Once again, we have another muscle group instead of a single muscle for our muscle of the month – the hamstrings. When we say hamstrings we are referring to a group of muscles at the rear of the upper leg. There are technically four hamstring muscles: semitendinosus, semimembranosus, and the biceps femoris long and short head; however, since the short head of the biceps femoris crosses only the knee joint and not the hip and is therefore not involved in hip extension, it is sometimes excluded from the ‘hamstring’ characterization. Much like how there are technically four Indiana Jones movies, but the Kingdom of the Crystal Skull was really bad and gets excluded (at least in my book).
All of the hamstring muscles flex (bend) the knee, while all but the short head of biceps femoris extend (straighten) the hip. This is because the three ‘true’ hamstrings cross both the hip and the knee joint and are therefore involved in knee flexion and hip extension. Specific functions for each muscle are as follows:
- Biceps femoris long head – extends the hip, flexes the knee, outwardly rotate the lower leg when the knee is bent.
- Biceps femoris short head – flex the knee, outwardly rotate the lower leg when the knee is bent.
- Semitendinosus – longest of the hamstrings, extends the hip, flexes the knee, inwardly rotate the lower leg when the knee is bent
- Semimembranosus – largest of the hamstrings, extends the hip, flexes the knee, inwardly rotate the lower leg when the knee is bent.
The hamstrings are used for walking, running, squatting, and jumping and they are antagonists (acting opposite) to the quadriceps muscles in the action of deceleration of knee extension. Because the hamstrings have their origin at the hip, they are stretched while sitting, and long periods of sitting may affect their function. Generally, we are lacking in hamstring strength due to our typically sedentary cubicle/seated lifestyle and the quad dominance tendency of Crossfit movements. In order to strengthen the hamstrings, we should focus on deadlifts, RDLs, back extensions, reverse hypers, and hamstring curls.