Every muscle has an equal and opposite muscle, called its antagonist. When a muscle contracts, its antagonist relaxes by the exact same amount; when a muscle relaxes, its antagonist contracts by the exact same amount. Biceps and triceps are the perfect example - when the biceps contract to bend the elbow, the triceps have to relax (see image)
Dysfunction starts to happen in a joint when one muscle begins to become much stronger than its antagonist. If you over-exercise your biceps, and under-exercising your triceps, you will end-up unable to straighten your arm. This isn't very likely in day-to-day activity, but is very likely if you're performing repetitive movements - like sport.
Sports often involve 1 or 2 specific movements that are repeated several times - even if the sport is otherwise quite varied. In water polo it's eggbeater kick, in wrestling it's grappling, in cricket it's side-bending, in cross fit it's gripping etc.
When athletes perform these repetitive movement without training the antagonist muscles, the joint range of motion decreases, injury occurs, and sports performance suffers. That is why we cross train!
Cross training is about finding a sport that exercises the antagonists of the muscles you over-train in your regular sport: if your regular sport involves pulling motions with the arms (eg. swimming, pole vault, rock climbing), pick a sport that involves pushing motions with the arms (eg. basketball, cross fit, shot put) etc.
For more information on cross-training, email me directly via the contact page. For specific information on an injury you have, or for treatment, contact one of my clinics directly. You can find that information here.
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Dr Mitch Clark