Plyometric and Resistance Training: A Dual Approach to Enhance Physical Fitness in 12–15-Year-Old Girls

1. Introduction

Plyometric and resistance training are powerful approaches for improving physical fitness, particularly in the development of muscle strength, power and overall athletic performance [1,2,3]. These training methods become more important during adolescence, a time characterized by rapid growth, hormonal changes and significant physiological adaptations [4,5]. For girls between the ages of 12 and 15, this phase represents a crucial opportunity to optimize their physical development through carefully structured training plans[2,6,7].

Plyometric training, which emphasizes explosive, high-intensity movements such as jumping, hopping and rapid changes of direction, is designed to improve the efficiency and responsiveness of the neuromuscular system [8,9,10]. This type of training improves the ability to generate power quickly, which is essential for activities that require speed, agility and strength [3,9]. By improving the stretch-shortening cycle of the muscles, plyometrics can lead to a significant increase in vertical jump height, sprint speed and overall athletic performance [11,12].

Resistance training, on the other hand, focuses on increasing muscular strength and endurance through the use of external loads such as weights or resistance bands [13,14]. This form of training not only builds muscle mass and strength, but also contributes to improved bone density, better body composition and a reduced risk of injuries [15,16,17]. For adolescent girls who are at a critical stage in their physical development, resistance training is particularly beneficial in laying the foundation for lifelong musculoskeletal health [13,18,19].

When combined, plyometric and resistance training are believed to create a synergistic effect in which the benefits of each modality are amplified, leading to greater overall improvements in physical fitness [18,20,21,22,23]. Plyometric exercises improve the rapid force production required for explosive movements [12,24,25,26,27], while resistance training provides the base strength that supports these dynamic actions [13,17,28,29]. Together, they provide a comprehensive approach to conditioning that can effectively address multiple aspects of physical fitness simultaneously [30,31,32].

Despite the clear benefits of these training methods, there is a need to better understand their specific effects on female adolescents. Girls between the ages of 12 and 15 undergo unique physiological changes, including hormonal fluctuations and shifts in muscle mass distribution [33,34,35], which may influence their response to various forms of physical training. These developmental factors suggest that adolescent girls may require tailored training programs that address their specific needs and growth patterns.

This study investigated the effects of an 8-week combined plyometric and resistance training program on the physical fitness of 12- to 15-year-old girls. The study focused on key fitness parameters such as muscular strength, power and sprint performance and provided valuable insights into how these training modalities can be optimized for young female athletes. The underlying hypothesis of this research was that the dual approach combining plyometric and resistance training would lead to significant improvements in all measured fitness parameters. The results of this study provide practical guidance for the design of effective, evidence-based fitness programs specifically tailored to the developmental needs of adolescent girls.

2. Materials and Methods

2.1. Study Design

This study was designed as a randomized controlled trial with a parallel group design. The study comprised an 8-week intervention period during which participants were exposed to different training protocols. The design ensured that all participants were randomly assigned to one of two groups, with each group completing a different training program. The study was designed to allow a controlled comparison of the results of each training approach, with pre- and post-intervention assessments conducted to measure the effects on various physical fitness parameters (i.e., a 20 m-sprint test and a squat jump test). The study design also accounted for potential confounding factors by keeping the timing and conditions under which all assessments and interventions were conducted consistent.

2.2. Participants

The study involved 47 adolescent girls between the ages of 12 and 15 who were actively engaged in competitive sports, particularly basketball, tennis and volleyball. Participants were recruited in May 2024 through local sports clubs and school athletic programs. To participate in the study, the girls had to be healthy and not have any chronic diseases or orthopedic conditions that could interfere with participation in the training exercises.

The participants were randomly assigned to one of two training groups. The first group (RT, n = 23) participated exclusively in a resistance training program, while the second group (CT, n = 24) completed a combined training program that included both plyometric and resistance exercises. In order to ensure a balanced group size, a block randomization was performed. Prior to the study, a power analysis was performed to determine the required sample size. This ensured that the study had sufficient power to detect significant differences between the groups, with additional participants included to account for possible dropouts.

Before the study began, all participants and their parents or guardians were informed in detail about the purpose of the study, the procedures and any associated risks. Written informed consent was obtained from both the participants and their legal guardians. The study protocol was approved by the responsible institutional committee for human experimentation and complied with the ethical guidelines of the Declaration of Helsinki. The study was conducted from May to June 2024.

2.3. Study Procedures

The study was conducted over a period of 8 weeks, with all sessions taking place in a school sports facility. Both the baseline and final tests were conducted under standardized conditions, with all measurements taken at the same time of day to compensate for possible diurnal variations. Prior to testing, participants were instructed to maintain their normal dietary habits but to refrain from physical activity for 24 hours prior to each measurement.

At the beginning of the study, all participants attended an orientation session in which the correct techniques for each fitness test were explained and demonstrated in detail. During this session, participants had the opportunity to practice the tests to ensure that they were familiar with the procedures before data collection began. Each testing session began with a standardized warm-up program that included light jogging and dynamic stretching exercises to prepare participants for maximal effort during the tests.

The fitness tests were spread over two days. On the first day, the participants completed tests to measure their sprint and vertical jump performance. On the following day, muscular strength and endurance were measured. To ensure that the results reflected the actual abilities of the participants, no verbal stimuli were given during the tests, allowing each participant to perform to their natural ability. This was to ensure uniform conditions for all participants and to enable an accurate assessment of their performance.

To ensure the accuracy of the results, the same researchers conducted all tests and monitored the training sessions. The pre-intervention tests took place the week before the training began, and the post-intervention tests were conducted the week after the 8-week program was completed. This allowed a clear comparison of the effects of the training protocols on the physical fitness of the participants.

3. Measures

3.1. 20-m Sprint Test

The 20-m sprint test was used to assess the participants' sprinting speed. Before the test, the participants completed a self-directed warm-up program consisting of 3 minutes of light jogging in preparation for the sprint. After the warm-up, they performed two practice sprints over the 20-meter distance at submaximal effort to familiarize themselves with the procedure. Each participant then completed three timed sprints at full speed, with a 2-minute walk break between each sprint to allow for adequate recovery.

The time for each sprint was measured using a hand-held stopwatch, with the fastest time recorded to the nearest 0.01 seconds to measure performance. The test was conducted on an indoor track to ensure consistent test conditions for all participants. The reliability of this test was confirmed by a test–retest analysis, which showed a high level of consistency with an intraclass correlation coefficient (ICC) of 0.88.

3.2. Squat Jump Test

The Squat Jump (SJ) test was used to measure the explosive power of the participants' lower limbs. Starting from a stationary position, with knees bent at a 90-degree angle and hands placed firmly on the hips, participants were instructed to perform a vertical jump with maximal effort, ensuring that no preparatory movements such as countermovements were involved.

In order to collect accurate data, each participant was fitted with a high-precision inertial measurement unit (IMU) attached to their hips. This device, which is capable of measuring acceleration and gyro data at a rate of 500 Hz, recorded the vertical displacement during each jump. The IMU transmitted the data wirelessly to a special software platform on a tablet, where jump height, peak velocity and generated force were calculated in real time.

Each participant completed three jumps, with a 2-minute break between attempts to ensure a consistent performance. The system recorded various measurements, including jump height (measured to the nearest 0.1 cm), peak power (in watts) and take-off speed (in m/s). The highest jump from the three trials was selected as the representative result for each participant.

The technology provided highly reliable measurements, which was confirmed by a test–retest reliability analysis that yielded an intraclass correlation coefficient (ICC) of 0.94, ensuring the accuracy and consistency of the data across all trials.

4. Training Protocol

The training program took place over 8 weeks, with participants attending sessions twice a week on non-consecutive days, e.g. Monday and Wednesday. Each session was carefully structured and lasted approximately 90 minutes to ensure a balanced workload and sufficient breaks between exercises.

All participants began each session with a standardized 10-minute warm-up program that included light jogging followed by dynamic stretching exercises to prepare the muscles for the activities ahead. The warm-up program was designed to increase the heart rate and increase blood flow to the working muscles to reduce the risk of injury.

After the warm-up, participants in the combined training (CT) group completed a 20-minute plyometric training segment. This segment included a series of progressively challenging exercises, such as jump squats, box jumps and lateral jumps, designed to improve explosive power and coordination. Each exercise was performed with maximal effort, focusing on proper technique to ensure safety and effectiveness.

Participants in the resistance-only training group (RT) completed a static stretching program targeting the major muscle groups during this time. These stretches were performed in a controlled manner, with each stretch held for 30 seconds to improve flexibility without compromising performance.

After the respective initial activities, both groups proceeded with the resistance training portion of the session. In this part of the protocol, three sets of 10 to 12 repetitions of the main strength exercises were performed, including barbell squats, lunges and calf raises. The load of the resistance training was gradually increased by approximately 5% each week to continuously promote the participants' strength development. Each session ended with a 5-minute cool-down period that included light stretching and relaxation techniques to aid recovery.

Throughout the program, participants were closely monitored by certified trainers who ensured that the exercises were performed correctly and safely. The trainers provided guidance on technique and adjusted the difficulty of the exercises to the participants' progress as required.

4.1. Static Stretching

The participants in the resistance-only training (RT) group integrated a static stretching program into their sessions during the 8-week program. The stretching exercises targeted the major muscle groups and were designed to improve flexibility and prepare the body for subsequent resistance training. Each stretch was performed in a controlled manner, with participants holding the position for 30 seconds before relaxing for 5 seconds and then repeating the stretch.

The stretching program included a range of exercises, e.g. hamstring, quadriceps, calf, hip flexor and shoulder stretches. These exercises were selected to ensure that all major muscle groups involved in the subsequent resistance training were adequately prepared. Participants performed the stretches on both sides of the body where appropriate to ensure balanced flexibility.

4.2. Plyometric Training

Plyometric training was a central component of the combined group's program, which aimed to optimize lower limb explosive power and neuromuscular responsiveness. The training protocol was carefully structured to increase intensity and complexity, with exercises selected to improve both vertical and horizontal force output, which is critical for athletic performance in competitive sports.

The protocol began with exercises focused on developing basic plyometric skills, such as squat jumps, and systematically progressed to more advanced exercises such as depth jumps and lateral jumps. Each exercise was performed in 3 sets of 8 to 10 repetitions, with rest intervals kept at 60 seconds to balance fatigue management and training intensity. The training load was increased fortnightly, not only by changing exercise parameters such as height or distance, but also by incorporating additional performance metrics such as rate of force development (RFD) and peak power output (PPO).

In the initial phase, participants worked on improving their concentric power development with exercises aimed at maximizing vertical jump height and horizontal displacement. As the program progressed, more sophisticated measures were introduced, including ground reaction force (GRF) monitoring using force plates, ground contact time (GCT) assessment with a target value of under 200 milliseconds, and reactive strength index (RSI) measurement to assess the efficiency of the application of the stretch-shortening cycle.

In the final stages, participants performed depth jumps from a height of up to 60 cm, focusing on minimizing GCT and maximizing RSI, with peak power exceeding 4000 watts during explosive movements. Trainers with expertise in sports biomechanics provided real-time feedback on kinetic and kinematic variables to ensure participants maintained optimal performance while reducing the risk of injury.

Each participant’s progress was closely tracked, with data collected on variables such as jump velocity (measured in meters per second), peak vertical force (measured in Newtons) and RSI. Based on these metrics, adjustments were made to training load and exercise complexity, allowing for highly individualized progression based on each athlete’s rate of adaptation.

Table 1. Plyometric Training Program for the Combined Training Group.

Plyometrics (20 min, 2 days week−1)
Weeks	Exercise	Sets × Repetitions	Distance/Height/Intensity	Rest Between Sets	Details/Focus
1-2	Squat Jumps	3 × 8-10 reps	Bodyweight	60 s	Emphasize maximal height and safe landing technique
1-2	Lateral Bounds	3 × 8-10 reps (each side)	1 meter	60 s	Focus on balance and lateral movement, controlled landings
3-4	Box Jumps	4 × 8 reps	20-25 cm box	60 s	Gradually increase box height, maintain proper form
3-4	Depth Jumps	3 × 6 reps	20-25 cm drop height	60 s	Minimize ground contact time, ensure quick rebound
5-6	Split Squat Jumps	4 × 8-10 reps (total)	Bodyweight	90 s	Maintain control, focus on explosive lift and landing
5-6	Lateral Bounds	4 × 10 reps (each side)	1.5 meters	90 s	Increased distance, focus on power and stability
7-8	Depth Jumps	4 × 6 reps	30 cm drop height	90 s	Emphasize quick transition and explosive jump
7-8	Lateral Bounds with Hold	4 × 8-10 reps (each side)	1.5 meters	90 s	Hold landing for 2 seconds to enhance control and balance

Rest Between Sets: Rest periods were set at 60 seconds for weeks 1-4, extended to 90 seconds for weeks 5-8 to support increased training intensity and volume. Progression Strategy: Exercises were designed to progressively increase in complexity, intensity, and volume over the 8-week period, ensuring safe and effective development of explosive strength and agility.

Table 1. Plyometric Training Program for the Combined Training Group.

Plyometrics (20 min, 2 days week−1)
Weeks	Exercise	Sets × Repetitions	Distance/Height/Intensity	Rest Between Sets	Details/Focus
1-2	Squat Jumps	3 × 8-10 reps	Bodyweight	60 s	Emphasize maximal height and safe landing technique
1-2	Lateral Bounds	3 × 8-10 reps (each side)	1 meter	60 s	Focus on balance and lateral movement, controlled landings
3-4	Box Jumps	4 × 8 reps	20-25 cm box	60 s	Gradually increase box height, maintain proper form
3-4	Depth Jumps	3 × 6 reps	20-25 cm drop height	60 s	Minimize ground contact time, ensure quick rebound
5-6	Split Squat Jumps	4 × 8-10 reps (total)	Bodyweight	90 s	Maintain control, focus on explosive lift and landing
5-6	Lateral Bounds	4 × 10 reps (each side)	1.5 meters	90 s	Increased distance, focus on power and stability
7-8	Depth Jumps	4 × 6 reps	30 cm drop height	90 s	Emphasize quick transition and explosive jump
7-8	Lateral Bounds with Hold	4 × 8-10 reps (each side)	1.5 meters	90 s	Hold landing for 2 seconds to enhance control and balance

Rest Between Sets: Rest periods were set at 60 seconds for weeks 1-4, extended to 90 seconds for weeks 5-8 to support increased training intensity and volume. Progression Strategy: Exercises were designed to progressively increase in complexity, intensity, and volume over the 8-week period, ensuring safe and effective development of explosive strength and agility.

4.3. Resistance Training

The resistance training component of the program was carefully designed to improve muscle strength, hypertrophy and endurance in the major muscle groups.

Participants performed a series of compound and isolation exercises, including barbell squats, deadlifts, lunges, bench presses and rowing. Each session consisted of 3 to 4 sets of 8 to 12 repetitions per exercise, targeting different muscle groups to ensure a comprehensive approach to strength development. The training intensity was initially set at approximately 70% of the participants' maximum repetitions (1RM), with the load being gradually increased by 2.5% to 5% every two weeks depending on individual progress.

Advanced periodization principles were used to structure the progression, with a focus on strength endurance in the initial phase, which then progressed to hypertrophy and culminated in the peak strength phases. In the final weeks, the training load was adjusted to reach up to 85% of the 1RM, with a focus on developing maximal strength and power.

Each exercise was performed with strict attention to form and technique, with rest periods of 60 to 90 seconds between sets to optimize recovery while maintaining high training intensity. In addition, kinetic and kinematic data was monitored using wearable sensors to capture variables such as bar velocity (measured in meters per second) and force production (measured in Newtons).

In the later stages of the program, exercises were further intensified using advanced resistance training techniques such as tempo manipulation (e.g., slow eccentrics), cluster sets, and resistance adaptation (with bands or chains) to further challenge participants' neuromuscular systems.

Participants' progress was systematically recorded, with strength gains regularly assessed, including through 1RM testing and analysis of peak power on key lifts. This data-driven approach ensured that resistance training was tailored to participants' individual needs and abilities, promoting optimal strength development throughout the program.

5. Statistical Analysis

Data analysis began with the calculation of descriptive statistics, including means and standard deviations, to summarize the main findings for each group. The normality of the data was checked with the Shapiro–Wilk test and the homogeneity of variances between groups was checked with the Levene test.

A multivariate analysis of variance (MANOVA) was performed to examine any baseline differences between the groups. The effect of the training interventions over time was then analyzed using a two-way ANOVA with repeated measures, considering the factors group (combined training vs. resistance training) and time (pre- and post- intervention). In cases where significant interactions between time and group were found, paired t-tests were then performed to identify specific changes within each group.

The consistency and reliability of key performance measures, such as 20-m sprint time and squat jump metrics (power, velocity, force and jump height), were assessed using intraclass correlation coefficients (ICC). The ICC values indicated the reliability of the measurements, which were categorized as good (0.8 to 0.9) or excellent (>0.9).

All statistical procedures were performed using [specific statistical software, e.g. SAS Jmp Statistics, version 14.1, Cary, NC, USA], with the significance level set at p < 0.05. To further illustrate the effects of the interventions, the percentage changes from pre- to post- training were calculated and reported.

6. Results

All participants successfully completed the study, and no significant differences were found between the CT and RT groups at baseline (Table 2).

A significant main effect of time was observed for the 20-m sprint, with the CT group reducing their time from 3.85 ± 0.26 s to 3.60 ± 0.25 s and the RT group from 3.88 ± 0.28 s to 3.72 ± 0.28 s, F_1,45 = 12.4, p < 0.001, although the between-group difference in sprint time reduction was not statistically significant.

In the squat jump test, the CT group showed a significant increase in jump height by 5.6 %, along with improvements in power output, velocity, and force, F_1,45 = 15.1, 13.2, 11.7, and 10.5, all p < 0.001. Significant Time × Group interactions were observed for jump height and power output, indicating a greater impact of the CT program, F_1,45 = 9.4 and 10.2, p < 0.01.

Vertical jump performance showed greater improvements in the CT group, with increases in both jump height (6.7 %) and power (6.8 %), F_1,45 = 20.2 and 17.5, p < 0.001. The RT group showed smaller increases, with no significant Time × Group interactions observed, indicating that the differences between groups were not statistically significant.

Handgrip strength increased significantly in both groups, with the CT group showing a more significant improvement (7.5 %) than the RT group (4.6 %) (F_1,45 = 11.3 and 4.9, p < 0.05), although the between-group difference was not significant. Flexibility, as measured by the sit and reach test, also improved in both groups, but no significant differences were found between them. These results are summarized in Table 3.

7. Discussion

The aim of the present study was to investigate the effects of an 8-week combined plyometrics and resistance training program on various parameters of physical fitness in adolescent girls aged 12 to 15 years. The primary hypothesis was that the dual approach would lead to significant improvements in all measured fitness parameters, including sprint performance, jump performance, handgrip strength, and flexibility. The results largely supported this hypothesis, with notable improvements observed in several key areas, although some results were less pronounced or did not reach statistical significance.

Sprint performance, measured by the 20-m sprint, showed a significant improvement in both training groups. The CT group showed a slightly greater reduction in sprint time compared to the RT group, which is consistent with previous research [36,37,38] suggesting that plyometric training improves sprint performance by increasing neuromuscular efficiency and force production required for explosive movements. However, the lack of a significant difference between the groups suggests that resistance training alone may also contribute to improving sprint performance, possibly by increasing muscle strength and coordination. This small difference could be due to the relatively short duration of the intervention, which may not have been sufficient to fully utilise the neuromuscular adaptations promoted by plyometric training. In addition, the age and developmental stage of the participants, who were still growing and undergoing hormonal changes, may have influenced the extent of the adaptations observed. Previous studies [1,22,39,40] have shown that plyometric training can significantly improve sprint speed in adolescents, but these effects are often more pronounced in older athletes or those with a more advanced training history, suggesting that developmental stage plays a crucial role in the response to such training interventions.

In terms of squat jump performance, the CT group showed significant improvements in jump height, power, velocity and force compared to the RT group. This result was expected as plyometric training specifically targets lower limb explosive strength by improving the stretch-shortening cycle, which is critical for jumping performance. The significant interactions between time and group observed for jump height and power output further support the hypothesis that the combined training program had a stronger effect on these parameters. These results are consistent with those of previous studies [41,42,43,44] who reported similar improvements in jump performance following plyometric training interventions. However, the relatively modest improvements in the RT group suggest that while resistance training alone may help to improve lower limb strength, it is less effective at improving explosive power without the inclusion of high intensity dynamic movements. The lack of significant improvements in some parameters in the RT group may be due to the nature of resistance training, which is effective in increasing muscle strength but does not target speed-strength characteristics to the same extent as plyometric training.

Vertical jump performance also improved significantly more in the CT group than in the RT group, particularly with regard to jump height and power output. This is consistent with the results of previous studies [11,45,46] that have emphasized the effectiveness of plyometric training in improving vertical jump performance through improved neuromuscular coordination and power development. For example, a study [47] found that plyometric training significantly increased vertical jump height in young athletes, highlighting the role of neuromuscular adaptations. The lack of significant interactions between time and group for some measures suggests that although the CT program was more effective overall, the RT group also benefited from the intervention. This could be due to the overall improvements in muscle strength and conditioning achieved by resistance training, which, although not as targeted as plyometrics, still contributes to overall athletic performance. However, the extent of these improvements may be limited by the specific nature of the resistance exercises, which generally do not utilize the stretch-shortening cycle to the same extent as plyometrics, resulting in less pronounced increases in explosive power and jumping performance.

The hand grip strength measured with a hand grip dynamometer, which measures the maximum isometric strength of the hand and forearm muscles, increased significantly in both groups, with the CT group showing a slightly greater improvement. This was somewhat expected, as resistance training is known to increase muscle strength, including the muscles involved in grip strength. The greater improvement in the CT group could be due to the additional neuromuscular load from the plyometric training, which may have contributed to the overall increase in strength. However, the lack of a significant difference between the groups suggests that the benefit of combined training on grip strength may be relatively modest compared to resistance training alone. This finding is consistent with previous research [8] who showed that while plyometric training can improve overall muscle strength, its effects on specific measures of strength such as grip strength may not be as pronounced as with targeted resistance training programs.

Flexibility measured by the sit and reach test improved in both groups, although no significant differences were found between them. This result is consistent with other studies [4,8] suggesting that, although these benefits tend to be less pronounced compared to those observed with specific flexibility or stretching programs. The moderate improvements observed in both groups may be due to an overall increase in muscle strength and joint stability, which may indirectly improve range of motion. However, the lack of a targeted flexibility training component likely limited the extent of the improvements. Research [48,49,50] has shown that while strength training can improve flexibility to some degree, stretching exercises are generally more effective in significantly improving flexibility, suggesting that a combination of training modalities may be necessary for optimal improvement.

Despite the promising results, the study has several limitations that should be taken into account. The relatively short duration of the intervention may have limited the extent of adaptations observed, particularly in the RT group where improvements in explosive power and flexibility were less pronounced. In addition, although the sample size was sufficient to detect significant changes, it may not have been large enough to fully explore the variability in response to training, particularly in a population of adolescent girls who are still undergoing significant physical and hormonal change. The study also lacked a control group, which would have provided a clearer baseline from which to measure the effects of the interventions. Future studies should consider longer intervention periods and larger sample sizes to more fully evaluate the effects of combined plyometric and resistance training in this population. In addition, including a control group and examining the effects of different training volumes and intensities could provide further insight into the optimal training strategies to improve physical fitness in adolescent girls.

8. Conclusions

This study demonstrates that an 8-week combined plyometric and resistance training program significantly improves key parameters of physical fitness in adolescent girls, including improvements in sprint performance, squat jump (height, power, velocity, and force), vertical jump (height and power), and handgrip strength. Although flexibility increased moderately, further improvements were likely limited by the lack of a targeted flexibility training plan. Overall, the combined training approach effectively supports physical development in this age group and provides valuable insights for optimizing training strategies for adolescent athletes.