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| Gym Exercise Physics Quesion |
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| thm_w:
--- Quote from: bostonman on May 21, 2021, 02:50:34 am ---I have done weight lifting for many years, however, I never thought of the angles of forces. Not to dive into weightlifting as I wanted this to be more about physics, but I was always taught to never straighten joints because it can damage them. So I got into the habit of (or at least trying) not to lock my arms/legs, and, when doing certain exercises, to keep a very small bend (and not moving it) on a joint (such as "shrugs"). --- End quote --- Don't believe random advice people give you in the gym. Do your own research to verify. You want to exercise full range of motion, at least some of the time, to improve strength in those end ROM positions (eg lockout). Of course in this case its unlikely to really matter, as you are just shifting the load slightly away from tricept onto something else (as others have explained). And you probably have other arm exercises you are doing anyway. |
| EEVblog:
--- Quote from: M0HZH on May 20, 2021, 10:29:31 am ---In bodybuilding there is a quest for the best "isolating" exercises, which are the exercises that will work ideally a single muscle group with minimal effort from others. While these are great for the purpose, sometimes they put additional strain on ligaments or joints. If you're just looking to stay fit, I wouldn't bother with these and just go for the safer form. --- End quote --- The pioneers of body building like Arnold have told stories of how they discovered these things. They simply did thousands of reps of each of the different angles of motion and they found out which muscles were being worked by the pain and spasm etc in the effected areas. |
| EEVblog:
--- Quote from: IanB on May 21, 2021, 03:00:28 am ---You can overanalyze this. If you are holding a 25 kg weight off the ground, the downward force exerted by that weight is always 25 kg (force). It doesn't matter how you bend or contort your arms, the downward force at your shoulder remains exactly 25 kg. Now, if the weight is not hanging directly below your shoulder there can be other forces too, such as bending moments, and maybe you don't want these due to unwanted stress on muscles and ligaments. But the fact remains that a 25 kg weight exerts a 25 kg downward force due to gravity, and no matter how it is suspended, this force appears in a downward direction at the suspension points. --- End quote --- But when you bend your elbow, even if the weight is still directly below your shoulder, you engage other muscles and have also rotated the shoulder a bit. It makes a difference. |
| eugene:
So a simple free body diagram might not be incorrect per se, but the actual load path of the forces through the shoulder joint depends on which muscles are being used, if any. |
| bostonman:
--- Quote ---Don't believe random advice people give you in the gym. Do your own research to verify. You want to exercise full range of motion, at least some of the time, to improve strength in those end ROM positions (eg lockout). --- End quote --- I agree, but this wasn't necessarily random advice, however, it's also not exactly about lifting weights. Technically it seems holding 25kg in each hand hanging vertical would exert the same force on each shoulder regardless if elbows are bent or not, but common sense, years of going to the gym, and this thread, it makes sense that it causes other muscles to change the direction of forces. Now if the arm was replaced with two pieces of metal, connected by a hinge for an elbow, and a ball/socket for the shoulder, things should be different. If the hinge could be tightened so it's being bent at a 45 degree bend, the ball/socket would rotate so the weight is still perpendicular to the ground, however, now force would need to be calculated on the hinge and the ball/socket. Unless I'm thinking about this wrong, the same force would be on both the hinge and ball/socket. In this example, it would be 25kg * 9.8 (rounding up to 10) 250N on the hinge (i.e. elbow) but at 125N on the bottom piece of metal and 125N on the upper piece of metal, and then another 250N on the shoulder; but it wouldn't have any angles because it's just rotating on a ball and the downward force would be the full 250N. I think in the case of a human, the back muscles and would pull on the arm muscles thus relieving some of the force on the shoulders. Same with the elbow, the bicept/tricept would pull on the forearm thus relieving stress on the elbow. Maybe I'm reiterating everything said here, but believe this is the most common and simple explanation. |
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