Squat Analysis Muscle & Motion

It is important to understand the anatomy of the lower limb to understand the complex movement that is a squat. You may need to refer back to the anatomy when reading about joints and muscles during the squat movement.

The primary hip muscle involved during a squat are the gluteus maximus and the hamstrings.
Gluteus Maximus: works eccentrically** to control decent and concentrically** on ascent. Helps to stabilise during a squat to prevent abnormal movement of the pelvis and knee.
Hamstring: Due to the muscle attaching over the hip and knee, the hamstrings are active during the full squat movement but are most active eccentrically at 10-70° of hip flexion.
Knee Joint: The 2 joints that make up the knee are tibiofemoral joint (where the tibia meets the femur) and the patellofemoral joint (where the patella meets the femur).
Tibiofemoral joint: hinge joint, working in the sagittal plane of movement with 0-160° of knee flexion
Patellofemoral joint: gliding joint where the patella slides over the surface of the femur to allow flexion and extension of the knee
The primary knee muscles during the squat are the Quadriceps group - they work concentrically during knee extension and eccentrically during knee flexion.

Performing a front squat reduces the compression forces in the knees and lumbar spine - therefore, if you suffer ligamentous or meniscal injuries this could be a better position to perform a squat in if you struggle with back squats.
However, shoulder and arm alignment with the bar must be correct otherwise the centre of gravity can shift forwards and is using the wrong musculature to perform these movements.
Ankle Joint:
There are 2 joints that make up the 'ankle' - sub-talar (inversion and eversion) and the talo-crural (inversion and eversion).
The primary ankle muscles that are involved during a squat are the soleus and gastrocnemius. They work eccentrically through dorsi-flexion and concentrically through plantar-flexion during dynamic movements.
Because of the attachments of these muscles over the knee and ankle, the medial head of gastrocnemius acts as a dynamic stabiliser for the knee during a squat, preventing knee valgus (medial movement of the knee) and posterior tibial movement.
Ankle mobility is important to perform a squat to help balance and allow full range of movement (without heels coming off the floor) to allow for depth in a squat.

References:

• A, Moss. R, Pellinger. T, Woodruff. K, Lewis. V, Booth. W, Khadra. T (2002) The Effect of a Back Squat on EMG Activity of 4 Superficial Hip & Thigh Muscles, Journal or Strength and Conditioning Research, 16(3): 428-432
• V. C, Almeida. G. L, Duarte. M, Hirata. R. P (2008) Kinematic, Kinetic & EMG Patterns during Downward Squatting, Journal of Electromyography & Kinesiology, 18: 134-143
• B. J (2010) Squatting Kinematics & Kinetics & Their Application to Exercise Performance, Journal of Strength and Conditioning Research, 24(12): 3497-3506