Effect of hip external rotator muscle contraction on pelvic floor muscle function and the piriformis

Introduction and hypothesis There is a lack of in-depth understanding of the muscles surrounding the pelvic floor muscle (PFM). The study aimed to investigate the functional relationship between PFM and hip external rotator muscles by observing the piriformis muscle thickness and displacement of the bladder base under different contraction conditions of the hip external rotator and/or PFM. We hypothesized that PFM function would be the strongest when the hip external rotator and PFM contracted simultaneously and that during PFM contraction, the piriformis muscle would act as an auxiliary muscle and show coactivation. Methods The participants included 13 healthy adult women (mean age 23.8 ± 2.8 years). The three contraction conditions were PFM contraction, hip external rotator contraction, and simultaneous contraction of the hip external rotator and PFM. During the tasks, the piriformis muscle thickness and bladder base displacement were measured using ultrasonography and compared among the contraction conditions using one-way analysis of variance. Results Significant differences were noted in bladder base displacement among the three contraction conditions ( P < 0.01), with PFM contraction having the highest value. The piriformis muscle thickness showed the greatest increase in simultaneous contraction. Conclusions In our study, we showed PFM contraction positively influences piriformis contraction, while PFM function operationalized by bladder base displacement was highest during PFM contraction alone.