Construction workers have a reputation for baring more than is easy on the eye. And they don’t all look like the man in the cola ad. Which makes the appeal of the latest fitness trend from America somewhat questionable for the fiterati are flocking to do builders’ workouts in a bid to tone up.

At the Crunch chain of gyms, the Sacked session invites members to “heave-ho their way to fitness’’ by hauling around bags of builders’ sand. Elsewhere, the CrossFit commando-style training, which has a cult following across the US, incoporates the sandbag-carry, an exercise in which a 40lb bag of sand is carried in any way but over the shoulder for a specified distance. Now the builders’ workout is reaching across the Atlantic with the UK launch of Outdoor Extreme, a tough circuit held in London’s parks in which participants lift sandbags or navigate wheelbarrows full of sand through obstacles.

Tommy Matthews, an instructor with Outdoor Extreme, believes that sandbags are set to become the fitness aid as they are heavy, hard to handle and never stay still for long. They are great for testing your concentration power when physical exhaustion is setting in. There are other benefits beyond what you get in a weight-training session at the gym.

The lifting of either sandbags or medicine balls (which are filled with sand) instead of working out on stationary equipment at the gym, means the body’s smaller and harder to reach muscles are brought into play, helping to improve balance and stability. Many proponents claim that because sandbag training programme is so demanding, an entire workout need last no longer than 20 minutes.

Researchers at McMaster University in Ontario have said in the Journal of Applied Physiology that just six minutes of intense exercise a week, such as that performed in CrossFit-type sessions, can be equal to an hour of daily moderate activity. Such bursts improved muscle health and performance comparable to several weeks of traditional endurance training. But don’t ever try to lift a 40lb bag straight away. With poor technique and limited strength, you will leave yourself vulnerable to injury. Therefore, start off with a lighter weight and progress gradually.