Kinesiology of Exercise


Because the axis of rotation of most back machines is closer to the hips than the waist, these machines are used mainly for back stabilization purposes. When you execute “back extension” using a back extension machine, the erector spinae muscles of your lower back contract isometrically to hold your spine in place. The axis is in the hip joints.

Major Muscles and Actions Involved

The erector spinae is contracted isometrically to stabilize the spine. In the hip joint the gluteus maximus and the upper hamstrings are involved in hip joint extension. In this action the trunk moves backward with the axis in the hip joint.

Sports Uses

Stabilization of the spine is very important in most lifting exercises. When the spine is stabilized, it is held in its proper alignment while some activity such as execution of an exercise or sports skill, takes place. An example of this is executing a jump, squat or overhead press with the spine held in place.

Exercise Analysis

  • In seated back machine exercises, the main action is in your hip joint. Your lower back remains basically rigid to push the resistance bar or pad backward. Because of this, the erector spinae muscles of your back remain under isometric contraction in their stabilizing action.
  • Your gluteus maximus and upper hamstrings are, however, dynamically contracting to move your trunk backwards. Thus, in this exercise the back muscles are developed with the trunk held in basically the anatomical position.
  • Technically the term back extensions can be substituted for back raises but only if the axis of rotation is in the waist during execution so that you execute spinal extension/hyperextension. However, in the back extension exercise on exercise machines, the axis is in the hip joint. 
  • Because of this I do not favor use of the term back extensions. This is misleading since you perform hip joint extension on the exercise machine. The erector spinae remain isometrically contracted. On some machines a little spinal hypertension is possible.
  • The hyperextension exercise is another very misleading exercise because many people use this name to describe the back raise exercise while others use this term to describe hip joint extension/hyperextension or hyperextensions. The term hyperextension is usually used for the exercise most commonly done on a Roman Chair in which you hang over from the hips. 

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