Taking a stroll on a grassy common these days, you'd be forgiven for thinking that National Service still existed.
Rather, it is a group of people participating in a military fitness class - and they're supposed to be having fun.
Over recent years, this no-frills bootcamp-style training has been adopted by fitness professionals and organisations everywhere.
Every day more people flock to join the sweaty throng, perhaps as a result of our increasing awareness of obesity, or because classes like these cost approximately a quarter of the average personal training session (about £10 a class in comparison to more than £40 for an hour's one-on-one training).
By far the best known of such outdoor fitness providers is British Military Fitness (BMF), running classes nationwide, with 100 a week in London alone. Dr Ralph Rogers, consultant in sports medicine at the London Orthopaedic Clinic, worries that these kinds of bootcamps result in injury, not weight loss and fitness.
He says: 'Exercise needs to be done gradually, with proper supervision, otherwise there is a risk of injury.
I would never recommend an overweight patient to do one of these military bootcamps.
When you overload the body, the result is injury - anything from shin splints to back problems - and, in this kind of environment, people make things worse by trying to soldier on.
'Neither the psychological aspect of why someone is overweight or the nutritional aspect is addressed by a bootcamp.
Even fit people can hurt themselves by being over zealous.' Osteopath Paul Raw agrees: 'I've seen a lot of ex-soldiers with bad backs because the idea of military-style training is to push yourself beyond your limit.