A friend of mine that is really into fitness but only does it in a competitive team manner has recently suffered an injury that is likely going to take him out of not just his sport of choice, but likely all sports that have a jerky movement to them. The sport he was playing was soccer, but it could apply to any sport where a person has to change directions at the drop of a hat.
A torn ACL is no joke, and even with millions of dollars worth of treatment, professional athletes more often than not do not recover from this. I'm not suggesting that people don't engage in these sports but I do think it is important to recognize that the human body, no matter how fit it is, will eventually give in for people that are over around 35.
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I don't know a huge among about the ACL or knees in general other than the fact that they are a pretty important part of most exercise. In a sport where you never know when you are going to have to turn around or head in another direction abruptly, you are going to be engaging the knees a lot more often than if you were just running, swimming, cycling, or something else similar.
My friend has tried to remain competitive with people 10 or ever 20 years younger than him for the past decade and I think that he and I can both agree that this was likely a mistake that he is going to be paying for, for a long time if not the rest of his life. There is a reason why leagues are made with age limitations and I think that my buddy is regretting that he didn't elect to become a part of these options.
He is now in recovery, has had surgery, and is now going to physio several times a week. This will have to continue for months and even once he is given the green light the levels of exercise that he is going to be able to engage in is going to be very limited. He would be foolish to rejoin playing soccer at a competitive level, but you never know how that is going to turn out, some people are stubborn and very competitive.
I'm not trying to tell people that they have to "give up" but I think it is important to recognize the fact that grandfather time has effects on all of us and it might not be a terrible idea to listen to your body when it is trying to tell you that you are out of your depth.
I think that if you want to remain active as you get older, and you really should do this, that you can still be involved in competitive sports, but you might need to dial it back a bit and focus on things that involve more exercises that can be controlled by the user. I don't know what my friend is going to do because he is quite disappointed that he will not be able to play for upwards of a year. After that, i think he will be very apprehensive to end up back in this situation again.
I'm hoping to get him involved in lifting weights and perhaps triathlons because while injury can and does happen in those things as well, at least it is a lot more controllable.