Stand tall and curl one leg to failure.
Keep your hips pinned to the pad, swinging your torso to move the weight is cheating the curl.
The standing leg curl trains one hamstring at a time, curling the heel toward the glute against the pad while the machine's thigh support holds the working leg still. Standing with the hip extended puts the hamstring in a longer position than a seated curl at the hip end, and the single-leg format means your stronger side can never quietly cover for the weaker one. It earns its slot as strict knee-flexion work that finds and fixes left-to-right gaps.
The seated curl trains the hamstrings at longer muscle lengths thanks to the bent hips, which is great for growth, while the standing version isolates one leg with the hip extended. If your gym has both, the seated is the staple and the standing is the imbalance fixer.
The weight is too heavy, so your body is generating momentum to help the hamstring. Drop the load, keep your thigh glued to the support, and make the heel travel from pure knee bend.
Curling with the ankle flexed, toes pulled up, keeps the calves from assisting and puts more of the work on the hamstrings. Pointing the toes brings the calves in, so flex if hamstring isolation is the goal.
