Fully dressed in regular running gear, complete with trainers, I’m submerged to chest level in a tank of water as I prepare to try the best-kept fitness secret of our leading sports stars.
Pressing the start button on a high-tech screen in front of me triggers the familiar whirring of a treadmill belt beneath my feet and I’m off on my sub-aqua jog.
As I get into my slow-motion stride, speed restricted by the water’s resistance, it feels like I’m running with cushions attached to my feet and suspended in a harness.
As wacky as it sounds, underwater treadmills are the most sought-after items of equipment among elite athletes and celebrities, too — Jennifer Aniston’s honed physique is reportedly down to aqua- running for 15 minutes every day.
It’s an extremely fast, effective yet low-impact way of getting into shape. Running on land is high-impact — each time a runner’s foot strikes the ground, a shock equivalent to three times his or her bodyweight reverberates from the feet, through the legs and into the spine.
But if you are submerged, the water acts as a giant cushion. And the deeper the water, the lighter your body becomes.The other key benefit is water resistance. Studies at the University of Virginia have shown water is 800 times denser than air and provides up to 12 times the resistance on land — meaning that you work harder and expend more energy pool-running.
While regular runners burn about eight calories a minute, aqua-runners use up 11.5.
It also boosts aerobic capacity. Since water lacks the gravitational pull of land, staying upright requires you to use opposing muscles for every action. It’s like resistance training without weights. I’m trying the HydroPhysio treadmill, in which the water depth can be regulated.
Beginners generally start by jogging in deeper water as it provides stability and although my feet strike the treadmill in the usual way, they do so with an unfamiliar softness. But I adapt my running style and the water around me remains almost still.
Since I did the London Marathon in April, the tendons in the backs of my legs have caused pain when I run. In water, I feel nothing and jog without any stiffness.
‘As you get fitter, the level of water can be reduced to give greater resistance,’ Chris Truman, of Hydro Physio, says. ‘It’s harder to run through water that’s at knee level. It is like weight training your legs as you jog.’
Although I’m not running as fast as usual, my lungs and legs tell me I am expending at least as much energy as I would on terra firma. Towards the end of my session, Truman begins to drain away the water. Gradually, my running action becomes more laboured. When the water hits calf level, I feel as if I am wading through a swamp and am reduced to a power walk.
By the time I emerge from the glass tank encasing the treadmill, I’m exhausted. Truman says the water temperature was a moderate 30C (significantly cooler than body temperature — 37C) and yet despite the cooling effect on my body, my face is beetroot red.
You don’t need to be dressed in skimpy swimwear — any running gear will do, although the tighter fitting the better. And, while you can run barefoot, a lot of aqua-runners feel more comfortable in trainers. There are even specialist aquatic running shoes called AQX Trainers — available. Although underwater treadmills are as yet only in private gyms of elite sports clubs, there are signs that they are about to catch on. Meanwhile, you can simulate the effects by ‘running’ in a swimming pool wearing a specialist ‘aqua-jogger’ inflated belt or a ‘zero gravity’ suit with in-built buoyancy panels to keep you afloat in the deep end.
Unlike underwater treadmill running, when your feet are in constant contact with the floor, aqua-jogging in a pool can be done in shallow water or afloat in the deep end of the pool.
If the way I feel when I finish my session is anything to go by, the effort will be worthwhile. I feel like I have run the Channel.