Nicole Thornley was never seriously out of shape. A former international swimmer who represented England and Great Britain in Commonwealth and European championships, she had long considered super-fitness her priority. Years of training meant that her 5ft 10in frame was just under 9 stones of toned muscle. On retiring from competitive swimming in 2005, however, she found herself facing the reality of less athletic mortals.

“I’ve never been fat, but I needed to tone up,” says the 31-year-old. “I joined a gym, but didn’t know which equipment to use as all I’d ever done was swim.” Needing direction, Thornley put her faith in a personal trainer she hired at her gym. It was a decision that she thought would re-ignite her passion for exercise; instead it left her barely able to walk.

Like the 80,000 people expected to join the gym rush in January — a figure that is 65 per cent higher than in any other month of the year — Thornley felt that the tailored fitness regimen offered by a personal trainer would benefit her far more than group classes. But her experience should serve as a warning to those who plan to take a similar route.

In recent years, attempts have been made to regulate the personal training business and weed out unscrupulous practitioners with the launch of the Register of Exercise Professionals (REPS). This government-backed programme provides a guarantee that those on its books meet required standards.

Jean-Ann Marnoch, chief registrar of REPS, says that there are 27,000 personal trainers on the books of REPS and adds “to be safe, no one should hire a trainer before checking that they are on the national register”.

The last thing Thornley expected was to end up seriously injured following advice she was given. “I had full confidence in my trainer as he seemed to know what he was doing. He took various assessments and then designed a programme specifically for me,” she says. “I signed up for six sessions and he introduced me to several new exercises and pieces of equipment in the gym.” One of those was the popular Smith machine, used to perform squat exercises with added resistance and consisting of a rack with a suspended barbell that moves up and down on steel runners.

“I had never used the Smith machine before,” Thornley says. “But by my fourth personal training session I was asked to squat with 145lb — much more than my total body weight.”

Finishing her session that Friday evening, Thornley sensed something wasn’t right. Later that evening she felt excruciating pain in her back. “I lost consciousness and by the following morning I could barely lift my head off the pillow,” she says. “I rang the hospital and was told to take painkillers and anti-inflammatories and to go to my doctors first thing on Monday morning.” It was the beginning of what Thornley describes as a spiral of despair. Over the next four months the pain persisted and she twice lost consciousness.

In February 2006 she was admitted to accident and emergency, where she was injected with morphine. “Since then I’ve had time off work, been unable to drive and have been dependent on painkillers,” Thornley says. “I had physiotherapy, chiropractic sessions, acupuncture, tranquil- isers and an epidural.” An MRI scan revealed that two intervertebral discs had been ruptured, including one prolapse disc and a segment of disc material lodged on the sciatic nerve. In the end, surgery was the only option.

“When I was just 29, I had an operation on my back,” Thornley says. “But I was recently told by my surgeon that the symptoms I've got now, including back and leg pain, are likely to be permanent.” With legal proceedings under way, the gym company denies all liability and responsibility for Thornley’s injury, although she and her medical team are convinced that her injuries resulted directly from her personal training session three years ago. Her experience is far from isolated. Physiotherapists increasingly find that their workload consists of clients who were injured during personal training sessions.

In one case, a woman practising a wide-legged stretch was pushed so forcefully by her personal trainer that she broke her pelvic bone. “So many people now hire a trainer to get them fit,” says Sammy Margo of the Chartered Society of Physiotherapy. “And so many patients are coming in for treatment for injuries that have taken place when a trainer has pushed the person too far or advised bad technique.”

In the UK, anyone can legally set themselves up as a personal trainer, regardless of knowledge and experience. Fitness qualifications vary enormously from weekend packages or correspondence courses to the rigorous YMCAFit courses for per- sonal trainers and exercise-related degrees that incorporate the study of anatomy, physiology, nutrition and stress management as well as exercise prescription.

Times recommends

• Who killed Jade Goody?
• Daisy Goodwin: This is better than real sex
• Foods that can help to prevent swine flu