Original Research
Hip Rotator Strength in Healthy Young Adults Measured in Hip Flexion and Extension by Using a Hand-held Dynamometer Nancy Bloom, PT, DPT, MSOT, Suzanne L. Cornbleet, PT, DPT Objective: The primary purpose of this study was to investigate the effect of hip joint position on hip rotator muscle strength of healthy young adults by using a hand-held dynamometer. Design: A cross-sectional cohort study. Setting: A university medical center. Participants: Thirty-four healthy (19 women, 15 men; mean [standard deviation] age, 25 2.3 years) participated in this study. Methods: A hand-held dynamometer was used to measure the strength of hip internal rotators and external rotators in 2 positions: hip flexion in sitting and hip extension in supine. The hip was tested in a neutral position with respect to rotation, abduction, and adduction. Isometric force in pounds was measured as the subject pushed against the device. Main Outcome Measurements: For each subject, hip rotator muscle strength measurements were taken during a single session. Hypotheses were developed before data collection. Results: Hip internal rotators were significantly stronger in hip flexion compared with hip extension (P < .01). There was no significant difference found in the hip external rotators between the 2 positions. Conclusions: Hip internal rotators and external rotators behave differently when comparing strength measurements between the positions of hip flexion and hip extension. A hand-held dynamometer provided an objective measurement of strength that was clinically feasible to use. Both muscle length and moment arms influence force production of the hip rotators as the hip position changes. Understanding these relationships may help clinicians interpret strength findings and direct intervention toward strengthening the appropriate muscles by using the most advantageous position. PM R 2014;0:1-6
INTRODUCTION Physical therapists must have a comprehensive understanding of the anatomy and kinesiology of the hip to skillfully examine movement in healthy individuals and those with lower quarter pain syndromes. Muscle strength is an important factor that contributes to movement and needs to be carefully considered when examining the hip. In particular, the strength of the hip rotators, specifically the gluteal muscles and the deep external rotators (ER), contribute to precise control and stability of the hip during a variety of functional activities [1,2]. During gait, for example, the ERs of the stance limb on the right act to control counterclockwise pelvic rotation and internal rotation of the right femur. Also, the internal rotators (IRs) of the stance limb on the right act to rotate the pelvis clockwise, thus assisting with swing of the left limb [2]. Measurements of hip muscle strength are influenced by factors such as moment arms and muscle length, which can vary, depending on the hip joint position [3-6]. Muscles that control hip rotation are an excellent example of the influence of these factors on muscle strength. Based on data from muscle modeling studies, moment arms for hip rotator PM&R 1934-1482/14/$36.00 Printed in U.S.A.
N.B. Program in Physical Therapy, Washington University School of Medicine, St Louis, MO. Address correspondence to: N.B.; e-mail: [email protected] Disclosure: nothing to disclose S.L.C. Program in Physical Therapy, Washington University School of Medicine, St Louis, MO Disclosure: nothing to disclose Poster presentation at the American Physical Therapy Association Combined Sections Meeting, San Diego, CA, 2013. Submitted for publication December 17, 2013; accepted June 9, 2014.
ª 2014 by the American Academy of Physical Medicine and Rehabilitation Vol. 0, 1-6, Month 2014 http://dx.doi.org/10.1016/j.pmrj.2014.06.002
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muscles vary between positions of hip flexion (HF) and hip extension (HE) [4,5]. Specifically, components of the gluteus maximus, medius, and minimus as well as the piriformis switch from having external rotation moment arms in HE to internal rotation moment arms in HF. Most of the deep ERs, such as the quadratus femoris and obturators, continue to have external rotation moment arms throughout the range of HF from 0 -90 [4]. These findings would lead the clinician to expect strength measurements for hip IRs to be stronger when the subject is tested in sitting with the hip flexed because of the increased contribution of the gluteal muscles. In contrast, one would expect the strength of hip ERs to be diminished in sitting due to the decreased contribution of the gluteal muscles. Only a few investigators have examined the relationship between hip position and hip rotator muscle strength in healthy human subjects [6-8]. Jarvis [8] used an electronic instrument to measure isometric hip rotator torque in sitting (HF) and supine (HE), with the hip in 2 positions of rotation (end range and near neutral). Lindsay et al [6] examined peak torque of hip rotation throughout the range by using an isokinetic dynamometer in sitting (HF) with the knee flexed and in supine (HE) with the knee flexed and extended. Johnson and Hoffman [7] examined isometric hip rotation torque by using an isokinetic dynamometer. The hip was placed in neutral hip rotation and tested in 3 positions: sitting in 90 HF, semireclined in 40 HF, and supine (HE). The electronic instrument used by Jarvis [8] was not referenced or described; therefore, it cannot be replicated. The use of an isokinetic dynamometer in the more recent studies by Lindsay et al [6] and Johnson and Hoffman [7] are useful tools for research but may not be available or feasible to use in many clinical settings. An alternative device that may be more feasible for clinicians to use is a hand-held dynamometer. The hand-held dynamometer has been used successfully to assess muscle strength in a variety of populations [9-11]. Although the methods and instrumentation varied among the 3 studies, all investigators reported similar findings for the strength of the hip IRs [6-8]. Torque values for the IRs were significantly stronger when tested in HF compared with HE, thus supporting the muscle modeling theory. In contrast, results among the same studies varied for the strength of the hip ERs. Jarvis [8] and Johnson and Hoffman [7] found no significant difference in the torque values for the hip ERs between positions. Lindsay et al [6], however, found that peak torque for the ERs was significantly stronger in the hip flexed position. These findings are in contrast to what would be predicted from the muscle modeling theory discussed previously. Therefore, other mechanisms need to be considered to explain the change in external rotation muscle strength between positions. When discussing these findings, Lindsay et al [6] suggested that a mechanism related to muscle length may have influenced the torquegenerating properties of the deep ERs. The hip flexed
position may place these muscles in a more optimal length, closer to their peak on the length tension curve, thereby potentially improving the strength of the contraction [12,13]. Although not mentioned by Lindsay et al [6], one could also consider the impact of a length change in portions of the gluteus maximus when the hip is positioned in HF. These muscles, acting as hip IRs, also may be placed at a more optimal length for force production. Overall, there is minimal evidence for the effect of position on either hip IR or ER strength in healthy human subjects, especially when using a simple clinical tool. Therefore, the purpose of this study was to compare hip rotator muscle strength in the position of HF to HE in healthy young adults by using a hand-held dynamometer. We hypothesized that (1) the hip IRs would be significantly stronger when tested in HF versus HE, and (2) there would be no significant difference for the ERs between positions.
METHODS Institutional review board approval at Washington University School of Medicine was obtained before the study was conducted. All the participants signed an informed consent form before data collection.
Subjects A convenience sample of 34 healthy individuals without pain in the low back or lower extremity (19 women and 15 men; mean [standard deviation] age, 25 2.3 years) participated in this study. Subjects were recruited from the physical therapy student body at Washington University in St Louis, Missouri.
Procedures The authors of this study were 2 experienced clinicians who performed all of the testing. Training sessions were held to establish consistency in testing methods between testers. Data from testing the first 18 subjects were used to determine intra-rater reliability. Strength measurements were taken for both the hip IRs and ERs in 2 positions for the subjects: (1) sitting with the hip and knee flexed to 90 (Figure 1A), and (2) supine with the hip in 0 of flexion and the knee in 90 of flexion (Figure 1B). In both positions, the hip was placed in neutral rotation, abduction, and adduction. For testing in the supine position, the untested extremity was positioned in hip and knee flexion with the foot on the table. This position for the untested extremity was selected for comfort and to decrease the stress on the lumbopelvic region. A hand-held dynamometer (Hoggan Health Industries, West Jordan, UT) was used to measure the strength of the hip IRs and ERs in both positions. The device was placed against the distal lower leg just above the lateral malleolus for testing the IRs (Figure 1A) and just above the medial
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Figure 1. Test positions. (A) Sitting with the hip flexed to 90 and placement of the dynamometer for testing hip internal rotators. (B) Supine with the hip in neutral or 0 flexion and extension, and placement of the dynamometer for testing hip external rotators.
malleolus for testing the ERs (Figure 1B). The examiner cued each subject to keep his or her thigh relaxed on the table and, as shown in Figure 1, the examiner manually monitored the distal femur to prevent substitution by the hip flexors. In addition, the examiners visually monitored the knee angle and verbally cued the subjects to maintain a position of 90 to prevent substitution by the hamstrings or sartorius. Straps were not used for stabilization so that these substitutions could be detected more easily. Before data collection, each subject was given a practice trial of pushing against the device in each direction. Instructions were given to push against the device as hard as possible without flexing the hip or moving the knee. Isometric force in pounds was taken as the examiner gradually applied pressure to meet the maximum force production achieved by the subject. The test ended after 2-3 seconds or sooner if the subject could not maintain the test position. Two strength measurements were taken for the IRs and ERs in each position on both limbs. The mean of these measurements was used for data analysis. The order of testing for position, side, and direction of pressure was randomized across the subjects. Although a separate person was not used to read and record the measurements, the testers were blinded to the digital readout during testing because of its location (Figure 1B).
A 3-way interaction effect was found for position, muscle strength, and side (Figure 2). Post hoc analysis revealed that both left and right hip IRs were significantly stronger (P < .01) when tested in HF versus in HE, which supports our first hypothesis regarding the hip IRs. The difference in
Data Analysis Reliability was estimated by using intraclass correlation coefficient (2,2). A 3-way (2 2 2), repeated measures analysis of variance was performed. The factors were position (HF, HE), muscle strength (IRs, ERs), and side (right, left). For all significant effects, a Bonferroni corrected post hoc test was performed for all pairwise comparisons. All significance testing was set at a ¼ .05.
RESULTS Both examiners demonstrated excellent intra-rater reliability (intraclass correlation coefficient ¼ 0.91 and 0.92; P < .01).
Figure 2. Mean strength values for hip internal rotators and external rotators in the hip flexed and extended positions for the left and right hips.
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the means between positions was 10.04 lb (4.55 kg) for the left hip and 11.99 lb (5.44 kg) for the right hip. There was no significant difference between positions for the left or right hip ERs, which supports our second hypothesis. The difference in the means between positions was 1.74 lb (0.79 kg) for the left hip and 1.37 lb (0.62 kg) for the right hip. The results are summarized in Table 1.
DISCUSSION The purpose of this study was to investigate the effect of hip joint position on hip rotator muscle strength in healthy young adults by using a hand-held dynamometer. The results of this study demonstrated that hip joint position affects hip IR strength measurements and that the use of a hand-held dynamometer provided a simple, efficient, and objective method to measure strength. The methods used in this study could be easily replicated by clinicians in any setting. We recommend using these methods, and we would like to highlight a few procedures used in this study that we believe improved patient comfort and the accuracy of the hip rotator strength measurements: (1) cue the individual to keep the thigh relaxed on the table and the knee at a 90 angle to help prevent undesirable movement during testing, such as hip or knee flexion, and (2) use the supine position to test hip rotator strength with the hip in extension as described in this study (Figure 1B). Rather than positioning both hips in extension as done in previous studies [6-8], we preferred placing the untested hip in flexion to avoid excessive lumbar extension and discomfort in the low back. We also prefer this position over prone, with the knee flexed to protect the knee joint from excessive stress. The IRs were significantly stronger in the position of HF (sitting) compared with the position of HE (supine) for all the subjects on both sides. These findings support the results of muscle modeling studies [4,5] as well as previous studies on healthy young adults [6-8]. Due to differences in methodologies and data analyses, it is difficult to directly compare the absolute values of our study with those of other studies performed with healthy adults. However, when the data were converted to a percentage change in strength between positions, the results for the IRs in our study (43%-55% stronger in HF) were similar to those reported by others investigators (34% [7], 44% [8], and 52%-54%) [6].
The change in strength between positions may be attributed to the change in moment arms for the piriformis and components of the gluteal muscles that favor internal rotation in HF. In addition, we believe that the position of HF may alter the length of these muscles. When considering muscle mechanics, specifically, the length tension relationship, the position of HF may place components of the gluteals and piriformis in a more optimal length, closer to their peak on the length tension curve, thereby enhancing force production [12,13]. In our study, the difference in strength for the IRs between positions was easily detected by the examiners. The subjects also commented that it was much easier to develop force when the hip was flexed compared with extended. Therefore, clinicians should expect hip IRs to test strong in sitting, the standard muscle test position [14]. When the IRs are found to be weak in sitting, the gluteal muscles should be considered as a contributing factor and may need to be addressed in rehabilitation. In contrast, when the hip was extended, the subjects commonly expressed difficulty generating force for internal rotation. The difficulty of generating force was most likely because there are no primary hip IRs with the hip in extension [2]. Therefore, clinicians should be aware that it is normal for IR strength to be less in positions of more HE, such as supine or standing. Although it may be normal for the IRs to test weaker in HE, it is important for the hip IRs to have sufficient strength for optimal function during standing activities such as gait, pivoting, and sports. Thus, strengthening the hip IRs in standing may be useful in rehabilitation. Next, we will discuss the results and clinical relevance related to the hip ERs. There was no significant difference in hip ER strength between positions of HF and HE for all the subjects, on both sides. These findings are similar to those reported by Jarvis [8] and Johnson and Hoffman [7] but differ from Lindsay et al [6]. When data were converted to a percentage change in strength between positions, the results for the ERs in our study (0.05%-0.06%) were similar to those reported by Johnson and Hoffman [7] (
Hip Rotator Strength in Healthy Young Adults Measured in Hip Flexion and Extension by Using a Hand-held Dynamometer Nancy Bloom, PT, DPT, MSOT, Suzanne L. Cornbleet, PT, DPT Objective: The primary purpose of this study was to investigate the effect of hip joint position on hip rotator muscle strength of healthy young adults by using a hand-held dynamometer. Design: A cross-sectional cohort study. Setting: A university medical center. Participants: Thirty-four healthy (19 women, 15 men; mean [standard deviation] age, 25 2.3 years) participated in this study. Methods: A hand-held dynamometer was used to measure the strength of hip internal rotators and external rotators in 2 positions: hip flexion in sitting and hip extension in supine. The hip was tested in a neutral position with respect to rotation, abduction, and adduction. Isometric force in pounds was measured as the subject pushed against the device. Main Outcome Measurements: For each subject, hip rotator muscle strength measurements were taken during a single session. Hypotheses were developed before data collection. Results: Hip internal rotators were significantly stronger in hip flexion compared with hip extension (P < .01). There was no significant difference found in the hip external rotators between the 2 positions. Conclusions: Hip internal rotators and external rotators behave differently when comparing strength measurements between the positions of hip flexion and hip extension. A hand-held dynamometer provided an objective measurement of strength that was clinically feasible to use. Both muscle length and moment arms influence force production of the hip rotators as the hip position changes. Understanding these relationships may help clinicians interpret strength findings and direct intervention toward strengthening the appropriate muscles by using the most advantageous position. PM R 2014;0:1-6
INTRODUCTION Physical therapists must have a comprehensive understanding of the anatomy and kinesiology of the hip to skillfully examine movement in healthy individuals and those with lower quarter pain syndromes. Muscle strength is an important factor that contributes to movement and needs to be carefully considered when examining the hip. In particular, the strength of the hip rotators, specifically the gluteal muscles and the deep external rotators (ER), contribute to precise control and stability of the hip during a variety of functional activities [1,2]. During gait, for example, the ERs of the stance limb on the right act to control counterclockwise pelvic rotation and internal rotation of the right femur. Also, the internal rotators (IRs) of the stance limb on the right act to rotate the pelvis clockwise, thus assisting with swing of the left limb [2]. Measurements of hip muscle strength are influenced by factors such as moment arms and muscle length, which can vary, depending on the hip joint position [3-6]. Muscles that control hip rotation are an excellent example of the influence of these factors on muscle strength. Based on data from muscle modeling studies, moment arms for hip rotator PM&R 1934-1482/14/$36.00 Printed in U.S.A.
N.B. Program in Physical Therapy, Washington University School of Medicine, St Louis, MO. Address correspondence to: N.B.; e-mail: [email protected] Disclosure: nothing to disclose S.L.C. Program in Physical Therapy, Washington University School of Medicine, St Louis, MO Disclosure: nothing to disclose Poster presentation at the American Physical Therapy Association Combined Sections Meeting, San Diego, CA, 2013. Submitted for publication December 17, 2013; accepted June 9, 2014.
ª 2014 by the American Academy of Physical Medicine and Rehabilitation Vol. 0, 1-6, Month 2014 http://dx.doi.org/10.1016/j.pmrj.2014.06.002
1
2
Bloom and Cornbleet
HIP RS MEASURED IN HF AND EXTENSION WITH DYNAMOMETER
muscles vary between positions of hip flexion (HF) and hip extension (HE) [4,5]. Specifically, components of the gluteus maximus, medius, and minimus as well as the piriformis switch from having external rotation moment arms in HE to internal rotation moment arms in HF. Most of the deep ERs, such as the quadratus femoris and obturators, continue to have external rotation moment arms throughout the range of HF from 0 -90 [4]. These findings would lead the clinician to expect strength measurements for hip IRs to be stronger when the subject is tested in sitting with the hip flexed because of the increased contribution of the gluteal muscles. In contrast, one would expect the strength of hip ERs to be diminished in sitting due to the decreased contribution of the gluteal muscles. Only a few investigators have examined the relationship between hip position and hip rotator muscle strength in healthy human subjects [6-8]. Jarvis [8] used an electronic instrument to measure isometric hip rotator torque in sitting (HF) and supine (HE), with the hip in 2 positions of rotation (end range and near neutral). Lindsay et al [6] examined peak torque of hip rotation throughout the range by using an isokinetic dynamometer in sitting (HF) with the knee flexed and in supine (HE) with the knee flexed and extended. Johnson and Hoffman [7] examined isometric hip rotation torque by using an isokinetic dynamometer. The hip was placed in neutral hip rotation and tested in 3 positions: sitting in 90 HF, semireclined in 40 HF, and supine (HE). The electronic instrument used by Jarvis [8] was not referenced or described; therefore, it cannot be replicated. The use of an isokinetic dynamometer in the more recent studies by Lindsay et al [6] and Johnson and Hoffman [7] are useful tools for research but may not be available or feasible to use in many clinical settings. An alternative device that may be more feasible for clinicians to use is a hand-held dynamometer. The hand-held dynamometer has been used successfully to assess muscle strength in a variety of populations [9-11]. Although the methods and instrumentation varied among the 3 studies, all investigators reported similar findings for the strength of the hip IRs [6-8]. Torque values for the IRs were significantly stronger when tested in HF compared with HE, thus supporting the muscle modeling theory. In contrast, results among the same studies varied for the strength of the hip ERs. Jarvis [8] and Johnson and Hoffman [7] found no significant difference in the torque values for the hip ERs between positions. Lindsay et al [6], however, found that peak torque for the ERs was significantly stronger in the hip flexed position. These findings are in contrast to what would be predicted from the muscle modeling theory discussed previously. Therefore, other mechanisms need to be considered to explain the change in external rotation muscle strength between positions. When discussing these findings, Lindsay et al [6] suggested that a mechanism related to muscle length may have influenced the torquegenerating properties of the deep ERs. The hip flexed
position may place these muscles in a more optimal length, closer to their peak on the length tension curve, thereby potentially improving the strength of the contraction [12,13]. Although not mentioned by Lindsay et al [6], one could also consider the impact of a length change in portions of the gluteus maximus when the hip is positioned in HF. These muscles, acting as hip IRs, also may be placed at a more optimal length for force production. Overall, there is minimal evidence for the effect of position on either hip IR or ER strength in healthy human subjects, especially when using a simple clinical tool. Therefore, the purpose of this study was to compare hip rotator muscle strength in the position of HF to HE in healthy young adults by using a hand-held dynamometer. We hypothesized that (1) the hip IRs would be significantly stronger when tested in HF versus HE, and (2) there would be no significant difference for the ERs between positions.
METHODS Institutional review board approval at Washington University School of Medicine was obtained before the study was conducted. All the participants signed an informed consent form before data collection.
Subjects A convenience sample of 34 healthy individuals without pain in the low back or lower extremity (19 women and 15 men; mean [standard deviation] age, 25 2.3 years) participated in this study. Subjects were recruited from the physical therapy student body at Washington University in St Louis, Missouri.
Procedures The authors of this study were 2 experienced clinicians who performed all of the testing. Training sessions were held to establish consistency in testing methods between testers. Data from testing the first 18 subjects were used to determine intra-rater reliability. Strength measurements were taken for both the hip IRs and ERs in 2 positions for the subjects: (1) sitting with the hip and knee flexed to 90 (Figure 1A), and (2) supine with the hip in 0 of flexion and the knee in 90 of flexion (Figure 1B). In both positions, the hip was placed in neutral rotation, abduction, and adduction. For testing in the supine position, the untested extremity was positioned in hip and knee flexion with the foot on the table. This position for the untested extremity was selected for comfort and to decrease the stress on the lumbopelvic region. A hand-held dynamometer (Hoggan Health Industries, West Jordan, UT) was used to measure the strength of the hip IRs and ERs in both positions. The device was placed against the distal lower leg just above the lateral malleolus for testing the IRs (Figure 1A) and just above the medial
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Figure 1. Test positions. (A) Sitting with the hip flexed to 90 and placement of the dynamometer for testing hip internal rotators. (B) Supine with the hip in neutral or 0 flexion and extension, and placement of the dynamometer for testing hip external rotators.
malleolus for testing the ERs (Figure 1B). The examiner cued each subject to keep his or her thigh relaxed on the table and, as shown in Figure 1, the examiner manually monitored the distal femur to prevent substitution by the hip flexors. In addition, the examiners visually monitored the knee angle and verbally cued the subjects to maintain a position of 90 to prevent substitution by the hamstrings or sartorius. Straps were not used for stabilization so that these substitutions could be detected more easily. Before data collection, each subject was given a practice trial of pushing against the device in each direction. Instructions were given to push against the device as hard as possible without flexing the hip or moving the knee. Isometric force in pounds was taken as the examiner gradually applied pressure to meet the maximum force production achieved by the subject. The test ended after 2-3 seconds or sooner if the subject could not maintain the test position. Two strength measurements were taken for the IRs and ERs in each position on both limbs. The mean of these measurements was used for data analysis. The order of testing for position, side, and direction of pressure was randomized across the subjects. Although a separate person was not used to read and record the measurements, the testers were blinded to the digital readout during testing because of its location (Figure 1B).
A 3-way interaction effect was found for position, muscle strength, and side (Figure 2). Post hoc analysis revealed that both left and right hip IRs were significantly stronger (P < .01) when tested in HF versus in HE, which supports our first hypothesis regarding the hip IRs. The difference in
Data Analysis Reliability was estimated by using intraclass correlation coefficient (2,2). A 3-way (2 2 2), repeated measures analysis of variance was performed. The factors were position (HF, HE), muscle strength (IRs, ERs), and side (right, left). For all significant effects, a Bonferroni corrected post hoc test was performed for all pairwise comparisons. All significance testing was set at a ¼ .05.
RESULTS Both examiners demonstrated excellent intra-rater reliability (intraclass correlation coefficient ¼ 0.91 and 0.92; P < .01).
Figure 2. Mean strength values for hip internal rotators and external rotators in the hip flexed and extended positions for the left and right hips.
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the means between positions was 10.04 lb (4.55 kg) for the left hip and 11.99 lb (5.44 kg) for the right hip. There was no significant difference between positions for the left or right hip ERs, which supports our second hypothesis. The difference in the means between positions was 1.74 lb (0.79 kg) for the left hip and 1.37 lb (0.62 kg) for the right hip. The results are summarized in Table 1.
DISCUSSION The purpose of this study was to investigate the effect of hip joint position on hip rotator muscle strength in healthy young adults by using a hand-held dynamometer. The results of this study demonstrated that hip joint position affects hip IR strength measurements and that the use of a hand-held dynamometer provided a simple, efficient, and objective method to measure strength. The methods used in this study could be easily replicated by clinicians in any setting. We recommend using these methods, and we would like to highlight a few procedures used in this study that we believe improved patient comfort and the accuracy of the hip rotator strength measurements: (1) cue the individual to keep the thigh relaxed on the table and the knee at a 90 angle to help prevent undesirable movement during testing, such as hip or knee flexion, and (2) use the supine position to test hip rotator strength with the hip in extension as described in this study (Figure 1B). Rather than positioning both hips in extension as done in previous studies [6-8], we preferred placing the untested hip in flexion to avoid excessive lumbar extension and discomfort in the low back. We also prefer this position over prone, with the knee flexed to protect the knee joint from excessive stress. The IRs were significantly stronger in the position of HF (sitting) compared with the position of HE (supine) for all the subjects on both sides. These findings support the results of muscle modeling studies [4,5] as well as previous studies on healthy young adults [6-8]. Due to differences in methodologies and data analyses, it is difficult to directly compare the absolute values of our study with those of other studies performed with healthy adults. However, when the data were converted to a percentage change in strength between positions, the results for the IRs in our study (43%-55% stronger in HF) were similar to those reported by others investigators (34% [7], 44% [8], and 52%-54%) [6].
The change in strength between positions may be attributed to the change in moment arms for the piriformis and components of the gluteal muscles that favor internal rotation in HF. In addition, we believe that the position of HF may alter the length of these muscles. When considering muscle mechanics, specifically, the length tension relationship, the position of HF may place components of the gluteals and piriformis in a more optimal length, closer to their peak on the length tension curve, thereby enhancing force production [12,13]. In our study, the difference in strength for the IRs between positions was easily detected by the examiners. The subjects also commented that it was much easier to develop force when the hip was flexed compared with extended. Therefore, clinicians should expect hip IRs to test strong in sitting, the standard muscle test position [14]. When the IRs are found to be weak in sitting, the gluteal muscles should be considered as a contributing factor and may need to be addressed in rehabilitation. In contrast, when the hip was extended, the subjects commonly expressed difficulty generating force for internal rotation. The difficulty of generating force was most likely because there are no primary hip IRs with the hip in extension [2]. Therefore, clinicians should be aware that it is normal for IR strength to be less in positions of more HE, such as supine or standing. Although it may be normal for the IRs to test weaker in HE, it is important for the hip IRs to have sufficient strength for optimal function during standing activities such as gait, pivoting, and sports. Thus, strengthening the hip IRs in standing may be useful in rehabilitation. Next, we will discuss the results and clinical relevance related to the hip ERs. There was no significant difference in hip ER strength between positions of HF and HE for all the subjects, on both sides. These findings are similar to those reported by Jarvis [8] and Johnson and Hoffman [7] but differ from Lindsay et al [6]. When data were converted to a percentage change in strength between positions, the results for the ERs in our study (0.05%-0.06%) were similar to those reported by Johnson and Hoffman [7] (
