What parts of the knee do squats strengthen?

What Parts of the Knee Do Squats Strengthen

Squats primarily strengthen the quadriceps muscles (rectus femoris, vastus lateralis, vastus medialis) and secondarily activate the hamstrings, gluteus maximus, and adductor muscles, with the depth of the squat significantly determining which muscle groups receive the greatest training stimulus.

Primary Muscle Groups Strengthened

Quadriceps Complex

• All four quadriceps muscles are activated during squats, with the vastus lateralis and rectus femoris showing particularly high activation levels 1
• The Spanish squat variation demonstrates the greatest activation of the rectus femoris and vastus lateralis compared to general or wall squats 1
• Quadriceps activity progressively increases as knee flexion deepens during the squat movement 2
• Quadriceps strengthening through squats provides an effect size of 1.05 for pain reduction in knee osteoarthritis, making it one of the most effective exercises for knee rehabilitation 3

Hamstring Muscles

• The hamstrings (biceps femoris, semitendinosus, semimembranosus) are co-activated during squats to provide dynamic knee stability 4, 2
• Hamstring activation increases significantly with resistance, rising from approximately 12% maximum voluntary contraction at low resistance to 27% at high resistance during single-limb squats 4
• This co-contraction reduces the quadriceps-to-hamstring ratio from 3.0 to 2.32 with increased resistance, which is crucial for reducing anterior tibial shear forces and protecting the ACL 4

Hip and Thigh Muscles

• Full depth squats produce significantly greater hypertrophy of the gluteus maximus (6.7%) and adductor muscles (6.2%) compared to half squats (2.2% and 2.7% respectively) 5
• The gluteus medius is also activated during squats to provide lateral hip stability 4

Depth-Dependent Muscle Activation

Full Squats (Deep Squats)

• Full squats are more effective than half squats for developing the adductor and gluteus maximus muscles while producing equivalent knee extensor hypertrophy 5
• Deep squats generate moderate to high quadriceps, hamstrings, and gastrocnemius activity throughout the movement 2
• The gastrocnemius (calf muscle) shows increased activation with deeper knee flexion 4, 2

Half/Quarter Squats

• Half squats produce similar knee extensor muscle volume increases (4.6%) compared to full squats (4.9%) 5
• Training in the 0-50 degree knee flexion range minimizes knee joint forces, making it appropriate for early rehabilitation when joint protection is prioritized 2

Clinical Applications for Knee Rehabilitation

Closed Kinetic Chain Benefits

• Squats are classified as closed kinetic chain exercises and should be prioritized in knee rehabilitation because they reduce patellofemoral pain compared to open kinetic chain exercises 6, 7
• Both open and closed kinetic chain exercises improve quadriceps strength post-ACL reconstruction, but closed chain exercises like squats produce less anterior knee pain 6

Rehabilitation Progression

• Leg press exercises (a squat variation) can be safely started at 3 weeks post-ACL reconstruction to improve subjective knee function and functional outcomes 6
• Isokinetic hamstring strengthening using a functional squat pattern (0-45 degrees) improves both quadriceps and hamstring strength after ACL reconstruction 6
• For patients who can squat 1.5 times their body weight, progression to plyometric strengthening and running drills is appropriate 6

Neuromuscular Control and Stability

• Squats enhance knee stability through improved neuromuscular control rather than compromising it when performed with proper technique 2
• The exercise strengthens periarticular muscles, which reduces mechanical stress on the joint and improves joint stability 3
• Squats improve sensorimotor function and address functional instability through enhanced neuromuscular control 3

Safety Considerations

Joint Forces

• Posterior shear forces (restrained by the PCL) are generated throughout all squat depths, while anterior shear forces (restrained by the ACL) occur only between 0-60 degrees of knee flexion 2
• Patellofemoral compressive forces peak near maximum knee flexion but are distributed over larger contact areas in deep squats due to the wrapping effect 8
• Deep squats do not increase injury risk to healthy knees and may actually provide better protection against degenerative changes than half squats with supramaximal loads 8

Optimal Training Range

• For healthy athletes, parallel squats (0-100 degrees knee flexion) are recommended over deep squats to balance muscle development with joint safety 2
• For rehabilitation patients, the functional range of 0-50 degrees minimizes knee forces while still providing therapeutic benefit 2

Common Pitfalls to Avoid

• Do not delay implementing squat exercises in rehabilitation, as early strengthening (starting at 3 weeks post-surgery) produces superior long-term outcomes 6
• Avoid relying solely on passive therapies instead of active squat-based strengthening, which shows significantly better results 7
• Do not assume half squats are safer than full squats for healthy knees—the evidence shows deep squats do not compromise joint integrity when performed correctly 8
• Monitor for anterior knee pain when introducing squat exercises, as this may indicate the need to modify depth or resistance 6, 7