A man performs an exercise called \"Good Morning\" [for reference: https://www.y

Question

A man performs an exercise called "Good Morning" [for reference: https://www.youtube.com/watch?v=YA-h3n9L4YU] with a barbell that weighs 225 lb force, which if executed safely can be an outstanding exercise for strengthening the lower back and adding power and safety to the squat and deadlift exercises. He made sure to use good form with multiple reps, as this exercise can be dangerous if done carelessly.

What was the collective muscle force required at the bottom of the movement when his upper body was parallel with the floor?

What was the joint reaction force and what was the angle of that joint reaction force?

Assumptions:

Distance from his hip axis of rotation to his upper body’s center of mass (~115 lbf) is ~14 inches.

Distance from where the barbell rested across his shoulders is ~23 inches from the axis of rotation.

The primary muscle groups involved in this exercise are the hamstrings and gluteus maximus (cause rotation) and erector spinae (keeps back straight). To simplify, assume they work synergistically with a common insertion point that is 4 inches from the axis of rotation with an angle of 11 from his spine.

There is no figure, only the video for a reference guide given:

https://www.youtube.com/watch?v=YA-h3n9L4YU

Explanation / Answer

The collective muscle force required at the bottom of the movement when his upper body was parallel with the floor=.Total mass X acceleration

where MASS =>Mass of the barbell + upper body centeral mass=>225+115=>340

Acceleration =9.8 (since the body is parallel)

Total muscle force required=>340 X 9.8=>3332 Newtons

The joint reaction force is same as the muscle force because to every action, there is an equal and opposite reaction which is 3332 Newtons

The angle of joint force is 180 degrees because the body is parallel to the floor.

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