Differences in hip and knee muscle strength between successful and unsuccessful performances of the single leg sit to stand test from a 20-cm-high box in healthy young adults

Abstract

[Purpose] To investigate differences in knee and hip muscle strength between successful and unsuccessful single leg sit to stand tests from a 20-cm-high box (SLST 20) in healthy young adults. [Participants and Methods] Sixty-six lower limbs from 33 healthy adults (20 males, 13 females; mean age 25.4 ± 3.4 years) were classified into successful and unsuccessful groups. Isokinetic strength of the knee (flexion/extension at 60°/s) and hip (flexion/extension/abduction/adduction at 30°/s) was measured. Statistical analyses included t-tests, Kruskal–Wallis tests, and analysis of covariance (ANCOVA) using sex as a covariate. Stratified analyses were also performed. [Results] The successful group had significantly greater strength in knee flexion/extension, hip flexion, and hip adduction. ANCOVA revealed that knee flexion and extension were significantly associated with SLST 20 performance, whereas hip strength was not, after adjusting for sex. Among females, hip adduction strength was significantly greater in the successful group, with no significant difference observed in males. [Conclusion] Knee strength is crucial for SLST 20 performance, and hip adduction strength may be important, particularly in females. Therefore, sex-specific assessments and training strategies should be considered in clinical practice.

INTRODUCTION

The single-leg sit-to-stand test (SLST) is a functional test used to evaluate the minimum height from which an individual can stand up on one leg¹⁾. This test involves a stepwise assessment using boxes of varying heights (40 cm, 30 cm, 20 cm, and 10 cm), with lower heights requiring greater neuromuscular control. Standing up from a 20-cm box (SLST 20) is considered a critical threshold as it is associated with dynamic movement abilities such as jumping and sprinting²⁾. In this study, we define the task of standing up from a 20-cm box as “SLST 20” and use this designation throughout the paper.

Previous studies have shown that SLST success at 40 cm, 30 cm, 20 cm, and 10 cm is strongly correlated with the Weight-Bearing Index (WBI), which normalizes knee extension strength to body weight^{1, 3)}. In Japan, SLST is commonly used as a clinical indicator during sports rehabilitation following knee surgery⁴⁾. However, in clinical practice, cases have been observed where individuals with sufficient knee extension strength still struggle to perform SLST 20, suggesting that factors other than knee extension strength may influence SLST 20 performance.

In contrast, studies conducted outside Japan have used SLSTs such as the Single-Leg Sit-to-Stand Test (SLSTS) and the One-Leg Sit-to-Stand Test (OLSTS) to evaluate lower limb function^{5, 6)}. The SLSTS assesses whether an individual can stand up once on one leg from a standard chair, typically approximately 46 cm in height⁵⁾. The OLSTS uses a chair of similar height but evaluates performance based on the time required to complete five single-leg sit-to-stand repetitions, which is then converted into a standardized rate per 30 seconds⁶⁾. Like SLST, these tests have been reported to correlate strongly with knee extension strength. However, the SLST 20 requires standing up from a significantly lower height—approximately half that of the SLSTS or OLSTS—which places greater demands greater hip and knee extension movements during the standing motion⁷⁾. Therefore, in addition to knee extension strength, the strength of muscles contributing to hip extension moments from lower heights may play a crucial role in SLST 20 performance.

Specifically, the hamstrings function as both knee flexors and hip extensors. During closed kinetic chain (CKC) activities, such as squatting and sprinting, the hamstrings generate substantial hip extension torque, ultimately contributing to knee extension^{8, 9)}. Additionally, the adductor magnus has been shown to exhibit strong hip extension capabilities when the hip is flexed beyond 90°, suggesting its contribution to successful SLST 20 performance¹⁰⁾. Furthermore, the gluteus maximus, the strongest hip extensor, is likely to play a key role when standing up from a low height.

This study aims to clarify the differences in knee and hip muscle strength based on SLST 20 performance. The findings of this study are expected to contribute to rehabilitation strategies by identifying factors that influence movement acquisition in patients with sufficient WBI but who struggle with CKC movements. In addition to the previously established importance of knee extension strength, we hypothesize that the successful SLST 20 group will demonstrate greater hip extension, hip adduction, and knee flexion strengths compared to the unsuccessful group.

PARTICIPANTS AND METHODS

A total of 66 lower limbs from 33 healthy young adults (20 males, 13 females; mean age: 25.4 ± 3.4 years; height: 164.9 ± 6.7 cm; weight: 57.6 ± 10.3 kg) participated in this study. Participants were recruited via volunteer posters displayed on campus, targeting healthy young individuals affiliated with Showa Medical University. Due to the lack of prior studies examining both hip and knee muscle strength in relation to SLST 20 performance, a formal power analysis was not feasible. Therefore, the sample size was determined based on the number of participants who could be assessed during the predefined data collection period (May to December 2024), which was established in accordance with the overall study timeline approved by the institutional ethics committee. All participants provided written informed consent prior to participation. This study was approved by the Ethics Committee of Showa Medical University (Approval No. 2024-062-A).

The exclusion criteria were as follows: (1) a history of lower limb surgery, (2) a history of lower limb fractures requiring alignment correction, (3) an acute musculoskeletal injury requiring at least one day of rest within the past 3 months, and (4) an acute lower limb fracture or injury within the past 6 months.

The SLST 20 was performed using a 20-cm box, following the methodology established in previous studies¹⁾. Participants began the task seated on the box in a slight forward-leaning posture with their arms crossed over their chest. The initial position was adjusted such that the lower leg of the supporting limb formed an angle of approximately 70° relative to the floor (20° dorsiflexion), while the non-supporting leg was extended forward without making contact with the floor. During the rising motion, participants were instructed to minimize momentum and maintain proper knee alignment, preventing excessive medial or lateral deviation of the supporting limb. Additionally, the non-tested leg was required to remain extended throughout the movement without touching the floor. Each participant performed up to two trials, and a trial was considered successful if they could complete the standing motion and maintain balance for 3 seconds in the final position. All trials were judged by the first author. A trial was deemed unsuccessful if postural instability was observed or if the non-supporting leg contacted the floor.

Isokinetic knee and hip muscle strength were assessed using an isokinetic dynamometer (Biodex System 4, Biodex Medical Systems, Shirley, NY, USA). Knee flexion and extension strength were measured at 60°/s in a seated position, consistent with the majority of previous studies that have employed this angular velocity for knee assessments¹¹⁾. In contrast, hip muscle strength was evaluated at 30°/s, as prior research has revealed that the lower angular velocity is more suitable for novice cohorts with a limited range of motion (ROM)¹²⁾. Specifically, hip flexion and extension strength were measured at 30°/s in a supine position with torso stabilization, whereas hip abduction and adduction strength were measured at 30°/s in a side-lying position with an elbow-supported posture. The elbow-supported configuration was adopted to minimize compensatory lateral trunk flexion, a common substitution for hip abductor muscle activity, and aligns with methodologies reported in previous studies. For each movement direction, measurements were performed five times, and the average peak torque (Nm·kg) was calculated. The value was then normalized to body weight for comparative analysis. The procedures for SLST 20 and muscle strength assessments are illustrated in Fig. 1. The SLST 20 evaluations and muscle strength assessments were performed by the first author to ensure consistency across measurements.

Fig. 1.

Experimental setup for SLST 20 performance and isokinetic strength measurements.

(A) Initial posture of the SLST 20 test. (B) Knee flexion and extension strength assessment using an isokinetic dynamometer. (C) Hip flexion and extension strength assessment in a supine position. (D) Hip abduction strength assessment in a side-lying position.

For statistical analysis, the normality of knee and hip muscle strength values was assessed using the Shapiro–Wilk test. Normally distributed variables were analyzed using Student’s t-test and Welch’s t-test to compare the two groups based on SLST 20 success. For variables that did not follow a normal distribution, the Kruskal–Wallis test was applied. Given the observed sex differences in SLST 20 performance, analysis of covariance (ANCOVA) was conducted with sex included as a covariate to examine its influence on knee and hip muscle strength. Additionally, stratified analyses were performed separately for male and female participants to further investigate sex-specific factors. The significance level was set at p<0.05, and all statistical analyses were performed using JMP Pro 17 (SAS Institute Inc., Cary, NC, USA).

RESULTS

The participants were classified into two groups based on their ability to successfully perform the SLST 20 task: the successful group and the unsuccessful group. Among the 66 lower limbs analyzed, 47 limbs were categorized as successful, whereas 19 limbs were categorized as unsuccessful.

Table 1 summarizes the characteristics of each group. No significant differences were observed in age, height, weight, BMI, tested limb dominance, or dominant foot distribution between both groups (all p>0.05). However, a significant difference in sex distribution was observed between both groups (p<0.01), with a higher proportion of males in the successful group.

Table 1. Participant characteristics by SLST 20.

Variable	Total (n=66)	Successful (n=47)	Unsuccessful (n=19)
Age (years)	25.42 ± 3.4	25.17 ± 3.6	26.05 ± 2.8
Height (cm)	164.89 ± 6.7	165.62 ± 6.5	163.10 ± 7.2
Weight (kg)	57.58 ± 10.7	57.53 ± 8.0	57.66 ± 14.7
BMI (kg/m2)	20.99 ± 2.6	20.87 ± 1.7	21.43 ± 4.0
Gender (Male/Female, n (%))*	40/26 (60.6/39.4)	33/14 (70.2/29.8)	7/12 (36.8/63.2)
Tested Limb (Right/Left, n(%))	33/33 (50.0/50.0)	25/22 (53.2/46.8)	8/11 (42.1/57.9)
Dominant Foot (Dominant/Non-dominant, n (%))	33/33 (50.0/50.0)	25/22 (53.2/46.8)	8/11 (42.1/57.9)

Data are presented as the mean ± standard deviation or number (%). *p<0.05.

SLST: single-leg sit-to-stand test from a 20-cm box; BMI: body mass index.

Table 2 presents the comparison of knee and hip muscle strength between the successful and unsuccessful groups after completion of the SLST 20. The Shapiro–Wilk test indicated that hip abduction and hip adduction strength did not follow a normal distribution; therefore, non-parametric tests were applied to these variables. The successful group exhibited significantly higher knee flexion strength (p<0.001), knee extension strength (p<0.001), hip flexion strength (p<0.01), and hip adduction strength (p<0.05) compared to the unsuccessful group. In contrast, no significant difference was observed in hip extension strength (p=0.12) and hip abduction strength (p=0.35) between the groups.

Table 2. Comparison of knee and hip muscle strength between the successful and unsuccessful groups during SLST 20.

Muscle strength	Successful (n=47)	Unsuccessful (n=19)
Knee flexion (Nm/kg, 60°/s)*	1.28 ± 0.26	0.99 ± 0.23
Knee extension (Nm/kg, 60°/s)*	2.52 ± 0.48	2.11 ± 0.31
Hip flexion (Nm/kg, 30°/s)*	1.55 ± 0.40	1.33 ± 0.21
Hip extension (Nm/kg, 30°/s)	1.95 ± 0.51	1.73 ± 0.51
Hip abduction (Nm/kg, 30°/s)	1.44 ± 0.35	1.34 ± 0.39
Hip adduction (Nm/kg, 30°/s)*	1.19 ± 0.34	1.05 ± 0.50

Data are presented as the mean ± standard deviation. *p<0.05.

SLST20: single-leg sit-to-stand test from a 20-cm box; Nm/kg: Newton meter per kilogram; °/s: degrees per second.

As presented in Table 3, ANCOVA with gender as a covariate revealed that knee flexion strength (p<0.001) and knee extension strength (p<0.01) were significantly associated with SLST 20 performance. In contrast, hip flexion strength (p=0.08) demonstrated only a trend toward significance, whereas hip adduction strength (p=0.44) was not significantly associated with SLST 20 performance. Additionally, neither hip abduction strength (p=0.85) nor hip extension strength (p=0.13) demonstrated significant associations. These findings suggest that although hip flexion and adduction strength initially differed significantly between the groups, their effects were no longer significant after controlling for gender using ANCOVA.

Table 3. Summary of ANCOVA results for knee and hip muscle strength.

Muscle strength	Gender	SLST20 Successful/Unsuccessful	Gender ×SLST20 Interaction
Knee flexion (Nm/kg, 60°/s)	*	*
Knee extension (Nm/kg, 60°/s)	*	*
Hip flexion (Nm/kg, 30°/s)
Hip extension (Nm/kg, 30°/s)
Hip abduction (Nm/kg, 30°/s)	*
Hip adduction (Nm/kg, 30°/s)

*indicates statistical significance at p<0.05.

SLST20: single-leg sit-to-stand test from a 20-cm box; ANCOVA: analysis of covariance.

To further investigate sex-specific factors affecting SLST 20 performance, a stratified analysis was conducted separately for males (successful: 33 limbs, unsuccessful: 7 limbs) and females (successful: 14 limbs, unsuccessful: 12 limbs), as shown in Table 4. The Shapiro–Wilk test revealed that hip abduction strength did not follow a normal distribution in males; thus, non-parametric tests were applied to these variables. In contrast, all muscle strength variables in females followed a normal distribution, so paired t-tests were applied.

Table 4. Stratified analysis of muscle strength by gender.

Muscle strength	Male participants	Female participants
	(n=33 Successful limbs, n=7 Unsuccessful limbs)	(n=14 Successful limbs, n=12 Unsuccessful limbs)
Knee flexion (Nm/kg, 60°/s)†	1.35 ± 0.24 vs. 1.20 ± 0.20	1.10 ± 0.20 vs. 0.87 ± 0.14
Knee extension (Nm/kg, 60°/s)	2.63 ± 0.47 vs. 2.28 ± 0.39	2.27 ± 0.43 vs. 2.00 ± 0.21
Hip flexion (Nm/kg, 30°/s)	1.59 ± 0.37 vs. 1.36 ± 0.23	1.46 ± 0.47 vs. 1.32 ± 0.20
Hip extension (Nm/kg, 30°/s)	1.93 ± 0.55 vs. 1.75 ± 0.76	1.98 ± 0.10 vs. 1.71 ± 0.34
Hip abduction (Nm/kg, 30°/s)	1.50 ± 0.36 vs. 1.57 ± 0.40	1.31 ± 0.22 vs. 1.20 ± 0.33
Hip adduction (Nm/kg, 30°/s)†	1.19 ± 0.38 vs. 1.28 ± 0.76	1.18 ± 0.23 vs. 0.92 ± 0.22

Data are presented as the mean ± standard deviation. ^†p<0.05 in female participants only.

SLST20: single-leg sit-to-stand test from a 20-cm box; Nm/kg: Newton meter per kilogram; °/s: degrees per second.

In males, the stratified analysis revealed no significant differences in hip flexion strength (p=0.12) or hip adduction strength (p=0.78) between the successful and unsuccessful groups. However, in females, hip adduction strength (p<0.01) was significantly higher in the successful group, whereas no significant difference was observed in hip flexion strength (p=0.33).

DISCUSSION

This study examined the relationship between SLST 20 performance and both knee and hip muscle strength. The results revealed that compared to the unsuccessful group, the successful group exhibited significantly greater knee flexion, knee extension, hip flexion, and hip adduction strengths. In contrast, no significant differences were observed in hip extension or hip abduction strengths. Additionally, ANCOVA revealed that knee flexion and knee extension strengths were significantly associated with SLST 20 performance, whereas hip flexion and adduction strengths exhibited no significant association after adjusting for gender. Stratified analysis further indicated that, among females, the successful group had significantly greater hip adduction strength than the unsuccessful group, whereas no such difference was observed among males.

The importance of knee extension strength in SLST 20 performance has been consistently emphasized in previous studies^{1, 3, 4)}. Our findings reaffirm this by highlighting its pivotal role in functional assessments involving sit-to-stand tasks. Additionally, knee flexion strength also showed a strong association with successful performance. The hamstrings, as primary knee flexors, are known to play a dual role under CKC conditions by assisting hip extension and stabilizing coordinated movement^{8, 9)}—a mechanism likely active during SLST 20.

Moreover, hip flexion strength was identified to be an important contributor to SLST 20 performance. The iliopsoas, a major hip flexor, plays a crucial role in controlling posterior pelvic tilt and facilitating the anterior movement of the center of gravity during rising motions^{13, 14)}. Given that the hamstrings generate forces that induce posterior pelvic tilt, activation of the iliopsoas is likely essential for counteracting this effect and maintaining postural stability during SLST 20 executions. In the context of the SLST, maintaining the non-supporting leg in an extended position may result in posterior pelvic tilt, particularly in individuals with shortened hamstrings. Previous biomechanical studies have shown that posterior pelvic tilt shifts the center of gravity backward, thereby making the rising motion more difficult. Thus, muscle flexibility and pelvic control—particularly the interaction between the hamstrings, iliopsoas, and gluteal muscles—may play a critical role in task performance. Our findings support this hypothesis, suggesting that adequate hip flexion strength is necessary for successful SLST 20 performances.

Interestingly, contrary to our initial hypothesis, no significant differences in hip extension strength were observed between the successful and unsuccessful groups. This suggests that hip extension strength may not have been a limiting factor in this cohort, as both groups demonstrated comparable and relatively high strength values. Additionally, the SLST 20 task requires a complex interplay of muscle coordination rather than relying solely on isolated hip extension torque. For instance, the hamstrings and adductor magnus—both of which exhibit hip extension capabilities in deep flexion—may contribute more prominently to generating the necessary hip extension moment during the task. These findings suggest that the role of hip extensors, such as the gluteus maximus, may be complemented or even compensated by synergistic muscle activity, particularly during tasks performed from low seated positions. Furthermore, although not directly assessed in this study, it is conceivable that the hip extension torque required for SLST 20 arises from the coordinated action of multiple muscles, including the gluteus maximus, hamstrings, and adductors. Given that successful performance requires the ability to rise and maintain balance for three seconds without compensatory movements, any imbalance among these contributing muscle groups may potentially disrupt motor coordination, leading to task failure. This hypothesis underscores the importance of muscle synergy, in addition to absolute strength, in accomplishing such functional tasks.

Furthermore, our findings suggest that hip adduction strength plays a role in SLST 20 success, particularly among females. The adductor magnus, a primary hip adductor, has been reported to exhibit strong hip extension capabilities when the hip is deeply flexed. This suggests that during SLST 20, the adductor magnus may assist in generating the hip extension moment required for standing. Given that females generally exhibit lower relative quadriceps strength compared to males, the involvement of hip adductors as a compensatory mechanism may be more pronounced in female participants¹⁵⁾. This finding underscores the potential role of gender-specific muscle activation patterns on SLST 20 performance.

This study has several limitations that should be acknowledged. First, the sample size used for stratified analysis was relatively small. In particular, SLST20 performance was unevenly distributed among male participants (successful: 33 limbs; unsuccessful: 7 limbs), potentially affecting the statistical power and generalizability of the findings. Future studies with larger and more balanced samples are warranted to enable more robust analyses, particularly regarding gender-specific muscle contributions.

Second, this study did not employ three-dimensional motion analysis to examine the biomechanical characteristics associated with SLST20 performance. Incorporating such techniques in future research would help facilitate a deeper understanding of the movement strategies underlying successful execution.

Additionally, ankle dorsiflexion ROM—previously reported to influence standing-up strategies—was not included among the primary measurements of this study. However, a supplementary analysis using available data revealed no significant difference in dorsiflexion ROM between the groups (Successful group: 43.1 ± 6.8°, Unsuccessful group: 46.9 ± 8.1°, p=0.06), suggesting that dorsiflexion may not have been a limiting factor in this sample. These findings imply that other factors, such as proximal muscle strength or postural control, may have played a more influential role. Future research employing multivariate approaches (e.g., multiple regression analysis) is needed to comprehensively examine the factors influencing SLST 20 performance. Despite these limitations, the present study offers important clinical insights. For example, some individuals with sufficient knee extension strength still failed to perform the SLST 20. Our findings highlight the importance of evaluating other muscle groups, such as those responsible for knee and hip flexion. Moreover, the observed significant role of hip adduction strength in females underscores the need for sex-specific rehabilitation strategies. Clinically, a stepwise assessment—beginning with joint ROM, followed by muscle strength and postural control—may serve as a useful framework for evaluating functional performance and designing individualized intervention plans.

This study demonstrated that knee flexion and extension strengths, as well as hip flexion and adduction strengths, contribute to successful SLST 20 performances. Notably, hip adduction strength was particularly important in female participants, suggesting the need to consider gender differences in lower limb function.

These findings suggest that assessing hip muscle strength and implementing targeted rehabilitation programs may benefit individuals who struggle with SLST performance despite having sufficient knee extensor strength. In particular, hip adduction strength appears to play a crucial role in females, whereas other unidentified factors may influence SLST performance in males, highlighting the need for further research on gender-based differences in lower limb function.

Future studies should employ three-dimensional motion analysis to elucidate the biomechanical mechanisms underlying SLST performance. In addition, it would be beneficial to assess motor control factors such as baseline balance ability and dynamic alignment at the spine, hip, knee, and ankle/foot during the task. Identifying compensatory movement patterns may refine the criteria for successful performance. Furthermore, considering the relationship between the participant’s height and box height (e.g., using varying step heights normalized by body height) may provide additional insights into task difficulty across individuals. These approaches will support the development of more effective and personalized intervention strategies.

Conflict of interest

The authors declare no conflicts of interest relevant to this study.