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The researchers note that a recent study by Hartmann (2012) found that partial squats were not effective at improving vertical jump performance, while parallel squats were effective. They also note that models have suggested that the knee extensors are more important in vertical jumping than the hip extensors, although other studies have suggested that this may depend on the actual jumping strategies of the athlete. Some athletes have been reported to use a hip-dominant jumping strategy while others have been found to use a knee-dominant strategy.

What did the researchers conclude?

The researchers concluded that:

Load had a more pronounced effect than squat depth on the force required from the ankle plantar-flexors

So load is mainly what dictates how hard the ankle-plantar flexors are working in comparison with their maximum possible force output. Going deeper makes little difference to how hard the ankle-plantar flexors have to work.

Depth was a more significant factor than load for the force required of the knee extensors.

So depth is mainly what dictates how hard the knee extensors (quadriceps) have to work. Adding more weight to the bar makes much less difference. This could be part of the reason that Olympic lifters have large quadriceps in comparison to other strength athletes, because they of all athletes have to squat deep and depth makes more difference to the quadriceps than load.

The force required from the hip extensors was influenced by both barbell load and squat depth.

So both depth and loading influence how hard the hip extensors have to work. Partial squats are therefore putting much more emphasis on the hip extensors and much less on the quadriceps, since depth is key for the knee extensors.

Both depth and load should be considered as variables in using squats depending on which muscle groups are to be strengthened.

So this means that the knee extensors can be strengthened most effectively by deep squats and can be performed with lighter loads. However, the hip extensors and ankle plantar-flexors can be trained using heavier loads with smaller ranges of motion, as these muscle groups are less sensitive to depth.

For athletes

Athletes wanting to develop vertical jump height, who have a knee-dominant jumping strategy, should squat deeper in order to maximize the stress on the quadriceps. Athletes wanting to develop hip extension power for sprinting and other movements could use partial squats for this purpose, although there are many other suitable hip extension exercises.

Additionally, athletes should be aware that, for all its great benefits, the squat does not use the leg muscles to their maximum capabilities, even at very high percentages of 1RM. This suggests that athletes should make use of a variety of exercises to develop the leg musculature to their full extent.

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For physique competitors

Individuals looking to improve their quadriceps size should employ deeper but lighter squats.