Question

When a weight-lifter lifts a weight equal to his upper body weight his trunk makes an angle of 40 degrees, with the vertical, as shown in the diagram below. In this position the erector spinae muscles exert a large force on the sacrum at an angle of 40 degree to the vertical with a moment arm of 50.0 mm about the mid-point between the fourth and fifth lumbar vertebrae. Assuming that the weight-lifter is quasi-static.
(i) Calculate the position and magnitude of the ground reaction force
where upper body weight is 430N
lower body weight is 370N
upper body mass moment arm is 23cm
lower ody mass m0ment arem is 15cm
weight mament arem is 35cm
muscle force moment ARM IS 50CM

Answer 1

The ground reaction force (GRF) is the force exerted by the ground on the body in response to the body's weight and muscle forces. The position and magnitude of the GRF can be calculated using the following equation:
GRF = (Upper Body Weight + Lower Body Weight + Muscle Force) x Moment Arm

GRF = (430N + 370N + 430N) x 50cm
GRF = 1,230N x 50cm
GRF = 61,500Nm

The GRF is 61,500Nm and is located at the mid-point between the fourth and fifth lumbar vertebrae.