You have more than 400 muscles in your body, making up nearly half your body weight, according to Dr. Susan Hall, author of "Basic Biomechanics." Your muscles attach to your bones via tendons, located at their ends. When they contract, your tendons pull on your bones, causing them to rotate at their joints with other bones. This allows you to perform movements, including bending over, flexing your elbow, reaching for an object, squatting down and walking.
Biceps Brachii
The biceps brachii is located on the anterior, or front, part of your upper arm. It is divided into two parts, the long head and the short head, both of which attach to the radius bone of your forearm. The two heads divide, however, near your shoulder joint, with the long head attaching to the top of the glenoid fossa, or shoulder socket, and the short head attaching to part of the scapula bone on your upper back. The biceps assists the brachialis and brachioradialis muscles with elbow flexion.
Deltoid
The deltoid is the most prominent muscle of your shoulder. It attaches proximally, or closest to your torso, to the clavicle bone and distally, or farthest from your torso, to the outside of the humerus bone of your upper arm. It is divided into three parts: the anterior deltoid flexes your arm, moves it horizontally toward the center of your body, and rotates it inwardly; the middle deltoid abducts your arm both horizontally and vertically, moving it away from the center of your body; and the posterior, or back, deltoid extends your arm and rotates it outwardly.
Hamstrings
Three muscles make up the hamstrings: the biceps femoris, consisting of a long head and short head, the semimembranosus, and the semitendinosus. All of these muscles attach proximally to a bone known as the ischial tuberosity, just below the hip socket. Both heads of the biceps femoris attach distally to the top of the fibula and tibia bones of your lower leg, and the semimembranosus and semitendinosus only to the tibia. The hamstrings are the primary flexors of your knee joint and also contribute to extension at your hip. Additionally, the biceps femoris outwardly rotates your thigh and the semimembranosus and semitendinosus inwardly rotate your thigh when your knee is flexed, according to the website "Get Body Smart."
Quadriceps
The rectus femoris, vastus lateralis, vastus intermedius and vastus medialis muscles form a muscle group known as the quadriceps on your anterior thigh. All four of these muscles attach distally to the base of your knee joint. The rectus femoris attaches proximally to the iliac spine, above and in front of the hip joint; the vastus lateralis to the top and lateral, or outside, portion of the femur bone of your upper leg; the vastus intermedius to the front of the femur; and the vastus medialis to the medial, or inside, part of the linea aspera, a bony projection that runs vertically along the posterior femur. The quadriceps function collectively to extend your knee joint, and the rectus femoris also assists with hip flexion.
References
- "Grant's Atlas of Anatomy (Eleventh Edition)"; Anne M.R. Agur and Arthur F. Dalley; 2005
- "Basic Biomechanics (Fifth Edition)"; Susan J. Hall; 2007
- Get Body Smart: Functional Anatomy of Skeletal Muscles
- Get Body Smart: Lower Limb Bones
