On how quads and hamstrings decompress the joints…

There probably is no greater anti-gravitational muscle we take for granted than the quadriceps. In fact many of us don’t even know when we are using them, because the events associated with the concentric and eccentric force they provide is overshadowed by the mindful result that occurred as they worked.   They are our “ing” muscle.   We use them in walking, squatting, jumping, running, swinging, dancing, lifting, reaching, shifting, and fighting.  Fighting to not fall forward or backward at the same time.  The lower quadriceps tendon pulls us forward so we don’t fall back and our upper quadriceps attachments allow us to extend backwards, with our back extensors, by keeping our pelvis and hips anteriorly rotated.  This concomitant hyperextension of the knee and back decompresses our joint surfaces associated with lateral movement, simply because the quadriceps neurologically and mechanically are associated with autonomic “fight” and “flight” subconscious over sagittal plane stabilization.  

There probably is no greater anti-gravitational muscle that we don’t appreciate for anti-gravitational transverse and lateral control during quadriceps co-contraction than the hamstrings.  The lower hamstrings attachments slightly move the tibia back under the femur as the knee decompresses.   This decompression occurs with hamstring lateral frontal and transverse plane directional, tuning and turning, guidance provided by two hamstrings; one on the outside and one on the inside, of the knee.   The outside hamstring has an external rotational influence on the knee and the one on the inside has an internal rotational influence on the knee.  Thus, both of them acting in unison, provide functional stability as the knee decompresses; while the central tendon of the anterior knee, the quadriceps tendon, destabilizes the joint, because of its lack of lateral joint attachment and its sagittal plane association. The proximal, or upper, attachment of the hamstrings also acts as destabilizer, since it also is central tendon, that attaches to a bone that will decompresses the anterior hip joint when the muscle antagonistically resists the pull of the upper quadriceps, that are acting more as stabilizers, because of their multiple attachment sites. 

There probably is no greater anti-gravitational muscle that stabilizes the femur during quadricep – hamstring co – contraction than the adductor magnus and the biceps femoris muscle.  The first serves as an external rotator of the linea aspera and the second as an internal rotator of the linea aspera.  It compliments the gluteals above the linea aspera and the hamstrings and gastrocnemius below the linea aspera.

Ron Hruska