The sight of two 130kg sumo wrestlers leaping towards each other, meeting with a resounding smack of flesh and a "whoomph" of breath forced out by the impact is scary stuff.
This isn't a sumo contest, it's a training session at the Dewanoumi stable in the heart of Tokyo's Ryogoku district, but it's still not for the faint-hearted.
Partly that's because these giant warriors are doing battle on a clay ring only a couple of metres from where I'm sitting on a cushion.
But it is also because it has been made clear that I am at this session under sufferance and only because of the influence of Doreen Simmons, a diminutive expatriate Englishwoman in her 70s, acknowledged sumo expert, television commentator on the sport and friend of the master of this stable.
Any behaviour that distracts the wrestlers - talking, noisy fidgeting, taking photos without permission - and it will be sayonara.
A couple of other foreigners turn up hoping to watch and are firmly shown the door, proving these rules are taken seriously.
This training session involves wrestlers from three or four stables and several - including two huge Russians and an enormous young Ukrainian - are standing on the street outside when we arrive.
When they are ready to enter they drop their coats on the doorstep and walk in, clad only in the sumo belts, which look a bit like giant nappies.
The colour of the belt denotes seniority, Miss Simmons explains. Those in black belts are the juniors and the white belts the seniors.
The junior bouts are underway as we are signalled to sit alongside the master, a grave, dignified former sumo champion, on a platform just above the ring.
Pairs of young men in black belts face each other a metre or two apart, in the centre of a circle of heavy rope, go through a series of manoeuvres until they are crouched ready to spring, and suddenly hurtle towards each other. The loser is the first one forced out of the ring or made to hit the ground.
After each bout other wrestlers shout "Watashi! Watashi!" or "Me! Me!" and the winner chooses which one he will meet next.
Size isn't everything, it seems, because the most successful of the juniors is the smallest man there, winning bout after bout by using speed to get under his opponents, get them off balance and push them out of the ring before they have a chance to use their greater bulk.
However, he quickly gets his comeuppance when bigger wrestlers join the training, most notably the giant Ukrainian who simply picks up the smaller man by his belt and throws him out of the ring.
Life is tough for these beginners who have to do all the dirty jobs in the stable - like cleaning the toilets - and act as personal servants to the seniors.
That, I guess, gives them a good incentive to succeed and the bouts are fought with tremendous intensity, sometimes over in seconds, but other times involving several minutes of grappling, pushing and straining to get an advantage, leaving the participants pouring with sweat and gasping for air.
Occasionally a wrestler is injured and - as Miss Simmons explains in a note she hands me - salt is thrown on the ring to purify it.
From time to time the master shouts comments to the wrestlers, mostly, by the look of it, demands to repeat the bout and try harder, occasionally an indication of praise.
Eventually, he decides that my rigid pose - agonising for my unsupple western knees - and utter silence merit some recognition and indicates that I can take pictures of the training so long as there is no flash and the fighters are not disturbed.
Then the senior wrestlers take to the ring with some promising juniors, such as the Ukrainian for whom hopes are high, Miss Simmons explains.
The seniors prepare by summoning selected juniors to charge into their chests and try to push them across the ring.
The impact as they hit would smash the average set of ribs, but at the end the seniors are just nicely warmed up while the juniors are exhausted from pushing all that meat.
Even then, however, there is no rest for the youngsters, who have to be on hand to pass towels or water to the seniors they served.
This stage of the training is simply awe-inspiring.
These are enormous men, the Japanese mostly with huge sagging bellies and chests which nevertheless clearly cover some very powerful muscles, the young Ukrainian and two Russian brothers more beefy. But on attack they move with extraordinary speed.
The younger of the two Russians dispatches one bulky Japanese opponent twice in succession with lightning fast hand thrusts which smash through to the throat and knock him backwards.
The Japanese wrestler seems angry at this indignity - though he might simply want to learn how to deal with this technique - and exercises his right to continue.
The result is a series of closely contested struggles, the two men wrestling back and forward, one seeming to have his opponent helpless at the edge of the ring, the other escaping with a quick swivel; one apparently in position for a throw, the other with a sudden surge of power pushing him back, each loser immediately demanding a rematch.
It is gripping stuff, like a battle between two elephants, with neither willing to concede though both are obviously exhausted.
Finally, I think the master decides enough is enough, the Japanese wrestler steps aside and the Russian chooses the Ukrainian as his opponent and allows himself to be shunted out of the ring.
During all the bouts the atmosphere is an intriguing mixture of extreme formality and amiable collegiality.
While a contest is taking place stringent rules apply to ensure nothing distracts the combatants. But between bouts the senior wrestlers offer the juniors advice on how to improve their style.
A strict protocol governs the privileges fighters enjoy.
The senior men have juniors to bring them towels to wipe off sweat or dirt from the clay ring, but rising juniors such as the Ukrainian must get their own towels, and the juniors are not allowed to use a towel at all and must stay dirty.
At the end of the training, the fighters join in a series of ritual exercises and then the seniors from this stable wander off for a bath and a meal while the luckless juniors, who would have been up since before dawn cleaning and preparing meals, busy themselves tidying the ring.
The visitors saunter outside chatting. One giant, still clad only in his belt, comes out carrying a set of golf clubs, tosses them in the back of a car and drives off.
The others put on the robes they discarded earlier on the step, hop on to their bicycles and head for their own stables.
It looks bizarre to me, but Ryogoku is well-used to it, because about half of the 50 stables are in the area, each of them with around a dozen wrestlers.
Wander through the district in the morning and you see plenty of huge, scantily clad figures hovering in the street outside their stables.
Sumo first came to this area as a result of a disastrous fire in which thousands died.
The charred bones were buried nearby and a shrine built to house the spirits.
But, as the local people were poor, the shrine needed a source of income and it was decided to copy another shrine further out of Edo - as Tokyo was then known - by leasing some of its land for sumo contests.
About the same time, the authorities built a bridge to provide an avenue of escape across the Sumida River in the event of another fire, and this made Ryogoku easily accessible to sumo fans from the rest of the city.
Today, more than half the wrestling establishments are in the district, as well as the huge green Ryogoku Kokugikan arena in which Tokyo's sumo tournaments are held.
Along the main road are statues of typical sumo poses with handprints of famous wrestlers around the bases.
Unfortunately, I wasn't able to go to a tournament because none were scheduled while I was in Japan. But having seen them in action at training I would certainly like to.
The Japanese Consulate in Auckland loaned me a video of a sumo contest and it looked fascinating. But I think my visit to a training camp was even more extraordinary.
Sumo is a unique Japanese mixture of beauty and ferocity, ritual and improvisation, involving size and speed, technique and raw power.
The nearest equivalent is perhaps when the first scrum goes down in a rugby test ... except in sumo the scrum is only two men.
Japan tournaments
Tournaments are held in Tokyo in January, May and September, in Nagoya in July and in Fukuoka in November.
Tickets
Standing tickets cost as little as 500 ($6.60) though the best seats can be up to 45,000 ($594).
Information
You can get tournament programmes in English and there is also a free English commentary (usually provided by Miss Simmons).
Sumo wrestlers do battle in Tokyo
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