Kinesiology of Exercise

Knee Curl

Most sports actions and normal everyday activities such as walking and running rely mostly on the anterior thigh muscles. However, in order to have a truly strong knee, which is involved in many activities, it is necessary to balance the development of the quadriceps muscles with development of the hamstrings on the posterior side. One of the best exercises for this is the knee curl. 

Major Muscles and Actions Involved

Knee curls involve the hamstring muscle groupl ocated on the back side of the thighs. The action in this exercise is knee joint flexion, in which the back of the shin moves toward the back of the thigh.

Sports Uses

Active knee joint flexion is used in running and kicking to "fold" the shin under the thigh as the leg is brought forward in the main power phase. It is needed in back kicks in soccer and in various wrestling moves. Aerialists use this action a great deal when hanging from their knees on the trapeze, and dancers need it to bring their shins up in various jumps.

Exercise Analysis

  • The knee curl exercise is very important for stabilizing the knee and preventing injury. In addition, hamstring strength is important to balance the quadriceps, which should always be stronger. However, the exact ratio of strength between the quadriceps and hamstrings has not been determined with any degree of accuracy.
  • It seems reasonable to expect greater strength of the quadriceps since this muscle group is substantially larger and used in more activities. Thus a 3:1 or even a 4:1 ratio is usually recommended. It is also interesting to note that some studies have shown that the stronger the hamstrings are, the more the quadriceps can be developed. Therefore, always complement quadriceps work with hamstring exercises.
  • In the lying knee curl exercise, when heavy weights are used the pelvic girdle has a tendency torise up (hip joint flexion). This happens naturally to raise the upper attachment of the hamstring muscles so that they are more taut and thus become more effective as knee joint flexors.
  • Excessive raising of the pelvic girdle may be potentially harmful to the lower back in some individuals. If you experience any problems, use an angled bench or do the free standing variant. If you use a use an angled bench, the bench automatically places the hip joint in flexion. Such positioning not only takes the pressure off the lumbar vertebrae and discs, but it also raises the attachment of the upper hamstrings to provide for a more effective pull of the
  • The knee curl exercise is most effective for strengthening the lower hamstring tendons and hamstring muscles, especially the short head of the biceps femoris which does not cross the hip joint. It is not effective for development of the upper tendons of the hamstrings.
  • The reason for this is twofold. First, as exercise intensity increases, hip joint flexion occurs and the upper hamstring stretches in its contraction. Second, because the upper end is responsible for hip joint extension, it must be exercised in this action. Therefore, you should do hip joint extensions to develop the upper tendon/muscle complex.
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