Male collegiate volleyball players’ depth jump biomechanical adaptations to eight-week complex–contrast training

Abstract

Although complex–contrast training (CCT) is widely used to improve depth jump performance in volleyball players, the effects of CCT on depth jump biomechanics are unclear. Thus, this study aimed to analyze CCT’s effects on depth jump biomechanics in volleyball players and to identify biomechanical factors that influence performance. Nineteen male collegiate volleyball players were randomly assigned to an experimental group (EXP: n = 9) or a control group (CON: n = 10). EXP underwent 8 weeks of twice-weekly CCT, while both groups completed three times weekly volleyball technical training. All participants’ depth jump performance and biomechanical characteristics were assessed during the braking and propulsion phases using motion capture systems and force platforms at three time points: pre-intervention and after 4 and 8 weeks of CCT. Repeated-measures ANOVA showed that the interaction effects of time × group were not significant for all variables. For variables with significant main effects, further post hoc comparisons revealed that, in the EXP, jump height (JH: 0.510 ± 0.024 m vs. 0.560 ± 0.042 m, p = 0.013, Δ = 9.911% [95% CI: 3.266% to 16.557%]), peak propulsive velocity (PPV: 3.136 ± 0.089 m/s vs. 3.303 ± 0.154 m/s, p = 0.009, Δ = 5.341% [95% CI: 2.019% to 8.662%]), energy release (ERE: 8.755 ± 0.375 J/kg vs. 9.604 ± 0.725 J/kg, p = 0.019, Δ = 9.839% [95% CI: 2.830% to 16.849%]), and net energy release (NERE: 3.398 ± 0.709 J/kg vs. 3.907 ± 0.610 J/kg, p = 0.021, Δ = 17.548% [95% CI: 2.318% to 32.778%]) significantly increased from pre- to posttest. Furthermore, posttest values of JH (EXP: 0.560 ± 0.042 m vs. CON: 0.520 ± 0.033 m, p = 0.029) and NERE (EXP: 3.907 ± 0.610 J/kg vs. CON: 3.150 ± 0.614 J/kg, p = 0.015) were significantly higher in EXP than in CON. The results indicated that 8 weeks of CCT improved biomechanical variables of the depth jump, including JH, PPV, ERE, and NERE, in male collegiate volleyball players, while variables related to the braking phase were largely unchanged. Although post-test differences between groups were observed for key performance-related indicators, these findings should be interpreted with caution due to the absence of a significant time × group interaction and may reflect limited statistical power. Overall, this study provides preliminary evidence for the potential benefits of CCT on certain biomechanical outcomes, although further investigation with larger samples is needed to confirm and extend these findings.

Introduction

During international volleyball competitions, elite male volleyball players execute 250–300 spiking or blocking actions per match¹. Notably, the player’s height relative to the net primarily determines the performance of these technical actions². For example, a higher blocking height can reduce the opponent’s offensive efficiency, and a greater spiking height can provide attackers with more favorable angles to score³. Therefore, jump height (JH) is a critical factor influencing the athletic performance of volleyball players⁴.

High-load training stimuli can enhance athletes’ explosive performance, such as JH. This phenomenon, characterized by a sharp increase in strength and explosive power following voluntary muscle contractions, is post-activation performance enhancement (PAPE)⁵. The PAPE induced by high-load squats is short-lived and may no longer positively influence JH as early as 9 min after the squat stimulus⁶. Although the specific duration and magnitude of the effect can vary depending on factors such as training level and stimulus intensity, the performance enhancement window generally closes within 20 min of the start of recovery⁷. As a result, athletes cannot derive sustained benefits from PAPE. However, if PAPE is strategically incorporated into training, it may be possible to repeatedly leverage the acute improvements in muscle contractile performance following high-load stimuli, thereby gradually promoting long-term adaptations in athletes⁸. This training method during periods of acute enhancement in muscle contractile performance is complex–contrast training (CCT)⁹. CCT consists of several complex pairs, each comprising two exercises with similar movement patterns but differing load and velocity. Typically, a complex pair consists of a high-load, low-velocity strength exercise, such as squats, paired with a low-load, high-velocity explosive exercise, such as depth jumps or countermovement jumps.

Numerous studies have extensively studied and shown the effectiveness of CCT in improving JH^10–12. A growing body of research has shown that training methods that inherently resemble adjusted CCT exercise sequences or simplified CCT content can achieve effects similar to those of CCT^11,13–15. Researchers have primarily used JH to evaluate the effectiveness of these training methods¹⁴. However, beyond JH, it remains necessary to examine biomechanical characteristics during the jump, as these variables can more specifically reflect changes in how force is applied during the movement¹⁶. For example, even if JH remains unchanged, notable changes in parameters such as peak ground reaction force (PF) or countermovement depth (CD) may indicate alterations in jumping strategy or force application patterns¹⁷. Furthermore, research has shown that jumps performed during the PAPE window not only improve JH but also lead to notable enhancements in biomechanical parameters such as PF, peak power (PP), and vertical impulse (VI)¹⁸. These findings suggest that CCT, which is based on the PAPE, may have certain potential in improving jump-related biomechanical characteristics. Therefore, analyzing variables such as PF, PP, VI, peak propulsive velocity (PPV), energy storage (EST), and energy release (ERE)^16,19–22 may provide a more comprehensive evaluation of the actual effects of CCT, rather than relying solely on JH as a performance metric.

As an important manifestation of jumping movements, the depth jump exhibits distinct stretch-shortening cycle (SSC) characteristics and is strongly correlated with spike jump performance in volleyball matches²³. Based on the direction of the vertical displacement of the body’s center of mass, the entire ground contact phase of the depth jump can be divided into the braking and propulsion phases²⁴. During the depth jump, the neuromuscular system prepares for ground contact through pre-activation and proprioceptive regulation prior to landing. Upon impact, the braking phase begins, during which the lower limb muscles undergo controlled eccentric contractions. This stretches the tendons, storing elastic potential energy, while simultaneously activating the stretch reflex to enhance muscular responsiveness²⁵. As the body descends to the lowest squat position, the movement transitions into the propulsion phase. Here, the muscle-tendon units perform concentric contractions, releasing the stored energy. This release synergizes with neural reflex mechanisms (e.g., stretch reflex), generating augmented extension force to complete the vertical jump. Throughout this process, mechanical factors (e.g., tendon elasticity) and neural factors (e.g., pre-activation and stretch reflex) interact coordinately to optimize the efficiency and performance of the depth jump²⁶. Compared to the countermovement jump, the depth jump can generate greater eccentric loading and faster eccentric velocity during the braking phase, thereby enhancing the utilization efficiency of the SSC and ultimately producing greater explosive force during the propulsion phase²⁷. Furthermore, the depth jump is not only a common training method in CCT but also an essential metric for evaluating the effectiveness of CCT. However, limited literature has reported the effects of CCT on biomechanical characteristics such as energy storage and release during depth jumps. Consequently, how CCT influences depth jump performance and its biomechanical characteristics remains unclear.

Therefore, this study conducted an 8-week CCT intervention on male collegiate volleyball players and compared changes in depth jump JH, as well as variables during the braking phase—including CD, peak braking force (PBF), peak braking power (PBP), vertical braking impulse (VBI), and EST—and variables during the propulsion phase—including peak propulsive velocity (PPV), peak propulsive force (PPF), peak propulsive power (PPP), vertical propulsive impulse (VPI), and ERE, along with net energy release (NERE) across both the propulsion and braking phases, before and after the intervention. The aim was to investigate the biomechanical mechanisms by which CCT affects depth jump performance. Based on previous studies^22,28–30, we hypothesize that eight weeks of CCT intervention could lead to increases in JH, PPV, PPP, ERE, and NERE (p ≤ 0.05). Specifically, JH is expected to show a large effect size (Hedges’ g ≥ 0.8), while PPV, PPP, ERE, and NERE are expected to show moderate effect sizes (0.5 ≤ Hedges’ g < 0.8).

Methods

Experimental approach to the problem

This study employed a randomized controlled trial design to investigate the effects of CCT using high-load squats as a conditioning stimulus on the biomechanical characteristics of the depth jump in male collegiate volleyball players to explore the biomechanical factors influencing depth jump performance under CCT. All participants underwent two familiarization sessions to become familiar with the experimental procedures, testing protocols, and training exercises before baseline testing. Participants randomly drew numbered slips (1–20) from a sealed box to determine their unique identifiers. Subsequently, the 20 numbers were randomly assigned to the experimental group (EXP) and the control group (CON) using SPSS 27. The allocation procedure was performed independently by a statistician not involved in the study implementation. Group allocation was concealed from the investigators during the assignment process. EXP underwent CCT twice a week for 8 weeks (16 sessions total), each spaced 48–72 h apart. At the beginning of the experiment, all participants maintained a routine of three 80-minute volleyball technical training sessions per week. While participating in the same volleyball technical training as EXP, CON refrained from additional physical training during the experimental period. Depth jump tests were conducted before, at the midpoint, and after the 8-week training intervention. These tests were completed over three days during the testing week, with a minimum 48-hour interval between the depth jump tests and training sessions. Additionally, one week before the training commenced, EXP underwent a 1RM squat test to determine the training load for subsequent sessions. A minimum 48-hour interval between the 1RM squat and first depth jump tests was maintained. To ensure consistency, the same staff member was assigned to the same tasks across all three depth jump tests. Blinding was not implemented during data collection. The experimental procedure is illustrated in Fig. 1.

Fig. 1.

Experimental procedure. Each depth jump test was conducted at least 48 h after the most recent complex–contrast or volleyball technical training session to minimize fatigue-related effects.

Subjects

The study utilized G*Power 3.1.9.7 for sample size calculation. The parameters were set as follows: β = 0.8³¹, significance level (α) = 0.05, effect size (f) = 0.35, with 2 groups and 3 repeated measurements. The results indicated that a minimum of 16 participants were required. The effect size was determined based on prior studies reporting moderate to large effects of CCT on jump performance in similar populations^28,29. To account for potential participant dropouts, 20 male collegiate volleyball players from Guangzhou Sport University were recruited for the study. The inclusion criteria for participants were as follows: (1) participation in regional volleyball competitions and at least one provincial-level or higher collegiate volleyball league; (2) male with at least two years of experience in generalized strength training; and (3) no history of injuries or health issues within the past six months. Participants were considered withdrawn from the study if they experienced discomfort or injury during the experiment, missed any test session, or missed two or more training sessions. During the experiment, one participant from EXP withdrew due to personal reasons after failing to attend the post-test on time. This resulted in 19 participants (EXP: n = 9; CON: n = 10) who completed the study and were included in the data analysis. There were no statistically significant differences between the two groups regarding age, height, body weight, and training experience (Table 1). All participants were fully informed of the experimental procedures and potential risks (including but not limited to muscle strains and joint injuries) prior to baseline testing. They were advised of their right to withdraw unconditionally at any time, and all voluntarily provided written informed consent. The study strictly adhered to the Declaration of Helsinki and was approved by the Ethics Committee of Guangzhou Sport University (Ethics Committee No. 2025LCLL-002), and registered with the Chinese Clinical Trial Registry under registration number ChiCTR2500095747, 13 January 2025.

Table 1.

Participant characteristic.

Characteristics	EXP (n = 9)	CON (n = 10)
Age (years)	19.9 ± 0.9	19.4 ± 1.1
Height (cm)	185.4 ± 4.6	187.8 ± 6.8
Weight (kg)	78.8 ± 11.1	76.7 ± 9.9
Training experience (years)	7.1 ± 3.2	6.2 ± 1.0

Procedures

Warm-up protocol^32,33. Before CCT or testing sessions, all participants performed the same standardized warm-up routine (Table 2), which included myofascial release, dynamic stretching, neural activation, and movement integration. The myofascial release component included one set of foam rolling or fascia ball pressing targeting major muscle groups. The dynamic stretching component included one set of exercises, such as knee-to-chest walks with heel raises, walking quadriceps stretches, and lateral lunges. The neural activation component included one set of exercises, such as fast high knees and short-distance sprints³⁴. The movement integration component involved two sets of barbell squats using a light load of 30% 1RM, with five repetitions per set³⁵. Throughout the experiment, all warm-up activities were conducted under the guidance and supervision of the same staff member.

Table 2.

Warm-up protocol.

Content	Exercise	Training volume
		Sets	Reps	Distance (m)
Myofascial release	Foam rolling or fascia ball pressing targeting major muscle groups	1	6	-
Dynamic stretching	Knee-to-chest walks with heel raise, walking quadriceps stretch, lateral lunge	1	6	-
Neural activation	Fast high knees, short-distance sprint	1	-	15
Movement integration	30% 1RM squat	2	5	-

A dash (–) indicates that the variable is not applicable for that exercise.

Training protocol^9,36. All participants engaged in volleyball technical training three times per week. The volleyball technical training included, but was not limited to, serving, passing, receiving, spiking, blocking, tactical coordination, and small-sided games (Table 3). The strength training component of CCT consisted of back squats at 85% 1RM⁹, while the plyometric training component included exercises such as countermovement jumps, depth jumps (30 cm), and alternating lunge jumps (30 cm box)³². Immediately after completing the back squats, the staff member initiated a timer for the participant. During this period, the participant rested for 5 min^9,37. Once the rest interval ended, the staff member supervised the participant in performing the corresponding plyometric exercises (Table 4). Upon completing the plyometric exercises, the participant was considered to have finished one set of CCT. The same staff member initiated the timer again, with a 3-minute rest interval between sets⁹. After the rest period, the participant immediately proceeded to the next set of CCT, and this process was repeated accordingly. CCT and volleyball technical training were scheduled on different days to ensure adequate recovery between sessions.

Table 3.

Example of volleyball technical training schedule.

Example 1		Example 2
Content	Duration (min)	Content	Duration (min)
Ball-handling warm-up	10	Ball-handling warm-up	10
Serving	10	Serve reception	10
Spiking	10	Blocking	10
Small-sided games	35	Spiking	10
Match analysis	10	Tactical coordination	35
Cool-down	5	Cool-down	5

Table 4.

Complex–contrast training schedule.

Weekly training sessions	Complex–contrast training	Sets	Intra-set rest (min)	Rest between sets (min)
Session 1	5 × 85%1RM squat + 8 × countermovement jump	3	5	3
	5 × 85%1RM squat + 8 × depth jump(30 cm box)	3	5	3
Session 2	5 × 85%1RM squat + 8 × alternating lunge jump	3	5	3
	5 × 85%1RM squat + 8 × split jump (30 cm box)	3	5	3

1RM squat test^35,38. This experiment conducted the 1RM squat test exclusively on the EXP before the training intervention to determine the load for the subsequent back squat exercises. After performing a warm-up of 10 loaded squats (estimated at 30% of their maximum weight) and resting for 1 min, the subjects performed six repetitions of squats (estimated at 50% of their maximum weight), followed by a 3-minute rest. They then performed three repetitions (estimated at 80% of their maximum weight), rested for another 3 min, and attempted a 1RM squat. If successful, the load was increased by 5–10% of the 1RM; if unsuccessful, the load was reduced by 5% of the 1RM. After a 5-minute rest, the process was repeated until the 1RM squat test was completed. All participants completed the test within five attempts. Throughout the process, all participants received the same verbal encouragement, spotting, and technical supervision from experienced personnel. To ensure safety, the test was immediately terminated in cases of abnormal pain, significant technical failure, or participant request due to excessive fatigue.

Depth jump test. Although this study primarily focused on pre- and post-intervention comparisons, we nevertheless incorporated a mid-term assessment after four weeks of intervention to dynamically monitor the progressive changes in training effects. The depth jump test for the subjects was scheduled on a different day from their 1RM squat test. The depth jump test was conducted in a three-dimensional motion capture environment with 10 cameras (Arqus, Qualisys corporation, Sweden), with kinematic data collected at 200 Hz. Following the completion of standardized warm-up activities and appropriate attire change, research personnel immediately placed 43 reflective markers (14 mm diameter) on the subjects according to the CAST model^39,40. To ensure consistency, the same technician was responsible for marker placement in identical anatomical locations across all testing sessions. Two independent technicians subsequently verified the accuracy of marker placement. Upon confirmation of proper marker placement, subjects stood on a Bertec force plate (FP4060-10, Bertec corporation, USA) with a sampling frequency of 1000 Hz. Static standing trials were conducted to record kinetic and kinematic data, with all subjects maintaining strictly standardized standing postures. At the start of the depth jump, the subjects stood on a 30 cm box with their hands on their hips throughout the test. They were allowed to step off the box with either foot but were not permitted to jump off it. Upon landing, both feet had to contact the force plate fully. During the test, the staff consistently provided cues such as “jump as high as possible” without imposing any requirements on CD. Each subject was required to complete three valid depth jumps per test, with a 90-second interval between each attempt³⁵. Throughout the study, all procedures for identical testing components were consistently administered by the same trained researcher to ensure procedural reliability.

Data processing. The collected data were processed using Visual3D software (C-Motion corporation, USA). The marker and force plate data were low-pass filtered (zero-lag, fourth-order, Butterworth) at an effective cutoff frequency of 15 Hz⁴¹. Ground contact was defined as when the force plate first detected a force and the vertical ground reaction force was ≥ 10 N, while takeoff was defined as when the vertical ground reaction force fell to ≤ 10 N⁴². The braking phase was defined as the period from ground contact to the lowest point of the center of mass, and the propulsion phase was defined as the period from the end of the braking phase to takeoff²⁴. The kinetic and kinematic variables selected for this study are presented in Table 5, with details of EST, ERE, and NERE illustrated in Fig. 2.

Table 5.

Calculation methods for kinematic and kinetic variables in the depth jump test.

Variable	Abbr.	Calculation
Jump height (m)	JH	The difference between the maximum height of the center of mass during the flight phase after the propulsion phase and the average height of the center of mass during static standing43
Countermovement depth (m)	CD	The difference between the maximum and minimum height of the center of mass during the braking phase
Peak propulsive velocity (m·s− 1)	PPV	The maximum velocity of the center of mass during the propulsion phase
Peak braking force (N·kg− 1)	PBF	The maximum vertical ground reaction force during the braking phase
Peak propulsive force (N·kg− 1)	PPF	The maximum vertical ground reaction force during the propulsion phase
Peak braking power (W·kg− 1)	PBP	The maximum value of the product of vertical ground reaction force and the velocity of the center of mass at each moment during the braking phase
Peak propulsive power (W·kg− 1)	PPP	The maximum value of the product of vertical ground reaction force and the velocity of the center of mass at each moment during the propulsion phase
Vertical braking impulse (Ns·kg− 1)	VBI	The sum of the product of vertical ground reaction force and time at each moment during the braking phase19,20
Vertical propulsive impulse (Ns·kg− 1)	VPI	The sum of the product of vertical ground reaction force and time at each moment during the propulsion phase19,20
Energy storage (J·kg− 1)	EST	The integral over the braking phase of the product of vertical ground reaction force and countermovement depth at each time point (Fig. 2)21,22
Energy release (J·kg− 1)	ERE	The integral over the propulsion phase of the product of vertical ground reaction force and countermovement depth at each time point (Fig. 2)21,22
Net energy release (J·kg− 1)	NERE	The difference between energy release and energy storage21,22

Fig. 2.

Relative vertical ground reaction force–countermovement depth curve. t_a denotes the initial ground contact; t_b indicates the lowest position of the body’s center of mass (COM); t_c represents the take-off moment. t_a corresponds to zero on the X-axis, and the curve begins at t_a and ends at t_c. The direction of the arrow indicates the path of the curve. An increase in X-axis values represents a decrease in COM height, while a decrease in X-axis values after t_b indicates an increase in COM height. The segment from ta to t_b corresponds to the braking phase of the depth jump, and the area under the curve represents energy storage during this phase. The segment from t_b to t_c corresponds to the propulsive phase, and the area under the curve represents energy release. The area enclosed between the t_b–t_c and t_a–t_b curves represents the net energy release.

The braking phase is the period from ground contact to the lowest point of the center of mass. The propulsion phase is the period from the end of the braking phase to takeoff. All force and power values are expressed as absolute peak magnitudes, regardless of direction. All force, power, impulse, and energy variables are expressed relative to body weight.

Statistical analyses

The experimental data were statistically analyzed using SPSS 27. The normality of the data was assessed using the Shapiro-Wilk test. For both kinematic and kinetic data, test-retest reliability was evaluated using the intraclass correlation coefficient (ICC, two-way random-effects model for absolute agreement) with 95% confidence intervals (CI), while their variability was quantified by the coefficient of variation (CV)⁴⁴. A two-factor repeated measures analysis of variance was performed on the experimental data with 2 (group: EXP vs. CON) × 3 (time: pre vs. mid vs. post) factors. When the interaction effect was not significant but a significant main effect was observed, post-hoc analyses were conducted to further explore within-group temporal changes and between-group differences at specific time points. Bonferroni correction was applied to adjust for multiple comparisons. In addition, principal component analysis (PCA) was conducted as a supplementary exploratory method to examine the interrelationships and independence among biomechanical outcome variables, and to identify potential coordination patterns between kinetic and kinematic parameters. The significance level was set at p ≤ 0.05. The effect size was calculated using Hedge’s g (g) to measure the magnitude of between-group and within-group differences for variables. The effect sizes were categorized as trivial (g < 0.2), small (0.2 ≤ g < 0.5), medium (0.5 ≤ g < 0.8), and large (g ≥ 0.8)⁴⁵. The effect size for the main effect of time, main effect of group, and time × group interaction were measured using partial eta squared (ηₚ²) and interpreted as follows: very small ( < 0.01), small (0.01 ≤ < 0.06), medium (0.06 ≤ < 0.14), and large ( ≥ 0.14)⁴⁵.

Results

ICC values were ≥ 0.9 for CD, PBF, PPF, PPP, VBI, EST, and NERE; 0.8–0.9 for VPI and ERE; and 0.75–0.8 for JH, PPV, and PBP. CV was ≤ 10 for JH, PPV, VPI, ERE, CD, PBF, PPF, PPP, VBI, EST, and NERE, and 11.114 for PBP. Detailed results are presented in Table 6.

Table 6.

Intraclass correlation coefficients and coefficients of variation for kinematic and kinetic variables.

Variables	ICC(95%CI)	CV
Jump height	0.787 (0.507,0.914)	5.278
Countermovement depth	0.901 (0.786,0.959)	7.790
Peak propulsive velocity	0.786 (0.518,0.912)	2.950
Peak braking force	0.939 (0.868,0.975)	3.953
Peak propulsive force	0.960 (0.915,0.984)	3.750
Peak braking power	0.791 (0.555,0.913)	11.114
Peak propulsive power	0.948 (0.889,0.978)	3.579
Vertical braking impulse	0.913 (0.815,0.964)	4.113
Vertical propulsive impulse	0.805 (0.578,0.919)	4.093
Energy storage	0.908 (0.800,0.962)	6.083
Energy release	0.817 (0.570,0.927)	5.522
Net energy release	0.916 (0.808,0.966)	9.724

ICC = Intraclass Correlation Coefficient; CV = Coefficient of Variation.

The PCA results showed that the Kaiser-Meyer-Olkin (KMO) measure was 0.66, suggesting that the data were marginally suitable for PCA. Bartlett’s test of sphericity was significant (χ² = 475.42, p < 0.001). Together, Principal Component 1 (PC1) and Principal Component 2 (PC2) accounted for over 80% of the total variance. PC1 included CD, PBF, PPF, PPP, VBI, VPI, and EST (loadings ≥ |0.3|⁴⁶) and was interpreted as the “force-impulse control factor,” representing how athletes modulate force, impulse, and energy absorption during the jumping. PC2 comprised JH, NERE, PPV, and ERE (loadings ≥ |0.3|⁴⁶) and was labeled the “explosive output factor,” emphasizing jump performance and propulsive output. Detailed results are presented in Table 7; Fig. 3.

Table 7.

Principal component loadings.

Variables	PC1 (48.67%)	PC2 (32.02%)
Jump height	−0.023	0.490
Countermovement depth	0.386	0.073
Peak propulsive velocity	−0.071	0.491
Peak braking force	−0.361	0.115
Peak propulsive force	−0.398	0.019
Peak braking power	−0.104	0.025
Peak propulsive power	−0.363	0.163
Vertical braking impulse	0.394	0.003
Vertical propulsive impulse	0.312	0.296
Energy storage	0.330	0.124
Energy release	0.135	0.477
Net energy release	−0.186	0.377

PC1 and PC2 refer to the first and second principal components obtained from principal component analysis. Factor loadings with absolute values ≥ 0.3 in PC1 and PC2 are shown in bold.

Fig. 3.

Principal component loading plot for Principal Component 1 and Principal Component 2. Each arrow represents a variable’s loading on the two components. The direction shows whether the variable is positively or negatively associated with the component, while the length reflects the strength of the contribution. The axes represent the loading coefficients of each variable on Principal Component 1 and Principal Component 2. JH = jump height; CD = countermovement depth; PPV = peak propulsive velocity; PBF = peak braking force; PPF = peak propulsive force; PBP = peak braking power; PPP = peak propulsive power; VBI = vertical braking impulse; VPI = vertical propulsive impulse; EST = energy storage; ERE = energy release; NERE = net energy release.

The time × group interaction for JH was not significant, while significant main effects of time (p = 0.002) and group (p = 0.050) were observed. For variables with significant main effects, further post hoc comparisons revealed that JH was significantly different between the pre- and post-tests in the EXP (p = 0.013), as well as between the EXP and CON in the post-test (p = 0.029). The time × group interaction for NERE was not significant, whereas significant main effects of time (p = 0.024) and group (p = 0.015) were found. Post hoc results indicated a significant difference in NERE between the pre- and post-tests in the EXP (p = 0.021), and between the EXP and CON in the post-test (p = 0.015).

In the braking phase, neither the main effects of group nor the time × group interactions were significant for any variables. However, significant main effects of time were observed for CD (p < 0.001), PBF (p < 0.001), PBP (p = 0.041), VBI (p < 0.001), and EST (p = 0.001). For variables with significant main effects, further post hoc comparisons revealed that in the CON, CD at pre-test differed significantly from both mid-test and post-test values (p < 0.05). In the EXP, PBF at pre-test differed significantly from mid-test (p = 0.014), while in the CON, PBF at pre-test was significantly different from both mid-test and post-test (p < 0.01). Additionally, VBI in the EXP showed a significant difference between pre-test and mid-test (p = 0.048), and in the CON, VBI at pre-test differed significantly from both mid-test and post-test (p < 0.05). A significant difference in EST was also found between pre-test and mid-test in the CON (p = 0.004).

In the propulsive phase, all time × group interactions were not significant. Significant main effects of time were found for PPV (p < 0.001), PPF (p < 0.001), VPI (p < 0.001), and ERE (p < 0.001), while a significant main effect of group was observed for PPV (p = 0.050). For variables with significant main effects, further post hoc comparisons revealed that PPV significantly differed between pre- and post-tests in EXP (p = 0.009). PPF showed a significant difference between pre- and mid-tests in EXP (p = 0.050), and also between pre- and post-tests in CON (p = 0.005). VPI in CON significantly differed between pre- and post-tests (p < 0.001). ERE significantly differed between pre- and post-tests in both EXP (p = 0.019) and CON (p = 0.007). Detailed results are presented in Table 8; Fig. 4, and Fig. 5.

Table 8.

Kinematic and kinetic variables of EXP and CON at pre-test, mid-test, and post-test.

	Pre	Mid	Post	Pre-Mid g (95%CI)	Pre-Post g (95%CI)	Pre-Mid Δ% (95%CI)	Pre-Post Δ% (95%CI)	Time effect ()	Group effect ()	Time × group ()
JH (m)
EXP	0.510 ± 0.024	0.521 ± 0.035	0.560 ± 0.042*	0.236 (−0.850,1.322)	1.139 (−0.099,2.376)	2.040 (−2.132,6.212)	9.911 (3.266,16.557)	0.002 (0.315)	0.050 (0.207)	0.525 (0.037)
CON	0.494 ± 0.049	0.492 ± 0.047	0.520 ± 0.033	−0.030 (−1.069,1.008)	0.598 (−0.488,1.685)	0.291 (−7.334,7.916)	6.015 (−1.528,13.558)
NERE (J·kg− 1)
EXP	3.398 ± 0.709	3.463 ± 0.777	3.907 ± 0.610*	0.088 (−0.743,0.919)	0.694 (−0.219,1.607)	3.689 (−11.107,18.485)	17.548 (2.318,32.778)	0.024 (0.196)	0.050 (0.208)	0.370 (0.057)
CON	3.014 ± 0.660	2.931 ± 0.597	3.150 ± 0.614	−0.114 (−0.916,0.688)	0.187 (−0.619,0.994)	−0.372 (−16.633,15.890)	6.493 (−7.477,20.463)
Braking phase
CD(m)
EXP	0.320 ± 0.061	0.357 ± 0.031	0.368 ± 0.038	0.648 (−0.374,1.669)	0.831 (−0.229,1.891)	14.488 (1.072,27.904)	18.328 (−0.139,36.795)	<0.001 (0.440)	0.476 (0.030)	0.433 (0.048)
CON	0.324 ± 0.043	0.363 ± 0.06*	0.400 ± 0.068*	0.686 (−0.311,1.682)	1.339 (0.163,2.514)	12.656 (2.385,22.927)	25.426 (6.677,44.175)
PBF (N·kg− 1)
EXP	28.357 ± 3.707	25.544 ± 2.622*	25.990 ± 2.531	−0.899 (−1.920,0.122)	−0.756 (−1.743,0.230)	−9.403 (−15.268,−3.538)	−7.738 (−14.025,−1.45)	<0.001 (0.473)	0.623 (0.015)	0.529 (0.037)
CON	28.243 ± 2.463	25.366 ± 2.316*	24.642 ± 3.135*	−0.931 (−1.933,0.071)	−1.165 (−2.237,−0.092)	−9.827 (−16.076,−3.578)	−12.54 (−19.707,−5.372)
PBP (W·kg− 1)
EXP	47.941 ± 9.911	43.555 ± 8.344	44.579 ± 7.601	−0.518 (−1.539,0.501)	−0.397 (−1.402,0.606)	−7.807 (−19.447,3.833)	−5.946 (−14.162,2.271)	0.041 (0.171)	0.121 (0.136)	0.977 (0.001)
CON	52.695 ± 6.585	47.692 ± 3.910	49.451 ± 8.437	−0.599 (−1.598,0.400)	−0.389 (−1.357,0.580)	−8.197 (−18.045,1.65)	−4.508 (−21.105,12.089)
VBI (Ns·kg− 1)
EXP	4.020 ± 0.365	4.264 ± 0.179*	4.301 ± 0.230	0.604 (−0.298,1.506)	0.695 (−0.226,1.617)	6.624 (0.528,12.720)	7.475 (1.521,13.430)	<0.001 (0.481)	0.303 (0.062)	0.147 (0.107)
CON	4.09 ± 0.358	4.332 ± 0.477*	4.624 ± 0.449*	0.606 (−0.268,1.481)	1.341 (0.247,2.437)	5.880 (1.369,10.391)	13.460 (5.172,21.747)
EST (J·kg− 1)
EXP	5.356 ± 0.682	5.531 ± 0.456	5.697 ± 0.560	0.234 (−0.654,1.123)	0.457 (−0.456,1.371)	4.484 (−5.996,14.964)	7.374 (−3.131,17.880)	0.001 (0.321)	0.401 (0.042)	0.268 (0.075)
CON	5.398 ± 0.724	5.677 ± 0.899	6.188 ± 0.602*	0.379 (−0.494,1.251)	1.073 (0.042,2.104)	5.092 (−0.429,10.613)	15.788 (5.723,25.853)
Propulsive phase
PPV (m·s− 1)
EXP	3.136 ± 0.089	3.173 ± 0.134	3.303 ± 0.154*	0.232 (−0.789,1.253)	1.066 (−0.098,2.229)	1.157 (−1.226,3.540)	5.341 (2.019,8.662)	<0.001 (0.347)	0.050 (0.208)	0.536 (0.036)
CON	3.074 ± 0.173	3.047 ± 0.152	3.169 ± 0.127	−0.176 (−1.157,0.805)	0.613 (−0.418,1.642)	−0.746 (−4.255,2.764)	3.277 (−0.417,6.970)
PPF (N·kg-1)
EXP	28.693 ± 4.362	26.337 ± 1.756*	26.620 ± 2.643	−0.692 (−1.554,0.171)	−0.609 (−1.451,0.235)	−7.139 (−13.643,−0.635)	−6.376 (−12.822,0.071)	<0.001 (0.413)	0.587 (0.018)	0.302 (0.068)
CON	28.152 ± 3.093	26.444 ± 3.012	24.971 ± 3.026*	−0.507 (−1.304,0.289)	−0.945 (−1.856,−0.034)	−5.826 (−11.446,−0.206)	−11.041 (−16.713,−5.369)
PPP (W·kg− 1)
EXP	67.959 ± 7.740	66.588 ± 2.943	69.892 ± 4.753	−0.211 (−0.944,0.522)	0.297 (−0.444,1.040)	−1.199 (−7.901,5.503)	3.473 (−2.630,9.576)	0.061 (0.152)	0.310 (0.061)	0.234 (0.082)
CON	66.994 ± 5.723	64.313 ± 5.831	65.478 ± 6.879	−0.418 (−1.153,0.317)	−0.236 (−0.946,0.474)	−3.947 (−7.338,−0.556)	−2.318 (−5.810,1.175)
VPI (Ns·kg− 1)
EXP	5.200 ± 0.291	5.373 ± 0.341	5.518 ± 0.283	0.425 (−0.569,1.417)	0.779 (−0.275,1.833)	3.388 (−0.087,6.864)	6.377 (0.453,12.301)	<0.001 (0.439)	0.684 (0.010)	0.318 (0.065)
CON	5.038 ± 0.405	5.271 ± 0.447	5.611 ± 0.399*	0.576 (−0.406,1.558)	1.418 (0.212,2.625)	4.890 (−1.08,10.86)	11.818 (4.781,18.854)
ERE (J·kg− 1)
EXP	8.755 ± 0.375	8.994 ± 0.812	9.604 ± 0.725*	0.286 (−0.775,1.348)	1.015 (−0.167,2.198)	2.618 (−2.181,7.417)	9.839 (2.830,16.849)	<0.001 (0.401)	0.228 (0.084)	0.952 (0.003)
CON	8.412 ± 0.930	8.607 ± 0.939	9.338 ± 0.558*	0.237 (−0.782,1.256)	1.122 (−0.055,2.298)	2.855 (−4.783,10.492)	12.062 (3.097,21.027)

JH = jump height; CD = countermovement depth; PPV = peak propulsive velocity; PBF = peak braking force; PPF = peak propulsive force; PBP = peak braking power; PPP = peak propulsive power; VBI = vertical braking impulse; VPI = vertical propulsive impulse; EST = energy storage; ERE = energy release; NERE = net energy release; * indicates a significant difference compared to the pre-test (p ≤ 0.05).

Fig. 4.

Effect sizes with 95% CI for comparisons between-group kinematic and kinetic variables in EXP and CON. * indicates a significant difference in between-group comparisons (p ≤ 0.05).

Fig. 5.

Within-group comparisons of kinematic and kinetic variables in EXP and CON. * indicates a significant difference compared to the pre-test (p ≤ 0.05). A = jump height; B = net energy release; C = countermovement depth; D = peak braking force; E = peak braking power; F = vertical braking impulse; G = energy storage; H = peak propulsive velocity; I = peak propulsive force; J = peak propulsive power; K = vertical propulsive impulse; L = energy release.

Discussion

This study investigated the effects of 8 weeks of CCT on the biomechanical characteristics of depth jumps in male collegiate volleyball players. The main results showed that, following CCT, the EXP exhibited increases in JH, PPV, ERE, and NERE, and these variables also demonstrated a pattern of coordinated changes. The observed effect sizes were generally in line with our hypotheses: JH and NERE matched the predicted magnitudes, while PPV and ERE exceeded the expected effects.

JH is a commonly used evaluation metric by researchers and coaches to assess athletes’ jumping performance. In the present study, EXP showed a notable increase in JH after 8 weeks of CCT intervention, which is consistent with previous findings that also targeted volleyball players. Berriel et al. reported that male volleyball players exhibited a notable improvement in JH after undergoing four weeks of CCT⁴⁷. Moreover, similar improvements have also been observed in athletes from other sports. Li et al. found that long-distance runners experienced marked increases in JH after 7 weeks of CCT⁴⁸, and Wang et al. reported marked JH growth in basketball players after an 8-week CCT intervention⁴⁹.

The present study found that the EXP showed no notable changes in biomechanical characteristics during the braking phase of depth jumps after CCT, whereas the CON exhibited notable alterations, specifically characterized by significant increases in CD, VBI, and EST, along with a significant decrease in PBF. Jidovtseff et al. found that even when performing jumps with similar movement patterns, modifications in jumping strategies (e.g., CD) can induce marked alterations in multiple biomechanical characteristics⁵⁰. This may suggest that CON possibly adjusted their jumping strategy progressively during the testing, potentially by increasing lower limb muscle work distance to enhance EST and VBI. The observed PBF reduction might also be related to CD changes, as Marshall et al. reported that compared to faster, smaller-CD jumping strategies, subjects adopting slower, larger-CD strategies exhibited lower vertical ground reaction forces at the end of the braking phase²⁷. In this study, subjects’ jumping strategies were guided by verbal instructions emphasizing “jump as high as possible.” Although identical instructions were provided during each test session, the influence of verbal cues on jumping biomechanics or strategy might vary over time⁴¹. Such variations could reflect the body’s attempt to optimize multiple factors during jumping, including the utilization of elastic potential energy⁵¹. Additionally, CON’s frequent participation in volleyball technical training involving substantial jumping exercises during the experimental period might have contributed to the observed changes in their jumping biomechanics, even without corresponding improvements in jumping performance. Although both groups participated in volleyball technical training, EXP’s additional CCT sessions included specific depth jump exercises. When athletes regularly perform training exercises that biomechanically resemble test movements, they tend to show improved motor coordination and control during those movements⁴¹, which may partially explain EXP’s ability to maintain their jumping strategy throughout the study period. In summary, these literature-supported interpretations should be viewed as potential explanations that provide a reasonable reference for understanding the observed changes in the CON and the relative stability in the EXP.

Following training, the EXP exhibited a marked improvement in PPV during the propulsion phase, with no concomitant changes in PPF, PPP, or VBI. Talpey et al. similarly observed that recreationally trained males showed marked improvement in countermovement jump PPV after 9 weeks of CCT using 3-8RM back squats as the conditioning stimulus, while PF and PPP showed no concomitant increases⁵². This phenomenon may be attributed to CCT’s unique ability to modify the force-velocity relationship through the combination of high-load (low-velocity) and low-load (high-velocity) exercises⁹, enabling athletes to achieve faster movement velocities while maintaining similar force output levels. PPP occurs at the moment when the product of force and velocity reaches its maximum value. Since the timing of PPV and PPF does not coincide (for instance, peak velocity may be achieved during the force decay phase), an increase in PPV does not necessarily lead to an increase in PPP. In contrast, Scott et al. observed that rugby players showed marked improvement in countermovement jump PPP after 6 weeks of CCT using 93% 1RM back squats as the conditioning stimulus³⁰. These divergent findings may be explained by differences in training load intensity, inter-set rest periods, and the type of jumping movement employed. Furthermore, the phenomenon of a marked increase in PPV without a notable change in PF following CCT has also been observed in some training methods that do not specifically utilize PAPE. For example, Sánchez et al. found that complex-descending training (CDT) (e.g., performing 3–4 sets of high-load squats followed immediately by 3–4 sets of countermovement jumps) effectively enhanced athletes’ JH and PPV without altering PF. However, unlike the results of the present study, their research also reported marked improvements in PBP and PPP⁵³. This outcome may be closely associated with the primary mechanism of CDT, which is more directly related to power enhancement¹⁵. The improvement in PPV observed with CCT could be associated with the training structure, which alternates between high-load, low-velocity exercises and low-load, high-velocity exercises. This repeated alternation may acutely induce PAPE, potentially resulting in myosin regulatory light chain phosphorylation, increased muscle or fiber water content, and enhanced muscle activation capacity⁹. Over the long term, these acute enhancements in muscle contractile performance might contribute to the observed improvements⁸.

The present findings show that CCT substantially enhanced athletes’ ERE during the propulsion phase. Unlike the CON, the EXP did not exhibit significant changes in EST during the braking phase. The increases in ERE and NERE observed in the EXP may be partly attributed to CCT-induced improvements in SSC energy transfer efficiency⁵⁴. Such efficiency gains may allow a greater proportion of stored elastic energy to be released during the propulsion phase rather than dissipated, thereby increasing net energy output. In addition, NERE may also be related to greater active muscle work during propulsion²¹. Training-induced adaptations, including increased antagonist neural drive, altered muscle activation strategies, changes in muscle fiber mechanics, and structural modifications in muscle morphology⁵⁵, could enhance the capacity for active work production, thereby further contributing to the observed increases in ERE and NERE. Currently, it remains unclear whether CCT differs from other single-modality training methods in modifying energy storage and release patterns during jumping movements. However, by promoting more comprehensive adaptations across components of the force-velocity relationship, CCT may elicit superior mechanical output during both training and competition⁹. In contrast, CON exhibited a different pattern - while showing notable ERE improvement during propulsion, it simultaneously showed increased EST during braking. This dual increase may be associated with CON’s adoption of a jumping strategy characterized by prolonged CD, which allows force application over an extended displacement range, thereby elevating EST. To date, no studies have investigated how CCT influences energy storage and release patterns during jumping movements, and therefore these findings should be considered exploratory.

The findings of the present study indicate that while the EXP showed improved JH following the training intervention, no notable changes were observed in VPI. Houlton et al. similarly found that countermovement jump VPI showed minimal changes within 5 min after PAPE induction using 3RM back squats during CCT¹⁶. However, not all studies support these findings. McCann et al. reported notable increases in JH and VPI during countermovement jumps 4–5 min after PAPE induction using 5RM back squats in collegiate volleyball players⁵⁶. Similarly, Poulos et al. found that elite volleyball athletes showed markedly greater JH and VPI during countermovement jumps following CCT with 87% 1RM squats as the conditioning stimulus, with this phenomenon being more observable in subjects possessing higher relative strength⁵⁷. Differences in factors such as stimulus intensity, athlete level, and relative strength may influence the effects of PAPE or CCT. For example, individuals with greater relative strength may experience greater benefits from CCT⁹. Therefore, variations in these factors among the aforementioned studies may account for the differences in results. Notably, while multiple variables may influence the outcomes, the current evidence suggests that JH enhancement does not necessarily coincide with notable VPI alterations.

Although in the present study we selected appropriate inter-set rest intervals for participants during CCT based on previous research⁹, as noted by McCann et al., no universal recovery duration exists that optimally induces PAPE in all individuals⁵⁶. In the current study, we did not assess PAPE responses for each participant during CCT, which may have resulted in the standardized 5-minute inter-set intervals failing to elicit optimal PAPE effects uniformly across all subjects, consequently leading to inter-individual variability in training adaptations. Furthermore, factors including sex, training experience, stimulation intensity, and sleep quality⁵⁸ may similarly contribute to differential responses to CCT. It should be noted that the present study exclusively examined male athletes and was conducted with a relatively limited sample size. Although significant between-group differences were observed at post-test for several key performance-related variables, the absence of a statistically significant group × time interaction suggests that these findings should be interpreted with caution and may reflect limited statistical power. Therefore, the observed effects should be considered preliminary. Future research should include more diverse populations, particularly female athletes, and employ larger sample sizes to further verify the generalizability and robustness of these findings. At the same time, it should be noted that employing higher loading intensities in CCT may pose potential injury risks for individuals with insufficient strength levels or limited training experience. Future studies may also consider integrating CCT within broader injury prevention frameworks⁵⁹ to better balance performance enhancement and joint safety. Although all measurements were conducted by trained personnel following standardized procedures to minimize potential bias, a methodological limitation of the present study remains that blinding was not implemented during data collection. Furthermore, future studies are recommended to incorporate surface electromyography (EMG) along with emerging technologies such as artificial intelligence and wearable systems⁶⁰ to comprehensively assess neuromuscular adaptations to CCT and reveal the interaction between neural drives and biomechanical features during depth jump.

Conclusion

This study found that 8 weeks of CCT improved biomechanical variables of the depth jump, including JH, PPV, ERE, and NERE, in male collegiate volleyball players, while variables related to the braking phase were largely unchanged. Although post-test differences between groups were observed for some key performance-related indicators, these findings should be interpreted with caution due to the absence of a significant time × group interaction and may reflect limited statistical power. Overall, the results provide preliminary evidence for the potential benefits of CCT on certain biomechanical outcomes. Further studies with larger sample sizes and more diverse populations are needed to confirm and extend these findings.

Author contributions

Yueming Li participated in topic selection, literature search, experimental design, experimental intervention, figure preparation, and editing. Weipeng Li was responsible for project administration and resource provision. Gesheng Lin participated in experimental design and implementation, topic selection, literature search, and editing. Ruixiang Yan participated in topic selection and experimental intervention. Jiaxin He guided topic selection, experimental design, data collection, and manuscript review. Jian Sun guided topic selection, experimental design, data collection, manuscript review, and resource provision. Duanying Li guided topic selection, experimental design, data collection, manuscript review, and resource provision.

Funding

Guangdong Provincial Philosophy and Social Sciences Regularization Project 2022 (GD22CTY09): Research on the Coordinated Development Path of International Competitiveness in Sports in the Guangdong-Hong Kong-Macao Greater Bay Area, GD22CTY09.

Data availability

Data of this study are available upon contacting the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.