You haven't increased intensity for ages

When you first donned your training shoes and hit the gym, that aerobics class had you drenched in sweat and your heart thumping. Six months later, it's a breeze and you know all the moves by heart. Trouble is, the class is no longer presenting a challenge to your fitness level, so you're gaining zilch by doing it. "You're no longer getting what's called 'overload'," says Dean Hodgkin, fitness consultant for Ragdale Hall Health Hydro. "Your body's response to overload is what makes you fitter. If you don't work harder or for longer, you'll hit a plateau and reap no further benefits."

Your weights are too light

Sorry, but the ubiquitous can of baked beans or bottle of mineral water, often suggested as a substitute for dumbbells, just doesn't cut it if your aim is to firm, strengthen and shape your muscles. Yes, we know you don't want to "bulk up", but in order to get muscles to lose their slack, you need to overload them with a weight that is heavier than they are accustomed to. And if you can do 25-plus repetitions, then you sure ain't overloading. The solution? "Select a weight that you can lift six to 12 times, with the last couple of reps of each set presenting a real challenge," says strength and conditioning specialist Pat Fox, from the Human Performance Centre at London South Bank University. "As soon as that becomes easy, increase the weight."

You hold the 'non-responder' gene

Research by Pennington Biomedical Research Centre at Louisiana State University suggests that some people simply don't respond to exercise. More than 700 people were put through a challenging 20-week endurance training programme and, while markers of aerobic fitness improved by 17% on average, some people showed no improvement at all, while others improved by as much as 40%. "The inheritability of responsiveness to exercise is at least as great as body weight, blood pressure and cholesterol levels," says Claude Bouchard, one of the researchers. He estimates that about 10% of the population will never get any fitter through exercise - neither losing weight, nor improving strength, speed or stamina. Even so, don't throw in your fluffy towel just yet - you get a lot more than a fitter heart and smaller waist from exercise. For instance, it can help stave off back pain, reduce stress, keep muscles toned and be a lot of fun. Oh, you've gone ...

You overcompensate with needless 'refuelling'

Just come out of the gym or running club? Before you reach for that sports drink or energy bar, ask yourself these questions. Have you worked out for fewer than 60 minutes? Are you having a day off from exercise tomorrow? Are you trying to lose body fat? "If the answer to any of those is yes," says Anita Bean, a nutritionist and author of the Six Week Workouts series (A&C Black, £5.99), "then stick to plain old water to rehydrate, bypass the post-workout bagels and muesli bars - or beer and crisps - and have your normal meal when you get home."

You're lazy the rest of the time

Research published in the Journal of Applied Physiology in 2002 revealed that the reason people's overall daily calorie expenditure doesn't increase when they take up exercise is that they unwittingly burn fewer calories during the rest of the day by being less active. Unfortunately, a 20-minute treadmill run does not give you carte blanche to put your feet up for the rest of the day. The study's authors suggest that newcomers to exercise may wear themselves out with over-tough workouts, forcing them to retreat to the sofa, so make sure you don't exhaust yourself.

You're not consistent

Getting fit is like climbing a staircase - each workout you complete contributes to your reaching the next step. Conversely, each session you miss is a step backwards. A study by scientists in Sweden found that after a lay-off of only six days, up to 10% of aerobic capacity can be lost. Strength, happily, lingers a little longer. If you have to cut back due to other commitments, try to maintain the intensity, rather than the frequency, of your workouts.

You don't have any goals

If you are one of those dazed individuals who make their way wearily around every single machine in the gym, listen up - it isn't law that you have to use all the equipment. In fact, you'll get a lot more benefit from a workout tuned to your specific goals. Not sure what they are? As the adage goes, if you don't know where you're going, how will you know when you've got there? Think about why you started exercising, about what you wanted to achieve - and get a professional trainer to overhaul your programme to ensure you're not wasting time or effort in the gym.