Exercise is often regarded as a panacea for many things..."if you want to lose weight, you must exercise"; "if you want to get those feel good endoprhins racing, take some exercise"; "if you want to help with stress, you should take up exercise".

All of this may be true but there are times when exercise is NOT a good thing and should be undertaken with caution or proper supervision. A good example of this is stress. Although exercise is often promoted as a means to work off all your stress, physiologically exercise actually puts stress on your body.

What happens when you get stressed about something? You might start to sweat; your heart races; your blood pressure goes up; you breathe a little heavier....what happens when you exercise? You sweat, your heart pumps faster, you breathe harder. Physically, your body's response to exercise is exactly the same response to a stressful situation.

So if you are in a stressed or even 'distressed' state and this has been the case for a while, your body will have been placed under enormous pressure to maintain its equilibrium (also called homeostasis) throughout this period. If you then add exercise to the mix, the additional stress that exercise puts on your body may just push it to breaking point. Your immune system could shut down and make you susceptible to illness, your blood pressure may rise dangerously high and you may feel dizzy or nauseous because your heart is struggling to pump blood fast enough to your muscles and brain.

And for the guys wanting to pump iron and bulge those biceps, if you are stressed (either from work or even from a cardio workout), you will have cortisol - a stress hormone - pumping round your blood. Cortisol inhibits the production of testosterone - another key hormone that is needed for muscle growth. If you pump iron with cortisol in your blood, your ability to produce testosterone is limited and you'll therefore be wasting your effort to bulk up your muscles. So if you've just done an aerobic workout and stressed your body and then go into the weights room, you're not going to see as much progress as you think you should.

But.....I do NOT want to put anyone off exercise, especially aerobic exercise that helps to keep your heart & lungs healthy. In fact, as a personal trainer that's the last thing I want to do!

Exercise can be a great stress reliever and the perfect way to wind down after a day of frustration but if you are stressed - and by this I mean truly & physically stressed out - try and de-stress first with some relaxation and meditation, before placing your body under any more stress with 10km on the treadmill. And if you want to build bigger muscles, don't do any aerobic exercise in the same workout - or even on the same day - as a resistance workout.

And if all else fails and you are unsure when and how to exercise safely, consult a fully qualified and registered exercise professional.

Lea Woodward is a qualified personal trainer in the UK. She runs a health & fitness company, activOne, that provides personal training, massage therapy and training resources to other personal trainers see http://www.activone.co.uk