Jockeys riding with a longer-stirrup in an upright position have been shown to slow a racehorse down, compared to the modern crouch posture.
"Major horse race times and records improved by 5% to 7% around 1900 when jockeys adopted a crouched posture," the researchers reported in their study, entitled "Modern riding style improves horse racing times".
The findings of Dr Thilo Pfau, Dr Andrew Spence, Dr Sandra Starke, Dr Marta Ferrari and Professor Alan Wilson were reported in the journal Science this week.
"When animals carry loads, there is a proportionate increase in metabolic cost, and in humans this increase in cost is reduced when the load is elastically coupled to the load bearer," they wrote.
"We show that jockeys move to isolate themselves from the movement of their mount.
"This would be difficult or impossible with a seated or upright, straight-legged posture.
"This isolation means that the horse supports the jockey's body weight but does not have to move the jockey through each cyclical stride path.
"This posture requires substantial work by jockeys, who have near-maximum heart rates during racing."
The old-fashioned riding style before 1900 involved long stirrups.
It was American jockeys who started to compete successfully in British racing using the now familiar shorter stirrups and a monkey-crouch position.
The style was popularised by US jockey Ted Sloan, which gave rise to the expression "monkey on a stick" to describe the position.
The 5%-7% time drop in times was huge, with race times since then only edging gradually down.
The researchers attached inertia sensors to the saddle and to the jockey to allow them to measure the movement of both the horse and jockey independently and then compare the data.
They speculated that the crouched riding style of modern jockeys allows them to avoid the acceleration and deceleration that the horse has to apply to an inert load, so that the jockey maintains a comparatively constant speed throughout each stride, thus making less work for the horse.
The results showed that the forwards and backwards, and up and down movements, of the jockey was much less than that of the horse.
This scientifically shows that a skilled jockey can lessen the work the horse has to do, resulting in an increase in speed.
The researchers said that the crouched style could also reduce aerodynamic drag, but their calculations indicated this would have accounted for just 2% of the 5%-7% improvement in racing times.