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Maher R1, Morrison C1, Zang Y1
1Creighton University, Physical Therapy, Omaha, United States
Background: Primary actions of the piriformis are hip external rotation (ER) and abduction; however studies suggest it may also perform hip internal rotation (IR) depending on the amount of hip flexion (HF). Studies using string models to determine findings have assumed universal tendinous attachment sites despite conflicting evidence. Additionally, the amount of HF may impact muscle length tension relationships resulting in differing strength capacity. Consequently, hip position may affect hip rotator strength, exercise selection, and efficacy of interventions.
Purpose: To determine the effect of hip position on piriformis action, assessed using ultrasound imaging, and to determine the effect of hip position on ER and IR strength.
Methods: Twenty-five healthy adults (8 males,17 females) enrolled in this study. The experimental limb was randomly selected and assessed under 4 randomly assigned testing conditions: prone ER/IR and seated 90/90 ER/IR. Ultrasound imaging (US) data were acquired using a curvilinear transducer. Participants performed 3 IR and 3 ER movements, at a rate standardized with a metronome, while US cineloops were recorded. A concentric contraction was defined as a shortening of the piriformis observed from its distal towards its proximal attachment while an eccentric contraction was defined as a lengthening from its proximal towards its distal attachment.
A hand-held dynamometer was used to assess strength across testing positions.
Results: The piriformis shortened during ER and lengthened during IR regardless of hip position.
A significant interaction effect was noted between hip position (prone, sitting) and strength (IR, ER) (p .001). Post hoc analysis indicated hip IRs (p .001) and hip ERs (p=.01) were significantly stronger in the sitting versus prone position. Furthermore, effect size (Cohen's d) values were small (d=.2) and large (d=.9) for differences between position for ER and IR respectively.
Conclusion(s): While strength of ER was stronger in sitting when compared to prone, the magnitude of this difference was small and piriformis action did not appear to change per US imaging. IR was significantly stronger in sitting, however this does not seem to imply a reversal of piriformis action but may be due to a positional effect on muscle length tension and/or moment arms both of which can affect force production. Furthermore, recent anatomical studies have shown variability in the dimensions and attachment of the distal tendon which prevents a reversal of action.
Implications: The results of this study are clinically significant in that they provide a functional dynamic visual record of the piriformis in vivo: the piriformis functioned as an ER in sitting position despite the effect of hip position on rotator strength. Stretching and exercises for the piriformis are based on the belief that it becomes an IR when the hip is flexed at or above 90°, however these findings are primarily based on cadaveric studies or recent static imaging and computational modeling studies. Further imaging studies across a variety of hip positions may provide a true representation of piriformis muscle function.
Keywords: Ultrasound imaging, piriformis, hip position
Funding acknowledgements: None
Purpose: To determine the effect of hip position on piriformis action, assessed using ultrasound imaging, and to determine the effect of hip position on ER and IR strength.
Methods: Twenty-five healthy adults (8 males,17 females) enrolled in this study. The experimental limb was randomly selected and assessed under 4 randomly assigned testing conditions: prone ER/IR and seated 90/90 ER/IR. Ultrasound imaging (US) data were acquired using a curvilinear transducer. Participants performed 3 IR and 3 ER movements, at a rate standardized with a metronome, while US cineloops were recorded. A concentric contraction was defined as a shortening of the piriformis observed from its distal towards its proximal attachment while an eccentric contraction was defined as a lengthening from its proximal towards its distal attachment.
A hand-held dynamometer was used to assess strength across testing positions.
Results: The piriformis shortened during ER and lengthened during IR regardless of hip position.
A significant interaction effect was noted between hip position (prone, sitting) and strength (IR, ER) (p .001). Post hoc analysis indicated hip IRs (p .001) and hip ERs (p=.01) were significantly stronger in the sitting versus prone position. Furthermore, effect size (Cohen's d) values were small (d=.2) and large (d=.9) for differences between position for ER and IR respectively.
Conclusion(s): While strength of ER was stronger in sitting when compared to prone, the magnitude of this difference was small and piriformis action did not appear to change per US imaging. IR was significantly stronger in sitting, however this does not seem to imply a reversal of piriformis action but may be due to a positional effect on muscle length tension and/or moment arms both of which can affect force production. Furthermore, recent anatomical studies have shown variability in the dimensions and attachment of the distal tendon which prevents a reversal of action.
Implications: The results of this study are clinically significant in that they provide a functional dynamic visual record of the piriformis in vivo: the piriformis functioned as an ER in sitting position despite the effect of hip position on rotator strength. Stretching and exercises for the piriformis are based on the belief that it becomes an IR when the hip is flexed at or above 90°, however these findings are primarily based on cadaveric studies or recent static imaging and computational modeling studies. Further imaging studies across a variety of hip positions may provide a true representation of piriformis muscle function.
Keywords: Ultrasound imaging, piriformis, hip position
Funding acknowledgements: None
Topic: Musculoskeletal: lower limb; Musculoskeletal; Women's & men's pelvic health
Ethics approval required: Yes
Institution: Creighton University, Omaha, Nebraska, USA
Ethics committee: Creighton University Institutional Review Board (IRB)
Ethics number: 646492-9
All authors, affiliations and abstracts have been published as submitted.
