As of now, the HPA is banning all cinch girths, the Argentine style girth common in Argentina, but increasingly rare in the UK and elsewhere outside South America. Because the cinch girth offers leverage, or gearing, it is quite easy to overtighten, causing discomfort and, as discussed, below, limiting performance.
But all girths can be overtightened, given sufficient will and manpower, and the sometimes acrobatic movements in the polo saddle, encourages tighter girthing than in other equestrian sports. But how tight is too tight?
HPA 2021 rule book, paragraph 2.4 on 'ponies and pony welfare', letter 'r.':
Growing up in the 70's, many years before getting into polo, the mantra of my riding instructors was always that you should be able to insert a couple of fingers behind the girth - following this rule would ensure that the girth was never too loose nor too tight. In everyday riding, you stay centered on your horse, thus keeping the saddle 'upright'. Although the same principle applies to polo, there are of course many circumstances, when we can't stay fully centered, placing more, or even much more, weight on one stirrup than the other. And there are few things in polo more disconcerting than an unstable saddle. The easy fix for most is to tighten the girth to the point that the saddle is absolutely secure (whatever the type of girth). And yet you can in many cases barely insert a coin behind the girth, let alone a couple of fingers.
In my experience, the need to overtighten the girth generally arises from poor saddle fit. If you think about it: to be stable, a saddle needs contact with angular surfaces on the horse's back. If, at the extreme, the cross-section of a mutton withered horse is completely circular, then there would be nothing to hold the saddle in place. By degrees, it's the same issue if the surfaces of the saddle panels do not match up with the surfaces of the horse's back. If they do, then a (polo) saddle will feel stable on the horse's back even before the girth is done up.
So, what are the consequences of overtightened girths? No scientific research appears to have been done in polo, but if you watch grooms, especially in high goal, leading their ponies back to the pony line, the first thing they do is usually to loosen the girth.
Various studies have been carried out in other equestrian sports, especially in racing where top speed and endurance are key, and I think we can glean some insights from these studies that are relevant for polo (see link to one article at the end of this post).
Girth tension converts to inward pressure
Studies performed on race horses have shown that overtightened girths interfere with the muscles involved in locomotion and respiration, particularly in the area just behind the horse's elbows (link to article below) and contribute to, or cause, pressure peaks under the saddle.
It's been shown that pressure under the saddle increases with speed and that peak pressure can cause soft tissue and follicle damage (often indicated by white hairs). Overtightening the girth further increases peak pressures under the saddle, resulting in increased risk of tissue damage.
The pressure under the girth itself may also interfere with the proper function of the intercostal and pectoral muscles, either directly limiting stride frequency and stride length (top speed) or indirectly limiting breathing at the canter and gallop (since breathing at canter and gallop is coupled with strides (one breath = one stride). Either way, your horses will fatigue sooner.
Horses develop compensating strategies to alleviate discomfort and these may over time cause performance and health issues. Trainer Magazine says: "Compensation strategies may manifest themselves with horses altering their gallop lead, demonstrating excessive lateral bending away from the leading leg, or stiffening of the thoracolumbar spine. In addition, there may be clinical signs of skin ulcers and bruising, along with muscle pain in and around the girth region."
My takeaway from this article and the research mentioned, is that your 100% horse in the yard, may only be a 90% horse on the field, if the girth has to be overly tight for your saddle to be secure. There is probably no specific tension that will work in all circumstances, but there are a few things we can do to avoid overtightening our girths:
1) Avoid mutton withered horses for polo - it is impossible to secure a saddle on a barrel, regardless of the girth tension applied
2) Play in well fitting saddles - the greater the inherent stability of the saddle (surface contact), the less the need for excessive girth tension (for example, the MVP, which is adjustable to fit differently shaped horses)
3) Stay centered in the saddle, as much as possible - centered riding keeps the saddle upright (and the rider connected to the horse)
4) Use girths with some degree of flex - the elastic ends of some of the elastic girths used in polo are so stretchy that all the elastic is taken out when tightening the girths even to relatively low tensions. If you can pull one of these elastic ends so far with your bare hands that the stretchiness is exhausted, then your girth will offer no flexibility at all.
5) Use girths with elbow cutouts - these girths are available in other equestrian sports, and I believe we will see them soon in polo (watch this space), because they have been found to alleviate pressure on the pectoral muscles giving our horses the opportunity to use these muscles as nature intended.