An athlete is doing a knee extension exercise using a 250 N dumbbell strapped to
ID: 1435212 • Letter: A
Question
An athlete is doing a knee extension exercise using a 250 N dumbbell strapped to her ankle which is 30 cm from her knee joint axis of rotation. She holds her leg so that the horizontal distance from her knee joint to the dumbbell is 15 cm.
a. For this position, what torque is created by the dumbbell about the knee joint axis?
b. If the moment arm of the knee extensors is 4 cm about the knee joint axis, what force muscle the muscles produce to hold the leg in the position described? Ignore the weight of the leg.
Explanation / Answer
a. Since gravity works vertically, the 15 cm distance should be used since it will be perpendicular to the force of gravity working through the weight. T = Fx d = 250N(0.15m) = 37.5 N-m
b. The torque due to the weight must be equal in magnitude, but opposite in direction. 37.5 N-m =Fmuscle*(0.04m)
Fmuscle= 37.5/0.04 = 937.5 N