Examining isotonic and isometric exercises for post-activation performance enhancement in kickboxers upper limb strength and power

Abstract

Maximizing muscular performance to improve upper limb strength and power can be advantageous in preparing kickboxers for training and competition. The objective of this study was to evaluate the influence of isometric and isotonic upper limb post-activation performance enhancement (PAPE) strategies on the maximal strength of young men kickboxers’ upper limbs, assessed through handgrip strength (HG) and the chest medicine ball throw test (CMBT). Fifty-three amateur men kickboxers, aged between 16 and 23, participated voluntarily in this regional-level study. Over two weeks, participants were randomly exposed to three conditions: isometric, isotonic, and control (no warm-up) conditions. Each condition was repeated twice during this timeframe, resulting in six data collection sessions. After each condition, participants underwent assessments for HG strength and CMBT performance. Repeated measures ANOVA revealed significant differences between conditions in HG (p < 0.001; ES = 0.740). Post-hoc analysis revealed that the isometric condition showed significantly better scores compared to both the isotonic condition (p < 0.001) and the control condition (p < 0.001). Significant differences were found in CMBT (p < 0.001; ES = 0.355). Post-hoc analysis revealed that the isometric (p < 0.001) and isotonic conditions (p < 0.001) displayed significantly superior outcomes in contrast to control. In conclusion, isometric exercise significantly improved maximal HG strength and muscular power in the CMBT, showing a potential effective strategy for implementing PAPE for kickboxers.

Introduction

Post-activation performance enhancement (PAPE) is a phenomenon observed in the acute augmentation of explosive neuromuscular capacity following specific conditioning activities performed at maximal or near maximal intensity, typically experienced within 3–10 min after warm-up¹. While the precise mechanisms underlying PAPE remain fully elucidated, they are thought to involve increased neural drive, activation of high threshold motor units, elevation of muscle temperature, and enhancement of muscle water content². Thus, PAPE can be particularly beneficial for kickboxing performance by temporarily boosting explosive power and speed, enabling quicker and more powerful strikes and movements during training or competition.

There has been a growing interest in understanding the implications of PAPE specifically for upper limb tasks³. While much of the existing literature has concentrated on lower limb activities like sprinting and jumping⁴, emerging evidence suggests that similar benefits can be observed in upper limb performance^5–7. For instance, studies have demonstrated that the bench press with loads of ≥ 80%1RM can be an effective conditioning activity to elicit improvements in subsequent power output in the BBT at 30–40% of one-repetition maximum (1-RM)^5,8,9. Moreover, the temporal dynamics of PAPE effects, with a peak typically observed between 8 and 12 min post-conditioning activity, underscore the importance of appropriate resting intervals between the warm-up protocol and subsequent upper limb tasks^7,8,10,11. This temporal specificity can be particularly interesting in activities requiring explosive upper limb movements, emphasizing the relevance of tailored warm-up strategies for optimizing performance in tasks such as throwing, or punching^12,13.

Designing an effective warm-up protocol necessitates consideration of various parameters beyond exercise intensity and resting time, including exercise volume, stimuli duration, recovery intervals, and notably, muscle contraction type (e.g., concentric, eccentric, or isometric)^14,15. Despite extensive investigation into dynamic isotonic contractions, there remains a dearth of evidence regarding the role of different muscle contraction types in eliciting PAPE^6,16. Notably, isometric contractions exhibit a lower metabolic demand than dynamic concentric contractions^17,18, suggesting that maximal isometric contractions during warm-up could improve subsequent explosive performance while mitigating fatigue accumulation¹⁹. However, the PAPE effects of isometric muscle contractions are subject to debate, with conflicting findings in the literature.

While some studies suggest superior PAPE effects with dynamic actions involving the stretch-shortening cycle compared to isometric contractions²⁰, the evidence is inconclusive since in another study the evidence is favorable to isometric in upper limbs⁶ In another study comparing isotonic versus isometric PAPE strategies, both approaches demonstrated similar effects and were not statistically significant in their impact on the countermovement jump of healthy individuals¹⁴. While studies comparing muscle contraction types concerning PAPE are not new, research focusing on the upper limbs is limited, especially in specialized populations such as martial arts athletes, particularly those practicing kickboxing.

Previous studies in other martial arts such as taekwondo or boxing have conducted experiments testing PAPE. However, most of these studies have focused on lower limb activities like kicking and jumping^21–23, with very few specifically comparing the effects of different muscle contraction types on the observed improvements. Understanding PAPE in the context of kickboxing, especially concerning the upper limbs, holds significant potential for enhancing athletic performance and optimizing warm-up protocols. Kickboxing is characterized by rapid and explosive movements of the arms and hands, highlighting the critical role of upper limb power and speed in executing techniques effectively²⁴. Investigating PAPE specifically in the upper limbs of kickboxing practitioners can provide valuable insights into optimizing warm-up strategies tailored to the demands of the sport.

Most research on performance enhancement in kickboxing through warm-up activities does not address PAPE. For example, a recent study showed that a regular warm-up improved punch and kick reaction times²⁵. In another study comparing different warm-up strategies for kickboxers, it was found that mean propulsive power improved significantly more with specific warm-ups than with low-intensity aerobic running²⁶.

By elucidating the effects of different conditioning activities on upper limb explosiveness, researchers and coaches can develop evidence-based training regimens aimed at maximizing performance gains while minimizing fatigue. Furthermore, understanding the mechanisms underlying PAPE in kickboxing may contribute to the development of targeted interventions to improve striking power, speed, and overall martial arts performance. Considering this, the aim of this study was to assess the impact of isometric and isotonic upper limb PAPE strategies on the maximal strength of young kickboxing men athletes’ upper limbs, as measured by handgrip (HG) and the chest medicine ball throw test (CMBT). It is hypothesized that the isotonic PAPE strategy will induce a greater increase in upper limb strength compared to the isometric PAPE strategy, as isotonic contractions are believed to facilitate more effective neural adaptations for explosive strength.

Methods

Study design

The study followed the STROBE guidelines for cross-sectional designs²⁷. A repeated-measures study design was employed, wherein each kickboxing athlete underwent three conditions: control (no warm-up), isometric PAPE, and isotonic PAPE. Athletes were randomly assigned to each condition to prevent any specific sequence bias. All potential sequences were generated to mitigate the influence of sequence on the observed effects. Furthermore, each condition was repeated twice, resulting in a two-week study duration with data collection conducted over three days each week, interspersed with 24-hour rest periods. Randomization was achieved using a simple method involving opaque envelopes distributed randomly, ensuring an equal chance for each kickboxer to be assigned to one of the six sequences. The study design is illustrated in Fig. 1.

Fig. 1.

Study design.

Setting

Recruitment took place over two weeks at the main kickboxing training centers and through social media channels. Initially, 61 volunteers expressed interest in participating in the study. However, after confirming their eligibility criteria, only 53 remained eligible. Eight volunteers were excluded due to upper limb injuries within the past month (n = 3) and a lack of availability to participate in all six assessment sessions (n = 5).

The inclusion criteria were as follows: (i) having more than 1 year of kickboxing experience, (ii) being men, (iii) aged 16 years or older, (iv) being healthy with no reported injuries in the month prior to evaluation, (v) abstaining from drug or illegal substance use, (vi) committing to participate in all six evaluation sessions; and (vii) having undergone familiarization with weightlifting, specifically the bench press, for at least three months of training.

The study was conducted during the mid-season, with evaluations spanning two weeks. Participants were required to participate in both evaluations and training sessions on the same day, with recovery days scheduled between these sessions. Therefore, on the day of exposure to either PAPE or control conditions, participants proceeded directly to their regular training session upon completing the post-intervention evaluation.

All assessments and exposure to PAPE conditions took place in the same facilities, maintaining a controlled temperature of 22 °C and relative humidity of 55%. Evaluation sessions occurred consistently at the same time of day for each participant, ranging from 5 to 7 pm.

Participants

Considering the findings of a previous study where a minimal effect size of 0.1 was observed when comparing the effects of isometric versus isotonic PAPE on muscle power¹⁴, an a priori effect size (f) of 0.35 was determined for the analysis of variance (ANOVA) of repeated measures within-factor, considering one group and three measurements. A power of 0.95 and a significance level of 0.05 were chosen, suggesting a total sample size of 23 participants. The a priori sample size calculation was conducted using G*Power software (version 3.1.9.6, Heinrich-Heine-Universitat Dusseldorf, Dusseldorf, Germany).

Fifty-three amateur men kickboxers at the regional level, aged between 16 and 23 years old (mean age: 19.0 ± 1.8 years; mean height: 170.4 ± 7.7 cm; mean body mass: 63.5 ± 9.7 kg; mean experience: 6.2 ± 2.1 years), participated voluntarily in this study. These participants followed training routines of 3 to 5 sessions per week, with each session lasting 100 to 120 min, and regularly competed at the regional level, including participation in regional tournaments.

Prior to participation, they received detailed information about the study design, protocol, associated risks, and potential benefits. Upon agreeing to participate, either they or their legal guardians (for those under 18 years old) signed an informed consent, which clarified their freedom to withdraw from the study at any time without incurring penalties or consequences.

The study adhered to the ethical standards outlined in the Declaration of Helsinki for research involving human subjects. Additionally, the study protocol received approval from the Ethics Committee of the Chengdu Institute of Physical Education, with the reference code 10/2024.

Procedures

A familiarization session was conducted the week prior to the start of the experiments. This session served to familiarize participants with the proper techniques for PAPE and assessment procedures (both HG and CMBT). Additionally, during this session, participants underwent a 1RM direct test for bench press. The 1RM was defined as the maximum load lifted by participants without any compensatory movements, only if they completed the pause on the chest correctly. Upon successful completion of an attempt, participants rested for 3 min before progressing to the next load until failure was reached. In the event of failure, the load was reduced by 2.5 kg, and another attempt was made following a 3-minute rest interval. The highest load successfully lifted was deemed the individual 1RM, later used to standardize isotonic intensity in PAPE.

Furthermore, to identify the sticking region for the isometric condition, movements during the 1RM test were recorded using a video camera (1080p Video Recording: 30 frames per second, version 1.0.8, Xiaomi 11i, China) positioned in the sagittal plane relative to the participants’ movements. Subsequently, the videos were uploaded to Kinovea software (version 0.8.15), which enabled determination of the sticking region, defined as the region of the lift between the first peak in velocity and the first minimum after the peak²⁸.

In alignment with prior methodological approaches²⁹, after identifying the sticking region, we proceeded to determine the height of the barbell at this juncture. Given that the sticking region encompasses a spectrum of motion rather than a precise point, we executed the protocol at the midpoint of this range to ensure that the isometric contraction influenced the sticking region effectively. Research indicates that isometric contractions can fortify muscles within a range of 20º–50º from the established joint angles³⁰.

After the first day of familiarization, the experiments proceeded in the sequence in which each athlete was assigned. For the days involving isometric PAPE, the protocol was as follows: five minutes of cycling on a stationary bike, followed by 5 min of specific shoulder mobility exercises. Following the general warm-up (uniform across all three conditions), the isometric PAPE was initiated by exerting maximal force on the Olympic bar against the J-hooks on the rack, performing 2 sets of 5-second presses. A rest period of 3 min was given between sets. The height of the bar was adjusted for each participant based on the values obtained from the sticking region analysis.

In the isotonic PAPE condition, the same general warm-up protocol was followed. Immediately afterward, athletes completed 2 sets of 3 repetitions each, using 75%RM for the full range bench press, executed with maximal intent and speed. A rest period of three minutes was allowed between sets. In the control condition, participants solely underwent the general warm-up (i.e., five minutes of cycling on a stationary bike, followed by 5 min of specific shoulder mobility exercises) before proceeding to the subsequent tests.

Physical assessments

The handgrip strength (HG) and chest medicine ball throw (CMBT) tests were conducted 8 min after the completion of each PAPE and control condition. This time frame was chosen based on previous research indicating that the effects of PAPE on upper limbs typically last between 8 and 12 min^7,8,10,11. Prior to the two weeks of data collection, on the familiarization day, all athletes were introduced to both the HG and CMBT tests. Each athlete performed each test three times under the supervision of researchers, ensuring the correct technique and readiness for the subsequent data collection days. No data were collected during the familiarization day for HG and CMBT, as the primary focus was on introducing the proper technique for the tests.

The assessments used for analysis were collected on the days when PAPE or control conditions were applied. Therefore, 8 min after the condition ended, participants were evaluated for maximal strength in the HG test, followed by an assessment of CMBT after a 3-minute rest period.

Handgrip strength

The HG assessment was conducted utilizing sealed hydraulic dynamometers with a precision of 0.1 kg to gauge grip force (TKK 5101 Grip-D, Takey, Tokyo, Japan). The instrument’s validity and reliability have been previously reported³¹. Participants maintained their elbows in full extension during the test, aligning with findings from previous research indicating significantly higher handgrip strength with extended elbows compared to flexed elbows at 90°³¹. Throughout the assessment, participants stood with their arms straight down, shoulders slightly abducted (around 10°), elbows fully extended, forearms in a neutral position, and wrists extended. They were instructed to gaze forward, keep their feet shoulder-width apart, and refrain from contacting any body part with the dynamometer except for the hand being measured. To ensure unbiased measurements, the dynamometer displays faced the examiner, providing blind assessments to the participants. Each participant completed two trials with each hand (alternating between right and left), with a 1-minute break between trials for the same hand. They were directed to gradually and consistently squeeze for a minimum of 2 s and were motivated to exert their best effort during the assessments. The peak result (the maximum force recorded, measured in kg) from each session was gathered for subsequent data analysis.

Chest medicine ball throw test

The CMBT test was performed using a 3-kg medicine ball (22 cm diameter). Participants assumed a seated position on the ground, back in the wall, legs extended in front, and hands placed on the chest. They received instructions to exert maximum effort and speed in propelling the medicine ball straight ahead from the chest. The researcher, focused in analyzing the throwing movements, oversaw the correct execution of throwing techniques throughout the trials. Two attempts were made with each ball and throwing style, with a 1-minute rest period between each attempt. The maximum release velocity of ball throws was assessed using a radar gun (Pocket radar, Model PR1000-BC, Inc. Santa Rosa, California) previously validated and deemed reliable for measuring ball displacement speed³². The radar was positioned at chest height of the participant, diagonally aligned from the participant’s direction to exclusively capture the ball’s trajectory. Velocity data was recorded by the radar gun, and the best trial (the fastest; measured in m/s) was selected for each day for subsequent data processing.

Statistical procedures

The data are presented as mean values accompanied by their respective standard deviations. Variability within participants was quantified using the coefficient of variation, expressed as a percentage, calculated for the average across all participants. The coefficient of variation, expressed as a percentage, was calculated to illustrate the variability among kickboxers within each condition and across analyzed repetitions. Mean values derived from data aggregated over all days within each condition were employed for subsequent inferential analyses. Following confirmation of normality and homogeneity assumptions, as determined by Shapiro-Wilk (p > 0.05) and Levene’s tests (p > 0.05) respectively, parametric tests were conducted. Specifically, repeated measures ANOVA was employed, supplemented by Bonferroni post hoc analysis. Partial eta squared was utilized to gauge the effect size of the ANOVA. Moreover, Cohen’s d was used to determine the magnitude of differences in pairwise comparisons, with the following interpretation: 0.0–0.2, trivial effect size; 0.2–0.6, small effect size; 0.6–1.2, moderate effect size; and 1.2–2.0, large effect size. Statistical analyses were conducted using SPSS (version 28.0, IBM, USA), with significance set at p < 0.05. The graphical illustration was conducted on JASP software (version 0.18.3, University of Amsterdam, The Netherlands).

Results

Within kickboxers, variability analysis for HG revealed a coefficient of variation of 1.3 ± 1.1% in the isometric condition, 1.5 ± 1.2% in the isotonic condition, and 1.5 ± 1.5% in the control condition. For CMBT velocity, the isometric condition exhibited a coefficient of variation of 2.0 ± 1.3%, the isotonic condition 4.7 ± 8.1%, and the control condition 1.8 ± 0.6%.

Descriptive statistics indicate that kickboxers showed an average HG strength of 44.2 ± 6.8 kg in the isometric PAPE, 43.6 ± 6.5 kg in the isotonic PAPE, and 41.5 ± 6.7 kg in the control condition. Analysis of variance across the three conditions revealed significant differences (F_1.789,93.023=147.882; p < 0.001; =0.740). Post-hoc analysis revealed that the isometric condition showed significantly better scores compared to both the isotonic condition (mean difference: 0.623 kg; p<0.001; d=0.091, trivial effect size) and the control condition (mean difference: 2.679 kg; p<0.001; d=0.393, small effect size). Additionally, the control condition exhibited significantly lower values than the isotonic condition (mean difference: 2.057 kg; p<0.001; d=0.304, small effect size). Figure 2 shows the intra-individual variations in HG strength observed across the three conditions.

Fig. 2.

The intra-individual variation of kickboxers across three conditions regarding handgrip strength. *: between-group significant differences (p < 0.05).

Descriptive statistics reveal that kickboxers had an average CMBT velocity of 0.49 ± 0.09 m/s during the isometric PAPE, 0.50 ± 0.10 m/s in the isotonic PAPE, and 0.47 ± 0.09 m/s in the control condition. A variance analysis across these three conditions disclosed significant differences (F_1.391,72.341=28.565; p < 0.001; =0.355). Further examination through post-hoc analysis revealed that the isometric (mean difference: 0.02 m/s; p<0.001; d=0.242, small effect size) and isotonic conditions (mean difference: 0.03 m/s; p<0.001; d=0.346, small effect size) displayed significantly superior outcomes in contrast to control. However, no significant differences were found between isometric and isotonic conditions (mean difference: 0.01 m/s; p=0.177; d=0.106, trivial effect size). The Fig. 3 illustrates the intra-individual variations in CMBT velocity among the three conditions.

Fig. 3.

The intra-individual variation of kickboxers across three conditions regarding chest medicine ball throw test. *: between-group significant differences (p < 0.05).

Discussion

Our study purposed to to assess the impact of isometric and isotonic upper limb PAPE strategies on the maximal strength of young kickboxing men athletes’ upper limbs, as measured by HG and the CMBT. Results showed that isometric PAPE significantly surpassed the isotonic condition and control in enhancing maximal HG strength. Furthermore, both isometric and isotonic PAPE were significantly superior to the control condition in this aspect. Additionally, both PAPE strategies exhibited significant improvements over the control in enhancing muscle power in CMBT, with no significant differences observed between the PAPE strategies.

Our findings suggest that isometric exercise produced significantly superior results compared to both isotonic PAPE and control conditions in terms of its impact on maximal strength, as measured by HG strength. These results contrast with those of a similar study that examined isometric and isotonic exercises in the lower limbs, albeit focusing on muscle power assessed through a vertical jump test¹⁴. It is noteworthy that HG predominantly involves isometric contraction. Conversely, our results align with another study⁶ that investigated the effects of isometric, concentric, eccentric, and concentric-eccentric conditions on ballistic bench press throws in ten men competitive rugby players. Such study⁶ found that only isometric contractions were effective in increasing upper body power output following rest periods.

Isometric contractions may elicit a specificity of adaptation whereby the neural pathways activated during the isometric exercise are highly specific to the subsequent task³³, in this case, handgrip strength³⁴. Consequently, the neural recruitment patterns primed by isometric PAPE align more closely with the demands of the maximal handgrip task, leading to improved performance³⁵. Additionally, isometric contractions have been shown to potentiate force production through increased motor unit synchronization and improved intramuscular coordination, augmenting the efficiency of muscle activation during subsequent efforts³⁶. Conversely, isotonic PAPE may lack the specificity required to fully optimize handgrip strength, as the dynamic nature of the exercise may recruit different motor units and movement patterns not directly applicable to grip strength, thereby reducing the priming effect.

Considering the impact of both isometric and isotonic PAPE strategies in CMBT, it was observed that both markedly outperformed the control condition in enhancing performance. However, no significant differences were detected between the efficacy of the two PAP strategies. Our findings contradict a previous study that examined the effects of isometric PAP compared to isotonic conditions on lower limb power, which revealed significant beneficial effects of isometric training¹⁶. Additionally, our results contradict another study focusing on upper limb performance, where isometric contraction significantly outperformed isotonic conditions in enhancing upper body power output⁶. Nonetheless, our results align with a study¹⁴ that compared isometric to isotonic training, showing similar effects on countermovement jump. However, in our study, both strategies significantly outperformed the control condition, whereas in the reported study¹⁴, no differences were found compared to the control.

Isometric exercises may induce potentiation through mechanisms such as increased motor unit recruitment and synchronization³⁷. These contractions stimulate the Golgi tendon organs, leading to a decrease in inhibitory signals and enhancing the excitability of the alpha motor neurons³⁸. Consequently, this results in heightened neural drive to the muscles involved in the subsequent explosive movement³⁹, such as the pectoralis major in the chest throw. Additionally, isometric exercises have been shown to improve the rate of force development⁴⁰, enabling individuals to generate greater force in a shorter time frame, crucial for explosive movements like the chest throw.

On the other hand, isotonic exercises may facilitate the recruitment of fast-twitch muscle fibers and improve intramuscular coordination⁴¹, crucial for explosive movements. Moreover, isotonic exercises lead to an increased production of metabolites such as lactate and hydrogen ions, which can stimulate muscle fibers and improve their contractile properties⁴². Furthermore, isotonic exercises induce changes in the muscle-tendon unit stiffness, optimizing the transfer of force during explosive movements like the chest throw⁴³.

Our study offers valuable insights into the effects of isometric and isotonic PAPE on upper limb muscular performance. However, several limitations warrant consideration. Firstly, our comparison was limited to strength and power tests, without incorporating individual techniques, which could have provided a unique analysis of their impact on real competitive scenarios, particularly relevant for kickboxers. Additionally, our study utilized the same period of rest from the PAPE to the assessment, limiting our understanding of the lasting effects over time. Future research should address this by implementing multiple observation points to comprehensively assess temporal effects. Furthermore, future studies must control for factors such as muscle temperature, diet, and hydration, as these variables could significantly influence study outcomes. Incorporating these recommendations into study protocols will improve the generalizability of findings in this field. Finally, incorporating electromyography during the PAPE implementation and understanding the potential contractile mechanisms that may depend on the stimulus and influence performance could be particularly valuable in exploring the processes that explain the findings.

Despite the study’s limitations, our findings provide practical implications for coaches and practitioners aiming to optimize upper limb muscular performance, especially in kickboxing. The showed superiority of isometric PAPE over isotonic conditions suggests that integrating isometric exercises into warm-up routines could augment maximal strength and power output in competitive settings. Coaches should prioritize incorporating targeted isometric exercises that engage the muscles involved in key movements. Furthermore, attention to factors such as rest intervals and temporal effects is crucial. Ensuring sufficient recovery between conditioning and performance assessments can maximize the effectiveness of PAPE strategies.

Conclusions

This study highlights that isometric PAPE strategies are highly effective for boosting upper limb strength and power in kickboxers. Both isometric and isotonic methods outperformed the control, with isometric exercises proving superior for grip strength. Given their lower metabolic demand and reduced fatigue, two 5-second maximal chest presses emerge as a practical, efficient way to optimize upper limb performance in training and competition.

Author contributions

LW designed and led the project. LW and AH collected and treated the data. LW made the statistical analysis. LW and AH authors wrote, reviewed and approved the manuscript.

Data availability

Data availability: the data is available upon reasonable request to the corresponding author.

Declarations

Ethical code

The study adheres to the ethical standards outlined in the Declaration of Helsinki. Approval for this study was obtained from the Ethics Committee of the Chengdu Institute of Physical Education, with the reference code 10/2024.

Informed consent

The participants signed a written informed consent.

Competing interests

The authors declare no competing interests.