Elbow Flexion Joint Reaction Force

[ziywang50]

Homework Statement:: This is not a homework, but I want to know how to find the joint reaction force of the 2d elbow flexion, if I know the angular acceleration of the forearm, and the mass of elbow with a known weight on hand. I assume the forearm is a simple cylinder. Suppose the angular acceleration is 0.6m/s^2. The mass of forearm is 10kg, and the mass of weight is 20kg. Suppose length of forearm is 30cm. Now 1) assume the upper arm is the same direction as gravity and hand is downward to the shoulder 2) assume the upper arm is 45 degrees to gravity and hand is downward to the shoulder I am confused with how to draw the free body diagram, since I read one introductory textbook but it only gives the stationary case and the arm is perpenticular to the force of gravity, and I am unclear about how to find the joint reaction force on the elbow joint. I want to find this force because I heard the joint reaction force is the major force to cause elbow injury.
Relevant Equations:: F=ma

I am guessing that the direction of the joint reaction force is just perpendicular to the forearm, but I am really uncertain about this. Can somebody help?

 

[berkeman]

Welcome to PF.

Can you upload a sketch of diagram of the system? Use the "Attach files" link below the Edit window to upload a PDF or JPEG version of rthe sketch. That would help a lot, IMO. Also include the muscle/tendon attachments that are generating the torque that results in the angular acceleration. Thanks.

I heard the joint reaction force is the major force to cause elbow injury.

Also, can you post a link to where you read this? And specifically what type of "elbow injury" are you concened with?

 

[ziywang50]

I read frm here. The diagram, maybe I will draw it later.

 

[berkeman]

I read frm here. The diagram, maybe I will draw it later.

That helps a little. Still, what kind of injuries are you primarily concerned with? If you are wanting to calculate the worst-case reaction forces, why are you not considering the case where the forearm is horizontal and the upper arm is vertical?

 

[berkeman]

[Thread moved from the schoolwork forums to the Biology/Medical forums]

 

[ziywang50]

@berkeman I am asking because I am not sure how to calculate the reaction force, and I do not know when the arm is horizontal it maximizes the reaction force. I mean it is shown on the paper but it is not really based on any theory and I am not going to use that simulation software they used. I am not sure what is the direction of that reaction force.

 

[ziywang50]

That helps a little. Still, what kind of injuries are you primarily concerned with? If you are wanting to calculate the worst-case reaction forces, why are you not considering the case where the forearm is horizontal and the upper arm is vertical?

View attachment 316680

Honestly any injury. But the paper gives elbow injury so I am considering elbow injury now

 

[berkeman]

Honestly any injury. But the paper gives elbow injury so I am considering elbow injury now

That doesn't really help. What is the overall purpose of your question? Do you have a medical worry, or is this for schoolwork after all?

There are a number of joint injury modalities, including chronic issues and acute injuries. Tendonitis is very different from compression fracture, for example.

 

[ziywang50]

@berkeman I just want to decide how to minimize the elbow injury from physics calculations.

 

[ziywang50]

@berkeman I just want to decide how to minimize the elbow injury from physics calculations.

Like an example would be which pose or which weight

 

[berkeman]

@berkeman I just want to decide how to minimize the elbow injury from physics calculations.

Like an example would be which pose or which weight

Sorry, still no help and too wide-ranging.

In addition to my EE full-time work, I work part-time in EMS and have actually used physical therapy for a couple joint injuries. So if you want help, you will need to say more please.

It sounds like you are wanting to avoid chronic elbow joint injuries (as opposed to acute injuries), possibly like tennis elbow and baseball throwing injuries? Or more weight lifter related injuries? I'm not the most patient medic in the world, so you have one or two chances left to explain what you want out of this thread...

(well, okay, I am the most patient medic in the world, but I'm still losing my patience)...

 

[ziywang50]

@berkeman I mean acute injuries, because I am looking at a weight lifting problem. Not chronic injuries.

 

[berkeman]

@berkeman I mean acute injuries, because I am looking at a weight lifting problem. Not chronic injuries.

So muscle strains and tendon sprains?

Can you upload a sketch of your scenario please? The more info you can provide us, the better we can try to address your questions.

Are you a weight lifter? Which lifts are you most concerned about? Should we assume bicept curls on a standard machine (Bowflex, etc.)?

 

[ziywang50]

So muscle strains and tendon sprains?

Can you upload a sketch of your scenario please? The more info you can provide us, the better we can try to address your questions.

Are you a weight lifter? Which lifts are you most concerned about? Should we assume bicept curls on a standard machine (Bowflex, etc.)?

Thank you for answering my post in detail. Yes, we can assume normal bicep curl for now. I am not a weight lifter, but I want to use code developed by other developers to find pose of a person then find if the weight is too heavy for that person by some physics calculation. The input should be a video, and my goal is to find which pose to minimize the chance of injury, given the weights are known. But I am not sure which force I should calculate and how to determine if the weight is too heavy to avoid acute injuries like tendon injuries and muscle strains as you mentioned.

 

[ziywang50]

@berkeman Dumbell lifting can be fine.

 

[ziywang50]

Here is my friend lifting, as an example

 

[BillTre]

I have seen some phone apps that are used to analyze body movements from videos, which you can take with your phone.
Did this with my daughter when she was doing track and field.
They can do a lot of analyses.
There might be some that do force calculations.

 

[Godot_]

[...]I assume the forearm is a simple cylinder. Suppose the angular acceleration is 0.6m/s^2. The mass of forearm is 10kg, and the mass of weight is 20kg. Suppose length of forearm is 30cm. [...]

...not based in reality.

The mass of the forearm is 3kg-ish, and the length is 40cm-ish.

 

[berkeman]

The input should be a video, and my goal is to find which pose to minimize the chance of injury, given the weights are known. But I am not sure which force I should calculate and how to determine if the weight is too heavy to avoid acute injuries like tendon injuries and muscle strains as you mentioned.

It sounds like you should be talking with a Physical Trainer or Physical Therapist about these questions. The mechanisms of injury (both for chronic and acute injuries) are not as straightforward as calculating reaction forces in joints. For example, when doing deltoid lifts with dumbbells, the degree of pronation of the hands makes a big difference in the likelihood of developing chronic injuries in certain muscle groups. PTs know this, and most people who don't use a PT do not know this.

 

[Fra]

I agree with previous speaker. Its the job of the senior trainer, sensei or whatever to make sure beginner dont start training the wrong way. Angles of limbs, knees and joints should be aligned to allow the intended muscles to take main load and minimize torque om joints. This goes also for training with hard punches and kicks. Different techniques may also be advised for speed vs force. I think after training you should have just normal sore muscles, not joint pain or overstretching something.

I learned this the hard way as well. Now i see i need to learn tecnique before going all in with power as the reactiona forces comes back to you when even hitting a heany bag. But thet may not tell you on first lesson.

/Fredrik

 

[pinball1970]

View attachment 316684
Here is my friend lifting, as an example

As per #19 & #20 Gyms usually give you a tour when you join for insurance purposes. This is not only to give you heads up on how best to avoid injuries but also to get the most out of your work outs in terms of muscle simulation.
They will also give recommendations on things like intensity and number of reps and sets, as well as best techniques.
Not just for your elbow but your back, shoulders and knees too (all important to you!)
Based on your age and weight they should be able to give recommendations on diet too, depends what you want to get out of it.