One area which often causes confusion or a lack of clarity at least, is the change created by moving the back-pad on the Eagle Leg Press. When you add the variety of foot placement options the area is ripe for misunderstanding. For many people the ‘explanation’ of this falls into the realm of opinion, personal preference and ‘gym lore.’
At the CYBEX Institute for Exercise Science we actually measured the forces involved at the variety of back pad positions and foot placement options in a number of different subjects. While there are some individual differences which may be attributable to leg length differentials and limb mass (more on that later), there are some common trends in the force analysis which may help clear things up.
Consistently, moving the back pad forward (i.e. position 1) slightly increases loading on the knee. Moving the back pad forward also increases the pre-stretch of the hip extensors (Gluteus Maximus) increasing its activation early in the movement. On the other hand, a forward back pad limits extension of the hip, so it remains somewhat flexed at the end of the movement. Conversely, moving the back pad back (i.e. position 5) reduces the loading at the knee and the associated contribution from the quadriceps, and increases the amount of hip extension achieved at the end position. The hip extensors don’t necessarily work harder, in fact they work slightly less with the pad all the way back; they simply work through different portions of the range of motion as the back position is altered.
Additionally, the lower the feet are placed on the footplate the greater the loading on the knee, and therefore, the greater the contribution from the quadriceps. Alternatively the higher the feet are place on the footplate the greater the load at the hips and the greater the contribution from the Gluteus Maximus and Hamstring group.
Lastly, the footplate may be adjusted forward and back in order to establish a comfortable starting position.
The upshot is that, on understanding this accurately, we can effectively manage muscle group activity and the stresses imposed on the involved joints.
International Director of Education and Training