The effect of box height on joint stiffness performance in drop jumping: a cross sectional study

Abstract

Background

This study aimed to examine the impact of performing drop jumps from different heights on joint stiffness, particularly focusing on the ankle and knee joints. In sports and training environments, joint stiffness plays a vital role in force absorption and performance output during jumping activities. Understanding how stiffness responds to varying jump heights can contribute to optimizing training strategies and minimizing injury risk.

Methods

A total of 32 athletes (16 males and 16 females; mean age: 21.6 ± 2.6 years; height: 178.1 ± 10.2 cm; weight: 68.7 ± 16.1 kg) voluntarily participated in this study. Each participant performed two drop jumps from three different box heights (26 cm, 35 cm, and 42 cm), executed on both the dominant and non-dominant lower limbs. Reflective markers were positioned on anatomical landmarks, and motion data were captured using the Qualisys Track Manager system. The collected kinematic data were processed with Visual 3D software to calculate gravity displacement and vertical stiffness values.

Results

The results demonstrated that knee joint stiffness was significantly greater than ankle joint stiffness across all jump heights. Moreover, peak knee joint moments increased with higher drop heights, with the highest values observed at the 42 cm box (p < 0.05). Vertical stiffness was higher at Box 3 than Box 1 (p < 0.05), but no other significant differences were observed.

Conclusions

The findings indicate that the knee joint serves as the primary contributor to force generation during drop jumps, while the ankle joint contributes mainly to stabilization. Although joint moments increased with drop height, vertical stiffness only differed between Box 1 and Box 3, suggesting adaptation occurs mainly through joint-level adjustments.

Introduction

Performance characteristics such as acceleration, deceleration, maximum speed, power and strength are critical to athletic performance and injury prevention, while dynamic movements such as jumping and leaping also come to the fore in many sports [1]. For athletes to perform optimally in various sports activities and to prevent injuries, an efficient stretch-shortening cycle (SSC) in the muscles is essential [2, 3].

Plyometric training contributes to strength performance by increasing the speed of stretch-shortening cycle (SSC) in musculo-tendon structure, therefore athletes and coaches apply plyometric training to improve stretch-shortening cycle performance [2, 4, 5]. In this approach, which is one of the strength training methods, it is aimed to improve power performance depending on the performance increase in the stretch-shortening cycle [6]. Stretch-shortening cycle exercises also increase stiffness, which is regulated through neural mechanisms, providing the necessary increase in stiffness to improve performance. Stiffness refers to resistance to deformation when exposed to force, and can be assessed at different levels of the musculoskeletal system, from a single joint to the whole body [7]. Vertical stiffness reflects resistance to vertical deformation during locomotion [8], while joint stiffness represents the resistance of a specific joint [9].

Joint stiffness and vertical stiffness are commonly measured to assess athletic performance, injury risk, and movement efficiency in various sports activities. Studies have indicated that vertical stiffness is associated with running performance [10–14]. It has been reported that vertical stiffness increases with running speed and stride frequency and can distinguish elite runners from low-performing athletes [11, 13]. In studies investigating joint stiffness, it has been reported that vertical stiffness provides more reliable results than joint stiffness [5, 12]. Some studies have reported that running economy improves with increasing knee joint stiffness, but it has been emphasized that this relationship cannot be directly linked to performance and further research is needed [11, 14].

Drop jump exercises are one of the plyometric exercises frequently performed by coaches and athletes to increase joint stiffness and vertical stiffness [15]. Research has shown that the magnitude of elastic force during drop jump is affected by the athlete’s box height [6], technique and verbal guidance [16] and the flexion angle of the joints [15]. Kim, Jeon [17], who investigated different techniques of drop jump exercise, reported that drop jump exercises are effective methods that accelerate the stretch-shortening cycle and improve jumping performances by utilizing the elastic energy stored in the muscle-tendon complex.

Studies on drop jump techniques have shown that different execution methods directly affect leg stiffness and strength output, and are associated with athletic performance in homogeneous groups [18, 19]. In a different study, it was suggested that leg stiffness is dependent on vertical stiffness and that joint stiffness should be investigated to understand the effect of joints on vertical stiffness [5]. Gohara, Yamamoto [20], investigated the effect of joint kinematics on vertical stiffness and reported that there was no relationship between vertical stiffness and joint kinematics. Studies comparing lower limb stiffness and joint stiffness have reported that ankle joint stiffness contributes more to lower limb stiffness than knee stiffness during drop jumps jumps [8, 10, 17].

As demonstrated in preceding studies, joint stiffness is dependent on muscle activation rate, range of motion and angular velocity. Nevertheless, the mechanism that governing these phenomena in multi-joint movements remains to be elucidated [9]. Vertical stiffness and joint stiffness performance were evaluated at different frequencies of hopping and running speeds and it was stated that more studies are needed to understand vertical stiffness performance [21]. In the literature, there are various studies on joint and vertical stiffness [5, 20, 21]. However, few studies have specifically examined how different drop jump heights influence knee and ankle joint stiffness in relation to vertical stiffness. Current findings remain insufficient to fully explain the biomechanical adaptations to varying jump heights.

Therefore, further investigation is warranted. In this context, the present study was designed to contribute to the existing body of knowledge by examining the effects of drop jump exercises performed from different heights on joint and vertical stiffness. It was hypothesized that (i) increasing drop height would lead to higher joint and vertical stiffness values, and (ii) the magnitude of these changes would differ across joints.

Materials and methods

Participants

Thirty-two athletes (16 males and 16 females; mean age: 21.6 ± 2.6 years, range: 19–32 years, and mean sports age 7.0 ± 1.71 years) voluntarily participated in this study. The participants were engaged in different sports, including football (n = 6), volleyball (n = 7), basketball (n = 10), handball (n = 2), tennis (n = 4), and athletics (n = 3). The mean height, weight, and sides of the dominant leg of the participants were 178.1 ± 10.2 cm, 68.7 ± 16.1 kg, and 13 left legs, 21 right dominant, respectively. Dominant leg was determined based on the participant’s preferred kicking leg, which is a widely accepted method in sports science research. Inclusion criteria required participants to be athletes engaged in sports that involve frequent jumping such as volleyball and basketball and to be familiar with drop jump exercises. Additionally, participants were required to be free of any lower extremity injuries for at least six months prior to the study. Exclusion criteria included any recent musculoskeletal injuries, neurological disorders, or any condition affecting jump performance. Sample size was calculated a priori using G*Power 3.1 for a repeated-measures ANOVA (f = 0.25, α = 0.05, power = 0.80). The effect size was derived from previous drop jump studies (e.g. [22, 23]),, even though these studies employed smaller sample sizes. Based on this analysis, a minimum of 30 participants was determined to ensure sufficient statistical power in the present study.

Study design and experimental procedure

This cross-sectional study was conducted in a biomechanics laboratory equipped with a seven-camera three-dimensional motion capture system (Qualisys AB, Gothenburg, Sweden). Participants attended a single measurement session during which reflective markers were placed on anatomical landmarks, and they performed drop-jump exercises from three different box heights: 26 cm (Box 1), 35 cm (Box 2), and 42 cm (Box 3). Drop heights of 26, 35, and 42 cm were chosen in line with previous studies using 20–45 cm boxes to examine stretch-shortening cycle performance, providing both safety and sufficient mechanical stimulus [24–26]. Prior to data collection, participants completed a standardized 10-minute dynamic warm-up. Drop jumps were performed twice for each height using both the dominant and non-dominant legs. Each trial began with the athlete standing with arms outstretched at their sides until a ‘Ready’ and ‘Start’ command was given. Participants then dropped from the box, landed on the force platform with hands on their waist to avoid marker occlusion, and immediately performed a maximal vertical jump. Athletes were instructed to minimize ground contact time and maximize jump height. Trials were accepted only if the movement technique was correct and marker data were complete; otherwise, the trial was repeated.

Instrumentation

Kinematic data were collected using 59 retroreflective markers placed on anatomical landmarks according to protocols established by [10, 27]. Markers were positioned on the head, trunk, pelvis, upper and lower limbs, including bony prominences such as epicondyles, malleoli, tibial tuberosity, and metatarsals. Participants wore tights for secure marker attachment; men were shirtless, and women wore sports bras. Motion data were captured at 500 Hz using seven high-speed Oqus 7 + cameras arranged around the athlete at 4–6 m intervals. Simultaneously, ground reaction forces (GRFs) were recorded using a Bertec multicomponent force platform, synchronized with the motion capture system via a 32-channel USB Interface Synchronisation Unit (Qualisys, Sweden). Calibration of the equipment was performed before each session.

Data collection

The previously described instrumentation (QTM 2020.3 system, Qualisys, Sweden) was used to record kinematic and kinetic data. After preparation, athletes were brought to the jumping area and positioned on the force platform. They waited in the ready position, with hands on their hips, until the start command was given. Ahead of each trial, it was verified that all markers were visible within the camera angles. With the start signal, the athlete performed the drop jump from a box, landing on the force platform with hands on the waist to avoid covering the markers. Each participant completed the jumps starting randomly with either the dominant or non-dominant foot, followed by the opposite foot. Athletes were instructed to minimize ground contact time and achieve the maximum possible jump height after landing.

Data capture

After the warm-up, and measurements were taken. The measurements consisted of two steps: pre-test swing capture and execution of the drop jump at three different heights. Once the athlete was ready, each trial was started with the commands ‘Ready’ and ‘Start’ and measurements were taken until all markers were captured. If the athlete performed the movement correctly and there was no missing data, the movement was accepted; if it was not performed with the desired technique, the athlete was asked to repeat it. Each athlete was required to repeat the trial once to ensure correct execution and complete data collection, resulting in one repetition per participant. The desired technique involved performing the drop jump with hands on hips, without stepping off the box, landing on a single foot into the force platform, and minimizing ground contact time on the force platform.

Data processing and analysis

The dynamic marker data recorded by the QTM was filtered using a 4th order Butterworth low pass filter with a cut-off frequency of 6 Hz to eliminate high frequency noise. Moments as first contact, take-off and second contact were marked based on the vertical ground reaction force data. GRF (x, y,z) were seperately filtered with a 4th-order Butterworth low-pass filter with a cutoff frequency of 15 Hz to eliminate high-frequency noise in Visual 3D software. Contact threshold is 40 N. All data obtained from the QTM, vertical ground reaction force, moments and marker positions, were imported into Visual 3D motion analysis software. This software calculated the parameters and positions of each solid body segment (hands, lower arms, upper arms, head, trunk, torso, pelvis, thighs, and legs) based on the athlete’s anthropometric data such as height and weight and estimated the athletes’ COM and hip joint centre during the drop jump.

The position data of COM and reflective markers (hip centre, sacrum) and COP data calculated in Visual 3D were transferred to Microsoft Excel version 16.84.

Joint moment, joint and vertical stiffness calculation

In this study drop jump exercise were defined as the time from initial contact with the force platform to the take-off sequence. When calculating the joint moments and joint stiffness during braking phase, the interval between initial contact and mid-stance was taken into account [28]. Joint angles were calculated for the foot and knee joint using motion capture data in the sagittal plane. Net joint moments were calculated separately for each joint using standard inverse dynamic techniques using specialized computer software (Visual 3D; C-Motion Inc, Rockville, MD, USA). All joint moments are reported as external moments and normalized to body mass. The average joint stiffness was calculated as the ratio of the joint moment change (ΔM) to the joint angle change (Δθ) during the braking phase, as in the formula shown below.

Vertical stiffness (N/m) was determined by dividing the peak vertical ground reaction force (GRFs) by the vertical displacement of the centre of mass (COM) [29]. The formula used to calculate the vertical stiffness is shown below.

The stiffness of the joint (Nm/rad) is the ratio of the joint moment to the change in the joint angle.

Jump height, ground reaction force and Reactive Strength Index (RSI) calculation

The jump height was calculated from the change in 3D displacement of the COM during the drop jump. Peak 3D Ground Reaction Force (kN) has been collected by Bertec Force Platform during drop jump. The Reactive Strength Index was calculated in Visual3D during COM’s drop jump. The formula used to calculate the RSI is shown below.

3-dimensional component of the GRF vector that is parallel to the leg length (F_leg) during the drop jumps. The F_leg calculation is explained in more detail in the previous study by Coleman [30].

Statistical Analysis

Statistical analyses of this study were performed using the open-source JASP (Jeffreys’s Amazing Statistics Program) version 0.18.1 developed by the University of Amsterdam [31]. Normality tests of all data were determined by the Shapiro-Wilk test. All analyzed variables met the normality assumption, as indicated by the following results: ankle maximum moment (W = 0.974, p = 0.616), knee maximum moment (W = 0.974, p = 0.602), ankle stiffness (W = 0.935, p = 0.054), knee stiffness (W = 0.960, p = 0.279), vertical stiffness (W = 0.935, p = 0.054), F3dmax (W = 0.965, p = 0.374), and RSI (W = 0.940, p = 0.074). Since all values were above the 0.05 threshold, parametric statistical analyses were considered appropriate. According to the test, parametric tests were selected for the analysis of normally distributed data. The study employed a repeated-measures 3 (height: 26, 35, 42 cm) × 2 (joint: knee, ankle) × 2 (extremity: dominant, non-dominant) factorial design. The dependent variables K_vert, K_joint, jump height, ground reaction force, and RSI were analysed by repeated measures ANOVA. Mauchly sphericity test was found significant and Greenhouse-Geisser correction was applied. Pairwise comparisons were made using Bonferroni correction to determine at which altitudes the differences between stiffness levels occurred [32]. Alpha level was set as p < 0.05.

Results

Table 1 shows the mean and standard error values of dominant and nondominant leg joint stiffness, ground reaction force, jump height RSI and Max Moment values at 3 different heights. At the same time, K_joint mean and standard deviation values and the rates of change according to the boxes are shown graphically in Fig. 1. According to the repeated measures ANOVA test results, different effects were observed in knee and joint K_joint values as the height increased (Table 2).

Table 1.

The mean±std values of joint stiffness, ground reaction force, jump height, RSI, and max moment parameters

Box Height	Extremity	Kankle (Nm/rad)	Kknee (Nm/rad)	K vert (N/m)	GRFp (kN)	Jump height (m)	RSI (m/s)	Ankle Max Moment (Nm)	Knee Max Moment (Nm)
		Mean±SD	Mean±SD	Mean±SD	Mean±SD	Mean±SD	Mean±SD	Mean±SD	Mean±SD
Box 1	D	3.51±1.33	5.56±2.02	16.27±5.17	2.06±0.45	0.10±0.03	0.31±0.14	2.60±0.58	3.55±0.61
	ND	3.26±1.35	5.48±2.19	16.41±5.68	2.04±0.44	0.10±0.03	0.31±0.14	2.56±0.45	3.84±1.52
Box 2	D	3.74±1.64	5.50±1.75	17.88±4.87	2.26±0.45	0.10±0.03	0.32±0.13	2.88±0.54	4.08±1.29
	ND	3.55±1.54	5.31±2.03	17.65±6.25	2.26±0.42	0.10±0.03	0.32±0.13	2.88±0.41	3.81±0.62
Box 3	D	3.93±2.20	5.37±2.14	18.23±6.25	2.34±0.45	0.10±0.03	0.33±0.13	3.04±0.51	4.12±0.59
	ND	3.51±1.29	4.98±1.72	18.76±6.23	2.37±0.41	0.10±0.04	0.33±0.15	3.03±0.49	4.08±0.75

Std Standart Deviation, D Dominant Leg, ND Nondominant Leg, K_ankle Ankle Stiffness, K_knee Knee Stiffness, K_vert Vertical Stiffness, GRFp Peak Ground Reaction Force, RSI Reactive Strength Index

Fig. 1.

Dominant and nondominant joint stiffness

Table 2.

The mean±std values of joint stiffness, ground reaction force, jump height, RSI, and max moment parameters

Box Heights	Extremity	Kankle	Kknee	Kvert	GRFp	RSI	Ankle Max	Knee Max
		(Nm/rad)	(Nm/rad)	N/m	(kN)	(m/s)	Moment (Nm)	Moment (Nm)
Box 2-Box 1	D	6,60	−1.01	9.90	9.70	3.20	10.80	14.90
	ND	8.90	−3.10	7.50	10.80	3.20	12.50	−0.80
Box 3-Box 1	D	11.90	−3.42	12.00	13.50	6.50	16.90	16.10
	ND	7.70	−9.12	14.30	16.10	6.50	18.40	6.30
Box 3-Box 2	D	5.10	−2.40	2.00	3.50	3.10	5.60	1.00
	ND	−1.00	−6.00	6.00	5.00	3.00	5.00	7.00

D Dominant Leg, ND Nondominant Leg, K_ankle Ankle Stiffness, K_knee Knee Stiffness, Kvert Vertical stiffness, GRFp Peak Ground Reaction Force, RSI: Reactive Strength Index

Table 3 shows the results of ANOVA test results and post-hoc comparison of box heights, joint stiffness and joint maximum moment values in which joint, extremity interaction was determined. As the height increased, knee K_joint values decreased and ankle K_joint values increased, but the difference due to height was not significant. Regardless of height, knee Kjoint values were significantly higher than ankle Kjoint values (p < 0.05). Although differences were observed between dominant and non-dominant max moment values, these did not reach statistical significance (p > 0.05). According to post-hoc analysis, knee joint stiffness values were higher than ankle joint stiffness values. Knee joint max moment values at Box 2 and Box 3 were significantly higher than those at Box 1 (p < 0.05).

Table 3.

Box heights, joint, extremity interaction of joint stiffness and joint max moment values

Variables	Box Heights	Joint	Extremity	Interaction Box Heights ✻ Joint	Interaction Box Heights ✻ Extremity	Interaction Joint ✻ Extremity	Interaction Box Heights ✻ Joint ✻ Extremity	Post-hoc
Kjoint (Nm/rad)	NS	F (1.31) = 52.004 p < 0.001 η²= 0.63	NS	F (2,62) = 10.507 p < 0.001 η²= 0.253	NS	NS	NS	Knee> Ankle
Joint Max Moment (Nm)	F (2.62) = 15.671 p < 0.001 η²=0.336	F (1.31) = 79.864 p < 0.001 η²=0. 720	NS	NS	NS	NS	NS	Knee> Ankle Ankle: Box2 > Box1 Box3 > Box1 Knee: Box3 > Box1

NS Not Significant

In Table 4, repeated measure ANOVA test and post-hoc results of the interaction of box height and extremity parameters of Kvert, jump height and RSI values are given. Vertical stiffness differed significantly across box heights [F (2, 62) = 5.35, p = 0.007, η² = 0.101], with post-hoc tests indicating higher values at Box 3 compared to Box 1. Jump height and RSI showed an increasing trend with box height, though these changes were not statistically significant (p > 0.05). According to post-hoc analysis, box 3 values were higher than box 1 values in K_vert values, and a significant difference was observed in GRF values depending on height (p < 0.05). No significant differences were found in jump height or RSI values across box heights, between extremities, or in their interaction (p > 0.05).

Table 4.

Box heights, Joint, Extremity interaction of joint stiffness, ground reaction force, jump height and RSI values

Variables	Box Heights	Extremity	Interaction Box Heights ✻ Extremity	Post-hoc
Kvert (Nm/rad)	F (2.62) = 5.345 p < 0.007 η²=0.101	NS	NS	Box3 > Box1
Jump height (m)	NS	NS	NS	NS
RSI (m/s)	NS	NS	NS	NS

NS Not Significant, K_joint Ankle and Knee Joint Stiffness, K_vert Vertical stiffness, GRFp Peak Ground Reaction Force, RSI Reactive Strength Index

Discussion

The objective of this study was to investigate the effects of drop jumps from different heights on ankle, knee, and vertical stiffness. The primary finding of this study was that, despite no significant difference in joint stiffness values between different heights, a significant difference was observed between box 1 and box 3 for K_vert values. Although no significant differences were found between box heights, Kankle and Kvert tended to increase at higher boxes, whereas Kknee showed a decreasing trend (Fig. 1). Knee joint stiffness was higher than ankle joint stiffness at all heights (box 1, box 2 and box 3). These findings provide partial support for our hypothesis: vertical stiffness was significantly higher at Box 3 compared to Box 1, although no consistent differences were observed across all conditions. However, the response of specific joints was heterogeneous, as ankle and knee stiffness demonstrated divergent trends.

In the current study, the effects of drop jump exercise performed from different heights on vertical and joint stiffness in athletes were investigated. Previous research has examined how vertical stiffness and jump height influence performance [33, 34]. It has been demonstrated that jumps performed from higher platforms require athletes to absorb more energy, resulting in a decrease in vertical stiffness [34].

Another study reported that vertical stiffness decreased with increasing jump height, attributing this decline to greater angular displacement of the hip and knee joints [33]. This observation indicates that jumps performed from higher platforms result in a softening effect, leading to increased resistance to kinetic energy. In line with previous research, our findings suggest that this shift in neuromuscular strategy is a key adaptation for managing higher impact forces and maximizing performance during drop jumps from greater heights. Specifically, Kubo et al. [35], demonstrated a mechanism supporting this strategy: The researchers reported that the activation of the quadriceps muscle is enhanced during the landing phase to increase leg stiffness rapidly, a critical factor for optimizing the storage and return of elastic energy. This mechanism directly explains why we observed an increase in vertical stiffness at higher drop heights, as the body utilizes a neural stiffening pattern to efficiently absorb and immediately redirect impact forces [36–39].

Whilst vertical stiffness is widely regarded as a general indicator of movement mechanics, a more specific subheading of this stiffness concept, that is joint stiffness, provides a detailed analysis of movement by examining the effects of individual joint regions on movement and stabilisation. It has been shown that ankle joint stiffness modulates leg stiffness at submaximal loads [9, 28]. Furthermore, the increase in Kankle values with increasing box height is associated with the stabilising function of the ankle joint [40]. This may explain the observed trend toward higher vertical and ankle joint stiffness at greater box heights increases in our study so that the stabilization role of the ankle joint becomes more pronounced. This may explain the observed trend toward higher vertical and ankle stiffness at greater box heights. The stabilizing role of the ankle joint may therefore become more pronounced.

Such adaptations are consistent with the ankle’s stabilizing role during SSC activities, as reported by Sugimoto et al. [41], where ankle stiffness was shown to regulate leg stiffness under increasing loads. When the k_vert was compared based on box heights, it was higher during box 3 than that of box 1, which may be related to the intensity of loading. However, like the study of Kosaka, Sasajima [2], no significant increase in joint stiffness was found with increasing box height. A possible explanation is that the jump heights selected (26–42 cm) may not have provided sufficiently high mechanical demands to elicit measurable differences, as previous studies observed declines in stiffness primarily at 50–60 cm and above [42, 43]. These findings suggest that the jump heights used in our study may have been insufficient to reveal joint stiffness differences in terms of exercise intensity [22, 44]. However, joint flexion was also examined in studies examining the effects of different heights on ankle stiffness values, and it was found that ankle joint stiffness was higher at 30 cm jump height compared to 40 and 50 cm [45], and higher at a jump height of 40 cm than 60 cm [46]. These results suggest that as the jump height increases, the stiffness of the ankle joint provides stabilisation by absorbing the impact during the drop jump.

Previous research suggests that ankle and knee joint stiffness may decrease with increasing jump height in drop-jump exercises. It has also been shown that joint stiffness decreases significantly, especially at heights of 40 cm and above [45, 47, 48]. At heights of 60 cm and above, no increase in biomechanical efficiency was observed, and these heights are reported to increase the risk of injury [47, 48]. Muscle-tendon characteristics and neuromuscular strategies are also important factors influencing joint stiffness [2]. Muscle coactivation plays a critical role in regulating leg stiffness at different jump heights, and leg muscle coactivation directly affects joint stiffness and force production [2, 49]. It may also indicate that muscle coactivation, which plays an important role in regulating joint stiffness, was not sufficiently triggered at the jump heights tested, limiting the observed stiffness response.

Studies measuring between-joint stiffness have suggested that the knee joint may have higher joint stiffness than the ankle joint because it contains large muscle groups of the body, can produce higher joint moments, and may be more active in lower extremity force generation [22, 50]. The results of our study support these findings, as knee joint stiffness was significantly higher than ankle joint stiffness across all heights. Different degrees of flexion are seen in the ankle and knee joints, particularly in exercises such as a drop jump [47, 48]. These differences are shaped by factors such as age, gender, training history and joint mobility of the athletes [21, 46]. At the same time, when joint stiffness and angular displacement are considered, the fact that joint stiffness in the knee joint (K_knee) was higher than that in the ankle (K_ankle) in our study can be explained by the consistent difference between knee and ankle flexion angles. This difference may also be influenced by the distinct flexion angles of the knee and ankle, with deeper knee flexion contributing to higher stiffness values [2]. This finding reveals that drop jump exercises place different loads on the knee and ankle joints and the effects of the difference in joint flexion angles on joint stiffness. Hobara, Muraoka [51], investigated the effects of hip, knee and ankle stiffness on leg stiffness during hopping exercise and reported that knee stiffness showed higher stiffness values than ankle joint. In our study, drop jump exercise was performed from 3 different heights. When the results of this study were compared with the study of Hobara, Muraoka [51], it was found that knee joint stiffness was higher than ankle joint stiffness in drop jump exercise. Similarly, Maloney and Fletcher [10] reported that knee joint stiffness was higher than ankle joint stiffness during jumping. This is likely due to the larger muscle groups surrounding the knee joint, which enable greater force production and joint moment generation [23]. The results of this study also show that knee joint stiffness is higher than ankle joint stiffness during jumping exercises performed from different heights. The finding of similar joint stiffness in different types of jumping (hopping and drop jumping) suggests that the effects of these exercises on joint stiffness are similar and that the knee joint is a critical factor in terms of leg stiffness in such exercises. This may indicate that knee joint stiffness should be improved in sports performance and injury prevention strategies. In addition to joint stiffness, joint moments have also been examined in drop jump exercises and knee joint moments have been reported to be high [28, 52]. Besides stiffness, knee moments are influenced by the forces generated by muscles around the joint. Knee joint moments, which are higher in exercises such as drop jump compared to the ankle joint, are due to the support of large muscle groups in the knee joint and its capacity to produce higher force [28]. In this study, knee joint moments were found to be higher than the ankle joint, which is in parallel with the literature. Researchers have reported that the knee joint plays the role of the main force generator in exercises such as drop jump, while the ankle joint is more involved in stabilisation [53, 54]. Peak ankle moments were higher at Box 2 and Box 3 compared to Box 1, while knee moments were higher at Box 3 than Box 1, indicating greater loading with increased drop height. Furthermore, the findings of the current study support the notion that joint moments are not directly associated with jump height, as no significant differences were observed in jump height, reactive strength index (RSI), or stiffness across different drop heights. Previous studies have proposed that the observed increases in joint moments may be attributed not to jump height per se, but rather to increased knee flexion during landing [22, 28]. In our study, although there was no significant difference between joint stiffness values at different jump heights, significant differences in knee and ankle joint moments were observed between box heights. The relationship between joint moment and joint stiffness is complex; joint stiffness is influenced not only by moment increase but also by other biomechanical and neuromuscular factors [54]. Joint stiffness is considered a parameter that indicates the resistance of the joint to external and internal forces, whereas joint moment is defined as an indicator of the force applied to the joint [50]. The differences between joint stiffness and moments show that joint stiffness is not only caused by changes in moment. In addition, previous studies explain that athletes may use different movement strategies at different heights and that these strategies may cause an increase in joint moments without causing a change in stiffness [50]. Biomechanical studies have shown that jumping from different heights can lead to muscle activation and kinematic differences, but these changes do not cause a change in joint stiffness despite the difference in muscle contraction rates [55].

The discrepancy between joint moments and joint stiffness in our study may be attributable to differences in muscle contraction strategies. This aligns with previous reports indicating that joint moments tend to increase primarily due to greater force requirements at higher loads, whereas stiffness is regulated not only by muscle force but also by the combined influence of passive tissues, active muscle control, and neuromuscular strategies [23, 42]. Prior studies have similarly suggested that increases in joint moments with greater drop heights may reflect elevated force demands, while joint stiffness can remain unchanged due to compensatory regulation through both passive and active musculature [50, 54, 55]. Moreover, as drop height increases, elastic energy recovery in the muscle–tendon unit becomes increasingly important for maintaining stable ankle and knee joint stiffness [35]. These findings suggest that stiffness regulation is a multifactorial process shaped not only by force production but also by neuromuscular coordination and the viscoelastic properties of the muscle–tendon complex. Finally, the absence of significant changes in joint stiffness may also be attributable to high inter-individual variability in neuromuscular strategies, which should be considered when interpreting stiffness outcomes.

Strengths and Limitations

This study possesses several strengths that contribute to the validity and relevance of its findings. First, the inclusion of a balanced number of male and female athletes (n = 16 each) enhances the representativeness and allows for potential sex-based interpretations. Second, the use of three distinct box heights (26 cm, 35 cm, and 42 cm) enables a comparative evaluation of joint stiffness under varying mechanical demands. The bilateral assessment (dominant and non-dominant legs) provides a more comprehensive understanding of lower limb function. Moreover, the application of advanced motion capture systems (Qualisys and Visual3D) ensures accurate and objective measurement of kinematic variables, thereby strengthening the methodological quality of the study. Finally, the differentiation between the roles of the knee and ankle joints highlighting the knee as the primary force generator and the ankle as a stabilizer offers practical implications for training and injury prevention in sports settings.

Despite these strengths, some limitations must be acknowledged. The cross-sectional design restricts the ability to draw causal inferences regarding long-term adaptations to drop jumping at different heights. Additionally, although three box heights were examined, the lack of significant differences in joint stiffness across conditions may indicate that the chosen heights did not generate sufficiently distinct mechanical stimuli. The absence of ground reaction force (GRF) data, which would typically be obtained via a force platform, limits the precision of the vertical stiffness calculations and the mechanical interpretation of joint loading. Furthermore, the study did not include electromyographic (EMG) analysis, which could have provided valuable insights into neuromuscular activation patterns contributing to joint stiffness. Injury and surgical history was self-reported and not medically verified, which represents an additional limitation. Lastly, the exclusive focus on young, physically active individuals limits the generalizability of the findings to other populations such as sedentary adults or older individuals.

Future research may benefit from employing a longitudinal design, incorporating wider box height ranges, integrating GRF and EMG data, and examining diverse participant groups to better understand joint stiffness adaptations across different contexts.

Conclusion

This study provides valuable information on the effects of jumping from different heights on vertical stiffness, joint stiffness, and joint moments. Our findings show that vertical stiffness was significantly higher at Box 3 compared to Box 1, although no consistent differences were observed across all heights.

Although no significant differences in joint stiffness were observed across box heights, ankle and vertical stiffness tended to increase, while knee stiffness showed a decreasing trend. In contrast, knee joint moments were significantly higher at greater heights, and consistently higher than ankle joint moments. These findings support the view that the knee joint primarily contributes to force generation, whereas the ankle joint plays a stabilizing role during drop jumps. When investigating the joint stiffness, the fact that the knee joint stiffness is higher than the ankle joint stiffness, in agreement with previous studies, reveals the influence of the knee joint on the overall leg stiffness.

The discrepancy between joint stiffness and joint moments suggests that strength demands may play an important role in the regulation of joint mechanics. In addition, neuromuscular control strategies are thought to shape joint responses independently of jump height. Furthermore, the role of the muscle-tendon unit in stabilising joint stiffness with increasing jump height highlights the importance of considering individual biomechanical strategies in performance and injury prevention. Future research should aim to explore the underlying mechanisms of these adaptations and their implications for enhancing athletic performance.

Abbreviations

JASP

Jeffreys’s Amazing Statistics Program

D

Dominant Leg

ND

Nondominant Leg

K_ankle

Ankle Stiffness

K_knee

Knee Stiffness

K_vert

Vertical stiffness

GRF

Ground Reaction Force

GRFp

Peak Ground Reaction Force

GRFs

Ground Reaction Forces

QTM

Qualisys Track Manager

COM

Centre of Motion

RSI

Reactive Strength Index

NS

Not Significant

SSC

Strech-shortening cycle

Authors’ contributions

AG: Led participant recruitment and marker placement during data collection, performed data processing and statistical analyses, and drafted the manuscript. MYK: Contributed to data collection, manuscript writing, and conducted the final manuscript review. AA: Conceived the study idea, assisted in data collection and raw data processing, and participated in the final evaluation of the manuscript.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Ethical approval was obtained from the Haliç University Non-Interventional Clinical Research Ethics Committee (approval date and number: 25.01.2023/05). All participants signed informed consent forms prepared according to the Declaration of Helsinki (WMA Declaration of Helsinki, 2013).

Consent for publication

Not Applicable.

Competing interests

The authors declare no competing interests.