Differences in lower-limb alignment and muscle strength between the sexes during distance-specific single-leg hop tasks

Abstract

[Purpose] This study examined the differences in lower-limb alignment and muscle strength between sexes during distance-specific single-leg hop (SLH) landings. SLH tasks are widely used in rehabilitation and return-to-sport decision-making; however, standardized progression criteria for hop distance are lacking. [Participants and Methods] A total of 59 healthy university participants performed SLH landings at distances set to 30, 50, and 80% of their height. The lower-limb alignment variables and isometric muscle strength were measured. Lower-limb kinematic variables were obtained using three-dimensional motion analysis, and isometric strength of the hip and knee muscles was measured using a handheld dynamometer. [Results] Females demonstrated greater hip flexion angles, lower hip adductor and abductor strength, and lower knee flexor and extensor strength compared to males. No differences were observed between the sexes in terms of vertical ground reaction force, knee valgus angle, or knee valgus moment. Females exhibited a hip-dominant landing strategy and lower strength in specific muscle groups. [Conclusion] The findings of this study highlight the importance of incorporating both movement instruction and strength training during rehabilitation for female athletes. These findings might establish clearer criteria for distance progression during SLH-based rehabilitation programs.

INTRODUCTION

Anterior cruciate ligament (ACL) injuries occur more frequently in females than in males, with two- to eight-fold differences observed between the sexes¹⁾. Approximately 70% of all ACL injuries comprise noncontact injuries that occur during jump landings and cutting maneuvers²⁾. Dynamic risk factors for noncontact ACL injuries include increased knee valgus angle and valgus moment³⁾ and elevated tibiofemoral joint loading associated with greater vertical ground reaction force⁴⁾. Females exhibit several biomechanical and anatomical characteristics that contribute to an increased risk of ACL injuries (e.g., decreased trunk flexion and increased knee valgus during injury scenarios⁵⁾ as well as greater pelvic width, Q-angle, and femoral anteversion⁶⁾). Greater generalized joint laxity⁷⁾ and reduced muscular and neuromuscular performance⁸⁾ were identified as sex-related risk factors.

Return-to-sport (RTS) decisions after ACL reconstruction incorporate a postoperative time that exceeds nine months⁹⁾, patient-reported outcomes¹⁰⁾ and functional performance tests (e.g., quadriceps strength and single-leg hop [SLH] tests^11,12,13)). The SLH performance, which includes the limb symmetry index (LSI) and hop distance, is widely used to evaluate dynamic knee stability and RTS readiness. An LSI of 90% or greater^{14, 15)} and a hop distance of at least 80% of height¹²⁾ are the commonly suggested benchmarks.

However, despite meeting the RTS criteria, reinjury remains a substantial concern. Ipsilateral graft rupture occurs in ~3–7% of the cases, and contralateral ACL injury occurs in 5–11% within two years after RTS^{16, 17)}. Further, females demonstrate a four-fold greater risk of ACL reinjury compared to males¹⁸⁾, highlighting the need for reinjury prevention strategies specific to female athletes.

The SLH test is widely used throughout rehabilitation and RTS progression; however, no established guidelines describe how the training volume or hop distance progress during recovery. Although many studies examined landing from a box or nonpredictive landing tasks, research using distance-specific SLH tasks remains limited. Clarifying the differences in SLH biomechanics and lower limb strength based on sex could provide essential information for developing reinjury prevention strategies for female athletes. However, no studies have systematically examined sex differences in distance-specific SLH landings; addressing this gap is essential to clarify how hop distance influences movement strategies in males and females. The purpose of this study was to clarify the differences in lower-limb alignment and muscle strength between sexes during distance-specific SLH tasks.

PARTICIPANTS AND METHODS

A total of 59 healthy university participants were included: 36 males (20.3 ± 1.0 years, 64.4 ± 8.0 kg, and 171.9 ± 5.2 cm) and 23 females (19.8 ± 1.2 years, 54.6 ± 5.7 kg, and 159.6 ± 4.5 cm). Participants were healthy university students with no history of ACL injury, no orthopedic or neurological disorders of the lower limbs, and no pain during testing. This study was approved by the Ethics Committee of Teikyo Heisei University (approval number: 2023-072-1), and written informed consent was obtained from all participants.

The SLH performance of the dominant limb and lower-limb muscle strength were assessed. The SLH task involved forward hops toward targets set at distances of 30, 50, and 80% of participants’ height. Before the task, participants were instructed to land on the same limb after hopping forward toward a target placed in front of them. Participants performed the task and landed on a force plate. Before testing, the participants performed three familiarization trials for each distance to minimize learning effects. Hop distances were performed in an order chosen voluntarily by each participant. The dominant limb was defined as the preferred leg used for kicking a ball. Each task condition was performed three times, and the mean value of the three trials was used for the analysis. A three-dimensional motion analysis system (NEXU2, Vicon, Oxford, UK) sampling at 200 Hz, and force plate (Kistler, Winterthur, Switzerland) sampling at 1,000 Hz were used for data collection. Furthermore, 51 reflective markers were placed on anatomical landmarks according to the Vicon CGM2.3 model. The initial contact was defined as the time point when the vertical ground reaction force dropped below −20 N. From the initial contact, we analyzed the maximum hip flexion angle, knee valgus angle, knee valgus moment, and knee flexion angle over a 3 s interval. The vertical ground reaction force was normalized to body weight, and its peak value was used for the analysis.

Lower limb muscle strength was assessed using a handheld dynamometer (μ-Tas MF-01; ANIMA, Tokyo, Japan). The isometric strengths of hip flexion, extension, adduction, abduction, internal rotation, external rotation, and knee flexion and extension were measured. Each test included three maximum effort contractions held for 5 s with 1-min rest intervals (Fig. 1). The mean of the three trials was divided by body weight (kg) and used for analysis. During testing, the participants folded their arms across the chest to avoid compensatory movements, and the dynamometer straps were adjusted to prevent slacking. The validity and reliability of handheld dynamometry for assessing muscle strength have been well established when standardized testing procedures are used, including in young adult populations^{19, 20)}.

Fig. 1.

Muscle strength measurement procedures.

A: hip flexion; B: hip extension; C: hip adduction; D: hip abduction; E: hip internal rotation; F: hip external rotation; G: knee flexion; H: knee extension.

Statistical analyses were performed using IBM SPSS Statistics (version 20.0; IBM Corp., Armonk, NY, USA). Sex (male or female) was entered as an independent variable, and SLH outcomes and lower limb muscle strength as dependent variables. Between-group comparisons were performed using independent t-tests. Statistical significance was set at p<0.05.

RESULTS

The female participants demonstrated greater hip flexion angles than the male participants during SLH landings at 30, 50, and 80% distances (Table 1). No differences were observed in the vertical ground reaction force, knee valgus angle, knee valgus moment, or knee flexion angle between the sexes (Table 1). Considering the lower limb muscle strength, females exhibited lower hip adductor and abductor strength, and lower knee flexor and extensor strength than males (Table 2). No differences were found in hip flexor, extensor, internal rotator, or external rotator strength between sexes (Table 2).

Table 1. Single-leg hop landing variables (males vs. females).

		Males	Females	p-value
vGRF (N/kg)	30%	−27.93 ± 4.13	−25.88 ± 7.07
	50%	−32.40 ± 5.04	−31.31 ± 5.43
	80%	−43.47 ± 8.22	−41.94 ± 6.61
Hip flexion angle (°)	30%	42.07 ± 8.15	48.06 ± 10.20	*
	50%	47.46 ± 8.28	54.11 ± 9.45	**
	80%	57.80 ± 10.97	66.34 ± 13.23	**
Knee valgus angle (°)	30%	−0.62 ± 4.87	−0.35 ± 4.49
	50%	−1.03 ± 5.03	−0.87 ± 4.82
	80%	−1.73 ± 5.81	−0.36 ± 4.69
Knee valgus moment (N·mm/kg)	30%	1,267.03 ± 367.94	1,134.50 ± 354.52
	50%	1,429.87 ± 451.10	1,276.09 ± 356.10
	80%	1,751.69 ± 614.11	1,580.14 ± 450.74
Knee flexion angle (°)	30%	50.70 ± 6.62	50.46 ± 5.78
	50%	55.62 ± 6.06	55.18 ± 5.92
	80%	62.89 ± 6.42	61.16 ± 7.77

Mean ± SD, independent t-tests: *p<0.05, **p<0.01. vGRF: vertical ground reaction force.

Table 2. Lower-limb muscle strength (males vs. females).

		Males	Females	p-value
Hip	Flexion	0.34 ± 0.11	0.34 ± 0.09
	Extension	0.50 ± 0.11	0.44 ± 0.11
	Adduction	0.28 ± 0.07	0.25 ± 0.05	*
	Abduction	0.43 ± 0.10	0.36 ± 0.09	*
	Internal rotation	0.30 ± 0.08	0.27 ± 0.06
	External rotation	0.35 ± 0.09	0.33 ± 0.08
Knee	Flexion	0.30 ± 0.06	0.26 ± 0.07	*
	Extension	0.47 ± 0.11	0.41 ± 0.09	*

kgf/kg, Mean ± SD, independent t-tests: *p<0.05.

DISCUSSION

This study summarized the differences in lower-limb alignment and muscle strength between sexes during distance-specific SLH tasks. Females demonstrated consistently greater hip flexion angles, lower hip adductor and abductor strength, and lower knee flexor and extensor strength than those of males, whereas no differences were observed in the vertical ground reaction force, knee valgus angle, or knee valgus moment between the sexes.

The increased hip flexion observed in females suggests that they adopted a hip-dominant landing strategy. Previous analyses of noncontact ACL injuries reported that females often present with reduced trunk flexion and increased knee valgus at the moment of injury^{3, 6)}. These characteristics reflect the injury mechanism; however, they do not necessarily represent landing patterns in healthy individuals. In this study, greater hip flexion might have contributed to reduced quadriceps loading and anterior tibial shear force⁸⁾, suggesting that females rely more on the hip to compensate for relatively lower muscle strength. This hip-dominant strategy was observed across all SLH distances, suggesting that it might represent a stable movement pattern among females. Previous studies have examined sex differences during landing tasks (e.g., box drops or unanticipated cutting maneuvers), which revealed reduced trunk flexion and increased knee valgus in females^{3, 5)}. However, these tasks differ from distance-controlled SLH landings, and direct comparisons remain limited. The findings of this study suggest that when the task is predictable and distance-specific, sex differences may manifest through hip-dominant strategies rather than frontal-plane knee mechanics. SLH distances in this study were set at 30, 50, and 80% of height; therefore, SLH is commonly used during rehabilitation after ACL injury. There is no established standard describing the strength level or landing capacity required to progress the hop distance^{11, 12)}. Clinical practice frequently relies on limb symmetry indices; however, objective criteria for distance progression remain unclear. Evaluating distance-specific landing mechanics might contribute to the development of clearer guidelines for stepwise progression during rehabilitation. Further research examining the relationship between specific strength measures^{13, 21)} and achievable hop distance could help refine these criteria.

Females showed lower hip adductor and abductor strength, and lower knee flexor and extensor strength, which is consistent with previously reported sex-related neuromuscular characteristics⁸⁾. Although females are described as being more likely to exhibit increased knee valgus³⁾, the predictable forward-landing task used in this study could have reduced the likelihood of detecting such sex differences. Differences between sexes tend to be more pronounced in unanticipated or multidirectional tasks⁵⁾. These findings highlight the importance of incorporating movement instructions that focus on coordinated hip flexion and trunk positioning in addition to strength training when designing rehabilitation programs for female athletes. The lower hip and knee strength observed in females was consistent with previous reports indicating sex-specific neuromuscular characteristics, including reduced hip abductor activation and decreased quadriceps strength⁸⁾. These factors were associated with altered lower-limb control during dynamic tasks and might explain the greater reliance on hip flexion observed in our study. In this context, appropriate movement instruction during hop tasks may be particularly important for female participants, in addition to strength training, to optimize movement strategies throughout the task.

This study has several limitations. The participants were healthy university students, which might limit the generalizability of the findings to competitive athletes and other age groups. In addition, this was a cross-sectional study; therefore, causal relationships between the observed movement characteristics and ACL injury risk could not be inferred.

The consistency between the present findings and previously reported sex differences^{3, 5, 6, 8)} supports the validity of the present results. Females adopted a hip-dominant landing strategy and demonstrated lower strength in specific lower limb muscle groups than males, regardless of the SLH distance. These findings confirm the importance of addressing both movement instructions and targeted strength training when planning rehabilitation programs for female athletes. Taken together, these findings indicated that rehabilitation programs for female athletes should emphasize strengthening of the hip and knee musculature and movement instructions that promote coordinated trunk and hip mechanics during distance-progressive SLH tasks^{11, 12)}. Clarifying these sex-specific patterns might contribute to establishing more objective progression criteria for SLH-based RTS testing. Future studies should examine muscle strength as joint torque and the hamstring-to-quadriceps ratio in relation to landing mechanics. In addition, the relationship between landing movement patterns and neuromuscular output during dynamic tasks should be investigated using electromyography.

Funding

No external funding was received for this study.

Conflict of interest

The authors declare no conflicts of interest.