Hip Abductor Power and Velocity: Reliability and Association With Physical Function

Abstract

Lanza, MB, Jin, KH, Karl, H, Myers, J, Ryan, E, and Gray, VL. Hip abductor power and velocity: reliability and association with physical function. J Strength Cond Res 37(2): 284–290, 2023—Muscle power, defined as the ability of the muscle to produce torque quickly, has received little attention and may be critical for understanding physical function and performance. Hip abductors' capacity to produce power through both torque and velocity is important for different human activities; hence, a reliable assessment of hip abduction is critical. The first aim of the study was to assess the intersession reliability of hip abductor muscles maximal torque and submaximal power and power during standing hip abduction in young adults using pneumatic resistance. A secondary aim was to investigate whether there was a relationship between hip abductor maximal torque and submaximal power and velocity with clinical assessments of strength and power in young adults. The subjects (n = 24; 26.0 ± 3.7 years) visited the laboratory 2 times. In the first visit, the subjects performed on a pneumatic resistance machine 1-repetition maximum (1RM) and submaximal tests (40, 60, and 70% of 1RM) of the hip abductors and clinical tests of lower-extremity strength and power (The 30-second chair stand test [30CST]; and stair climb power test). During the second visit, all tests were repeated except the clinical tests. One-repetition maximum torque and submaximal power and velocity (at all levels) had excellent reliability (intraclass correlation coefficient ≥ 0.943) with absolute reliability of 13.5% up to 28.3%. The agreement between days from Bland-Altman plots for power and velocity was near 0 for all levels. Hip abduction velocity had a significant positive correlation with 30CST at 60% (r = 0.416; p = 0.048) and 70% of 1RM (r = 0.442; p = 0.035). In conclusion, we showed an excellent intersession reliability of the hip abductor muscles 1RM torque and submaximal power and velocity using pneumatic resistance. Furthermore, we demonstrated that hip abduction velocity might be important for the performance of the 30CST.

Introduction

Muscle strength (or torque) commonly evaluated is essential for developing treatment plans and tracking progress in rehabilitation and sports performance (³⁶). However, muscle power, defined as the ability of the muscle to produce torque quickly (torque × velocity) (²⁹), has received less attention and may be just as crucial for understanding physical function and performance. Previous research has demonstrated that muscle power is important for athletic performance (⁴⁵) and the recovery of balance (^4,25). More importantly, muscle power is a better predictor of functional performance than muscle strength in older adults (¹²). Although muscle strength has been the focus of assessments and interventions and studies show that it is a reliable measure (⁴³), there is limited evidence to support the reliability of assessing muscle power. Thus, investigating the reliability of evaluating muscle power, and the components that make up muscle power (torque and velocity) is necessary.

Muscle power depends on a quick movement velocity, which is critical to achieving better performance in young (¹⁶) and older adults (¹⁵). Clémençon et al. demonstrated that older adults with slower joint velocity movements (i.e., knee extension) took longer to complete the 6-m walk test and the stair climb test. Given the importance of muscle power to physical function and performance, interventions that use methods for improving power would be important. A review of velocity-specific training found that improving the movement velocity resulted in overall higher functional performance (e.g., higher torque production) and greater adaptations after training (e.g., increased neuromuscular activation) (¹⁶). Thus, evaluating power and the components of power is critical for understanding a person's physical function and performance. In addition, it can provide valuable information for assessing function and tracking progress during an intervention. Thus, determining whether power can be reliably assessed over multiple sessions is critical.

Physical function in relation to static and dynamic balance has focused on the capacity of the hip abductor muscles to produce power (^10,37) because declines in physical function are associated with fall risk in older adults (^42,44). Hip abductors capacity seems to be important not only as an agonist muscle group (e.g., hip abduction task) but also as stabilizers of the task (e.g., walking or stepping). For instance, it was demonstrated that individuals with higher hip abduction power (measured on an isokinetic machine at different velocities [30 and 90°·s⁻¹]) had a faster preferred walking speed (¹⁷). Additionally, individuals with a slower rate of force development were more likely to have patellofemoral pain (³⁹), and following a stroke, there was a significant relationship between improvements in hip abductor power of the impaired and nonimpaired limb and recovery of walking speed (¹¹). Thus, reliable assessment of hip abduction is critical in different contexts.

Hip abductor power and its components are typically assessed using an isokinetic dynamometer (^2,3,28), a device that limits the contraction speed. Muscle assessments in this manner do not reflect movements of the body in real-life situations (⁷), where movement velocity is variable and task dependent. A method of assessment that uses pneumatic resistance allows the movement to be performed without constraining the speed. Therefore, pneumatic resistance may be more insightful when trying to mimic a real-life task (e.g., lateral step or raise from a chair). In this context, pneumatic resistance allows an individual to perform the task without constraining speed and may be a better method to assess rapid contractions. Previous studies have used pneumatic resistance to assess the pectoralis major and quadriceps femoris muscle power and its components (^{6,21,27,30,41}). However, only a few studies have assessed the reliability of pneumatic resistance during a leg press, chest press, knee flexion, knee extension, and lateral pull-down (^13,33,43). The reliability of power and other components that contribute to power has not been established between sessions for hip abductors.

Considering the importance of the hip abductors muscles for balance recovery (^31,32,34) and rehabilitation (²⁴), investigating the reliability of the hip abductors muscle power is warranted. Therefore, the purpose of this study was to assess the intersession reliability of hip abduction maximal torque, velocity, and power during maximal and submaximal contractions. Based on previous research in other muscle groups using pneumatic resistance (^21,27,30), it was hypothesized that pneumatic resistance would be a reliable measure of hip abduction muscle power and its components of velocity and maximal torque. A secondary aim of the present study was to investigate whether there was a relationship between muscle power, velocity, and torque with clinical assessments of strength and power.

Methods

Experimental Approach to the Problem

This cross-sectional study examined the intersession reliability of hip abductors muscle maximal torque, velocity, and power during maximal and submaximal contractions. Additionally, we also investigated whether there was a relationship between muscle power, velocity, and torque with clinical assessments of strength and power. Subjects visited the laboratory 2 times. Subjects performed 1-repetition maximum (1RM) and submaximal tests (40, 60, and 70% of 1RM) of the hip abductors and clinical tests of lower-extremity strength and power.

Subjects

A convenience sample of 24 subjects between the ages of 18 and 35 years (Table 1) (Maryland, United States) without a history of the neurological or muscular disorder or serious injuries or surgery of the lower extremity volunteered to participate. Subjects were selected by advertising across the University of Maryland, Baltimore, and the investigator pretest verified study criteria by asking the subject to fill an eligibility checklist. The sample size for the reliability was calculated by the software GPower (version 3.1) (²⁰), and the following inputs were used: (a) tails (2); (b) effect size (1.08), based on the mean difference of muscle power reported somewhere (⁴³), (c) alpha (0.05); (d) power (0.8). A sample size of 9 subjects was suggested by the program. This study was approved by the University of Maryland School of Medicine Institutional Review Board, and all subjects provided written informed consent for participation after being informed of potential benefits and risks of the study.

Table 1 - Demographics, clinical tests, and power assessment.^*

No. of subjects	24
Females	15 (63%)
Age (y)	26.0 ± 3.7
Height (m)	1.69 ± 0.89
Mass (kg)	75.0 ± 14.9
30CST (count)	25.4 ± 5.02
SCPT (W)	461.0 ± 76.0
1RM (Nm)	48.4 ± 12.2
Power (W)
40% of 1RM	164 ± 50
60% of 1RM	198 ± 63
70% of 1RM	207 ± 77
Velocity (m·s−1)
40% of 1RM	5.55 ± 0.83
60% of 1RM	4.94 ± 0.81
70% of 1RM	4.59 ± 0.96

^*1RM = 1-repetition maximum; 30CST = 30-second chair stand test; SCPT = stair climb power test.

Procedures

Subjects visited the laboratory 2 times to perform the same tests at a similar time of the day. In the first visit, the anthropometric measurements (height and weight) were performed first. After that, the subjects performed 1RM and submaximal tests (40, 60, and 70% of 1RM) of the hip abductors and clinical tests of lower-extremity strength and power. In addition, the hip abductor muscles of the dominant leg were tested, and testing was performed in standing on a Keiser 250-A pneumatic resistance machine (Keiser Sports Health Equipment, Fresno, CA). The dominant leg was determined by asking the subjects 3 questions, (a) “Which leg do you use to kick a ball?” (b) “Which leg do you step with first when you start walking?” and (c) “Which leg do you use to squish a bug?” The leg selected for 2 out of the 3 questions was considered the dominant leg.

Before the 1RM test, the task was demonstrated and followed by a familiarization (6 repetitions) using a low resistance. The 1RM test was performed with the subject in standing, legs fully extended, and the trunk upright. The subjects were provided with verbal instructions to execute the contraction as hard and fast as possible, without any other compensatory body movements. If the body position was not maintained throughout the trial, the trial was considered invalid. There was a gradual increase in the external resistance with each repetition until the 1RM was reached. The 1RM was reached when the subject rated their perceived rate of exertion at 17 or more using the Borg Rating of Perceived Exertion Scale (¹⁸). There was a maximum of 6 attempts with a 1-minute rest between each trial. After the 1RM test, subjects were given a 3-minute rest before starting the submaximal assessment. The torque value of their 1RM test was used for the subsequent submaximal tests performed at 40, 60, or 70% of the 1RM value. Each submaximal test consisted of 6 repetitions, where the first 3 repetitions were performed at 10% of 1RM (lower-level repetitions). The following 3 repetitions were performed at 40, 60, or 70% of 1RM (upper-level repetitions). The machine provides an average value from the 3 repetitions performed at each tested level. There was also a 10-second interval between each repetition, followed by 3 minutes of rest between the lower- and upper-level repetitions. The order of the submaximal tests (40, 60, and 70% of 1RM) was randomized for each subject (Figure 1). Subjects were provided with verbal instructions to execute the contraction as fast as possible without moving the trunk.

Figure 1.: Experimental design.

The 30-second chair stand test (30CST) (²⁶) and stair climb power test (SCPT) (⁸) were performed at least 3 minutes after the submaximal assessment and were performed only on the first day. The 30CST is a measure of lower-extremity strength. A chair with a straight back without armrests (seat 17″ high) and a stopwatch were used as equipment for the test. The instructions given were to rise from the chair and sit down on the chair as fast as possible, with their arms crossed and hands place on the opposite shoulder. The subject was encouraged to perform as many repetitions as possible in 30 seconds. After a “Ready, and Go,” the number of times they completely stood up in 30 seconds was recorded and used for analysis. The SCPT is a clinical measure of lower-extremity power that measures the time required to complete a flight of stairs with 10 steps. Subjects were instructed to ascend the stairs as fast as possible and only use the railing if they thought it was necessary for safety purposes. Two trials were completed, and the average time of the 2 trials was used. The power during the SPCT was calculated as (⁸),$\text{Power}=\text{particpant}\text{mass}\times \text{acceleration}\text{due}\text{to}\text{gravity}\times \left(\frac{\text{vertical}\text{height}\text{of}\text{stairs}}{\text{time}\text{required}\text{to}\text{climb}\text{the}\text{stairs}}\right).$

The second visit was approximately 1 week later. Again, the subjects performed the 1RM test and the submaximal tests of 40, 60, and 70% of the 1RM. The ordering of the submaximal test was the same order as the first visit for each person. The pneumatic machine provided velocity and power outcomes. The velocity was calculated at the clevis of the air cylinder in SI units and calculated in meters per second (in meters per second), whereas power was calculated as the product of velocity (in meters per second) and torque (in newton meters) and presented in Watts.

Statistical Analyses

All statistical analyses were performed using SPSS version 26.0 (IBM Inc., Chicago, IL). Data were first examined for normality (Shapiro-Wilk test). All the tests were assessed at the α = 0.05 level of significance. The relative reliability of the hip abduction outcomes, which was the torque during the 1RM, power, and velocity was determined by the intraclass correlation coefficient (ICC 2,1) and coefficient of variation (CV%, [SD/mean] × 100%). The ICC values were interpreted as weak (<0.4), moderate (0.4–0.59), good (0.6–0.74), and/or excellent (0.75–1.0) (¹⁴). The absolute reliability was calculated as the percent standard error of measurement (SEM%) using the following formula: SD × $\sqrt{1-\text{ICC}/\text{Mean}\left(\text{day}1\text{and}\text{day}2\right)}$ × 100, where SD is the standard deviation, and ICC is the ICC (²²). The reliability (or agreement) of the variables (e.g., 1RM power on day 1 vs. 1RM power on day 2) was assessed through the Bland-Altman procedure (⁹). In order to address the secondary aim of the study, Spearman or Pearson's product-moment bivariate correlations were used to assess the correlations between power and velocity at submaximal torque (40, 60, and 70% of 1RM) and torque of the 1RM with the clinical tests (30CST and SCPT).

Results

The clinical outcome measures and the maximal and submaximal assessment of torque, power, and velocity are reported as mean ± SD and are listed in Table 1.

Intersession Reliability

Excellent reliability was found in the hip abductor torque of 1RM and the power and velocity of the submaximal tests (40, 60 and 70%) with ICC values ranging from 0.857 to 0.976 (Table 2). The CVs were less than 10% for the 1RM torque, submaximal tests (40, 60 and 70%) of power and velocity, ranging from 5.9 to 9.3%. The absolute reliability (SEM%) of the power during the 40 and 70% of 1RM was 13.5 and 14.1%, whereas the torque at 1RM increased to 28.3%, as did the 60% of 1RM increased to 25.2%. Interestingly, the velocity SEM% decreased as the submaximal tests 1RM increased from 40 to 70% with values of 39.2, 32.3, and 23.7, respectively (Table 2).

Table 2 - Between session interclass correlation coefficient (ICC) with 95% confidence interval (CI), coefficient of variation (CV), and the percentage standard error of measurement (SEM%) from 1-repetition maximum (1RM), power, and velocity.^*

	ICC	95% CI	CV (%)	%SEM
1RM	0.943	0.847–0.977	7.1 ± 1.5	28.3
Power
40% of 1RM	0.955	0.894–0.981	8.3 ± 1.1	13.5
60% of 1RM	0.94	0.861–0.975	9.3 ± 2.0	25.2
70% of 1RM	0.976	0.944–0.990	6.1 ± 1.1	14.1
Velocity
40% of 1RM	0.864	0.669–0.944	6.3 ± 1.3	39.2
60% of 1RM	0.857	0.660–0.940	7.2 ± 1.2	32.3
70% of 1RM	0.927	0.822–0.970	5.9 ± 1.0	23.7

^*CV presented as mean ± SEM.

The agreement between days for power based on the (Bland-Altman plots, Figure 2), was near 0 for 40–70% of 1RM (bias values [d] ranging from −2.5 to −8.3). Similarly, hip abduction velocity also had excellent intersession reliability for all resistances, with ICC ranging from 0.857 to 0.927 and CV = 5.9–7.2% (Table 2), with an absolute reliability ranging from 23.7 to 39.2% (Table 2). The velocity showed a bias of almost 0 for all resistances (40% of 1RM; d = 0.006; 60% of 1RM; d = −0.002; and 70% of 1RM; d = 0.19; Figure 3).

Figure 2.: Bland-Altman plots for the intersession assessment of power at 40% of 1RM (A), 60% of 1RM (B), and 70% of 1RM (C), with limits of agreement (horizontal dashed line), from −1.96 seconds to +1.96 seconds. 1RM = 1-repetition maximum.

Figure 3.: Bland-Altman plots for the intersession assessment of velocity at 40% of 1RM (A), 60% of 1RM (B), and 70% of 1RM (C), with limits of agreement (horizontal dash line), from −1.96 seconds to +1.96 seconds. 1RM = 1-repetition maximum.

Bivariate Correlations

The hip abduction power, across all submaximal resistances, showed no significant correlations with the 30CST or SCPT tests (Table 3). However, hip abduction velocity showed a significant and positive correlation with 30CST at 60% (r = 0.416; p = 0.048), and 70% of 1RM (r = 0.442; p = 0.035) (Table 3). Nevertheless, no significant correlations were found between velocity and SCPT. There were also no significant correlations between maximal (1RM) and submaximal torque (40, 60, and 70% of 1RM) with 30CST or SCPT tests (r ≤ 0.306; p ≥ 0.155).

Table 3 - Bivariate correlations between power, velocity, and 1-repetition maximal (1RM) with the 30-second chair stand test (30CST) and stair climb power test (SCPT).^*

	30CST		SCPT
	r	p	r	p
Power
40% of 1RM	0.217	0.331	0.066	0.772
60% of 1RM	0.156	0.479	0.134	0.542
70% of 1RM	0.202	0.355	0.320	0.137
Velocity
40% of 1RM	0.324	0.142	0.174	0.439
60% of 1RM	0.416	0.048 †	−0.021	0.925
70% of 1RM	0.442	0.035 †	0.185	0.397
1RM	−0.077	0.722	0.138	0.520

^*r = correlation coefficient; p-values = level of significance.

^†Significant correlation with p < 0.05.

Discussion

The present study aimed to assess the between-session reliability of the hip abductors 1RM torque and the power and velocity during submaximal contractions in young adults using pneumatic resistance. As a result, we demonstrated excellent intersession reliability of the 1RM torque and the power and velocity at 3 submaximal resistances of the hip abductor muscles. Furthermore, the study revealed that hip abduction velocity at 60 and 70% of 1RM positively correlated with the performance on the 30CST. Thus, the pneumatic machine used in the present study provides a reliable way to assess 1RM torque and submaximal power and velocity of the hip abductor muscles.

We demonstrated excellent reliability for all the variables assessed in the present study (maximal torque, and submaximal velocity, and power) of the hip abductor muscles using pneumatic resistance. The benefit of pneumatic resistance is that the resistance or tensions through the movement remain the same regardless of how quickly one moves through the movement. The pneumatic resistance provides a constant resistance while the velocity of movement varies. Although no other studies have examined the reliability of velocity at submaximal resistances, 2 studies assessed the reliability of maximum torque (^33,43), and 1 investigated the reliability of power (¹³). A previous study (³³) found excellent reliability for 1RM torque during a seated leg press and a chest press using pneumatic resistance (both ICC = 0.99), with a small bias (near to 0) on Bland-Altman plots in young and older adults. Similarly, another research (⁴³) also showed excellent reliability for maximum torque (ICC ≥ 0.84) using pneumatic resistance with the leg press, chest press, knee flexion, knee extension, and latissimus pulldown in older adults. Additionally, the reliability of power at 40 and 70% of 1RM performed on a pneumatic leg press extension also had excellent reliability (¹³). However, in the present study, we found higher reliability of hip abduction power assessment compared with the study of Callahan et al. (¹³), which may be to the result of differences in single vs. multiple joint movement and protocol. As we and others have demonstrated, pneumatic resistance seems to be a reliable way to assess maximum torque and submaximal power and velocity. More importantly, as we demonstrated in this study, it is a reliable method for evaluating torque, power, and velocity of the hip abductors in standing.

The excellent reliability seen in the present study might be to the result of the strict protocol adopted here, which allowed us to replicate the same position and possibly generate the same effort on different days. Moreover, the use of percentage loads in relation to 1RM torque may also contribute to consistent values for the submaximal contractions between days. Nonetheless, the subjects of the present study performed the task with the dominant side, which may lead to a functional specialization on that side (³⁵) and may also positively influences the reproducibility of results. Therefore, the hip abduction pneumatic machine used in the present study provides a reliable way to assess maximum torque, and submaximal power, and velocity in healthy young subjects, allowing the method to be used to quantify declines in function or performance and assess the effects of training interventions.

We demonstrated for the first time that hip abduction velocity at 60 and 70% of 1RM explained 17% up to 19.5% of the variance of the 30CST. Although the agonist/antagonists muscle groups involved in the sit-to-stand task are the knee flexors/extensors and hip flexors/extensors, the hip abductor muscles work as stabilizers of the movement. Nonetheless, our data suggest that if an individual can quickly generate hip abductor torque, this may impact the ability of the same individual to use the hip abductors as stabilizers during other tasks, such as sit to stand. The 30CST is a test commonly used in older adults for fall risk (^5,26); improving hip abduction velocity may be a good strategy to improve the performance of the 30CST test and may decrease the risk of fall.

The present study did not find significant correlations between hip abductor power and 1RM torque with the clinical tests (30CST and SPCT). Opposite to that, Allen et al. (⁴) demonstrated that peak power at 30% of 1RM and maximal force, measured on a pneumatic leg press machine, explain up to 33% of the variance during a walking test (comfortable speed) and up to 54% during maximal walking speed in Parkinson's disease. Those with lower power at 30% 1RM were more likely to experience a fall than those with a higher power. Additionally, another study showed that power at 40 and 70% of 1RM (pneumatic leg press machine) explained 72 and 76% of the variance (respectively) during the SPCT test (³⁸). Differences between studies may be to the result of the different muscles used to assess power and hence a different role in the task, considering that the SPCT, and 30CST, predominantly used knee and hip extensors/flexors as agonists and antagonists of the movement during the entire task. In contrast, the hip abductor muscles act only as stabilizers, and this may explain the differences in results in the present study. Thus, it is possible that the capacity of the knee and hip extensor muscles to produce muscle power may have a more important role during the SPCT test than hip abductors.

This study has strengths and limitations that are important to highlight. First, we recruited 24 young, healthy subjects, which is not a large sample size. Although the sample had excellent reliability and significant correlations between velocity and the 30CST. A larger sample size would provide more confidence in our results, at least for the correlations. Additionally, given the novelty of our study, we could not compare our results with previous research. Therefore, other researchers are encouraged to replicate our study to confirm (or not) our results. Finally, our study was performed in young healthy adults, and extrapolation to other populations should be done with care.

In conclusion, we demonstrated excellent inter-session reliability of the hip abductor muscles, 1 repetition maximum torque, and the power and velocity generated during submaximal contractions in young adults using pneumatic resistance. Furthermore, we demonstrated that hip abduction velocity in young adults might be important for the performance of the 30CST.

Practical Applications

The findings in this study are important, particularly when assessing physical capacity or changes in physical capacity that require muscle power in a patient, athlete, and/or healthy individuals, to be confident that the variables collected are reliable across time. The present results provide coaches, trainers, and clinicians important information about the reliability of assessing power and velocity of the hip abductors in younger adults. More importantly, the significance of the hip abductor muscles in relation to physical performance is sometimes overlooked. The importance of these muscles should be considered with regard to physical capacity, not only as an agonist muscle but also as a stabilizer. Although the 30CST test is predominately used in older adults, the results of our study are still important for younger adults. The performance of the test depends on speed and the ability to generate force with each chair rise. The positive correlation of hip abduction velocity with the 30-second chair stand test in younger adults indicates a critical aspect assessed with this clinical measure beyond lower extremity strength. Thus, when prescribing exercises, the performance on the 30CST may demonstrate the ability of the hip abductors to produce quick movements against greater loads of force. This information is important because the performance of the hip abductor muscles is essential for sports and daily life situations as both an agonist and stabilizer. Nonetheless, the present results may also have practical applications to other populations since because power training is effective for eliciting improved functional performance in older adults (^23,40) and neurologic populations, such as individuals with Parkinson's disease (⁴¹) and stroke (¹). However, caution should be taken when translating these findings to other populations. Finally, considering the 30CST is a well-used test in older adults (^5,26) related to falls, the velocity of movement might be an important variable related to fall prevention (¹⁹). It would be ideal for future research to investigate whether these findings are similar in a population with a high rate of falls (e.g., older adults and stroke population).