Muscles flexing the knee joint

The hamstrings( semitendinosus, semimembranosus and biceps femoris)

Gastrocnemius

Gracilis

Sartorius

The hamstrings

Functional activity

It must be emphasized that, with the exception of the short head of the biceps femoris, these three muscles pass over, and act upon both the hip and knee joints. Their action is therefore extremely complex. Although details of their attachments and much of their action are described, the functional activities of these muscles with respect to the knee joint have yet to be considered.

Rotation of the knee joint by the hamstrings is usually considered to take place when the foot is off the ground. However, this is not exactly true. It is certainly easier to describe the rotation that occurs when the feet are firmly on the ground. For example, consider moving sideways from seat to seat. The feet are fixed and body weight is taken on to them; however, the person stays in a sitting position just allowing the buttock to come clear of the seat. The trunk is then moved to one side by a swevelling of the femur on the upper surface of the tibia. This is achieved by the combined action of the medial rotators of the knee and the lateral rotators of the other.

Finally, the simultaneous action of the hamstrings on both the hip and knee joints must be considered. Such a situation arises in the athlete who is accelerating towards a bend. Here the hamstrings are functioning to lift the trunk into a more upright position as well as to flex the knee of the leg that is being swung-through ready for the next stride. As the body is being forced around the bend, the hamstrings will also have to produce a rotation of the knee in order to produce this turning force. It is thought by some that the hamstrings act as a tie between the back of the pelvis and the tibia, which can adjust the relationship between the two bones. This is particularly important when the body is changing posture during active movement as there are additional forces due to the acceleration of body segments. This concept would certainly go some of the way to explain why there are so many injuries of the hamstrings in the athletes.

Gastrocnemius

Gastrocnemius is mainly a strong plantar flexor of the ankle joint and is dealt with in that section. Nevertheless, it is also a strong flexor of the knee.

The medial and lateral heads of gastrocnemius cross the knee joint on their respective sides. The muscle appears to come into action when the foot is fiexed and the body is being pulled forwards on the feet. This is best seen when pulling forwards on the slied of a rowing body seat, or manoeuvring the fully reclined body. The turning and pulling down of the body when in bed or on a plinth is very important considering that we spend one-third of our lives lying down.