a warm-up sequence. See the hamstring exercises guide for the full hip-extension versus knee-flexion split and a no-machine substitution path for knee-flexion training.