Cross-training workouts are extremely popular. These programs typically involve performing high reps of three to five exercises as explosively as possible. The safety of these programs has been questioned because form usually breaks down with fatigue, which increases the risk of injury. A University of Connecticut at Storrs study led by David Hooper and Bill Kraemer found that squat biomechanics deteriorated during a 55-rep squat workout. Hip involvement decreased with fatigue, which placed greater loads on the knee joint and spine. These changes diminish the training effect of the exercise and increase the risk of injury.
The safety of squats has been questioned for more than 50 years. The National Strength and Conditioning Association, in a position stand, stated that squats are safe if performed correctly. Clearly, technique breaks down during high-rep squat workouts, which place the spine and knees at risk and reduces the effectiveness of the exercise for building lower body strength. (Journal Strength Conditioning Research, 28: 1127-1134, 2014