This is a concidence, i just got back from training, and was working on single rep benchpress in preparation of the powerlifting meet i'm competing at the 27th.
The reason very high weight benchpressers arch their back, is range of motion. In a competitive meet, you will have to drop the weight till it touches your chest, then pauze, then press out. An arched back, reduces the distance the bar has to drop.
But arching is the least of your concent, if you arent doing pauzed reps.
To start off, the most important is the position of the shoulders, and the inner pressure.
Arching a back helps that last one a bit aswel. SImply lie flat on the bench, put your feet down the ground.grab the bar with your hands and lift your ass of the bench. then push your ass closer to your shoulders with your legs. You will arch, but you will also be alot more locked in.
Next, kick your legs under the bench, and try to find a position, in wich you can put pressure on the legs towards your shoulders.
THAT, is the key of the positionning.
Next, shoulders. You should grip wide enough, for me its ring finger on the rings.(but i'm 1m91 w long arms). When you push out the weight from the rack, try to press your back into the bench. Your BACK muscles should be moving the weight, like you are doing a pulldown. Meanwile, keep shoulders tight, like you want to squeeze a can of soda when placed between shoulderblades.
Now, when lowering the weight move your head into your chest, lifting it off the bench a bit. WIth your hands, try to bend the bar. Because of this, the ellbows will move inwards. THIS IS VERY CRITICAL. if your ellbows are sticking out, you put huge stress on the rotator cuffs, wich is the most commen injury. I had that twice before i trained at a decent gym.
Lower the weight till your chest. Try to keep pressing shoulders into the bench with your legs, keep that pressure on.
When your weight hits the chest, push out, trying to pull the bar apart to the sides. your ellbows will go out now, wich will help the movement of the bench. Mean time, whilst the head was tucked in while decending, when pushing the bar back up, move your head back onto the bench, this helps the first 10 cm when leaving the bar from the chest.
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Having said that, still looking for someone with experience in endurance events, or sport events in general that lasted over 3 houres