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Abstract

This study aims to determine the effects of asymmetry on kinematics, kinetics, and changes in the center of pressure of the dominant and non-dominant legs during single-leg landings in female athletes. Fifteen healthy female collegiate soccer players performed dominant and non-dominant single-leg landings from a height of 40 cm. The three-dimensional kinematics and kinetics of the lower extremities were collected simultaneously with a 16-camera motion capture system and one force platform, respectively. Moreover, joint angle, joint range of motion, ground reaction force, joint moment, center-of-pressure, and absolute symmetry index (ASI%) were analyzed. Results corroborated that the range of motion in the knee and hip joints were significantly lower in the non-dominant than in the dominant leg. Meanwhile, the non-dominant leg exhibited higher medial–lateral center-of-pressure displacement than did the dominant leg. In addition, the ASI% for the peak ground reaction force and the loading rate exceeded 10% (peak ground reaction force: 11.31%, loading rate: 19.28%), which indicated asymmetry between the two legs during the single-leg landing impact. The findings also imply that for the female soccer players, the risk of injury (e.g. anterior cruciate ligament injury) can be higher in the non-dominant than in the dominant leg during unilateral dynamic movements.

Keywords

Asymmetry unilateral landing kinematics absolute symmetry index female

Introduction

Jump landing is one of the most common functional activities and has been previously confirmed to cause a conversional form of non-contact anterior cruciate ligament (ACL) injury.¹ In addition, this movement is common in sports such as soccer, basketball, and volleyball, in which the incidence of ACL injuries is two to eight times greater among female than male athletes.² ACL injuries can cause other pathological changes to the knee joint, such as reduced knee joint stability,² meniscus and cartilage damages,³ and arthritis.⁴ Although numerous studies have investigated the mechanisms of ACL injuries, such injuries continue to occur frequently. Particularly, the annual incidence of ACL injuries has increased by 1.3% in jump-landing sports.²

From a biomechanical perspective, a large knee valgus angle, valgus moment (or rotation) on the knee joint, and a small knee flexion angle are the main reasons that induce the occurrence of non-contact ACL injuries during landings.⁵ However, recent epidemiological studies have corroborated that the symmetry that occurs during lower extremity injuries might also be an important cause of non-contact ACL injuries among athletes, particularly in female soccer players.⁶ The main indications of the phenomenon known as asymmetry or laterality include the degree of the symmetry exhibited in the body posture when exercising and a bias toward the limbs on one side of the body during autonomic nervous activities.⁷ Asymmetry has long been examined in neurophysiological and motion control studies.^8,9 However, the relationship between the effects of asymmetry and injuries to the ACL during jump landings has not received sufficient research attention.

Most landing-related studies have assumed that the lower extremities are symmetrical.^10,11 Therefore, these studies have only utilized single-side data from the subjects’ lower extremities to analyze the risks of landing-related injuries.^10,11 Moreover, certain investigations of single-leg landings have only chosen the dominant side to represent the overall performance of the subjects’ bilateral lower extremities.¹⁰ Considerable studies have nonetheless noted the presence of symmetry that affects the lower extremities during landing movements.^12–14 For instance, studies have examined the lower extremity asymmetry in the context of several types of landings, including forward jump landing,¹⁴ vertical landing,^13,14 and sudden stop-jump landing.¹² The above-mentioned studies have utilized a double-leg landing approach to observe the asymmetrical phenomenon. Results have confirmed the manifestations of lower extremity asymmetry during the double-leg landing task, of which knee moment in the dominant leg was larger than that in the non-dominant leg. Moreover, the dominant leg knee joint angle also appeared less flexed during the initial contact, and females exhibited higher asymmetry than males during double-leg landing tasks, especially for knee valgus in the initial contact.¹⁴ Previous studies have concluded that the dominant leg might undertake more injury risks than the non-dominant one because of lower extremity landing asymmetry.¹³ However, scholars have not studied the biomechanical characteristics of the asymmetries of the lower extremities during landing tasks that only involve a single leg. Particularly, in female athletes, few reports have addressed the existence of differences between the dominant and non-dominant legs in the biomechanical characteristics during single-leg landings.

Therefore, this study aims to determine the effects of asymmetry on kinematics, kinetics, and changes in the center-of-pressure (COP) of the dominant and non-dominant legs during single-leg landings in female soccer players. The hypothesis is that the biomechanical characteristics of the lower extremities during single-leg landings will exhibit asymmetry, particularly with respect to the indicators of joint angle, joint moment, and lower extremity landing stability.

Methods

Participants

Fifteen healthy female collegiate soccer players (age: 20 ± 1 years, height: 167 ± 6 cm, mass: 56 ± 6 kg) with an average of 9 ± 2 years of experience in soccer events volunteered to participate in this study. All the participants had no history of lower extremity injuries in the previous 6 months and vigorous exercise within 24 h before the experiment. Furthermore, the informed written consent approved by the Institutional Review Board of the Shanghai University of Sports was obtained from each participant.

Experimental protocol

Prior to the test, all the participants were asked to kick a ball to determine which leg was their dominant leg.¹⁵ The leg they chose to kick with was deemed the dominate leg, whereas the plant leg was deemed the non-dominate leg.^12,14 Each participant was required to complete two testing procedures: step-off dominant and non-dominant single-leg landings. The participants began with a warm-up by running on a treadmill for 5 min (speed: 6.5 km/h) and stretching in both testing sessions.

In the landing tests, 38 reflective anatomical and tracking markers were placed on the foot, ankle, shank, knee, thigh of both legs, as well as on the pelvis, trunk, and head.^12,14 The anatomical markers were then removed before the landing trials began, and only tracking markers were left in data collection. A 16-camera motion capture system (100 Hz, T40; Vicon Motion Inc., Oxford, UK) and one force platform (1000 Hz, 9287B; Kistler Instruments AG Corp., Winterthur, Switzerland) were used to obtain the three-dimensional (3D) kinematics and ground reaction forces (GRF) simultaneously during the step-off landing testing. The participants were required to perform five trials of successful step-off single-leg landings from the height of 40 cm in either the dominant or the non-dominant leg condition. The testing order of the two landing conditions was randomized.

To maintain a consistent and reliable landing technique during the testing session, the participants were instructed to (1) step off with one leg on the platform and then landing, (2) keep their hands on each side of their hips, and (3) land as comfortably and normally as possible.¹⁶ All the participants used toe-heel landing strategies. A trial was considered successful if they were able to complete the step-off landing without losing their balance. They wore the uniform standard lab shoes with a rubber outsole and a thin foam insole but no midsole (Shanghong Shoes Co., Ltd., ClassyVast, China).

Data processing and analysis

Visual 3D biomechanical analysis software (v5, C-Motion, Inc., Germantown, MD, USA) was used to compute the 3D kinematic and kinetic variables of both sides of the lower extremities in single-leg landings. The 3D angular kinematics were computed using a Cardan (X-Y-Z) rotation sequence. A right-hand rule was used to determine the polarity of the 3D angular kinematic and kinetic variables. Moreover, the 3D marker coordinates and GRF signals were smoothed using a fourth-order Butterworth low-pass filter with cutoff frequencies of 7 and 100 Hz, respectively. The landing phase was determined from the initial foot contact to the maximum flexion angle of the knee.

The main kinematic variables of the hip, knee, and ankle joints included the following: (1) joint flexion/ankle plantarflexion angle at the initial contact; (2) ankle eversion, knee adduction, and hip abduction angle at the initial contact; and (3) joint range of motion (ROM), which were determined by calculating the changes in the angles of the three joints in the sagittal and frontal planes.

In addition, the main kinetic variables included the following: (1) peak vertical GRF, which was normalized to body mass; (2) the GRF loading rate, which was calculated from the maximum GRF value and time to the maximum GRF; and (3) normalized peak moments of the three joints in the sagittal and frontal planes.

Meanwhile, the absolute symmetry index (ASI) was used to analyze the landing impact symmetry between the dominant and non-dominant legs during the single-leg landing (equation (1))^17,18

ASI (%) = | \frac{D - N}{0.5 \times (D + N)} | \times 100

(1)

where D refers to the variables of impact forces or loading rate for the dominant leg, while N refers to the same variables for the non-dominant leg. Symmetry can be considered acceptable for an ASI <10%.¹⁹

In addition, COP data (in cm) and COP velocity (in cm/s) were included in the analysis, and the COP was defined as the distance between the medial to lateral (M-L) and anterior to posterior (A-P) during single-leg landings. Meanwhile, COP velocity was defined as the sum of the cumulated mean of M-L and A-P COP displacement divided by the total time in seconds.²⁰

Statistics

The mean and standard deviation (SD) were calculated for each variable of the participants’ dominant and non-dominant legs during the single-leg landings. Moreover, paired t-tests were used to determine the significant differences between the dominant and non-dominant legs in each biomechanical variable (p < 0.05). All the statistical procedures were conducted using SPSS Statistical Software (Version 19; SPSS, Inc., Chicago, IL, USA).

Results

Joint kinematics

At the initial foot contact, no significant differences were observed between the dominant and non-dominant legs with respect to any of the lower extremity joint angle variables (Figure 1). With respect to the ranges of the motion of the lower extremity joints, the hip and knee joint ranges of motion in the sagittal plane were smaller in the non-dominant than in the dominant leg. No differences were observed in the frontal plane ROM for each joint. The ankle joint results were similar to the findings in the hip and knee joints. However, the differences in the sagittal plane were not statistically significant (Figure 1).

Figure 1.

Lower extremity joint angle at the initial contact and joint range of motion (ROM) between the dominant and non-dominant legs during single-leg landings.

COP displacement and velocity

The examinations of COP displacement and displacement velocity during single-leg landings affirmed that the M-L COP displacement was significantly larger for the non-dominant than for the dominant leg (Figure 2). No significant differences were observed between the two legs with respect to the A-P COP displacement or the COP velocity.

Figure 2.

Center of pressure (COP) displacement and displacement velocity between the dominant and non-dominant legs.

Dynamics and symmetry

The peak GRF values of the dominant and non-dominant legs were not significantly different between the two sides. No difference emerged between the two legs with respect to either the time to peak GRF or the GRF loading rate. Moreover, the examinations of the peak moment of the ankle, knee, and hip joints during single-leg landings revealed no significant differences between the two legs (Figure 3).

Figure 3.

Ground reaction force (GRF) and lower extremity moment during single-leg landings.

With respect to the ASI (%) for the GRF and the loading rate, the symmetry indices for the GRF and the average loading rate exceeded 10% (Figure 4). The ASI (%) obtained from calculations that involved the dominant and non-dominant sides revealed asymmetry in the kinetic parameters of the lower extremities on the dominant and non-dominant sides during the unilateral landing task.

Figure 4.

Absolute symmetry index (ASI) for the peak ground reaction force (GRF) and loading rate during single-leg landings.

Discussion

The findings of this study contended that during single-leg landings, asymmetries existed between the lower extremities with respect to the ranges of the motion of the hip and knee joints, the M-L COP displacement, and the landing impact symmetrical indices. These results were consistent with those of the previous studies,^12,14 which confirmed the existence of symmetry between the dominant and non-dominant lower extremities in female soccer players during single-leg landings. In addition, the lower extremity on the non-dominant side exhibited worse stability than the dominant side during the landing stage.

Joint kinematics

The examinations of the ranges of motion during single-leg landings revealed statistically significant differences between the dominant and non-dominant legs for the hip and knee joints in the sagittal plane during the landing phase. These findings were consistent with the results of a prior study.²¹ Relative to male soccer players, female soccer players have a smaller range of joint flexion in their non-dominant leg, particularly when compared with the hip and knee joints in the dominant leg. Consequently, female soccer players have a more “rigid” landing posture. This landing pattern is often closely associated with the occurrence of lower extremity injuries.^22,23 Reductions in the range of joint motion in the dominant and non-dominant legs might correlate with asymmetries in the lower extremity muscle strength on either side of the body.²⁴ Particularly, muscle strength is typically higher in the dominant than in the non-dominant leg.²¹ A relevant muscle strength study has confirmed that in strength tests, female athletes with a history of ACL injuries exhibit decreased hamstring muscle strength relative to male and female athletes who have not experienced ACL injuries.²⁵ Given that the hamstring muscles play an important role in knee flexion, weakness in these muscles might be a reason for the reduced joint flexion in the non-dominant relative to the dominant leg. Increased hamstring muscle strength during the joint flexion stage of jump landings can markedly reduce ACL strains. Moreover, the main mechanism of this phenomenon is the co-contraction of the quadriceps and hamstring muscles during dynamic movements.²⁵ This mechanism increases the joint flexion, thereby reducing the risk of ACL injuries.

Regarding the joint angles at the initial contact, although the ankle dorsiflexion angle was greater in the dominant than in the non-dominant leg, a statistical difference was not observed. The similar joint angles in lower extremity indicated that during the initial contact phase, the athletes utilized the alike strategy in the dominant and non-dominant legs. During landing tasks, landing height and difficulty are important factors when determining whether the performances represent the athletes’ maximum capabilities.²⁶ Certain studies have verified that in landing studies, increase in the landing height will increase the observed ranges of motion and the GRF values of the lower extremity joints.^23,26 Therefore, whether differences in the joint angles between the dominant and non-dominant legs would arise if the landing height was increased beyond 40 cm that was applied in this study is unclear.

COP displacement and displacement velocity

Effective movement control during the completion of a specific task reflects the capacity for stable postural control.^27,28 The time to stabilization,²⁸ COP displacement, COP displacement velocity,²⁷ and dynamic postural stability index^20,28 are commonly used to evaluate the body’s ability to maintain a stable posture. COP dispersion is a frequent manifestation of instability or a lack of balance.²⁹

The COP displacement results deduced that the M-L displacements were significantly larger in the non-dominant than that in the dominant leg. Although the COP displacement velocity was not statistically different between the two legs, results showed there was a trend toward that the COP displacement velocity of non-dominant leg was larger than the dominant leg (p < 0.1). In prior studies, the COP was generally measured in a static standing posture,^21,30 while in this study, the COP displacements and velocities were calculated dynamically to measure the stability during the landing. Compared with the dominant leg, the non-dominant leg exhibited 40% greater M-L COP displacement, indicating that the female soccer players’ non-dominant legs were more unstable during the landing.

In human beings, body posture is mainly controlled through nerve center responses and proprioceptive feedback-related adjustments.^21,27,31 Reduced stability control correlates with worse proprioception. Studies have demonstrated specific links between inadequate stability control in a single-leg posture and the incidence of lower extremity injuries,^27,32 particularly ACL injuries.²⁷ Non-contact ACL injuries primarily result from increases in ACL loading consequent to reduced joint flexion in combination with a large knee abduction angle and abduction moment³³ on the knee joint. Meanwhile, Durall et al.²⁷ inferred that the changes in the COP are correlated significantly with the peak knee abduction moment and that in female athletes, the relatively large knee moment in the frontal plane is correlated with a lack of postural control. In addition to central nervous system control and proprioceptive feedback, the ability to stabilize the body posture might also correlate with muscle strength and the joint flexion ROM.^21,25 Therefore, we presumed that large M-L COP displacements in the non-dominant legs during unilateral landings might be induced by a worse proprioception, joint flexion, and muscular control.

During landings, non-contact lower extremity injuries, particularly knee and ankle sprains, are primarily attributable to a small range of joint motion and large initial contact angles along with medial and lateral joint torsion.³³ These factors cause landing-related injuries by preventing the joints from effectively absorbing energy and reducing the impacts of landings.³³ We proposed that large M-L COP displacements in the non-dominant leg combined with less knee flexion may cause less impact absorption in the knee joint in the frontal and sagittal planes than in the dominant leg. Therefore, the non-dominant leg may suffer a greater risk of injury than did the dominant leg while attempting to provide support and stability during single-leg landings.

Dynamics and symmetry

A study on landing and ACL injuries has corroborated that such injuries frequently occur during the early stages of landing, that is, approximately 40–60 ms shortly after the initial contact.³⁴ Typically, the peak GRF and maximum ACL loading were achieved during this period.³⁵

Prior jump-landing studies have reached inconsistent conclusions regarding the GRF values and times to the peak GRF during the bilateral lower extremity landings.^12,21,36 In addition, a prior investigation confirmed that the brain center-controlled activation of the lower extremity protective mechanisms was synchronized during bilateral landing tasks, with the landing shocks randomly being distributed between the bilateral lower extremities.¹³ In this study, the dominant and non-dominant legs of the female soccer players exhibited no differences in the peak GRF, time to peak GRF, or loading rate. These findings corroborated that the dominant and non-dominant legs experienced approximate magnitude and rate of loading within similar time periods, which was consistent with prior findings that have focused on ankle joint biomechanics during a bilateral landing task.¹³ Compared with the dominant leg, the non-dominant leg exhibited reduced ranges of motion but withstood similar landing impacts. With regard to the non-dominant leg, a relatively rigid landing posture and a similar GRF to the dominant leg, combined with large M-L COP displacements during landing, might increase the injury risk of female soccer players’ non-dominant leg when involved in jump-landing activities.

In recent years, symmetrical indices have been utilized to evaluate the symmetry of impacts during landings.³⁷ Milner et al.³⁷ observed that during double-leg landings, the subjects exhibited significantly greater landing symmetry if provided with verbal instructions to land with their centers of gravity evenly distributed across both lower extremities than in the absence of verbal cues. This finding implied the potential existence of lower extremity asymmetry during bilateral landing tasks. In this study, the symmetrical indices also demonstrated the presence of asymmetries, for example, >10% of the ASI, in the impact variables, particularly with respect to the loading rate. The loading rate is typically used to represent the speed at which the GRF is absorbed by the body within a certain period, and injury risks increase as greater amounts of energy are absorbed by the body within a shorter time period.³⁸ An analysis of the landing-related ACL injuries in female athletes indicated that the body weight was unevenly distributed during landing, and the resulting asymmetry in the loading rate was a major risk factor to ACL injuries.³⁹ Therefore, in the evaluations of landing impact symmetry, simple GRF and time to peak GRF examinations cannot be enough to elucidate whether the landing impacts are symmetrical; symmetry indices could be important reference indicators.

Limitations

In the current landing study, only one landing height (40 cm) was used. Therefore, whether increases in drop height would have caused differences in the other variables used to assess landing asymmetries is unknown. More drop heights can be involved in the future study. In addition, when the landing height is sufficient to evoke the athletes’ maximum athletic capabilities, the athletes will use a toe–heel landing approach to protect themselves. The athletes examined in this study were instructed to utilize the toe–heel landing approach, which might have influenced the landing approaches of individual high-level athletes. Finally, all the participants were female soccer players with varying positions. In this study, however, we did not consider the effects of different soccer positions on bilateral landing biomechanics, which might occur individually.

Conclusion

The examinations of the ranges of joint motion, COP displacements, and symmetrical indices in the dominant and non-dominant legs of female soccer players during single-leg landings confirmed the existence of asymmetry between the two lower extremities. Specifically, compared with the dominant leg, the non-dominant leg exhibited smaller ranges of motion in the knee and hip joints, larger medial and lateral COP displacements, and >10% of the ASI in the peak GRF and loading rate. These findings implied that for the female soccer players, the risk of injury could be higher in the non-dominant than that in the dominant leg during unilateral dynamic movements. In addition, in clinical landing studies, both legs are suggested to be considered in evaluating the risks of lower extremity injuries.

Footnotes

Handling Editor: Prashant Jamwal

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (11772201, 81572213); National Key Research and Development Program of China (2018YFF0300500); and the Talent Development Fund of Shanghai Municipal (2018107).

ORCID iD

Weijie Fu

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Asymmetry between the dominant and non-dominant legs in the lower limb biomechanics during single-leg landings in females

Abstract

Keywords

Introduction

Methods

Participants

Experimental protocol

Data processing and analysis

Statistics

Results

Joint kinematics

COP displacement and velocity

Dynamics and symmetry

Discussion

Joint kinematics

COP displacement and displacement velocity

Dynamics and symmetry

Limitations

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References