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