A student attempts a back extension exercise using two different machines. In th

Question

A student attempts a back extension exercise using two different machines. In the first machine, the student performs the back extension until the body is completely horizontal. With the second machine, the student is positioned at an angle of 30 degrees and performs the back extension. If the weight of the student’s upper body is 400 N located 0.40 m from the axis of rotation at the hips, what is the torque that the back muscles need to generate in order to stabilize (i.e. not move) the body in the extended position on each of the machines?

Explanation / Answer

given that

weight = m*g = 400 N

r = 0.40 m

theta = 30 degree

we know that

torque = F*r*sin (theta)

for the first machine when body is completely horizontal

theta = 90 degree

F = m*g = 400 N

r = 0.40

torque = 400*0.40*sin(90)

torque = 160 N*m

for the second machine when body positioned at an angle of 30 degrees

so

torque = m*g*r*sin(30)

torque = 400*0.40*0.5

torque = 80 N*m

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