ok today i rode my local trails in my fastest time ever and it felt great uintill i got home and off the bike, my legs then turned to jelly and i fell over!! can anyone think of any causes for this, i was drinking plenty of water so doubt it was dehydration

when you do physical exercise especially arduous exercise, your muscles can shorten and snap, basically this is what happens leading up to an injury. take my advice.... stetch off!!! you dont wanna be like me, ive got runners knee. the tendon on the side of your thigh, the ITB pulls your kneecap over and makes the kneecap rub on its seat. roughing the edges on your thigh and shin bones, it ends in much pain and swelling which equals time off bike. Gymnasium bikes did that to me with a combination of sit ups and rowing.

peace.

Not quite the case,

When your ripping your trails you get a release of a chemicals in the brain called endorphins, these are a feel good horomne.  They only last for as long as your doing something that feels good. 


When you've finished ripping and got home you experience a "come down", which can be psychological, or in your case physical.  This is also the same with adrenaline.

Other reasons can be lack of NaCl (sodium chloride, basically salt) as it affects nerve conduction and can make you twitchy, also lack of carbohydrates or build up of lactic acid.

Jus get home, chill and grab some munch...you'll be fine dude!

strecth before you ride.... helps alot... thats all i reccomend, and take it easy in the heat

build up pof lactate in your blood system mainly legs and not enough ATP production to carry on exersize meaning you have to replace the oxygen debt and AD and Pi in your blood system to be able to work again this can take from 30 seconds to 48 hours to be full replenished.