In this study we report the detailed the architectural properties of the rotational muscles of the hip, including pectineus, piriformis, gemelli, obturator internus and externus, quadratus femoris, and gluteus minimus, corrected for muscle sarcomere length. The key findings were the relatively small individual PCSAs, short fiber lengths, short sarcomere lengths, and uniformly low pennation angles. To our knowledge there are no prior publications on the properties of these selected hip muscles utilizing sarcomere length to provide normalized values for fiber length and PCSA.
In a comprehensive literature review of the short external rotators of the hip, Yoo et al.  found only one published article containing quantitative values of hip muscle architecture . In that study, Friederich and Brand reported architectural measurements from two cadaveric specimens without sarcomere normalization. Regarding the short external rotators, they found the PCSAs of the piriformis (20.54 cm2) and quadratus femoris (21.00 cm2) were the largest, by approximately a 4 fold greater magnitude. Our data showed a similar range of distribution of PCSAs between the short external rotator muscles, but our PCSA values were significantly smaller (Fig. 2). The lack of sarcomere length measurements from their study makes it difficult to reconcile our data, highlighting the importance of this normalizing measurement for comparing muscle architecture because most sarcomere lengths they measured were below optimum.
This concept may be supported by comparing our data to the normalized data of Wickiewicz et al. . In their pilot study, they report normalizing muscle architecture data to sarcomere length. Although their sarcomere length was based on an average from a separate study and not a direct measurement, it still provides a baseline for comparison. Their average measurements for pectineus muscle length (12.30 cm), fiber length (10.43 cm), pennation angle (0 degrees), and PCSA (2.9) are nearly identical to our values (Table 1). Although pectineus was the only muscle available for direct comparison, these findings underscore the importance of normalizing data in comparing muscle architecture.
Our data provide a number of insights into the design of the hip short external rotators. All muscles exhibited an almost parallel pennation angle of 0 degrees, suggesting force generation is maximized to act in a single axis of rotation. This may be helpful in maintaining low muscle mass and PCSA in constrained regions of the hip while still allowing sufficient force generation. From gross dissection, we know that the superior gemellus, obturator internus, inferior gemellus, and obturator externus are essentially fused. If these muscles are considered as a single functional unit, their collective PCSA becomes functionally relevant (Fig. 2). In fact, their combined PCSA exceeds that of gluteus minimus. With the addition of quadratus femoris and piriformis, the collective “short external rotators” become a substantial force-producing unit. Considering them as a unit with a large PCSA and short fiber lengths, their design features correspond to a stabilizing role .
Although we did not directly measure joint geometry, these architectural data may be combined with known values previously reported to evaluate their role in muscle-joint kinematics. Due to the short external rotators’ close proximity to the axis of rotation, muscle length does not change substantially relative to joint position and moment arms remain oriented toward external rotation . Unlike the gluteal muscles, the short external rotators may therefore rotate the hip relatively independent of sagittal and coronal motion. Such independent movement provides valuable rotational control without otherwise affecting joint position. When combined with their rotational antagonists (gluteus minimus, pectineus, and adductors), these muscles appear to provide a stabilizing role to the hip joint . With simultaneous internal and external rotational contraction, a medial compressive force is created to balance the lateralizing force of the abductors. Such balance may facilitate dynamic stabilization of the hip joint, though further studies are necessary to validate these hypotheses.
Additionally, our findings have implications for current surgical approaches to the hip. Decisions to release the short external rotators during hip surgery should represent a balance between achieving adequate surgical exposure and preserving soft tissue anatomy, which may lead to less post-operative pain, faster rehabilitation, and a more stable joint . During traditional posterior approaches to the hip, the short external rotators are often sacrificed. Early in the practice of total hip arthroplasties, leaving the short external rotators unrepaired was believed to have no adverse effect on hip stability [8, 31]. The importance of these structures has become apparent in the recent years, however, as other reports have shown that adequate repair of the posterior structures greatly decreases the future risk of hip instability caused by soft-tissue attenuation [32, 33]. While recent meta-analyses have shown that surgical approach does not affect dislocation rate , few studies directly measured muscle function following surgery. Evolving techniques including the direct superior approach, which spares the external rotators, may offer a functional advantage , but long-term comparison studies are lacking. Further research should be dedicated to assessment of hip muscle function following hip surgery.
This study has several limitations. Fixation position was in an externally rotated joint configuration and may not reflect the clinically accepted definition of a neutral hip joint angle which is 0° abduction, flexion, and rotation. However, normalization of results with sarcomere length removes variation associated with position and therefore positioning should not significantly affect the results. Second, the advanced age of the cadaveric specimens may have led to lower PCSAs than would otherwise be observed in younger patients, but still likely provide a baseline for functional predictions and are comparable among muscles. Future studies may expand on these data for functional evaluation such as electromyographic studies of activation patterns during various movements and activities.