Hamstrings

Hamstrings: what are they?

Biceps femoris on the outside, semitendinosus and semimembranosus on the inside: all three start on the ischial tuberosity, the bone you sit on, and attach below the knee. Because they cross both the hip and the knee, they can never be fully lengthened or fully shortened at both joints at once. That single fact explains most of hamstring training: an exercise loads them either by flexing the hip with a straight knee, or by bending the knee with an extended hip, and a complete program needs both.

Two jobs, two families of exercise

Hip extension is the hamstrings pulling the torso upright, or the leg back, from a hinged position. This is the Romanian deadlift, the good morning, the hip thrust with a straight leg, the sprint drive phase. Knee flexion is the hamstrings bending the knee: leg curls, Nordic curls, sliders. Training only one family leaves half the muscle undertrained, which is why so many strong deadlifters have poor leg-curl strength.

The third job is the one that hurts people. In sprinting, the hamstrings act as a brake at the end of the swing phase, lengthening under high force to stop the shin before the foot lands. Nearly all hamstring strains happen there, in the stretched position under speed, which is why eccentric work at long muscle lengths is the central piece of hamstring injury prevention.

How to train the hamstrings

Cover both joint actions in the same week, and make sure at least one movement loads the stretched position. Load and rep ranges differ noticeably between the two families: hip-dominant work tolerates heavy weight, knee-dominant work rarely does.

Seated versus lying leg curls

The seated leg curl holds the hip flexed, which lengthens the hamstrings before the knee even moves; the lying curl works them in a shorter position. Both are useful, but if a client only has time for one knee-flexion exercise, the seated version trains the muscle in the longer position that also matters most for sprinting. It is also the easier of the two to progress, because the machine holds the pelvis in place.

One practical note: cramping in the hamstrings during curls is common and usually about position and warm-up rather than a problem with the muscle. Starting with a lighter set, and letting the client find a hip and foot angle that does not pinch, resolves it most of the time.

What goes wrong in practice

The first pattern is a client who hinges with their back rather than their hips. If the lower back rounds and the hamstrings never report a stretch, the RDL is training the spinal erectors instead. Cue the hips backwards with the ribs stacked over the pelvis, cut the range to where the back stays neutral, and let the range grow.

The second is the quad-dominant client with a large gap between knee extension and knee flexion strength. You will not see it on a squat, but it shows up as a hamstring that gives out in sport. A leg curl and a hinge in every lower-body week closes it.

The third is a previous hamstring strain that never got rebuilt. Scar tissue and a shortened functional length leave the muscle vulnerable to a second strain in the same spot. Slow, heavy eccentric work at long lengths, added gradually, is the piece that is usually missing.