It's common for masters runners who've been running for 2-3 years to beat those who've been running 20-30 years and who used to be extremely fast. It would appear that, after a point, running makes people worse at running. The reason usually attached to this is "tissue elasticity" - which is never defined and may be meaningless.
What I think happens is that, while exercised muscle fibers become stronger after slight tears in the tissues from effort, the tendons attached at either end of them, when torn, develop scar tissue cross-linking; this makes them stronger but less pliable and makes one's range of motion decrease. There's a strong correlation with top running speed and hamstring inflexibility, though inflexibility does not lead to speed.
The standard "solution" to this problem is stretching, which is itself problematic. Runners who stretch are more likely to get injured, rather than less likely; for a long time, I thought that this was because runners tend not to stretch if they think they don't need to, but once they get injured, start a stretching routine. It turns out, however, that competitive athletes also tend to stretch competitively, over-doing it and hurting themselves. After a year of classes where instructors told me I'd become more flexible if I went at it slowly and consistently - and didn't - I've come up with a process that's worked for me.
"Tendons shouldn't hurt if you press on them." That's one of the most important lessons I've learned in treating running injuries. If you press on a tendon and it hurts, continued pressure for 20-40 seconds (or deep tissue massage) often causes it to stop hurting; if it hurts so much that you cannot press on it for more than a second or two, you've got an injury. A month ago, I had more than a dozen spots like that, but have worked them out. Foam rolling is a decent way to deal with very minor issues, but I found I need much more pressure (accidentally bruised myself once) to make real change.
Once the tendons have been freed up, the muscles are easier to stretch. The procedure I used was to look up which muscles were connected to the tendons that hurt and then look up stretches recommended specifically for those muscles. The exercises tend to fall into groups, where you could change from one to another in a single motion; I'd move from one to the other and back slowly, which increases range of motion as well as anything I've found.
Does it work? Well, I can touch my toes now and couldn't before and I find that I'd been changing the way I move to compensate for tightness and pain but now can move more freely. There's an odd trade-off: what one gains in mobility, one loses in stability, so it's important to work on balance. Also, fixing one problem often discloses another. I've gone through more than 60 issues in the past month.
Over the past decade, I'd lost my back-kick when running fast because I'd lost some range of motion and my running stride became more stereotyped - if you always run in a slow shuffle, you become efficient at that, but lose some ability to run fast.
Only time will tell if increased mobility will allow me to compete better against the neophyte master runners.
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