The primary muscles which contribute to the squat, in no particular order, are the quadriceps, hamstrings, hip flexors/extensors, abdominals, and spinal erectors. When an athlete fails to rise from the bottom of a squat, it is important to note that not all of the muscles are failing simultaneously. Rather, a specific muscle will fail, and the key to progress is identifying the weakness, then strengthening it. While it is impossible to simply state that if x happens when squatting, it is muscle y that is causing the problem, some general guidelines follow. If a lifter fails to rise from the bottom of a squat, it generally indicates either a weakness in the hip flexors and extensors, or a lack of acceleration due to inhibition of the golgi tendon organ (no stretch reflex – train with lighter weight and learn to accelerate if this is the case). If an athlete has a tendency to lean forward and dump the bar overhead, it generally indicates either weak hamstrings or erectors. If an athlete has trouble stabilizing the bar, or maintaining an upright posture, it is often due to a weakness in the abs.