Overload. This is probably one of the most important, if not the single most important fitness principle out there. Look up any fitness program or talk to any personal fitness trainer you will always see and hear that in order to see real results you need to push yourself a little harder at each an every workout. Essentially, an overload principle states that in order for your body and muscle groups get stronger and better, you need to add more than normal (or previous) level of stress/load.
For example, let’s say you do a bench press of 135lbs today for 10 repetitions. The next time you do the same exercise, overload it. Add-on extra 10lbs and try to do 145lbs for 10 repetitions. This principle applies to any sport. The key thing is to make your muscles work harder each and every time than they’re previously used to either by lifting more weight, running faster or longer, or stretching further.
By overloading you’re letting your body and muscles adapt to new levels of stress. If you systematically do this that you’ll be able to reach your fitness and performance goals much faster.