Effect of 10-Week Plyometric Training on Anaerobic Performance and Biomechanical Properties of the Muscles in Football Players: Randomized Controlled Trial

Abstract

This study investigated the effects of a 10-week plyometric training program on sprint performance, reactive power, and biomechanical muscle properties in soccer players. Twenty soccer players were randomly assigned to an experimental group (n = 10) or a control group (n = 10). Both groups maintained their regular weekly training, with the experimental group performing additional plyometric sessions twice weekly. Pre- and post-intervention assessments included 5 m and 30 m sprint times, Reactive Strength Index (RSI), and biomechanical properties (tension, stiffness, elasticity) of the rectus femoris (RF) and vastus lateralis (VL). The experimental group demonstrated significant improvements in 5 m (p < 0.01; ES = 1.44) and 30 m (p < 0.01; ES = 1.11) sprint times and RSI (p < 0.05; ES = 0.87). No significant changes were observed in muscle tension, stiffness, or elasticity at the group level. However, correlations indicated that higher baseline elasticity in the VL was linked to greater 5 m sprint improvements, while changes in RF elasticity were negatively associated with 5 m sprint gains. These findings suggest that plyometric training effectively enhances short-distance sprint performance and reactive power in soccer players. Although group-level biomechanical properties did not change significantly, individual variability in muscle elasticity may modulate training outcomes, supporting the integration of plyometric exercises into soccer training regimens.

1. Introduction

Plyometric training is a widely used method that utilizes a biomechanical mechanism called the stretch-shortening cycle [1]. In this way, we can develop the ability of the neuromuscular system to generate the maximum force in the shortest time possible [2]. Plyometric exercises are characterized by high speed, short duration, and maximum effort, requiring the activation of motor units associated with fast-twitch type II muscle fibers [3]. A study that used a plyometric exercise protocol consisting of 10 sets of 10 jumps in opposing movements showed that this type of exercise caused preferential damage to type II muscle fibers. These results concluded that plyometric training also causes more significant hypertrophy of type II muscle fibers than type I [4]. It usually involves various jumps, leaps, or ballistic throws with the shortest possible contact phase with the ground/object and the most significant intention of jumping or throwing the object. Such training is a common strategy in various sports disciplines [5]. The impact of plyometric training on skeletal muscle hypertrophy is often described as relatively minor. In a comprehensive review of neuromusculoskeletal adaptations and performance to plyometric training from 2010, Markovic and Mikulic [1] concluded that plyometric exercises can potentially induce muscle hypertrophy and are generally weaker than resistance-induced.

Plyometric training can positively affect professional and amateur soccer players [6]. Positive effects have also been observed in the form of increased strength among men and women, provided that it is performed with appropriate progression, respecting the principles of tissue adaptation time and at a sufficiently high intensity [7]. In many sports, the sprint component is crucial in the context of reaching a specific spot on the field faster than the opponent [8], as well as, in the case of soccer, winning the running duel for the ball or reorganizing the formation in defensive play. It has also been shown that sprinting (45%) and jumping (16%) are the two motor actions of a football player that most often lead to goal-scoring situations [9].

Moreover, it has been confirmed that combining plyometrics and body mass, including counter-movement jumps, depth jumps, and squat jumps, increases vertical jump height [7]. This type of training increases neuromuscular coordination by training the nervous system, thereby enabling the muscles’ stretch-shortening cycle [10]. Properly programmed plyometric training positively affects sprint values, with athletes improving their times over distances of 20 and 30 m. Moreover, no significant differences were found in intensity and the addition of external resistance; however, regarding the results achieved, the selection of exercises proved significant.

Combining several exercises yielded better results than prescribing only one type of plyometric exercise [7,11]. Exercises aimed at reactive strength, such as the drop jump, which involves jumping off a platform and then jumping upwards with maximum intent, also significantly improved RSI values [12]. Increased pennation angle, tendon thickness, and increased stiffness are the changes in parameters observed after applying plyometric training [13].

Adaptive changes during training also concern the biomechanical properties of muscles, such as muscle tension (F), dynamic stiffness (S), and elasticity (D). Deficits in these properties can cause weakened athletic performance and be a reason for injuries in athletes [14]. In the case of the quadriceps muscles, they are one of the risk factors for developing knee joint disorders [15]. The primary function of tendons is to store and transmit the mechanical force of muscle contraction to the bones [16]. High values of dynamic stiffness in tendons due to adaptation to the type of training should allow an individual to withstand greater mechanical loads.

The study examined whether significant biomechanical changes occur in the femoris muscles following a training program, using the Myoton device for measurement. Consequently, a study enhances the range of measurement techniques available by incorporating this device. There is a need for more evidence regarding the methodology of this training, the types of exercises, and the assessment of changes in the biomechanical properties of muscles and their impact on the athlete’s motor skills. Therefore, this experimental study aimed to assess the impact of a 10-week plyometric training program on running speed, selected parameters of explosive muscle strength, and changes in the biomechanical properties of the quadriceps muscles (muscle tension, flexibility, muscle stiffness) in amateur football players.

2. Materials and Methods

2.1. Participants

The G* power (v3.1.9.6, Kiel University, Kiel, Germany) software was employed to determine the a priori sample size. Recruiting at least 16 participants was necessary to achieve an effect size level of 0.6 (α = 0.05; power = 0.96) [17]. Twenty male football players (n = 20) from the KS Polonia Łaziska-Górne football club competing in the IV league (age: 23.5 ± 10.5 years, BMI: 23.97 ± 3.96 kg/m², training experience: 14.5 ± 5.95 years), were randomly divided into two groups: the experimental group (eG-n = 10) and the control group (cG-n = 10) according to the following inclusion criteria: age 16–38 years, with a minimum of 5 years of training experience, training at least 3 times a week. The different ages and experiences of the players taking part in the study represented most of the football teams. All players had current medical examinations, allowing them to participate in football competitions. Exclusion from the examination concerned elevated blood pressure occurring before the examination (pressure >140/90 mmHg) in individuals treated after injuries, damage, or unspecified skin and myofascial musculoskeletal system changes. Exclusion could occur at any time during the study at the participant’s request. All participants actively and regularly participated in training sessions and competitions during the study. The study was approved by the ethics committee of the Polish Physiotherapy Association (RESOLUTION No. 3 March 2024 dated 27 March 2024) and conducted by the Helsinki Declaration.

2.2. Study Design

The study was designed as a prospective experimental study with a control group (Figure 1). The experimental group underwent a 10-week plyometric training program. In contrast, the control group did not undergo plyometric training and followed a standard football training program. The group assignment was done through simple 1:1 randomization with a random sequence using the website ranomizer.org. The group assignment was independent of the treatment time and the research staff. Each participant underwent an introductory intervention, plyometric training, receiving a 10-min briefing seven days before the study.

The control group (n = 10) followed the previously planned training by the coach for the duration of the study (10 weeks), which was typically football training encompassing the physical, technical, and tactical preparation of the football team players (

Table 1. Standard training program followed by the control and experimental groups.

Day of the Week	Type of Training	Exercise	Workload/Intensity	Number of Repetitions/Working Time	Number of Series
Tuesday	Aerobic capacity and power with football elements	Running warm-up	≤60% HRmax	15 min	1
		Continuous run	70–80% HRmax	16 min	2
		Games in small groups, e.g., “rondo”	≤50% HRmax	10 min	1
		Patterns of play in offense or defense	≤50–80% HRmax	15 min	1
		Small football games on a scaled-down field	-	8 min	2
		Stretching + mobility	≤50% HRmax	5 min	1
Wednesday	Muscle strength and football tactics	Running warm-up	≤60% HRmax	15 min	1
		Barbell glute bridge	60% 1RMmax	×10	4
		“Plank” position	-	1 min	4
		Dumbell split squat	60% 1RMmax	×10 per side	4
		Tactics and shooting	≤50–80% Hrmax	15 min	1
		Formations movement	≤50–80% HRmax	15 min	1
		Stretching + mobility	≤50% HRmax	5 min	1
Thursday	Glycolytic capacity and power + set pieces	Running warm-up	≤60% HRmax	15 min	1
		Shuttle run	90–100% Vmax	30 s	6
		Set pieces	≤50% HRmax	20 min	1
		Stretching + mobility	≤50% HRmax	5 min	1
Friday/Saturday	Friendly game/league game	Football game		45–90 min	1
		Stretching + mobility	≤50% HRmax	5 min	1

HRmax—maximal heart rate.

). The experimental group (n = 10) incorporated a plyometric training protocol into their planned training, having previously conducted an introductory session with a research assistant who explained the exercises, demonstrated proper movement patterns, and provided feedback on the performed exercises. Each athlete participating in the study warmed up according to their training habits for about 10 min. The athletes performed the plyometric training protocol twice a week before the central part of the training. For 10 weeks with a minimum of 48 h between training sessions, they performed 60 to 72 jumps by the end of the protocol during one session in 2–4 sets. The break between each series was 120 s. Each competitor was equipped with a heart rate watch to ensure that their heart rate returned to readiness values. The intensity with which the athletes performed the exercises was increased; at the beginning of the protocol, the hurdles they jumped over and the box they jumped off were lower, and by the end, they were higher. The exercises included two-footed hurdles in a straight line, multiple jumps, and a drop from a platform with a reactive rebound. The training protocol is presented in

Table 2. Training program implemented by the experimental group.

Exercise	Number of Jumps	Number of Series	Rest Time Between Series	Intensity	Comment
Hurdles jump	8	3	120 s	Maximum	Hurdles set at the height of 30 cm, successively overcome reactively with the shortest possible contact of the feet with the ground
Multi-jumps	10	3	120 s	Maximum	The athlete performs the longest possible multi-jumps from a short run
Drop jump	6	3	120 s	Maximum	Reactive vertical jump after jumping off a box set at a height of 40 cm

. The training was conducted on the field and in the indoor hall at the Municipal Stadium in Łaziska Górne, at 3 Sportowa Street. Sprint measurements, reactive strength indicators, and tissue properties were conducted in the exact location in the afternoon from 5:00 PM to 6:00 PM. For each subject, the same conditions for task execution and measurements were maintained. All participants were tested under the same conditions (5:00 PM to 6:00 PM). The essential characteristics of the research group are presented in

Table 3. Essential characteristics of study group (n = 20).

Variable	Mean ± SD	Range
Age (year)	22.45 ± 3.67	16–30
Height (cm)	181 ± 4	173–192
Weight (kg)	78.15 ± 7.76	64–93
BMI (kg/m2)	23.82 ± 1.81	20.45–26.86
Training experience. (year)	14 ± 2.6	10–20

. All participants underwent the following measurements: sprint time for 10 and 30 s (s), reactive strength index (RSI-[m.s⁻¹]), muscle tension (F-[Na Hz]), stiffness (S-[N/m]), and elasticity (D-[NaN]). The measurements were taken during the following periods: (1) before the implementation of the programmed “pre” training; (2) in the 10th week of the programmed “post” training.

2.3. Measures

2.3.1. Sprint

For sprint time measurements according to the “gold standard” [18], fully automatic photoelectric cells (Microgate Witty System, Bolzano, Italy, 2019) were used (Figure 2). According to various studies, a setup at intervals of 0 m, 5 m, and 30 m on a natural grass football field does not constitute a significant difference in measurements compared to hard surfaces [19,20] (Figure 3). Each athlete performed two trials at the maximum possible speed, with complete rest between trials. Previously, each athlete warmed up individually for about 10 min, where each ended the warm-up with a few faster sprints as a precaution before the upcoming test. All athletes were trained in the starting procedures, which are strictly standardized—a standing start and push-off from the more muscular trailing leg (Figure 4). The photoelectric cells measured the sprint time over two segments: the time for 5 m and the time for 30 m expressed in seconds.

2.3.2. Reactive Strength Index (RSI)

Reactive Strength Index (RSI) RSI describes a person’s ability to transition from eccentric to concentric muscle contraction quickly and aims to assess the athlete’s reactive strength—muscle strength. RSI was determined using the drop-jump method on the Optojump Next System. The drop jump is reliable for assessing RSI and related kinematic parameters (42). The athletes had to jump down from a height of 40 cm and, upon landing, perform a vertical jump with maximum intent (Figure 5). The entire procedure was performed with hands placed on the hips. Before the study, participants warmed up by doing ten squats and passive thigh muscle stretching for one minute. A research assistant trained all athletes in jump technique, and three attempts were made before measurements were taken. The test included two different drop heights and assessed how high the athlete could raise their center of mass. To calculate the RSI for each participant, we used the equation RSI = jump height [m]/contact time [s] [21].

2.3.3. Biomechanical Properties of Muscles

Biomechanical properties were measured using a myometer (MyotonPRO AS, Myoton Ltd., Tallinn, Estonia, 2021). Myoton is a digital device with a body and a deep probe (Ø 3 mm). An initial pressure (0.18 N) is applied to the surface through the probe, which causes the material underneath to compress. Next, the device releases a mechanical impulse (0.4 N, 15 ms) that temporarily deforms the medium. Myometry is a reliable measurement method and allows for the detection of differences in physical properties compared to stretched muscle fibers [22]. The measurement method involves recording the damped natural vibrations of soft biological tissue in the form of an acceleration signal and then simultaneously calculating the parameters of the stress state and biomechanical properties. Thanks to the device, we can assess, among other things, the state of resting muscle tone (F), which is defined as the suppressed EMG signal and dynamic stiffness (S). Stiffness assessed using myotonometry is based on the theory of free oscillations and results from the natural oscillations of tissues in response to brief mechanical exposure of the skin. Tissues can also regain their original shape after deformation. This property, measured in this study, is called elasticity (D). The greater the elasticity, the faster the tissue returns to its original shape. Measurements with the myotonometer were performed in a standardized, supine resting position. The myotonometer was applied perpendicularly to the tissue (Figure 6). The broadest cross-sectional area of the muscles was determined and marked, meaning that the measurement location for the rectus femoris muscle was the midpoint of the distance from the anterior superior iliac spine to the upper border of the patella. For the vastus lateralis muscle, the segment’s midpoint is from the patella’s upper edge to the greater trochanter [14,23] (Figure 7).

2.4. Statistical Analysis

Statistical analyses were performed using Statistica 13.1. (StatSoft, Tulsa, OK, USA) and PRISM 9.4.1. (GraphPad, La Jolla, CA, USA). Results are presented as means with standard deviations and 95% confidence intervals. Normality, homogeneity, and sphericity of variance of sample data were verified using Shapiro–Wilk, Levene, and Mauchly tests, respectively. Differences between the variables considered were compared using ANOVA with repeated measures. In the case of a significant main effect, the Bonferroni post hoc test was used for post hoc comparisons. The association’s strength in the variance analysis was calculated using ƞ2. The strength of the effect was calculated using the coefficient ƞ2. The strength of the effect was classified as weak if ƞ2 ranged from 0.01 to 0.059, medium from 0.06 to 0.137, and large > 0.137. For pairwise comparisons, effect sizes (ES) were determined by Cohen’s d and characterized as significant (ES > 0.8), moderate (0.5 < ES < 0.8), small (0.20 < ES < 0.49), and trivial (ES < 0.05). Spearman’s rank correlation coefficient was used for the correlation analysis. Statistical significance for differences was set at p < 0.05. Basic descriptive statistics are presented in

Table 4. Basic descriptive statistics.

	Experimental Group		Control Group
	Pre	Post	Pre	Post
	M ± SD (−95%; +95%)	M ± SD (−95%; +95%)	M ± SD (−95%; +95%)	M ± SD (−95%; +95%)
RSI †,*	1.48 ± 1.42 (0.15; 0.40)	1.69 ± 1.68 (0.17; 0.46)	1.19 ± 1.35 (0.22; 0.58)	1.24 ± 1.28 (0.21; 0.57)
5 m [s] †	1.09 ± 1.11 (0.04; 0.12)	1.01 ± 1.02 (0.03; 0.10)	1.10 ± 1.12 (0.05; 0.15)	1.06 ± 1.07 (0.04; 0.13)
30 m [s] †	4.30 ± 4.33 (0.10; 0.28)	4.13 ± 4.18 (0.10; 0.28)	4.36 ± 4.43 (0.14; 0.38)	4.28 ± 4.30 (0.09; 0.25)
RF F [Hz]	15.61 ± 15.30 (0.88; 2.36)	15.45 ± 15.25 (0.81; 2.16)	15.39 ± 15.55 (0.59; 1.58)	15.21 ± 15.20 (0.60; 1.61)
RF S [N/m]	281.90 ± 274.00 (26.91; 71.45)	271.90 ± 274.00 (22.87; 60.71)	275.70 ± 275.50 (12.28; 32.60)	273.90 ± 276.00 (12.27; 32.58)
RF D [N/m] †	1.51 ± 1.54 (0.17; 0.47)	1.40 ± 1.40 (0.19; 0.53)	1.42 ± 1.43 (0.20; 0.53)	1.38 ± 1.33 (0.16; 0.43)
VL F [Hz]	18.74 ± 3.31 (2.52; 4.84)	15.87 ± 14.38 (1.43; 3,79)	19.02 ± 18.40 (2.47; 6.55)	17.04 ± 15.56 (1.41; 3.76)
VL S [N/m]	346.30 ± 321.50 (52.00; 138.03)	291.60 ± 264.48 (26.07; 69.19)	345.40 ± 333.00 (35.71; 94.78)	311.70 ± 291.90 (19.03; 50.52)
VL D [N/m]	1.59 ± 1.47 (0.11; 0.29)	1.35 ± 1.14 (0.19; 0.52)	1.70 ± 1.47 (0.01; 0.31)	1.53 ± 1.40 (0.12; 0.33)

RF—rectus femoris; VL—vastus lateralis; F—tension; S—stiffness; D—elasticity; ^†—intragroup difference; *—intergroup difference.

.

3. Results

The first part of the analysis involved intergroup comparisons. In most cases, the data distributions are normal (p > 0.05), and in a few cases, they are close to normal. Therefore, a parametric analysis of variance with repeated measures was used. Due to extreme asymmetries in the data distribution, a non-parametric equivalent was used for some of the viscoelastic properties of muscle.

Significant differences were found for the main effects of the 5 m variable pre-post (F = 16.476; p < 0.01; ƞ2 = 0.47). In contrast, no significant differences were found for the group effect (F = 1.090; p > 0.05) and the higher-order pre-post*group interaction (F = 2.128; p > 0.05). To determine which groups had significant differences, further analyses were conducted using Bonferroni’s post-hoc multiple comparison tests. Significant differences were found in the 5 m [s.] variable after and before the intervention in the experimental group (p < 0.01; df = −0.087 (−8.6%); ES = 1.44). These results are supported by the data presented in Figure 8.

Significant differences were found for the main effects of the 30 m variable pre-post (F = 37.81; p < 0.01; ƞ2 = 0.67). In contrast, no significant differences were found for the effect group (F = 2.14; p > 0.05). However, for the interaction group*pre-post, analysis of variance allowed us to reject the null hypothesis of no difference (F = 5.5; p < 0.05; ƞ2 = 0.23). There was a significant difference in the 30m variable after and before the intervention in the experimental group (p < 0.01; df = −0.1720 (−4.15%); ES = 1.11) after applying Bonferroni post-hoc multiple comparison tests. These findings are further supported by the data illustrated in the accompanying Figure 9.

Analysis of the dependent variable RSI results showed significant differences for the group (F = 10.15; p < 0.01, ƞ2 = 0.36) and pre-post (F = 8.58; p < 0.01; ƞ2 = 0.32) factors. There were no significant differences in the group*post-pre interaction (F = 3.33; p < 0.01). There was a significant difference in the RSI variable after and before the intervention in the experimental group (p < 0.05; df = 0.2070 (14.3%); ES = 0.87) after using Bonferroni’s post-hoc multiple comparison tests. These findings are further supported by the data illustrated in the accompanying Figure 10.

Significant differences were found for the main effects of the RF elasticity (D) variable pre-post (F = 11.76; p < 0.01; ƞ2 = 0.39). In contrast, no significant differences were found for the effect group (F = 2.43; p > 0.05). and for the group*pre-post (F = 0.35; p > 0.05;). However, there were no significant differences after applying post-hoc comparisons (p > 0.05). These results are supported by the data presented in Figure 11.

The second part of the analysis examined the correlation between the following assumptions: (1) Change in test (Δ(Post: Pre) [RSI/5 m/30 m]) vs. baseline level of viscoelastic muscle properties (Myoton pre) (Figure 12), (2) Change in test (Δ (Post: Pre) [RSI/5 m/30 m]) vs. susceptibility of tissue to training (Δ(Post: Pre) [myoton F/S/D]) (Figure 13). (3) Baseline results: RSI/5 m/30 m vs. tissue susceptibility to (Δ(Post: Pre) [myoton F/S/D]) (Figure 14). The results are presented as a correlogram with Spearman’s rank correlation coefficient r∈(−1;1). The degree of correlation between the evaluations according to the grading standards [24] is shown in

Table 5. Grading table of Spearman correlation coefficient.

Grading Standards	Correlation Degree
r = 0	No correlation
0 < |r| ≤ 0.19	Very week
0.20 ≤ |r| ≤ 0.39	Weak
0.40 ≤ |r| ≤ 0.59	Moderate
0.60 ≤ |r| ≤ 0.79	Strong
0.80 ≤ |r| ≤ 1.00	Very strong
r = 1.00	Monotonic correlation

r—correlation coefficient.

.

Correlation analysis revealed a statistically significant moderate positive correlation between VL D pre and Δ5 m variable (r = 0.65; p < 0.05). A statistically significant, strong negative correlation exists between RF ΔD and Δ5 m variables (r = −0.78; p = 0.01).

4. Discussion

The conducted study aimed to evaluate the impact of a programmed 10-week plyometric training on two physical aspects, speed and power, as well as its effect on and changes in the biomechanical properties of the quadriceps muscles (muscle tension, flexibility, muscle stiffness) in amateur football players. The study’s results indicate that the programmed additional plyometric training significantly improved sprint performance over both 5 m and 30 m in the experimental group (EG). Analysis of Bonferroni test results showed significant differences in performance before and after the intervention in the experimental group, confirming the hypothesis that plyometric training can contribute to improving starting speed and overall sprint speed in amateur football players.

Plyometrics, as one of these methods, is considered a potentially effective tool for improving the functional performance of soccer players [25,26]. In the works of other authors, improvements in sprint results over short distances (5 m) and longer distances (40 m) have been observed, which may suggest that plyometric training translates into acceleration as well as the maximum speed achieved by the athlete [27]. A low or moderate training volume is recommended for plyometric training based on the drop jump exercise. In the 20 m sprint results, Eduardo Sáez Sáez de Villarreal and others observed the most excellent effectiveness in groups following a 7-week program based on training twice a week. Additionally, compared to a group with twice the training volume, the measured results were weaker at the end of the study (approximately 120 jumps per week vs. 240 jumps per week) [28]. In the literature, one can also encounter research protocols similar to the one proposed in our study—encompassing both forms of vertical jumps (drop jump) and horizontal jumps (multiple jumps and hurdle jumps). Such a protocol was implemented in the study by Loturco et al. [29], where improvements in sprint performance along with reactive strength were observed among professional football players, who achieved statistically significantly better results regardless of whether they performed exercises with vertical or horizontal work specificity.

Each sprint phase is characterized by a different component of the direction of the interaction forces between the foot and the ground during the acceleration phase and the phase of reaching maximum speed [30,31]. Different placements of the center of mass and the direction of force vectors from the ground in various sprint phases can represent different forms of exercise. For the acceleration phase, horizontal jumps are recommended; beyond a distance of 10 m, where the athlete gains speed, becomes more upright, and the force vector starts to become more perpendicular to the ground—vertical exercises such as drop jumps [32,33,34]. Such a sequence of events may suggest that it is worthwhile to implement various forms of plyometric exercises with different force orientations to maximize the physical potential of football players and develop overall fitness. In the study by Ramirez-Campillo et al. [35], it was found that regardless of the type of additional intervention—plyometric training, strength training targeted at sprinting specifics, or a combination of plyometric and strength methods—similar improvements in sprinting results were observed among male and female football players.

In contrast, the work of Zghal et al. [36] negates the effectiveness of isolated plyometric training in improving sprint performance. Only the research group implementing a mixed protocol—lower limb resistance training combined with plyometric training based on hurdle jumps—showed positive adaptations in the form of improved 5-m sprint results; no statistically significant differences were observed at 10 and 20 m. Even though explaining the neurophysiological phenomena that influence sprinting performance through plyometric training still presents many challenges for authors, the scientific literature offers numerous explanations. Adaptation within the main biomechanical components: parallel elastic tissue structures (Parallel Elastic Component, PEC) and contractile structures (Contractile Components, CC). In the muscle stretching phase, potential kinetic energy is stored in elastic tissues, which is released to generate concentric force when the muscle returns to its standard length (Rebound Force Response). Such a phenomenon accounts for 70–75% of the concentric increase in force, making plyometrics so efficient [37,38]. In the context of the rebound phase during a sprint, force generation is one of the key variables for running speed.

Plyometric training significantly improved the experimental group’s reactive strength index (RSI). Significant differences in RSI values before and after training suggest that regular execution of plyometric exercises, such as drop jumps, improves muscle ability to quickly transition from the eccentric to the concentric phase, which is crucial for the dynamic movements characteristic of soccer. In many studies, it can be concluded that the most effective training protocols for building, among other things, reactive strength include 1–2 sessions of plyometric exercises per week [28]. The topic of plyometric training periodization was more thoroughly examined in a study by Guiyang Liu et al. [39], who concluded that a microcycle of plyometric training (<20 jumps/session) performed 4 times a week (using the so-called micro-dosing) with appropriate exercise selection is as effective a tool for building reactive strength as a protocol performed 2 times a week with twice the volume per training session (approximately 40 jumps/session).

In comparison with the meta-analysis by Peter A. van de Hoef et al. [40], it can be concluded that plyometric training can be an effective tool for improving RSI results among soccer players. A common denominator in many studies is the selection of exercises—countermovement jump (CMJ) is one of the most frequently chosen plyometric exercises in research protocols. In our training regimen, the role of CMJ was performed by hurdle jumps. The authors of the studies have repeatedly indicated that one of the main physiological factors behind the body’s adaptive mechanisms is the stimulation of the central nervous system, which leads to increased recruitment of motor units and their synchronization, which can prove to be one of the key aspects in optimizing force production [41].

The sensitivity to speed improvement over 5 m decreases with increasing RF flexibility. Therefore, our research shows that training-induced adaptations in the biomechanical properties of RF play a key role in short-distance sprints. Additionally, higher baseline elasticity values in the VL muscle may be associated with more significant improvement in 5 m sprint performance following programmed plyometric training. This indicates that the initial mechanical properties of the VL muscle may serve as a determining factor for the ability to adapt to short-distance sprints. Despite the use of advanced measurement methods (myometry), no statistically significant changes were observed in the biomechanical properties of the quadriceps muscles, such as muscle tone, elasticity, or stiffness, in both the experimental and control groups. The lack of significant changes may be due to the limited duration of the study or the need for a more extended period of muscular adaptation at the tissue level. The impact of plyometric training on biomechanical changes at the tissue level was studied by Zubac et al. [42], who noted, among other things, a decrease in VL muscle tension after an 8-week plyometric training program. In various articles and scientific studies, it can be found that plyometric training has a marginal impact on biomechanical changes, except for improvements in flexibility.

Presumably, changes in muscle stiffness can be regulated through changes in the viscoelastic properties of intramuscular and tendinous tissues, which may require a long adaptation period. The study by Moran et al. [43] concluded that considering the number of available training methods, plyometric training is not necessarily the best choice for improving muscle stiffness; a potentially better choice could be traditional resistance training or eccentric muscle training. The measurement results of the elasticity of large-volume muscles such as the vastus lateralis and rectus femoris may be unreliable when using the MyotonPRO tool. Davidson et al. [44] reached such a conclusion in their work, where they investigated the measurement reliability of the myotonometer for selected biomechanical parameters in skeletal muscles of smaller and larger volumes. The aim of such studies is, among other things, the objectification of measurements for assessing the effectiveness of specific training protocols.

Although this study may provide valuable insights into the effectiveness of plyometric training in improving the athletic performance of soccer players, there are limitations to the study and opportunities for future research. Firstly, the study focused exclusively on measurements in a heterogeneous group of football players. Therefore, the results cannot be interpreted for other sports groups. Moreover, although the effect on running speed and muscle power was emphasized, further research is needed to explain the exact mechanisms underlying their impact on football players.

Future research should also examine individual differences in athletes’ responses to plyometric training interventions, considering factors such as training experience or morphological and genetic predispositions. Moreover, comparative analyses using other forms of plyometric training with different populations of athletes and other volunteers can provide a more comprehensive understanding of the optimal approach. Gaps can be observed in the literature regarding studies on the impact of plyometrics on biomechanical changes in tissues measured in football players using the MyotonPRO device. Further research on the impact of plyometric training in football should focus on the relationships between the duration of training protocols and the recorded changes in muscle biomechanical parameters, as well as the selection of exercises, intensity, and volume. It would be worthwhile to study the response of other muscles in the context of biomechanical changes following plyometric training. The results of this study offer practical implications for optimizing training strategies that could potentially improve performance in sports. Athletes can potentially improve the performance of important motor parameters used during the game and enhance overall athletic readiness. The training program we proposed improves the motor skills of football players. However, it should not be directly implemented by coaches and players. We suggest possibly familiarizing yourself with the recommended guidelines in the context of programming such training to increase its effectiveness and success.

5. Conclusions

Our study indicated the effectiveness of implementing an additional 10-week plyometric training program for the football team in improving sprint performance and reactive strength. The program, which is performed twice a week with intervals of 36–48 h between training sessions, includes exercises such as hurdle jumps, triple jumps, and drop jumps. It is a practical training solution that enhances physical potential. No statistically significant parameter changes were observed among the experimental group regarding biomechanical measurements of muscles, tension, flexibility, and stiffness. Despite the absence of statistically significant group-level alterations in biomechanical properties, including muscle tension, stiffness, and elasticity, the findings underscore the promise of leveraging baseline muscle elasticity as a potential biomarker for predicting training adaptation. This underscores the significance of personalized training regimens in optimizing athletic performance.