Caused by: Running
Pounding the pavement has loads of health positives, but it can't half play havoc with your face. "Running can cause a loss of volume in the cheeks, especially if done for long periods and after the age of 35, when the skin begins to lose elasticity and collagen breaks down," says Dr Nick Lowe, a dermatologist at London's Cranley Clinic. "Running can accelerate this process because those who do it tend to have low fat levels and this leads to a drawn, hollow look and sagging facial tissue."
What to do: Dr Christine Lydon's 10 Years Thinner offers a great alternative workout. Running helps shift fat from your hips and thighs, and her regime - which includes intense killer squats, lunges, press-ups and light weightlifting in short bursts - gives the same results, minus all that pavement plodding.
Caused by: Being too fit
If you're aiming for a superwoman-style body to get baby-ready, beware. "Women who are extremely fit will have little fat tissue - and fat is the building block of reproductive hormones like oestrogen," says Dr Gillian Lockwood, Medical Director of Midland Fertility Services. According to fertility expert Zita West, for a fertility-friendly body, your body mass index (BMI) should be between 19 and 25. "A BMI of 18 or less is too low if you are trying to conceive," she says.
What to do: Keep exercise moderate. "Any stressful situation can prevent you conceiving, and that includes over-exercising," says fitness consultant Jane Wake. "Work out at a light to moderate level. The ideal is 30-45 minutes of brisk walking or swimming five times a week."
Caused by: Core stability exercises
Sit-ups are the answer to abs of steel only if done properly. "The popularity of techniques to tone the muscles around the abdomen have become a dangerous fad," says Dr Morc Coulson, author of The Fitness Instructor's Handbook. "In the hands of the wrong instructor - or no supervision - core exercises can not only exacerbate back pain by confusing the muscles in the tummy and back, they could cause it too."
What to do: If you have back pain, see a physiotherapist who can prescribe safe spinal exercises (see Csp.org.uk). Avoid Swiss balls without the help of a qualified trainer, and make sure you find a qualified instructor if doing Pilates. Visit Pilatesfoundation.com.
Caused by: Your kit
Is your post-workout complexion more lumpy than luminous? Pimples on your forehead, thighs, bottom or back could be down to what you're wearing. "When headbands and tight tops rub against the skin and stop it from breathing, you can get blockage of the oil ducts in the skin's surface," says Dr Nick Lowe. "This leads to what doctors call acne mechanica, because its cause is outside the body."
What to do: Remove make-up before your workout. Dr Lowe recommends using a foaming facial cleanser and putting some spot gel on susceptible areas just before the workout. Try Origins Spot Remover, £10, or Dr Lowe's Spot Gel, £8.76. Exercise-wise, try something less sweaty but with the same impact, for example swimming.
Caused by: Yo-yo exercise
Suddenly decided to get fit? Don't overdo it straight away! "If you're inactive for long periods, then suddenly put your body under extreme stress by
What to do: "Keep exercise regular and build it up over time," says Professor Whyte.
"If you're training for a specific run, don't stop exercising after the run is completed, but switch to walking or a lighter workout." If you find yourself getting bored, update your iPod! Studies at Brunel University show that tunes of 120-180 beats per minute (BPM) are the most motivational. Pump It Up by Ministry of Sound, £6.32, shows the BPM next to each song.