In horse racing, what is the so-called ‘poverty line’?

In horse racing, what is the so-called 'poverty line'?  The term ‘poverty line’ is believed to date from the eighteenth century, when it was first used by horsemen to describe a deep depression between two large muscles in the hindquarters of horses that were extremely underweight and/or generally in poor condition,

Despite the negative connotations of the name, the poverty line is evident in most horses, with the exception of those that are obese, so is no hindrance in the sport of horse racing. In the latter case, fat coverage may obscure the muscles of the hindquarters – the biceps femoris, semitendinosus and semimembranosus muscles, collectively known as the ‘hamstrings’ – rendering the poverty line invisible.

On the other hand, a pronounced poverty line may not, necessarily, indicate that a horse – especially a thoroughbred racehorse – is receiving anything less than premium care and nutrition, or suffering from an ailment. Indeed, thoroughbred racehorses are typically maintained in lean condition and tight, well-defined muscles in the hindquarters demonstrate that a horse is ‘hard fit’ and ready to run to the best of its ability, conditions permitting. For this reason, experienced paddock watchers pay close attention to the poverty line, or the lack of it, in horses that are making their seasonal debut, or returning from a break, by way of establishing their likely fitness levels.