The hamstring is the back portion of your leg and is actually made up of three different muscles: the semitendinosus (the middle muscle), bicep femoris (the outer muscle), and the semimembranosus (the inner muscle). They are composed of fast twitch fibers and initiate walking and running movements.
Most people’s hamstring muscles are underdeveloped because a majority of training for general population in the legs is done by the quadriceps. A common phrase in the health & fitness industry is, “You’re only as strong as your weakest part.” When you have well-balanced legs with the proper quad to hamstring ratio, it will make activities like walking, running and jumping much easier.
The semitendinosus is used the most in everyday life. If you were to be training on a lying leg curl machine, your feet must remain neutral having your toes pointed straight down to target your semitendinosus.
The bicep femoris is many times the strongest of the three hamstring muscles in untrained individuals. If you were to be training on a lying leg curl machine, your feet must be turned out, with your toes pointing away from each other in order to target the bicep femoris. But with the majority of the population having tight external hip rotators, this creates an externally rotated femur and foot, making a stronger biceps femoris muscle.
Many times the weakest hamstring muscle is the semimembranosus. If you were to be training on a lying leg curl machine, your feet must be turned inward, with your toes pointed at each other in order to target your semimembranosus. This muscle is paramount to train as one of its main functions is to stabilize the knee.
It is very important to change up the way you position your feet because in order to grow strong hamstrings you must target all three of the hamstring muscles. Just as in training the upper body you need to switch your grips and angles to target different muscles, the same goes for hamstrings.
You can also train the three hamstring muscles by going plantarflexed (toes pointed away from the body) or dorsiflexed (toes pulled towards the body). If you are training plantarflexed, then you have a lot less calf involvement whereas the opposite is true if you are training dorsiflexed, making plantarflexed the more difficult variation.