Knee pain or hip pain after crouching or squatting is common in those with tight lower back, hip and knee muscles. Most of us who sit all day long have tight muscles in the lower back and lower limbs. So when we perform crouching or squatting movements whether done in a prolonged fashion or repetitively, you may feel weakness, discomfort or pain in the hips, groins, front of the thighs as well as along the inner aspect of the thighs and perhaps even knee pain. This should tell you that the muscles in these areas were abused by the crouching or squatting positions.
The muscle responsible for discomfort or pain at the front of the thighs or pain in the front of the knees is the rectus femoris muscle and the muscle at the inner aspect of the thigh and knee that has been stressed is the adductor magnus muscle.
If pain is in the outer aspect of the knees, the pain is likely to be from stress to the tensor fascia lata muscle and if pain is at the back of the knees, the pain is from the hamstrings muscles.
Although the other quadriceps muscles are important for producing anterior thigh and knee pain, they are not the main muscles to be injured since the other quadriceps cross only one joint namely the knee joint whereas rectus femoris (which is also a quadriceps muscle), tensor fascia lata and hamstrings cross both the hip and knee joints and are thus more likely to be abused with crouching or squatting.
Initially, anterior thigh and knee pain is more common than posterior thigh pain. Because of our sedentary positions involving sitting for prolonged periods, the muscles in the front of the hip such as rectus femoris, tensor fascia lata and psoas major may be chronically shortened while the muscles in the back of the hip, the gluteus maximus (buttock muscle), hamstrings and adductor magnus are chronically over- stretched and weakened.
At the knee joint, sitting places the knees in a bent position, therefore the rectus femoris and the tensor fascia lata whose functions are to extend the knee are over stretched and weakened whereas the hamstring muscles whose function is to bend the knee become short and tight due to the imbalance of muscle power as in the see-saw principle.
Upon crouching or squatting, the sitting position is exaggerated with excessive hip and knee bending. Therefore, the rectus femoris and tensor fascia lata muscles become shorter and tighter at the hip and more long and stretched out at the knee. Excessive shortening contraction in bending (flexion) at the hip and excessive lengthening contraction in straightening (extension) at the knee injure both rectus femoris and tensor fascia lata muscles.
To maintain the person in a crouched position or squatting position, muscles at the back of the hip have to undergo an excessive lengthening contraction in addition to an excessive shortening contraction at the knee. Since muscles at the back of the hip that have to undergo a lengthening contraction such as the gluteus maximus and adductor magnus are huge and very strong, the hamstring muscles which also does the same action at the hip can now have more concentrated power for bending the knee. The stronger the pull of the hamstring muscles to actively bend the knee, the more power the rectus femoris and tensor fascia lata muscles have to exert to counteract this force.
Therefore, when you try to stand erect after crouching or squatting, the first weakness, discomfort or pain will be felt in the front of the thigh and front of the knee giving rise to thigh pain and knee pain.
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