An agent, action, or condition that elicits or accelerates a physiological or psychological activity or response.

When we put the body under conditions that are more stressful than it is used to, it responds in a myriad of ways. The mechanisms within the body - cardio vascular, muscular, neural, skeletal and hormonal (CMNSH) - are pre-programmed to protect and nurture our precious bodies.

It is exactly this response that we are trying to elicit in our training pursuits, whether they are sporting, health, weight loss or any other type of physical pursuit where a physical change is the goal.

It does not matter if you lift weights or train for marathons. Every time you train the goal is to get the body to respond to the stimulus you are giving to it. For the greatest improvements all the parts of CMNSH have to be subjected to something greater than before. When this occurs the body adapts to this extra stress by building its self bigger stronger and healthier.

Sleeping is essential for the rebuilding process as this is when it all happens. After your eyes shut the pineal gland (situated behind your eyes) releases melatonin. This starts a whole hormonal cascade of events with all the important hormones like testosterone and human growth hormone being released.

This is called the overload principle and is undisputed as the system by which we progress. When you are actually training or lifting weights you are not changing your body you are actually damaging it (in a good way). That is why athletes are told to: train, eat and sleep, then they grow!

However, as soon as your body gets used to the stimulus it stops. In other words your body has stopped adapting as the stimulus is not great enough to elicit the responses that it once did and adaptation has halted.

The body actually adapts quicker than most people think and keeping a training programme in the same mode for to long will either end with a plateau or possibly with some form of overuse injury.

For continued changes in all the systems of CMNSH it is essential to change the stimulus.

Below is just a small sample of the different types of components:

* Strength * Hypertrophy * Muscular endurance * Cardiovascular * Power

And these all have sub components that are interrelated, such as:

* Enzymic changes * Rate coding * Fibre type * Motor unit synchronicity

Add to all that plyometrics, lactate threshold, hill sprints, intervals, bodyweight....the list goes on.

Well I hope you're still following me but what I'm trying to say is - If you don't continually change the type of stimulus you give the body then you will stop changing.

Let's look at a practical example. Most people know some one who either plays tennis 3 times a week or goes out for a 3 mile run every day. The one thing you notice is they don't seem to be changing shape at all. Maybe they did when they started but not for a long time. Now the individuals body has become accustomed to it. By changing what they are doing the resumption of their progress is almost guaranteed. That doesn't mean they have to stop what they enjoy doing they just need to make some changes.

One word of caution though. Try not to change your training every time you do a session. In order to get the most consistent changes it is important that some sort of structured programme which changes regularly but follows a system is adhered to.

This is the basis of cycled training - also know as periodization - that optimises progression and avoids plateaus.

My clients and I will not spend any longer than 6 weeks doing the same programme. In most cases they are between 2 and 4 weeks. As I said earlier it is important to keep to some structure otherwise it is hard to monitor progress and progress will be slow.

To help explain this further I am going to look at one weight training exercise and explain how you would progress with it.

Example: Squats Weeks 1-2 Strength: 4-6 reps Weeks 3-5 Strength Hypertrophy: 6 to 10 reps Weeks 6-8 Muscular Development: 10 to 15 reps Weeks 9-10 Endurance: 15-50 reps

The weight lifted during each phase would try to be more on each successive session in that phase. This means you may squat 100kg for 6 reps in week one but by the end of week 2 you might be able to do 120kg for 4 reps. In week 3 you would obviously not be able to lift 120kg for 6 to 10 reps so you would select a lighter weight. Maybe 85kg, which you would challenge on each successive session until the phase changes.

This continues until you arrive back at week 1 again. At this point you will not be able to do 120kg for 4 reps as before but you wouldn't need to start back at 100kg and by the end of the week 2 you should be lifting more than you did at he end of week 2 the first time round.

Now I have massively oversimplified the concept (remember CMNSH systems are complex) and I probably wouldn't choose to do either all squats for every phase or return to squats again the second time. Hopefully you will have a better understanding of the key concept within cycled training of having a different focus.

It depends on your goals as to what you need to work on but I can almost predict that everyone reading this will avoid the things that they probably need the most. It's human nature, you can't avoid it.