noun
martingale
A piece of horse tack: a strap running from the girth to the bridle, used to control the horse's head carriage. Domain-specific equestrian term.
競走馬には鞅を付けることが義務付けられている。
Racehorses are required to wear a martingale.
この鞅は馬が頭を上げすぎるのを防ぐ。
This martingale prevents the horse from raising its head too high.
Descriptive spelling using 胸 (chest) and 懸 (hang); less common than 鞅.
Variant using 繋 (connect); rarely encountered.
Variant using 掛 (hang); rarely encountered.
腹帯 is a girth (belly band) that secures the saddle, while 鞅 is a martingale strap connected to the bridle for head control.
The kanji 鞅 is a specific character for 'martingale'. The alternative spellings 胸懸, 胸繋, and 胸掛 are descriptive compounds using 胸 (chest) and verbs for hanging/connecting. The reading むながい is standard; むなかき is obsolete.