JOGGING was never particularly good for you, so doctors said: its jarring action could damage the knees and the back. But if cycling your way to health was ever an alternative, it may not be now.

American doctors have just put a question mark against mountain biking. Almost all the cyclists interviewed by one Californian doctor had suffered at least one injury in the past 12 months.

Dr Robert Kronisch, a sports medicine consultant in San Jose who recently completed a study of serious mountain biking injuries, discovered an incidence of damage far higher than for jogging or most other sports. Interviewing 265 riders, he found that more than 20 per cent had suffered injuries serious enough to warrant medical attention in the last year.

Most of the injuries were fractures, lacerations and shoulder injuries, Dr Kronisch reported. Almost all the cyclists questioned had suffered some injury in the past year.

Most of his interviewees for the Clinical Journal of Sports Medicine were from US mountain bike clubs, which suggests that they were advanced riders, possibly more likely to take their machines over punishing terrain than novices. Most beginners, like Britain's four-and-a-half million mountain bikers, are happy to cycle in parks or on simple country paths.

On the other hand, as members of a club, they are more likely to be experienced and to know their limits: beginners may be at more risk of injury.

Dr Kronisch is not alone in his concerns. Ron Phiffer, who lectures in cycling medicine, spoke at a conference on 'The Role of Medicine and Science in Cycling' in Colorado this summer. He said that mountain biking should definitely not be taken up as a means of getting fit. Beginners should get fit first, and then buy an appropriate bike, he said.

'For beginners it is crucial that the bike fits properly. One of the most common injuries is patella tendinitis, causing painful and inflamed knees. This happens when too high a gear is used (meaning more effort is required to push the pedals round) and when the saddle is too low.' Back pain and saddle sores are other common ailments.

Dr Ed Burke is a physician at the Colorado Springs base of the US Olympic cycling team. He says that most mountain bikes are never used on difficult terrain, but that 'the roads of London or New York can be just as rough as some of the tracks here in the Rockies'.

He says that people who want to get fit on a bike must cycle systematically. 'It is only a good way to get fit if you do it regularly, at least four times a week, and get your cardiovascular activity up for at least 20 minutes a time.'

Ron Phiffer, who used to race for one of mountain biking's top teams, advises beginners to take the sport slowly for the first few months and to get a full medical if in doubt about fitness. 'People take a while to learn to do it properly,' he said. 'It is while they are learning that they are most susceptible to injuring wrists and clavicles (collar bones) because they can't fall properly.'

Getting healthy on a mountain bike is one thing: staying so is another. In America last year more than a thousand cyclists died from head injuries. The 'beer cooler', the styrofoam crash helmet, is now mandatory in many North American cities.

Dr Burke makes no bones about recommending them: 'Wherever you go, cycling is only a low impact activity until you crash.'

(Photograph omitted)