A person lifts a heavy bag that has a weight of 275 N. In case A (see below) the

Question

A person lifts a heavy bag that has a weight of 275 N. In case A (see below) the line of action of
the force representing the weight of the bag is 0.32 m from the L5/S1 joint in the lumbar spine, while in case B it is only 16 cm from the L5/S1 joint. How much torque does the weight of the bag create about the L5/S1 joint in these two cases? If the back extensor muscles operate with a moment arm of 0.05 m relative to
the L5/S1 joint (in both cases), how much force do the back extensor muscles need to generate for the person to hold the bag in these two static postures? (excluding the weight of the person’s torso)

Case A

Torque about L5/S1 joint __________________

Muscle force __________________

Case B

Torque about L5/S1 joint __________________

Muscle force __________________

Explanation / Answer

case A ,

the torque about L5/S1

torque about L5/S1 = Force * distance

torque about L5/S1 = 275 * 0.32 N.m

torque about L5/S1 = 88 N.m

Muscle force = F

Muscle force = 275 N

------------

for the case B

torque about L5/S1 = Force * distance

torque about L5/S1 = 275 * 0.16 N.m

torque about L5/S1 = 44 N.m

Muscle force = F

Muscle force = 275 N

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