The squat is a well-known exercise for the knee and hip muscles and is commonly used in rehab programmes.
The advantage of the squat is that it is a closed-chain exercise where ankle, knee and hip joints must be co-ordinated, developing a functional movement pattern as well as training the muscles.
The squat can be performed in various ways, in terms of weight with or without a barbell, in terms of knee angle with the degree of knee flexion, and in terms of foot position with wide or narrow stance. This variation of loads and in exercise technique has an impact on the resultant knee joint forces and knee muscle activity. In turn this affects the suitability of the squat as a rehabilitation exercise. For example, very deep squats involve high compression forces making them unsuitable for patients suffering with knee injuries.
In a comprehensive review of the available research of the biomechanics of squat, (Rafael Escamilla (2001) Medicine & Science in Sports & Exercise, 33(1), 127-141), Escamilla provides a complete picture of the forces and muscle activity involved in the variations of the squat exercise. This is very helpful information for practitioners and trainers to make informed decisions about prescribing the squat exercise.
The tibiofemoral joint
A range of studies have calculated the shear forces acting on the tibiofemoral joint. These are the forces which draw the tibia forwards or backwards relative to the femur. A posterior shear force draws the tibia back, placing strain upon the posterior cruciate ligament (PCL) and an anterior shear force draws the tibia forward, placing strain upon the anterior cruciate ligament (ACL).
The studies find that there is a moderate load placed on the PCL during the squat, which increases as the knee flexes. The PCL force occurs after 600 of knee flexion - when the quadriceps force exerts a posterior force on the joint - with a magnitude in the range of 1000-2000 N. The maximum load of the PCL has been estimated at 4000 N, and so the squat exercise is perfectly safe for athletes with a healthy PCL. Those recovering from PCL injury should restrict the range of movement to no greater than 600 of knee flexion, as this is when PCL loading begins.
Contrary to the PCL, ACL forces were generated between 0 - 600 of knee flexion - when the quadriceps force exerts an anterior force on the joint. However the ACL loads are found to be low, at a peak of 500 N. Considering that the maximum load of the ACL is around 2000 N, it would seem squats involve little strain on the ACL and should be safe for ACL patients to include in their programmes. By increasing the forward lean of the trunk during the squat, ACL stress can be reduced to zero due to the increased hamstring activity providing extra posterior force on the joint. However, by increasing the forward movement of the knees the shear forces can increase and so rehabilitation patients should avoid this position by keeping the knees behind the toes.
The exercise speed can also affect shear forces. One study showed that a fast cadence squat (one second ascent and one second descent) produced up to 30% greater shear forces than slower cadence squats (two seconds each phase). Therefore slow and controlled technique will safeguard the cruciate ligaments.
The studies also calculate the compression forces in the tibiofemeral joint. This is the load due to the articulating surfaces of the tibia and femur. These compressive forces calculations range from 500 - 8000 N, where compression increases with the amount of weight lifted during the squat. There is no data concerning the maximum compression force that is safe for the joint, but one can assume that if very high loads ( > 7000 N ) are produced on a regular basis then miniscus and cartilage injury risks will increase.
Interestingly, increases in weight lifted do not effect the ACL or PCL forces in the same way as compression forces. One study involved lifters squatting 250kg, and although the compression and quadriceps muscle forces were very high (8000 N) the shear forces for ACL and PCL were in the normal range described above. This data suggests that a greater compression force may be important for the stability of the joint, helping to control shear forces.
The width of stance during the squat was also shown to increase compression forces, with wider stance increasing compression force by 15%. Stance width had no effects on shear forces, but shear forces are greater during the ascent phase of the squat exercise.
One important finding on a practical level is that as fatigue increases so do the shear and compression forces in the tibiofemoral joint. Most biomechanical studies will analyse a few repetitions; however, in reality patients and athletes will complete a few sets of a number of repetitions, e.g., 4 x 8. A study involving 50 repetitions of the squat showed that shear and compression forces increased from 25-85% from the first to last repetitions. This suggests that if your clients complete a number of sets of the squat, the joint stress may be much greater towards the end. This is why a cautious and progressive approach to the exercise prescription is important.
Patellofemoral compression force
Patellafemoral compression force is caused by the contact between the underside of the patella and its articulation with the femur. Calculations of the patellofemoral compression forces during the barbell squat with a weight of around 70% of maximum show that the joint force is 4-7 times bodyweight (about 4000 - 5000 N). These are generally greater loads than many patients will lift in the initial stages of a rehab programme, and so patients are unlikely to load the joint as much in a rehab context until the injury is healed and strength is regained.
The peak compression force occurs at the greatest knee flexion angles, generally around 900 and beyond. Patellofemoral patients (eg chrondomalacia patella) need to perform squats in the 0-500 range as the loads are moderate in this range.
The compression force increases with stance width. A study showed that the wide-stance squat increased patellofemoral compression force by 15% during the descent. Additionally, if the squat is performed in the low bar position, with the barbell held below the acromium, then greater trunk and hip flexion occurs in the movement and the patellofemoral forces are reduced. Patellofemoral patients should use a narrow-stance low-bar technique to minimise patellofemoral compression.
All the knee muscles - quadriceps, hamstrings and gastrocnemius - are involved in the squat to a greater or lesser extent. The quadriceps are the prime movers, particularly the vasti muscles, which show significantly greater activity than the rectus femoris. The peak quadriceps activity occurs at 80-900, with no further increases with greater knee flexion. This supports the idea that the half squat (to 900 of knee flexion) is the preferred technique as no further quadriceps activity will result from performing a full squat movement.
Hamstring activity is greatest during the ascent phase of the movement and is strongly related to weight lifted. During the bodyweight squat, hamstring activity is minimal and not until loads of around 12 RM are lifted do the hamstrings show significant activity, presumably to enhance
knee stability. Gastrocnemius activity is moderate during the squat.
Low stress. Heavy weights are safe. Slow and non-fatigued is best. Forward lean, low bar technique is best.
Moderate stress. Limit to 600 flexion. Slow and non-fatigued is best.
Tibiofemoral compression stress
Stress high with high weights and deep flexion. Very heavy & deep lifting should be limited. Narrow stance is best.
Patellofemoral compression stress
Stress high with increased knee flexion. Limit to 500 flexion and avoid heavy weights. Forward lean, low bar technique is best. Narrow stance is best.
Quads activity increases with knee flexion, peaking at 900 flexion.
Increased weight increases quads activity Hamstrings require heavy weights for significant activity. Half (900) squats are best.