When was the last time you changed your conditioning program? When was the last time you added more weight, more miles, different exercises? It was more than a month ago, your body has probably stopped progressing. In fact, consider yourself lucky if you haven't gone BACKWARDS in your conditioning. And if you've hit an athletic plateau, your unchanging training may be the reason why.

Many recreational athletes just don't realize how quickly the human body adapts to repeated movement. In a way, this millennia-old trait is a good thing; it's a survival mechanism, left over from our ancient days of nomadic wandering to find food, water and shelter.

If the food source dried up, only those who could walk to find a new supply would survive. The first few days of walking 20 or more miles before dark, carrying babies and weapons, must have been horrifically exhausting. Our ancestors probably took some rest days. The next trek may have been a little less physically draining, and the next, even less so.

Over time; many, many generations of time, the human body's quick adaptation to physical stress allowed the human species to live long and prosper. Its main advantage is the saving of energy. While it takes the organism a lot of work to do something the first time or two, after the adaptation, the body can do the same amount of work without using the same amount of resources. Less nutrients are required to power the same amount of physical stress. The body isn't exhausted as quickly. There is faster recovery.

That quick adaptation, also common in many other species such as canines, is now ingrained, and with us still. The human body has become adept at adjusting fast to whatever physical stress is placed upon it.

That's why you have to keep changing your workout to keep progressing, especially athletically. The first time you try a mountainous hill on your bike, it may take you an hour of heavy gasping to get to the top. But as every athlete knows, if you keep practicing, it gets easier and easier. Soon you'll be able to do it in 40 minutes without even panting hard. The same goes for running around bases, swimming in a triathlon, even lifting a heavier weight.

That's also exactly why it's essential to frequently change your training routine. But the most important part of beating adaptation is not just to do MORE, it's also to do DIFFERENT. If the leg press machine is one of your constant exercises, don't just increase the weight — change to a different type of leg press, one that uses muscles somewhat differently. Use the smith machine or the hack squat. If you sit on a bench to do biceps curls, change to doing it standing while bent over the bench, supporting yourself on one arm while curling the weight with the other.

Because everyone's body adapts at a different rate, you might need to change your routine every three weeks or every six weeks instead of the average of every four weeks. Test yourself — your legs may adapt faster than your upper body. The most accurate test is the resting heart rate. Take your heart's beats-per-minute before getting out of bed in the morning. If it goes up, that may indicate overtraining (another, and important, subject). But if it goes down, even slightly, it indicates your body has adapted to your current training.

Once a physical stress like resistance training or longer mileage gets easy, without next-day soreness, it's probably time to change it up; use a different exercise in the gym or a different type of mileage terrain such as hills instead of flatland, cracked irregular concrete instead of smooth asphalt. Constant change will always beat the constant adaptation factor, making your progression to a stronger and faster athlete continue to expand.