Your hamstring muscle is actually made up of three individual muscles: biceps femoris, semimembranosus and semitendinosus. These muscles are named according to their location, number of divisions and attachments. Your hamstrings cross two joints, enabling them to pull your thigh bone backward and also to bend your knee, drawing your foot toward your butt.
Biceps Femoris Muscle
The biceps femoris has two divisions, the long and short head. The long head originates on the bone you feel under your gluteal muscles and then inserts on the now outer, top end of your shin bone, your tibia. The short head originates at the middle of your thigh bone and then inserts on the smaller bone of your shin, your fibula. The muscle cells of all your biceps femoris run at a diagonally straight line and are parallel to each other.
Semitendinosus Muscle
Your semitendinosus is a single muscle with a very long tendon. It originates on the same bone as your biceps femoris, your ischium, and then inserts toward the medial, posterior border of your shin bone.
Semimembranosus Muscle
The semimembranosus muscle has more of a flat, membrane-like origination off of the same butt bone, then inserts on the medial side of your shin bone, just below your knee. All your hamstring muscles work together to bend your knee and straighten your hips.
Slow-Twitch Muscle Fibers
The muscle cells and fibers of your body have protein molecules specifically arranged to shorten and lengthen in order for you to move. These protein molecules are called contractile units, which are made primarily of actin and myosin proteins. Slow-twitch muscle fibers contract at a slow rate because they have a slow form of myosin. Muscle cells have mitochondria, cell structures that make energy your cells can use. Slow-twitch muscle fibers have a large amount of mitochondria and myoglobin, a pigment used to store oxygen in the muscle cells. If you have plenty of slow-twitch fibers in your hamstring and quadriceps muscles, you will be a better long-distance runner because your leg muscles have a large amount of mitochondria to make energy and store oxygen over a long period of time.
Fast-Twitch Muscle Fibers
Fast-twitch fibers have a fast form of myosin. These fibers have few mitochondria and little myoglobin, which is why these cells cannot contract over a long period of time. If your hamstring and quadricep muscles have plenty of fast-twitch fibers, you will be better at sprinting or lifting heavy weights compared to running a 3.1-mile race.
References
- "Anatomy & Physiology"; Gary Thibodeau, Ph.D., and Kevin Patton, Ph.D.; 2007
- "Exercise Physiology, Energy, Nutrition & Human Performance"; William McArdle, Frank Katch and Victor Katch; 2007
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