ALONGSIDE the boom in activity holidays, from sailing to rock climbing, a less attractive pastime has developed - hours spent in the glum atmosphere of a doctor's surgery.

All too often injuries are ruining special interest holidays, and the cause is predictable. People who lead largely sedentary lives suddenly see themselves, for two weeks of the year, as the heroes of their chosen sports - and all too often come a painful cropper. They forget the watchword of most sports clubs: you get fit to play sport, not play sport to get fit.

It is also a maxim at the London Clinic, which has started, in its physiotherapy department, a fitness assessment unit for people planning active holidays. The centre is run by Jim Brennan, an affable ex-serviceman and physiotherapist who helped to set up the training programme for the television series The Krypton Factor. It caters for many sports, including skiing, canoeing, rock climbing, abseiling and adventure training.

The aim is straightforward: the prevention of injuries through safety and knowledge. 'We try to get people fit so they don't come back as another injury statistic. It is better to spend six weeks with us before the holiday, and enjoy it, than six weeks' physiotherapy with us after it because you got injured,' says Mr Brennan.

The unit gives new arrivals a basic fitness assessment, including aerobic or cardiovascular fitness, which tests the strength of the heart and lungs, and anaerobic fitness, which measures the ability of tissue to survive without oxygen.

The unit's physiotherapists work out a programme to bring each person up to a basic level of fitness. From that platform, a further plan is worked out for special needs, depending on the activity planned.

'These programmes are specifically tailored to each person's needs. For example, a woman of 45 who is an enthusiastic hill walker will not need the same vigorous exercise that a 24-year-old downhill skier requires. So they get different exercise programmes,' says Mr Brennan.

Initial programmes involve building up aerobic fitness to give stamina and endurance and to exercise the heart. 'The heart is a muscle, albeit a specialised one, and it still needs exercise. If it is not stressed except for the odd run for the bus or taxi, and you are contemplating going on a holiday where it will be continually stressed for up to four or five hours a day, then it is going to require some pre-running and some pre-programming.'

Would-be rock climbers are taught to check the progress of their fitness by pulse-reading at the end of each session. The greater the difference between your pulse rate immediately after exercise, and one minute later, the fitter you are.

Many of Mr Brennan's clients are in quite good condition when they arrive for assessment, possibly because many of them are, as he describes them, 'young businessmen about town' who may already have attended the clinic for squash or tennis injuries.

But when some 'suspect' individuals turn up, they are referred to their own GPs for medical checks to see if they are fit enough to go on the planned holiday or to do the sometimes strenuous exercises. Some people have to work within maximum and minimum pulse rates for which, again, a special programme is worked out.

The next step is to toughen up the muscles needed for the particular sport chosen. For example, someone planning a water-skiing holiday will not be given the same exercises as a snow-skier. 'The upper body and back are used completely differently in each sport, although the legs

are used very similarly,' says Mr Brennan.

'The body coming down over the ski slopes and over the water is very similar. So are the injuries. But the upper body in water skiing obviously requires a lot more strength, because it is there and in the torso that a lot of the technical strength is needed.

'We would therefore recommend a programme of specific fitness for that area after we had looked at your general fitness. If we build up straight from nothing to what we consider you need for your water-skiing holiday, we risk the chance of injuring you because you are not basically fit enough to do the exercises we have given you. So the first step is essential.' The whole process for the water-skier could take up to two months of regular exercise.

The situation would be easier if a 40-year-old woman, a regular badminton player, asked to be made fit for a walking holiday in Scotland. 'We would have less of a problem from her basic fitness, her cardio-vascular fitness. But from the point of view of leg endurance and strength in the ankles, because she is going to be wearing boots, the mechanics are going to be different.

'The movement of the ankle and knee joints is different, so she will require leg endurance to tone and protect both joints as well as the hip from the new ranges of movement she will be pushed into by the undulating terrain.

'So what we would do is specifically exercise for muscle endurance through the maximum range of both ankle and knee joints, so she is prepared for any movement. It would take two to three weeks.'

Mr Brennan stresses that the unit, which has been running for a year, is not interested in body-building for its own sake. 'The general opinion of a lot of young people today is that big muscles make you fit. That is not strictly true. It is about supply and demand: the bigger the muscle, the more blood is required to re-supply, and therefore the more fit and efficient the heart and lungs have to be.'

'What we do is ask: what type of fitness do you want? Do you want muscle tone, muscle bulk or muscle power, and why do you want it? Is it geared to your basic sport? If it is not, we advise on the way in which you can turn the programme around to make a more efficient human body rather just bulk for the sake of appearance.'

An initial assessment and programme at the clinic costs pounds 64. After that the customer can either do the exercises, for which no equipment is required, at the unit, which will cost more, or wherever it is convenient.

'People enjoy their holiday much more if they feel confident,' says Mr Brennan. 'Their fitness will sustain them through it and become a springboard to a fitness-conscious future which will help them through any stress at work. We hope it will not just last for their holiday, but become a continued awareness of the need to be fit throughout life.'

London Clinic, 20 Devonshire Place, London W1N 2DH, 071-935 4444.

The National Sports Medicine Institute (St Bartholomew's College, Charterhouse Square, London EC1, 071-251 0583) charges pounds 30- pounds 60 for fitness assessment and a specific exercise programme.

The New Cavendish Centre (114 Harley Street, London W1N 1AG, 071-224 4566) offers an in-depth assessment, including ECG stress tests and oxygen take- up levels. It will also prescribe the relevant exercises. The cost is pounds 120- pounds 210.

City Health Care (36 Moorgate, London EC2R 6EL, 071-638 4988) has two- hour fitness assessments, including ECG tests and oxygen take-up levels. It will also provide an exercise programme which would have to be done at a gymnasium. The cost is pounds 318.

Bupa runs pre-ski fitness classes at many of its hospitals. It also does full medical checks at which patients are given exercise programmes for specific activities. The cost of the checks is pounds 342 for men and pounds 366 for women.

(Photograph omitted)