The hamstrings are at work during various phases of walking.
Walking is a basic human movement that people don't tend to give a lot of thought to. The fact is, though, that even a simple walk can be an effective workout for building both cardiovascular and muscular endurance. Included in the many muscles involved in your casual walk is the muscle group known as the hamstrings. Although they are normally thought of as one muscle, the hamstrings are actually three separate muscles that work together during different phases of walking.
The hamstrings -- individually called the biceps femoris, semimembranosus and semitendinosus -- are located on the back of your thigh. All three of these muscles begin at the ischial tuberosity, at the very base of your butt, and separate as they make their way down the leg. The semimembranosus and semitendinosus stay along the inside of the thigh and cross over the knee joint to attach at the inside of the lower leg. Meanwhile, the large biceps femoris continue on the outside of the leg and attach below the knee on the lower leg.
As a group, your hamstrings control movement of both your hips and knees. Working together, these muscles can either move your hips backwards or bend your knee toward your body. Individually, these muscles assist in the rotation of your thighs. The semimembranosus and semitendinosus muscles rotate your thigh toward your body, while the biceps femoris moves it away. While you're standing still, your hamstrings activate to keep you stable and upright.
During the Stance Phase
Exercise scientists have divided the action of walking into several phases, including the stance and swing phases. As the name implies, the stance phases is the portion of walking during which you are standing with your feet planted on the ground. This phase also includes all of the motion leading up to and after that point, when your heel hits the ground, your foot flattens and your heel leaves the ground. Throughout the stance phase, your hamstrings are active to stabilize your hips and move them backwards. This backwards movement of your hips propels you forward.
During the Swing
As your foot leaves the ground and swings out in front of you, you are in the swing phase of walking. At this point, your hamstrings are active to flex your knee and move your foot away from the ground. The hamstrings in your stationary leg are also active at this point, since that leg is in the stance phase. During both phases, hamstring activity increases as you move more quickly.