Effects of plyometric training on heading jump performance and motion in collegiate soccer players

Abstract

This study aimed to determine the effects of plyometric training (PT) on heading jump performance and motion in collegiate soccer players. Seventeen male soccer players belonging to the university soccer club were included in the study. The subjects were divided into PT (n = 8) and control (n = 9) groups. The PT group performed PT twice a week for 7 weeks. Before and after the intervention period (pre- and postintervention), vertical jump, heading jump in place, heading jump with a run-up, and rebound jump (RJ) were measured. Jump motions were analyzed using 3D motion analysis. Significant interactions were detected between height of the vertical jump, heading jump in place, heading jump with a run-up, and RJ index (p < 0.05), and these values after the intervention period (post) were significantly higher in the PT group compared with the values before the intervention (pre). The rate of change in the PT group was 10.0% for the height of the heading jump in place and 11.9% for the heading jump with a run-up. Although no significant differences were observed in most jump motion variables, the hip joint flexion angles in the starting phase for vertical jump and heading jump in place were lower after the intervention compared with before the intervention (p < 0.05). Our results indicate that sinking before jumps increased and recoil motion was used. PT improved height of the heading jump in soccer players and may be useful for improving performance during games in soccer games.

Keywords:

• Plyometrics
• football
• vertical jump
• explosive power
• jump motion

Reviewing Editor:

• Stirling Sharpe, University of Canberra, Australia

Subjects:

• Sport and Exercise Science
• Sports Development
• Sports Coaching

Introduction

A technique that soccer players use in soccer play is jump heading. Players sometimes jump and head the ball to win aerial battles with their opponents. Although jump heading is not performed often by soccer players during games (Lago-Peñas et al., 2023), jump heading is an important technique contributing to victory or defeat. Modern football has become faster and more intense (Barnes et al., 2014; Wallace & Norton, 2014), and tactics that apply high pressure from the front side are sometimes used. Ball holders often have no choice but to kick long balls against high pressure, and players compete with jump headings against the long balls. A previous study analyzing professional football matches in Spain (Lago-Peñas et al., 2023) reported an increase in aerial battles. In a previous study (Andrzejewski et al., 2022), the technical performance of football players in 612 games in the German Bundesliga was analyzed, focusing on the relationship between individual technical performance and team performance. The study found that losses in aerial battles correlated negatively with winning points (Andrzejewski et al., 2022). Namely, jump heading is an important technique determining victory or defeat. Both jump heading technique and height are important to win aerial battles. To make a high heading jump, it is also important to learn appropriate jumping techniques, such as the arm swing and body sinking (Harman et al., 1990; Luhtanen & Komi, 1978; Matsuda et al., 2015; Shetty & Etnyre, 1989). Higher jumps increase the chances of winning the aerial battle. In addition, longer flight times and better posture while heading demonstrate inherent heading skills.

Plyometric training (PT) improves jumping ability in soccer players (Bedoya et al., 2015; Ramírez-Campillo et al., 2014; Sedano et al., 2011). Sedano et al. (2011) reported that plyometrics performed on elite soccer players three times a week for 10 weeks increases their jumping ability more than from general physical training. In a study by Bedoya et al. (2015), in which seven studies on the effect of plyometric training for youth soccer players was reviewed, six of the seven reported that plyometrics improved CMJ. In addition, PT is effective not only for jumping ability, but also for the soccer players’ agility and endurance. Meylan & Malatesta (2009) reported that soccer players who engage in plyometrics twice a week for 8 weeks improved their agility test results in contrast to those who did not. Ramírez-Campillo et al. (2015) found that soccer players who practiced plyometrics had better endurance test results compared with soccer players who did not. Thus, PT not only improves jumping ability in soccer players but also improves a range of physical fitness attributes, such as agility and endurance (Hammami et al., 2016; Meylan & Malatesta, 2009; Negra et al., 2020; Ramírez-Campillo et al., 2014, 2015, 2015). However, jumping ability, agility, and endurance are elements that are included in physical fitness, but do not directly indicate performance during a soccer match. Several studies have shown that PT improves ball speed when a soccer player kicks (Michailidis et al., 2013; Rubley et al., 2011; Sedano Campo et al., 2009; Sedano et al., 2011). For example, Sedano et al. (2011) found that soccer players who performed plyometrics three times a week for 10 weeks increased their ball speed for instep kicks compared with soccer players who did not. Because the instep kick is a technique that is often used during matches, increasing the ball speed of instep kicks may be directly linked to performance during matches. However, few studies have investigated the effects of PT on abilities and techniques that are closely related to performance in soccer matches, apart from ball speed after kicks. This study investigates the effects of PT on soccer players’ heading jump performance and movement. As mentioned above, PT is effective at improving jumping ability, so it can be expected to positively affect jump heading height, which is an action that takes advantage of an individual’s own jumping ability. However, the effects of PT are unclear because the heading jump motion is different from the motion of a simple vertical jump. In addition, recoil movement (SSC: stretch-shortening cycle) is emphasized during PT (Haff & Triplett, 2015) and heading jumps may also utilize recoil motion. Therefore, jumping movements may change depending on PT. However, the effects of PT on jumping movements, including vertical jumps, have not been studied. If PT positively impacts not only each element of physical fitness (power, agility, etc.) but also performance during matches, PT will have even greater importance for soccer players. Thus, the purpose of this study was to determine the effects of PT on heading jump performance and motion in collegiate soccer players.

Methods

Experimental approach to the problem

To examine the effects of PT on heading jump performance and motion in soccer players, we measured vertical jumps, heading jump in place, heading jump with a run-up, and RJs before and after the intervention period (pre- and postintervention). The subjects were divided into a PT group and a control group, so that the mean height of the heading jump, which was a primary endpoint before intervention, was similar between the groups. As a result, there was no significant difference between the two groups in the mean heading jump height before the intervention. During the 7-week intervention period, the PT group performed PT twice a week before club activities. The control group underwent regular soccer training instead of PT. The soccer training consisted primarily of individual practice skills (e.g. dribbling, traps, kick practice), which were not of high intensity. The training time was the same as that of the PT performed by the PT group (approximately 15 to 30 min). Both groups participated in club activities four times a week as usual. The club activities included individual skill training, group tactical training, and games, and the training time was about 2 hours per day.

In this study, only the subjects were blinded. This was unavoidable in terms of conducting the experiment. The assessors took great care to avoid bias during measurements and data analysis. Thus, we considered that there was almost no assessment bias; however, the possibility of some bias cannot be denied. Studies using a double-blind research design will be done in the future.

Subjects

Subject characteristics are shown in Table 1. The subjects included 17 male soccer players belonging to the university soccer club, including 8 subjects in the PT group (PT group) and 9 subjects in the control group (C group). The university team participated in the regional soccer league. The subjects’ length of playing experience time was 11.7 ± 2.4 years. As mentioned above, the groups were divided so there were no intergroup differences in the height of the heading jump in place before the intervention. In addition, no significant intergroup differences in age, height, body weight, and years of soccer experience were detected. None of the subjects experienced full-scale PT.

Table 1. The characteristics of the subjects.

		PT	C	t	p
		(n = 8)	(n = 9)
Age	M	19.9	19.7	0.32	0.75
	SD	1.5	1.3
Height (cm)	M	170.9	171.1	0.09	0.93
	SD	5.2	5.2
Body mass (kg)	M	67.3	64.9	0.72	0.48
	SD	7.5	6.1
Soccer experience(y)	M	11.2	12.2	0.86	0.40
	SD	1.8	2.7
Jump height (cm)	M	49.7	51.5	0.63	0.54
	SD	6.4	5.5

PT: Plyometric training group; C: Control group.

Jump height: jump height of heading jump in place.

Ethical approval and informed consent

All subjects were fully informed of the purpose, experimental procedures, advantages, and risks of this study in advance, and they provided informed consent. The experimental protocol for this study was approved by Shiga University Research Review Board (approval number: A220305).

Procedures

Measurements

Before and after the intervention, vertical jumps, heading jumps in place, heading jumps with a run-up, and rebound jump were measured. The vertical jump is frequently used to evaluate explosive power and has been used in many studies to examine the effects of plyometrics on soccer players (Meylan & Malatesta, 2009; Michailidis et al., 2013; Ramírez-Campillo et al., 2014, 2015, 2015; Sedano Campo et al., 2009). The heading jumps made during play can be roughly divided into two types: jumps on the spot, without a run-up, and heading jumps with a run-up. The measurement of heading jumps on the spot and with a run-up are included in the measurement items. Rebound jump is often used to evaluate SSC ability (explosive muscle performance and spring ability) (Zushi et al., 2022).

SSC ability was evaluated during rebound jumps using Optojump Next (Microgate, Italia). Vertical jumps, heading jumps in place, and heading jumps with run-up motions were analyzed using three-dimensional motion analysis (OptiTrack Flex3, Acuity Inc., Japan).

Vertical jump

The subject performed a double-legged vertical jump with recoil and arm swing. The subjects were instructed to jump as high as possible with whole body recoil and arm swinging. Jump height and jump motion were analyzed using motion analysis. Measurements were performed twice, and the subjects rested sufficiently between measurements. The reliability of the jump height was ICC = 0.91 for pre-intervention and ICC = 0.95 for postintervention. The better result between the two measurements of jump heights was used for the analysis.

Heading jump in place

The subject performed the jump in place with both feet, using recoil and arm swing, and performed a soccer heading motion when reaching the highest point. The subjects were instructed to jump as high as possible with whole body recoil and arm swinging and perform a heading motion while imagining that there is a ball. Jump height and jump motion were analyzed using motion analysis. Measurements were performed twice, and the subjects were adequately rested between measurements. The reliability of the jump height was ICC = 0.91 for pre-intervention and ICC = 0.94 for postintervention. The better jump height result of the two measurements was used for the following analysis.

Heading jump with a run-up

After taking three steps, the subject jumped with a recoil and arm swing, and when he reached the highest point, he performed a soccer heading motion. If the run-up was more than 4 steps, the motion could not be recorded; therefore, the run-up was set at 3 steps. Takeoff was performed with one foot (dominant foot when jumping). The subjects were instructed to jump as high as possible with whole body recoil and arm swinging and perform a heading motion while imagining that there is a ball. Jump height and jump motion were analyzed using a motion analysis system. Measurements were performed twice, and the subjects were allowed to rest adequately between measurements. The reliability of the jump height was ICC = 0.81 for pre-intervention and ICC = 0.91 for postintervention. The better jump height result of the two measurements was used for the analysis.

Rebound jump

The subjects performed a two-legged takeoff jump five times in a row from a standing posture while placing their hands on their hips. The subjects were instructed to make the contact time as short as possible and jump as high as possible while placing their hands on their hips. Optojump Next (Microgate, Italy) was used for this measurement. Measurements were performed twice. If the subject went out of the device during the measurement, the attempt was considered a failure, and the measurement was performed again. Sufficient rest was given between measurements. The RJ index was calculated by dividing the jumping height by the contact time. The reliability of the RJ index was ICC = 0.90 for pre-intervention and ICC = 0.92 for postintervention. The highest RJ index value was used for the analysis.

Three-dimensional analysis of jump motion

The experimental coordinate system was a right-handed system, with the direction of movement as the X axis, the lateral direction as the Y axis, and the vertical direction as the Z axis. Subjects had a total of 39 dedicated reflex markers (front head (left, right), back head (left, right), upper sternum, xiphoid process, lateral elbow joint (left, right), forearm (left, right), radial side of wrist joint (left, right), ulnar side of wrist joint (left, right), anterior superior iliac spine (left, right), thigh (left, right), lateral knee joint (left, right), lower leg (left, right), ankle lateral malleolus (left, right), 2nd metatarsal (left, right), posterior head (left, right), 7th cervical vertebrae, acromion (left, right), right scapula, 10th thoracic vertebrae, posterior superior iliac spine (left, right), 2nd metacarpal (left, right), heel (left, right)) while performing vertical jumps, heading jumps in place, and heading jumps with run-ups. Using a 3D motion analysis system (OptiTrack Flex3, Acuity Inc., Japan) consisting of 8 optical motion capture cameras, the 3D coordinates of the reflective markers in each jump were measured. The frame rate of the camera was 100 Hz. The calibration results were exceptional, which was the highest grade on the 5-point scale. SKYCOM3.5 (manufactured by Acuity Inc., Japan), a dedicated motion analysis software, was used to analyze each jump motion.

Analysis target phase

For the vertical jump and heading jump in place, the analysis target phase was from the time when the knee joint angle reached the maximum flexion angle before the jump to the time when the positional displacement of the body’s center of gravity reached the maximum value. In the heading jump with a run-up, the analysis target phase was from the time when the knee joint angle of the supporting leg reached the maximum flexion angle in the third step of the run-up to the time when the displacement of the center of gravity of the body reached the maximum value during the jump.

Parameters

The following parameters were analyzed: jump height (cm) defined as the maximum displacement of body center of gravity when jumping; joint angle (deg) and angular velocity (deg/s) of each phase; hip joint angle defined as the angle formed by the vector from the greater trochanter point to the acromion point and the vector from the greater trochanter point to the midpoint of the knee joint; knee joint angle defined as the angle formed by the vector from the midpoint of the knee joint to the greater trochanter and the vector from the midpoint of the knee joint to the midpoint of the ankle joint; ankle joint angle defined as the angle formed by the vector from the midpoint of the ankle joint to the midpoint of the knee joint and the vector from the midpoint of the ankle joint to the second metatarsal bone; trunk angle defined as the angle formed by the vector from the midpoint of the left and right greater trochanter to the seventh cervical vertebra and the z axis; the degree of bending of the body during heading motion; arm swing angular velocity defined as the maximum value of the three-point angular velocity of the angle formed by the vector from the midpoint of the shoulder joint to the midpoint of the elbow joint and the vector from the midpoint of the shoulder joint to greater trochanter from the starting phase to the takeoff phase; and the angular velocity defined as the temporal differentiation about the angle of each part and each joint.

Training program

Table 2 shows the training program for the PT group. This program was administered by a university professor certified as a Strength and Conditioning Specialist by the National Strength and Conditioning Association. There are many types of plyometrics. Various types have been used in previous studies, although the types used in each previous study are different. In the present study, we selected a common plyometric exercise to increase the explosive power of the subjects. The following six types of jumps were used for training: CMJ, box jump, split jump, tuck jump, one-leg vertical jump, and depth jump. CMJ and box jumps were low-intensity exercises. Split jumps and tuck jumps were medium-intensity exercises, and the one-leg vertical jump and depth jump were high-intensity exercises. Based on the principle of progressive loading (Haff & Triplett, 2015), low-intensity and medium-intensity PT events were included in the first and second weeks, and high-intensity PT events were included from the third week onwards. The number of jumps was gradually increased from 80 to 120. In the middle of the program, PT with one leg was added. The training was performed at intervals of at least 2 days. Since proper landing posture is important in the PT (Haff & Triplett, 2015), instruction on landing form was given before the first PT. Subjects were instructed to try to jump as high as possible during the PT. The height of the plyo box during depth jumps was set at 0.75 m. For the box jump, the height was set at 0.75 m (0.4 m + 0.35 m) based on preliminary experiments. During the 7-week training period, all subjects participated in club activities four times a week as usual. PT was performed before club activities and was performed while the athletes were not fatigued.

Table 2. Plyometric training program.

	Week 1	Week 2	Week 3	Week 4	Week 5	Week 6	Week 7
CMJ	10×２	10×２	10×２	10×２	10×２	10×２	10×２
Sprit jump	right 5 × 2 left 5 × 2	right 5 × 2 left 5 × 2	right 5 × 2 left 5 × 2	right 5 × 2 left 5 × 2	right 5 × 2 left 5 × 2	right 5 × 2 left 5 × 2	right 5 × 2 left 5 × 2
Tuck jump	10 × 2	10 × 2	10 × 2	10 × 2	10 × 2	10 × 2	10 × 2
Box jump	10 × 2	10 × 2	10 × 2	10 × 2	10 × 2	10 × 2	10 × 2
One-leg vertical jump			right 5 × 1 left 5 × 1	right 5 × 1 left 5 × 1	right 10 × 1 left 10 × 1	right 10 × 1 left 10 × 1	right 10 × 1 left 10 × 1
Depth jump			5 × 2	5 × 2	10 × 2	10 × 2	10 × 2
Contact number	80	80	100	100	120	120	120
Intensity	Low Medium	Low Medium	Low Medium High	Low Medium High	Low Medium High	Low Medium High	Low Medium High

Note. CMJ means counter movement jump.

Note. 〇×〇 means number × set.

Statistical analyses

Basic statistics (means and standard deviations) were calculated for all variables. Two-way analysis of variance was performed to compare groups and compare parameters before and after training. When significant differences were identified, multiple comparison tests were performed using the Bonferroni method. The rate of change was calculated for each evaluation variable, as follows: Rate of Change = ((Value in Post − Value in Pre)/(Value in Pre)) × 100. Unpaired t-tests were performed to examine intergroup differences in rates of change for each evaluation variable. The statistical significance level was set at p < 0.05.

Results

Jump performance

Significant interactions were observed in the jump height of the vertical jump, heading jump in place, and heading jump with a run-up (Tables 3–5), and the postintervention measurements in the PT group were higher than the pre-intervention parameters. The vertical jump height change rates were 6.5% ± 4.0% for the PT group and −1.7% ± 5.0% for the C group (t = 3.46, p < 0.01). The heading jump in place change rates were 10.0% ± 10.4% for the PT group and −4.9% ± 3.8% for the C group (t = 3.74, p < 0.01). The heading jump with a run-up change rates were 11.9% ± 9.5% for the PT group and 1.9% ± 6.9% for the C group (t = 2.36, p = 0.03). Change rates between groups were significantly different. A significant interaction was also observed in the RJ index. The post-RJ index in the PT group was significantly higher than the RJ index for the pre-intervention value (Table 6). The change rate was 37.8% ± 23.5% in the PT group and 2.6% ± 19.4% in the C group (t = 3.17, p < 0.01).

Table 3. The results of two-way ANOVA in vertical jump.

		PT group		C group		Two-way ANOVA						Multiple comparison test
		(n = 8)		(n = 9)		group		Pre-Post		interaction
		Pre	Post	Pre	Post	F	P	F	P	F	P
Jump height(cm)	M	50.5	53.6	50.7	49.7	0.57	0.46	3.43	0.08	15.2	0.00*	PT: pre < post
	SD	4.7	4.0	5.9	5.5
Arm swing angular velocity (deg/s)	M	691.0	661.4	596.7	542.1	3.87	0.06	2.03	0.17	0.17	0.68
	SD	154.9	105.3	135.8	108.5
Ankle joint angle	M	72.7	74.6	76.6	74.6	0.50	0.49	0.02	0.90	1.28	0.27
(Start phase) (deg)	SD	7.0	7.5	7.0	4.9
Ankle joint angle	M	83.0	104.4	89.4	93.8	0.15	0.71	7.56	0.01*	3.73	0.07
(Take-off phase) (deg)	SD	14.1	10.1	13.9	15.6
Ankle joint angle	M	139.1	136.3	147.4	136.1	0.79	0.39	8.87	0.01*	2.30	0.15
(End phase) (deg)	SD	9.3	9.0	13.4	11.7
Knee joint angle	M	76.0	62.3	78.5	74.8	1.05	0.32	6.24	0.02*	2.99	0.10
(Start phase) (deg)	SD	17.6	9.6	18.5	15.6
Knee joint angle	M	98.1	125.9	103.8	109.5	0.54	0.47	6.76	0.02*	3.27	0.09
(Take-off phase) (deg)	SD	22.2	13.7	20.7	18.8
Knee joint angle	M	162.6	166.0	167.5	159.4	0.03	0.87	0.44	0.52	1.85	0.19
(End phase) (deg)	SD	16.5	7.4	8.7	19.2
Hip joint angle	M	61.9	48.1	63.3	65.7	3.61	0.08	2.88	0.11	7.03	0.02*	PT: pre > post
(Start phase) (deg)	SD	8.0	8.7	14.8	15.2
Hip joint angle	M	125.5	124.7	129.7	131.5	3.21	0.09	0.03	0.87	0.12	0.73
(Take-off phase) (deg)	SD	7.1	4.7	16.4	6.8
Hip joint angle	M	153.6	159.0	158.8	156.3	0.07	0.79	0.12	0.73	1.37	0.26
(End phase) (deg)	SD	14.4	7.2	7.4	16.7
Trunk angle (deg)	M	4.3	3.5	3.6	2.2	2.97	0.27	2.98	0.26	0.58	0.46
	SD	1.7	1.6	1.3	1.5
Ankle extension velocity (deg/s)	M	680.6	667.8	707.4	682.5	0.47	0.50	1.04	0.32	0.10	0.75
	SD	667.8	66.8	74.5	92.9
Knee extension velocity (deg/s)	M	709.4	775.7	700.7	740.9	0.51	0.49	7.90	0.01*	0.48	0.50
	SD	108.5	46.9	70.8	66.5
Hip extension velocity (deg/s)	M	543.5	601.8	579.5	581.9	0.08	0.78	3.00	0.10	2.83	0.11
	SD	85.9	63.7	69.5	61.1

M: Mean; SD: Standard deviation, *:p < 0.05.

Table 4. The results of two-way ANOVA in heading jump in-place.

		PT group		C group		Two-way ANOVA						Multiple comparison test
		(n = 8)		(n = 9)		group		Pre-Post		interaction
		Pre	Post	Pre	Post	F	P	F	P	F	P
Jump height (cm)	M	49.7	54.2	51.5	48.9	0.50	0.49	0.91	0.36	20.55	0.00*	PT: pre < post
	SD	6.1	4.5	5.3	4.7
Arm swing angular velocity (deg/s)	M	669.6	664.7	659.9	566.3	0.91	0.35	3.03	0.10	2.21	0.16
	SD	196.1	92.7	123.3	96.3
Ankle joint angle	M	74.2	74.9	78.4	77.7	1.12	0.31	0.00	0.99	0.20	0.67
(Start phase) (deg)	SD	7.3	6.4	9.0	10.9
Ankle joint angle	M	92.9	100.7	98.6	99.1	0.13	0.73	0.72	0.41	0.62	0.44
(Take-off phase) (deg)	SD	17.8	13.4	15.1	16.6
Ankle joint angle	M	141.0	138.5	139.3	136.0	0.32	0.58	2.10	0.17	0.03	0.86
(End phase) (deg)	SD	7.5	8.0	10.9	9.4
Knee joint angle	M	85.5	72.2	88.0	85.1	2.30	0.15	5.50	0.03*	2.41	0.14
(Start phase) (deg)	SD	16.7	11.2	12.9	12.5
Knee joint angle	M	114.0	126.8	120.4	117.7	0.04	0.84	0.59	0.45	1.68	0.21
(Take-off phase) (deg)	SD	24.4	11.8	22.0	17.9
Knee joint angle	M	158.7	165.1	159.2	155.5	0.62	0.44	0.09	0.77	1.92	0.19
(End phase) (deg)	SD	18.2	6.5	14.3	18.4
Hip joint angle	M	78.5	60.2	77.1	78.4	2.11	0.17	3.96	0.06	5.99	0.03*	PT: pre > post
(Start phase) (deg)	SD	15.3	13.3	16.0	14.9
Hip joint angle	M	127.5	130.4	132.1	131.1	0.55	0.47	0.13	0.72	0.67	0.43
(Take-off phase) (deg)	SD	11.0	6.0	8.3	9.8
Hip joint angle	M	157.3	151.9	152.8	159.3	0.09	0.77	0.05	0.83	2.1	0.17
(End phase) (deg)	SD	13.2	16.4	13.6	10.8
Trunk angle (deg)	M	9.6	7.1	7.0	6.2	1.38	0.26	3.32	0.12	1.36	0.26
	SD	5.3	3.7	3.3	2.4
Ankle extension velocity (deg/s)	M	746.8	688.5	747.0	704.5	0.11	0.75	5.2	0.04	0.13	0.72
	SD	56.9	58.7	62.2	94.8
Knee extension velocity (deg/s)	M	712.6	787.9	723.3	710.9	1.29	0.27	1.47	0.24	3.38	0.08
	SD	98.8	34.7	72.3	96.0
Hip extension velocity (deg/s)	M	567.7	553.5	579.1	546.3	0.00	0.96	1.33	0.26	0.20	0.66
	SD	72.9	162.2	79.0	66.6

M: Mean; SD: Standard deviation, *:p < 0.05.

Table 5. The results of two-way ANOVA in heading jump with run-up.

		PT group		C group		Two-way ANOVA						Multiple comparison test
		(n = 8)		(n = 9)		group		Pre-Post		interaction
		Pre	Post	Pre	Post	F	P	F	P	F	P
Jump height(cm)	M	46.6	51.6	46.8	47.4	0.42	0.52	9.56	0.01*	6.62	0.02*	PT: pre < post
	SD	7.5	4.9	7.6	6.3
Arm swing angular velocity (deg/s)	M	685.9	691.0	601.1	561.7	2.42	0.14	0.16	0.70	0.23	0.64
	SD	172.3	86.4	203.6	191.4
Ankle joint angle	M	98.9	96.9	92.6	91.3	1.28	0.27	0.30	0.59	0.00	0.96
(Start phase) (deg)	SD	22.0	8.7	9.9	9.0
Ankle joint angle	M	127.2	114.3	122.1	106.1	1.89	0.19	9.06	0.01*	0.13	0.73
(Take-off phase) (deg)	SD	21.8	8.5	14.3	11.3
Ankle joint angle	M	143.7	135.5	131.4	137.2	1.33	0.27	0.22	0.64	7.50	0.01*	PT: pre > post
(End phase) (deg)	SD	12.5	12.9	12.1	8.3
Knee joint angle	M	121.1	113.7	107.3	114.2	2.15	0.16	0.13	0.72	3.07	0.10
(Start phase) (deg)	SD	21.5	11.9	13.0	11.8
Knee joint angle	M	147.2	140.7	156.3	132.9	0.00	0.96	6.99	0.02*	1.90	0.19
(Take-off phase) (deg)	SD	23.0	6.0	10.9	26.6
Knee joint angle	M	158.6	151.4	155.0	157.9	0.02	0.89	0.04	0.85	1.10	0.31
(End phase) (deg)	SD	16.4	21.7	16.7	11.2
Hip joint angle	M	116.1	117.2	114.9	117.5	0.00	0.95	0.69	0.42	0.11	0.74
(Start phase) (deg)	SD	11.2	12.0	15.0	11.8
Hip joint angle	M	147.0	137.5	149.3	139.6	0.44	0.52	14.44	0.00*	0.00	0.96
(Take-off phase) (deg)	SD	7.0	8.2	9.2	10.2
Hip joint angle	M	136.9	145.2	149.2	150.8	2.00	0.18	1.08	0.31	0.51	0.48
(End phase) (deg)	SD	12.2	13.1	19.2	19.9
Trunk angle (deg)	M	8.2	9.0	6.9	6.9	0.01	0.91	2.12	0.23	1.44	0.25
	SD	4.9	4.4	2.4	4.6
Ankle extension velocity (deg/s)	M	660.0	592.3	585.6	624.2	0.18	0.68	0.04	0.84	0.94	0.35
	SD	171.2	177.4	154.0	131.1
Knee extension velocity (deg/s)	M	620.2	595.5	532.7	555.9	1.34	0.26	0.00	0.99	0.33	0.57
	SD	174.2	143.2	145.5	118.2
Hip extension velocity (deg/s)	M	480.4	477.8	425.3	440.0	1.84	0.19	0.06	0.81	0.11	0.74
	SD	101.1	107.4	87.9	68.1

M: Mean; SD: Standard deviation, *:p < 0.05.

Table 6. The results of two-way ANOVA in RJ index.

		PT group		C group		Two-way ANOVA						Multiple comparison test
		(n = 8)		(n = 9)		group		Pre-Post		interaction
		Pre	Post	Pre	Post	F	P	F	P	F	P
RJ index	M	1.4	1.8	1.3	1.3	4.13	0.06	16.5	0.00*	17.4	0.00*	PT: pre < post
	SD	0.4	0.2	0.3	0.3

M: Mean; SD: Standard deviation, *:p < 0.05.

Jump motion

For the vertical jump, a significant interaction was observed in the hip joint flexion angle at the starting phase, with post-intervention measurements exhibiting lower values compared with pre-intervention measurements in the PT group (Table 3). No significant differences were observed in other movement variables of the vertical jump. For heading jump in-place, a significant interaction was observed for the hip joint flexion angle at the starting phase, with post-intervention measurements exhibiting lower values compared with pre-intervention measurements in the PT group (Table 4). No significant differences were observed in other movement variables of heading jump in-place. In heading jump with run-up, a significant interaction was observed in the ankle joint angle during the final phase, which was lower after the intervention compared with before intervention. No significant differences were observed for other movement variables in heading jumps with run-up.

Discussion

In the PT group, the jump height of the vertical jump was higher after the intervention than the height before the intervention, unlike the C group. The rate of change was also significantly higher in the PT group (6.5% improvement) compared with the C group. Thus, PT twice a week for 7 weeks improved the explosive power of collegiate soccer players. The purpose of PT is to improve explosive power (Haff & Triplett, 2015). Multiple studies showed that PT positively affects the jumping ability (explosive power) of soccer players (Bedoya et al., 2015; Ramírez-Campillo et al., 2014; Sedano et al., 2011). For example, plyometrics three times a week for 10 weeks affected the performance of elite male soccer players (Sedano et al., 2011). In the previous study (Sedano et al., 2011), CMJ with and without arm swing improved significantly in the PT group but not the C group. A review of seven studies on the effects of PT on youth soccer players also demonstrated that PT improves explosive power (CMJ) (Bedoya et al., 2015). For example, Ramirez-Campillo et al. (Ramírez-Campillo et al., 2014) examined the effects of plyometrics on 76 soccer players with an average age of 13 years. The results indicated that CMJ was significantly improved in the plyometrics group. Many studies have shown that plyometrics improve the vertical jump of young and elite soccer players. Although this study targeted university soccer players, our findings regarding CMJ are consistent with those of previous studies for different populations. During the CMJ, the player crouches down and then jumps; thus, CMJ includes SSC (Haff & Triplett, 2015). In this study, the plyometric exercise performed was jump training, which was expected to improve SSC execution ability. The results revealed that the rebound jump index, which is used to evaluate SSC ability, improved in the PT group (37.8% ± 23.5%). Furthermore, a previous study on soccer players (Ramírez-Campillo et al., 2014) examined the effect of PT on the reaction strength index (RSI), which is used to evaluate SSC ability, and revealed that PT significantly improved the RSI. Thus, the PT group was assumed to improve their SSC ability and CMJ.

Although the effects of PT on vertical jump height in soccer players were previously investigated, the effects of PT on heading jump height were not investigated. In this study, we investigated the effects of PT on heading jump height in place and heading jump with a run-up. Previous studies that have examined the effects of PT on soccer players have determined the effects on explosive power, which is one of the elements of physical fitness; however, the effect of PT on heading jump height and movement, which are directly linked to performance in soccer matches, has not been examined. This is a major difference between previous studies and this study. Both parameters (heading jump height in place and heading jump height with a run-up) were significantly improved by the PT intervention. A significant intergroup difference was also observed in the rate of change; the heading jump in place improved by 10.0% and the heading jump with a run-up improved by 11.9% in the PT group. Thus, PT greatly improved the jump height of college soccer players during heading jumps. Heading jump in place is not the same as the CMJ; however, both jumps are the same in terms of crouching down in place and then jumping, i.e. both jumps use SSC. As mentioned above, the PT group was speculated to have improved SSC ability and explosive power; thus, this improvement was also speculated to be utilized during heading jumps in place. A heading jump with a run-up is a one-legged jump after the run-up; therefore, the movement is very different from that in the CMJ. However, when heading jump with a run-up, SSC is used because a player crouches slightly before jumping. Moreover, in the PT group, the jump height of heading jumps with a run-up was assumed to improve because of improved SSC ability. In addition, a single-leg vertical jump was included in the PT (Table 2). The PT group was assumed to become accustomed to single-legged jumping by repeating it. Familiarity with one-legged jumps may have led to improved heading jumps with run-up in the PT group. The height of the heading jump improved significantly by about 10% after PT training in soccer players, and this change may have a positive effect on performance during games. PT improves physical fitness, including power, agility, endurance, and change of direction, in soccer players (Hammami et al., 2016; Meylan & Malatesta, 2009; Negra et al., 2020; Ramírez-Campillo et al., 2014, 2015, 2015); however, except for kick performance, the impact of PT on performance during games (Michailidis et al., 2013; Rubley et al., 2011; Sedano Campo et al., 2009; Sedano et al., 2011) has not been fully investigated. Our results suggest that PT contributes to improvements in the competitiveness of soccer players, increasing the value of PT.

Changes in jump motion due to PT have not been thoroughly investigated but were investigated in this study. This point is also the originality of this study. Verifying whether the effect of PT on jump ability results from changes in movement or other factors (e.g. changes in the internal muscle tissue characteristics) would be extremely important for understanding PT. No significant interactions were observed in most of the movement variables in the three jumps. Thus, PT may not affect jump movement overall. In order to jump high, the initial velocity of the vertical direction during the jump is important (Feltner et al., 1999). The initial velocity can be improved by 6–10% using the arms (Feltner et al., 1999; Harman et al., 1990; Lees et al., 2004; Shetty & Etnyre, 1989). Harman et al. (1990) reported that using arm swing during the CMJ increased the vertical ground reaction force and the jump height. Because arm swing is important for improving jump height (Harman et al., 1990; Luhtanen & Komi, 1978; Shetty & Etnyre, 1989), it was examined in this study. Since plyometrics often involve jump training with arm swing, it was considered that it may affect the angular velocity of the arm swing; however, the results were different. Although the exact cause is unknown, because the subjects had been playing soccer for many years and may have become accustomed to the arm swing while jumping, it is possible that no significant changes were observed after 7 weeks of training. Furthermore, the PT group was instructed to shorten the contact time when landing and to jump as high as possible; however, no special instructions were given regarding arm swing. The results may have been different if arm swing instructions had been provided.

However, significant differences in the hip joint angle at the beginning of the vertical jump and the heading jump in place and the ankle joint angle at the end of the heading jump with a run-up were detected. Those variables were improved after the PT intervention compared with before the intervention. Since athletes in the PT group were conscious of jumping high during the PT, they were also naturally conscious of sinking just before the jump. Matsuda et al. (2015) investigated the association between the jump height and jumping motion during vertical jumps and head jumps with a run-up and discovered an association between jump the height and crouches during both jumps. It is presumed that by reducing the hip flexion angle, the hip extensor muscles are stretched, which enables better use of neuromuscular function (SSC). It is also possible that the use of recoil results in increased jump height. The ankle joint angle in the ending phase of the heading jump with a run-up is difficult to interpret. The action of raising the toes when performing box jumps during the PT program may be involved in the ankle joint angle. No significant differences in the ankle joint angle at the end of the vertical jump and heading jump in place were detected between the groups, but the ankle joint angle was lower after the PT intervention compared to before the PT intervention. Since the exact interpretation of these results is difficult at this point, further studies are necessary. As there was almost no difference in motion variables, improvements in jumping height obtained in this study were not due to changes in jumping technique, but rather changes in internal muscle tissue characteristics (Fouré et al., 2012), increased tendon cross-sectional area (Barnes et al., 2014; Houghton et al., 2013), and improved neuromuscular function (SSC).

During the heading jumps in this study, we asked the subjects to jump and perform a heading motion, assuming that there was a soccer ball. Since it was not possible to supply uniform balls during heading jumps, we could not adopt the experimental setting of heading the ball. Therefore, it should be noted that the athletes did actually jump and header the ball. Future research is needed.

This study targeted university soccer players who had no PT experience. The training effects are likely to appear in the early stages of training, as the training effect appeared in this study. We do not know whether the same results would be obtained if the subjects were soccer players who have experienced PT or soccer players with sufficient explosive power. Further investigation is necessary for soccer players with different subject characteristics.

Practical applications

Our results show that PT for collegiate soccer players conducted twice a week for 7 weeks greatly improved not only the vertical jump height and RJ index, but also the heights of the heading jump in place and heading jump with a run-up. In other words, PT for soccer players may be useful for improving performance during games. In the future, soccer and training leaders can expect PT to improve the jump height of heading jumps, increasing the significance of PT for soccer players. In the future, coaches should make better use of PT for soccer players.

Acknowledgments

The authors disclose professional relationship with companies or manufacturers who benefit from the results of this study. The results of this study do not constitute an endorsement of the products by the authors of the National Strength and Conditioning Association.

Disclosure statement

The authors declare no conflicts of interest associated with this manuscript.

Data availability statement

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