Collectively, the group of four long, slender but powerful posterior thigh muscles are known as the hamstrings. A strain occurs when a muscle or tendon is stretched beyond its physiological limits causing the fibers of the muscle or the tendon to stretch or tear with varying degrees of accompanying pain, weakness and loss of function.
Assuming a moderate or severe strain, the injured state of the affected muscle or tendon (in this case, the hamstrings) does not tolerate isotonic contractions (either shortening or lengthening) as this causes pain or aggravates existing pain. The hamstrings are chiefly responsible for knee flexion which is required to lift the referenced limb in order to ascend a stair (this is a concentric or shortening contraction which causes pain). Furthermore, when the now-flexed foot is planted on the stair, an eccentric or lengthening contraction of the same hamstrings is required to elevate the entire body including the contralateral limb to the level of the stair ascended which causes yet more pain.
On the other hand, there is an effect of gravitational assistance while descending stairs that results in the reduction of the contractile force needed from the muscles leading to less pain. In addition, for descending stairs, knee extension is required which is effected by the quadriceps and not the hamstrings.
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