The semimembranosus is one of the three hamstring muscles on the back of your thigh. It sits on the inner side of the hamstrings, running from the bottom of your pelvis down to the upper part of your shin bone near the knee. You can think of it as a strong strap that helps you bend your knee and extend your hip, meaning it helps pull your leg backward behind you. It also helps stabilize the knee, especially when you change direction, decelerate, or hinge at the hips.
For visible abs, the semimembranosus matters more than most people realize because it supports the movements that let you train hard, burn calories, and keep your posture solid. When your hamstrings are weak or tight, your pelvis can tilt forward more than it should, which often increases lower back arch and makes it harder to brace your core properly. That can limit the quality of your squats, deadlifts, lunges, and sprint work, and it can make direct ab training feel like it’s going into your hip flexors or lower back instead of your midsection.
You train the semimembranosus through hip hinge patterns and knee bending patterns. Hip hinges like Romanian deadlifts, good mornings, and hip thrust variations hit it hard because the muscle is lengthened under load. Knee flexion work like leg curls also targets it, especially if you control the lowering phase. You’ll feel it most when you keep your ribs down, brace your core, and move through the hips without excessive lower back motion. Keeping this muscle strong and mobile helps you train consistently, protect your knees, and create the full body strength base that supports a lean, defined midsection.