Badly designed and fitted tack induces physiological stress responses in horses, and may compromise animal welfare. Moreover, horses, just like humans, comprise a series of interconnected myofascial lines. However, to date there are no measurements of the effects of horse tack, such as bridles on muscle parameters. This study used acoustic myography to test whether two commercial anatomically designed and fitted bridles, have a measurable and positive effect on both equine muscle-function and performance. A Quantum bridle was tested on 12 Icelandic horses, whilst a Finesse bridle was tested on 8 Icelandic horses, and results compared with data from a standard bridle (n = 12) tested at the same time and under identical conditions. Sensors were placed on M. Brachiocephalicus and M. Splenius, and the horses exercised following a set protocol at three speeds. The results revealed statistically significant improvements in muscle performance as assessed by both the regularity of the recording at each speed, and the number of transient powerful contractions (spikes) for the anatomically designed and fitted bridles, compared with the standard bridle. It is concluded that the effect of anatomically designed bridles can not only be measured in the neck muscles of exercising horses, but that they appear to have a positive effect on muscle performance.