Question

Imagine someone going from a shoulder width standing double leg stance position to a single leg stance position. Please describe what happens to their base of support as well as their center of gravity, and where the line of gravity moves to.

Please explain what muscles (at least one that crosses each joint in the kinetic chain) have to work harder in this single leg stance position.

Lastly, pick one of these muscles and explain why it has to work harder in single leg stance, include in this explanation its moment arm relative to the line of of gravity.

Answer 1

Please answer explained below.

If a person is standing in single leg stance position the base of support (area inside greater toe anteriorly and heel posteriorly ) is less compared to double leg standing position ,because in double leg standing position area between both legs is the base of support .

In single leg standing center of gravity shift to non-supporting leg and it causes downward force to the tilt the pelvis .center of gravity is about 5/9 of the distance from the greater trochanter of the supporting leg .During single leg standing the force applied on head of the femur is very large ie; 2.4 times the weight of a person and the force exerted by the abductor muscle on the trochanter is also large .These two forces are in opposite directions and little away from each other .This causes development of compression and shear in femur.

the line of gravity passes posterior to the hip joint and anterior to the knee and anterior to the ankle .

In single leg standing body weight act eccentrically on the hip and try to tilt the pelvis in adduction . this is balanced by the abductors of supporting leg .Major abductor is gluteus Medius .The downward counterbalancing force exerted by gluteus Medius on the supporting hip act as a fulcrum.