source: this website
The squat movement can be described as a compound exercise which involves multiple groups of muscles. It is usually performed by recreational and professional athletes to strengthen hip, knee and ankle muscles. The squat exercise consists of two main phases, lowering and standing.
The lowering phase
The body starts from a standing position and, replicating the motion performed while sitting on a chair, it is lowered until the squat configuration is achieved. All the lower limb joints are involved, with several groups of muscles that contract as they lengthen. This results in eccentric contractions.
- Hip: flexion movement. The hip extensors (gluteus maximus, semimembranosus, semitendinosis and biceps femoris) mainly control the speed of the body, whose lowering is naturally supported by gravity.
- Knee: flexion movement. The knee extensors (rectus femoris, vastus medialis, vastus intermedius and vastus lateralis) mainly allow to tune the knee bending speed.
- Ankle: dorsiflexion movement. The plantarflexor muscles (gastrocnemius and soleus) mainly counteract the pull of gravity and provide a stable support on the ground.
The standing phase
The body leaves the squat configuration and returns to an upright position. The speed of this movement is continuously controlled, as well as the stable support provided by the feet. Once again, this is ensured by the combined action of all the lower limb joints. The same groups of muscles as for the lowering phase now shorten as they contract. This produces concentric contractions.
- Hip: extension movement. The hip extensors mainly bring the trunk back to an upright position.
- Knee: extension movement. The knee extensors help contracting and smoothly straightening the knee joints.
- Ankle: plantarflexion movement. The plantarflexor muscles push down against the ground and are responsible for the overall stability of the body.