Seated Leg Curl.

The seated position is why a lot of lifters get more out of this than the lying version, and it comes down to muscle length. Sitting keeps your hips flexed, which puts the hamstrings on stretch before you even start curling, and training a muscle in that lengthened position tends to pay off in growth. A well known trial that pitted the seated curl against the lying leg curl found more hamstring growth from sitting, so if you only run one curl, this is a sensible pick. The evidence is not a mountain, but it points the same way the mechanics do.

It also covers what hip hinges leave out. A romanian deadlift trains the hamstrings as hip extensors but barely touches their second job, bending the knee. A leg curl is pure knee flexion, so it trains exactly what your hinges miss. Round out the hamstrings with both, and the seated curl is the curl half done in its most productive position.

Control the lowering and quit heaving. The usual mistake is slamming the weight down and letting it snap the legs straight on the way back. Curl under control, hold the squeeze, and fight the return so the hamstrings keep working as the lever opens. Pin your hips and torso to the pads, and if your butt is lifting off the seat, the load is too heavy.

Treat it as isolation. A full range and a clean tempo will always beat another plate on the stack. Ten to fifteen reps with a pause at peak contraction suits it well, and it counts toward your weekly sets for the hamstrings when you map out volume.