Receipt date: 01/20/2022 / Revision date: 03/18/2022 / Acceptance date: 04/28/2022

Introduction

Flexibility, along with Strength, Endurance, and Speed, is one of the biomotor capacities that condition sports performance (Hohmann et al., 2005). However, throughout history, its development has been disadvantaged because of studies that generated a poor perception about the effects of its implementation (Chaabene et al., 2019). In this context, it becomes important to reconsider the benefits of the use of stretching work in the preparation of athletes.

In Combat Sports, the development of Flexibility exercises, besides being proposed due to a custom of historical nature (Chaabene et al., 2019), would be considered as a conditioner of performance in most of its modalities (Basar et al., 2014; El-Ashker, 2018; Franchini & Herrera-Valenzuela, 2021; Lima, 2017; Lenka & Shah, 2019; Sánchez-Sánchez et al., 2014; Saraiva et al., 2014; Schwartz et al., 2015; Slimani et al., 2016; Wongputthichai & Ketchatturat, 2017). 

Several authors have postulated about the benefits of having adequate levels of this capacity, from its positive impact on the strength-velocity productions during muscle contractions (Del Rio Valdivia et al., 2015; Hunter & Marshall, 2001; Kokkonen et al., 2007 and 2010), which is why it was determined to know the levels of this capacity in a group of Boxing and Muay Thai athletes.

After evaluating the Flexibility from the Flexitest method (Araujo, 2005 and 2008) in a sample composed of 10 competitors of these disciplines, and after the analysis of the results, the lowest levels of range of motion (ROM) in the ankle, wrist and shoulder areas were evidenced. 

According to the problems encountered, the implementation of a program for the development of flexibility with the sample of athletes mentioned was determined and the results obtained were analyzed in order to evaluate its effect on ROM and speed of execution of straight fist strikes. 

Flexibility and sports performance

In the field of sport, improvements in Flexibility could be related to increases in the ability to apply muscular force and achieve more powerful actions (Kokkonen et al., 2007 and 2010; Shrier, 2004).

Sporting gestures, and particularly those of a ballistic nature, involve muscular chaining actions in which a series of shortening and lengthening reactions of various muscles are combined to varying degrees, which occur in different planes of movement at the same time following a spiral-diagonal activation pattern (McAtee & Charland, 2010). Thus, when the athlete moves by activating his antagonist musculature to accumulate energy to be used later in the agonist action, he can thus achieve a greater ROM and travel for his acceleration, which may allow him to apply a greater amount of Force (Weineck, 2005). 

Taking the above into consideration, improvements in the Flexibility of athletes could offer benefits on the production of Strength-Speed, from its incidence on the stretch-shortening cycle (SSC).

The SSC, present in most sports actions, makes it possible to link an eccentric muscle stretching action with a concentric muscle shortening action performed at high intensity, so as to develop a large amount of force in a very short period of time from the use of the potential energy stored in the elastic components (Copoví Lanusse, 2015).

Improved Flexibility could have a positive impact on SSC due to a reduction in passive stiffness or resistance of structures to deformation, allowing optimal stretching with a greater reserve of potential kinetic energy by a reflex accumulation of a greater number of muscle fibers in the motor sequence (Gleim & McHugh, 1997; Kim, 2006; Medeiros & Lima, 2017; Rodriguez Casallas & Gracia Diaz, 2015; Weineck, 2005).

The benefits of improved flexibility on SSC performance would be in contrast to the role of active muscle stiffness in isolated isometric or concentric actions. This is so since in these cases a higher stiffness would have been shown to be positively related to isometric force production, rate of isometric, and concentric force development (See Figure 1), which would be explained by a more efficient contractile force transmission in the muscle-tendon units.

Figure 1. Force-time curve and muscle stiffness.

Note: representation of the oscillations generated by applying forces on two systems. A system with higher stiffness (dotted line) would allow a higher initial force transmission to occur and will oscillate at a higher frequency. Taken from Gleim, G. W. & McHugh, M. P. (1997). Flexibility and Its Effects on Sports Injury and Performance (p. 290). Sports Medicine.

Following the research work carried out by Rees et al. (2007), Flexibility training through the PNF (Proprioceptive Neuromuscular Facilitation) method, not only could generate improvements on ROM but also an increase in muscle-tendon stiffness with improvements on Strength development.

According to Medeiros & Lima (2017), another viscoelastic property that has an influence on the improvement of muscle performance is the reduction of hysteresis or loss of energy as heat. According to these researchers from a stretching regimen, this parameter can be positively influenced allowing a reduction in energy dissipation in the muscle-tendon unit.

Data acquired on punch kinematics from studies by Cheraghi et al. (2014) indicated that boxers partially flex their upper extremity joints, especially the elbow joint, at the onset of the gesture. Piorkowski et al. (2011) also found from the analysis of video recordings, that during the onset of a fist strike the performers performed knee flexion/extension counter movements. Apparently, these athletes would intuitively be using the SSC to throw punches, allowing to think that the development of Flexibility could be related to improvements in the ability to throw fist punches in a faster and more powerful way.

Method

The methodology used for this research was quantitative since it was based on the collection and analysis of data composed of a score for twenty passive joint movements and the peak velocity in different combinations of straight fist strikes. 

Research Design

The research design was pre-experimental and action-research type, considering that it was a problem evidenced in a specific group of athletes and for whom an intervention proposal was carried out, which was included as part of their daily training. 

The development presented a longitudinal character since the data were collected in a pre-test and after the conditioning activity (of 6 weeks duration) in a post-test. 

Population and Sample

The study was carried out with a non-probabilistic sample composed of 6 boxing athletes (1 woman and 5 men) and 4 Muay Thai athletes (4 men), who carried out their training together at the Integral Fitness training center located in the Autonomous City of Buenos Aires, Argentina. The inclusion criteria consisted of having more than one year of training experience in these activities and having participated in some amateur or professional competition. In addition, they should not be recovering from any injury that could compromise the results of the tests.

Variables

The dependent variable was composed of the participants' levels of flexibility and the speed achieved in straight fist strikes, while the independent variable was the conditioning activity, that is, the training program for this physical capacity (see Figure 2).

Figure 2. Study variables.

Note: IV - independent variable, DV - dependent variable.

Measuring Instruments and Techniques

The Flexitest method (Araujo, 2005) was used to evaluate the flexibility levels of the participants. This tool allows the measurement of 20 joint movements performed passively and has been used in athletes from different disciplines (Farinatti et al., 2014; Marinho et al., 2011; Montealegre Suárez & Vidarte Claros, 2019; Roa López, 2009). The maximum ROM achieved by each joint was compared with a list of images showing different positions, allowing a score to be assigned for each one according to its amplitude. According to the score given, the following rating scale was established: 0 very poor, 1 poor, 2 average, 3 good, and 4 very good. In addition, the sum of the results of all the movements provided an indicator of the overall Flexibility level of the subject called flexindex. As there were no significant differences between the two sides of the body except for pathological conditions (Braganca de Viana et al., 2008), the measurement of the limbs was carried out only on the right side of the participants. Regarding the reliability of this test, high intra- and inter-observer levels were determined for Flexitest (Araujo, 2005 and 2008).

Hykso Punch Trackers accelerometers were used to determine the speed of execution of isolated straight punches and in combinations. These devices are placed on the wrists of the athletes and allow knowing the peak velocity developed during specific punching gestures (Omcirk et al., 2021). Its design is specifically prepared for this task, allowing data collection from remote synchronization with mobile devices such as smartphones or tablets. A good level of intra-tool reliability of these sensors was determined by comparing them with the data obtained from the analysis of gestures through the Kinovea software (López et al., 2020). 

Procedures

Initial evaluation

First of all, the flexibility levels of the athletes were evaluated through the Flexitest method (Araujo, 2005). For this, a session was organized specifically for this purpose in the morning hours and distanced with a minimum of thirty minutes from any physical exercise, considering that in the early hours of the day flexibility levels are higher (Rodríguez Casallas & Gracia Díaz, 2015), and that the increase in body temperature could modify muscular and joint resistance (Bishop, 2003).

Once all the individual scores were obtained, they were also grouped according to body zones; thus, forming seven averages: ankle, knee, hip, trunk, wrist, elbow, and shoulder.

After analyzing the results, it was concluded that the lowest levels of flexibility were in the ankle, shoulder, and wrist areas.

During the evaluation day, the peak velocities achieved in straight punches thrown in the air with both hands (jab and direct) and in combinations of these movements (jab-direct and jab-direct-jab-direct) were also recorded using Hykso Punch Trackers. The objective of this was to obtain a performance indicator of straight punches, and then compare them after the application of the program for the development of Flexibility.

Training periodization

Considering the results of the evaluations, a plan for flexibility training was developed, focusing only on the 3 areas where the lowest ROMs were observed: ankle, wrist, and shoulder.

This periodization was based on the model presented in the work of Lima et al. (2019), where the training load was dosed following an incremental staggering throughout the weeks. The work was programmed with a frequency of three weekly stimuli, located at a time of the day immediately after the strength training was developed (Leite et al., 2017); thus, to ensure greater adherence and achieve completion.

The macrocycle, with a duration of 6 weeks (Franchini & Herrera-Valenzuela, 2021), was divided into two mesocycles of three weeks each (See Tables 1 and 2). During the first mesocycle, exercises under the dynamic and static stretching methods were used. When moving to the second mesocycle, dynamic work was maintained, but with the objective of increasing the intensity of the work, static exercises were developed under the FNP modality in its two variants: agonist tension-relaxation and antagonist tension-relaxation (Franchini & Herrera-Valenzuela, 2021; Hohmann et al., 2005; Kim, 2006; McAtee & Charland, 2010; Peck et al., 2014; Weineck, 2005).

Table 1

 1st Mesocycle

No.	Exercise	Method	MICROCYCLE 1		MICROCYCLE 2		MICROCYCLE 3
			Reps/Seg	Series	Reps/Seg	Series	Reps/Seg	Series
1	Ankle rotations	Dynamic	12c/l	3	12c/l	4	15c/l	4
2	Dorsiflexion of ankles	Static	20"	3	30"	3	30"	4
3	Plantar flexion	Static	20"	3	30"	3	30"	4
4	Shoulder circles with elastic bands	Dynamic	12c/l	3	15c/l	4	15c/l	4
5	Internal rotation of shoulders	Static	20"	3	30"	3	30"	4
6	External shoulder rotation	Static	20"	3	30"	3	30"	4
7	Posterior shoulder abduction	Static	20"	3	30"	3	30"	4
8	Posterior shoulder adduction	Static	20"	3	30"	3	30"	4
9	Wrist rotations	Dynamic	12c/l	3	12c/l	4	15c/l	4
10	Wrist flexion	Static	20"	3	30"	3	30"	4
11	Wrist extension	Static	20"	3	30"	3	30"	4

Note: model of the first mesocycle of the periodization of the Flexibility training. 

Table 2 

2nd Mesocycle 

No.	Exercise	Method	MICROCYCLE 1		MICROCYCLE 2		MICROCYCLE 3
			Reps	Series	Reps	Series	Reps	Series
1	Ankle rotations	Dynamic	15c/l	4	15c/l	4	20c/l	4
2	Dorsiflexion of ankles Plantar flexion	FNP agonist	5 "x20"	1	5 "x20"	2	5 "x20"	3
		FNP antagon	5 "x20"	1	5 "x20"	2	5 "x20"	3
3	Shoulder circles with elastic bands Internal rotation of shoulders	FNP agonist	5 "x20"	1	5 "x20"	2	5 "x20"	3
		FNP antagon	5 "x20"	1	5 "x20"	2	5 "x20"	3
4	External shoulder rotation	Dynamic	15c/l	4	15c/l	4	20c/l	4
5	Posterior shoulder abduction Posterior shoulder adduction	FNP agonist	5 "x20"	1	5 "x20"	2	5 "x20"	3
		FNP antagon	5 "x20"	1	5 "x20"	2	5 "x20"	3
6	Wrist rotations Wrist flexion	FNP agonist	5 "x20"	1	5 "x20"	2	5 "x20"	3
		FNP antagon	5 "x20"	1	5 "x20"	2	5 "x20"	3
7	Wrist extension Ankle rotations	FNP agonist	5 "x20"	1	5 "x20"	2	5 "x20"	3
		FNP antagon	5 "x20"	1	5 "x20"	2	5 "x20"	3
8	Dorsiflexion of ankles Plantar flexion	FNP agonist	5 "x20"	1	5 "x20"	2	5 "x20"	3
		FNP antagon	5 "x20"	1	5 "x20"	2	5 "x20"	3
9	Shoulder circles with elastic bands	Dynamic	15c/l	4	15c/l	4	20c/l	4
10	Internal rotation of shoulders External shoulder rotation	FNP agonist	5 "x20"	1	5 "x20"	2	5 "x20"	3
		FNP antagon	5 "x20"	1	5 "x20"	2	5 "x20"	3
11	Posterior shoulder abduction	FNP agonist	5 "x20"	1	5 "x20"	2	5 "x20"	3
		FNP antagon	5 "x20"	4	5 "x20"	2	5 "x20"	3

Note: model of the second mesocycle of the periodization of the Flexibility training.

Final evaluation

For proper control of exercise intensity, athletes were asked to reach moderate pain levels during their execution (See Figure 3), equivalent to 5-6 points on a perceived exertion scale (Apostopoulos, 2015).

Figure 3. Perceived exertion scale.

Note: pain level that athletes should aim for (4 to 6) when performing the Flexibility exercises. 

Once the six weeks of periodization were completed, the athletes were summoned again following the same criteria established for the initial evaluation: morning schedule and prior to the development of their training sessions.

The twenty joint movements corresponding to the Flexitest protocol were evaluated, as well as the speed of execution of straight strokes.

Statistical Analysis

Categorical variables were reported as number of presentation and percentage. Continuous variables that assumed a normal distribution were reported as mean and standard deviation (SD). Otherwise, median and interquartile range (IQR) were used. Statistical tests (Shapiro-Wilk test) and graphical methods (histograms and quantile-quantile) were used to determine the sampling distribution of continuous variables.

Pre- and post-implementation changes in the flexibility program were explored using statistical inference tests. For this purpose, the Student’s t-test for paired samples or the Wilcoxon signed-rank test was used.

On the other hand, the strength of association between changes in Flexibility and changes in Velocity production in straight fist strikes was evaluated.  For this purpose, Pearson's r correlation coefficient or Spearman's rho correlation coefficient was used, as appropriate. The magnitude of correlation was considered very high (0.9 to 1.0), high (0.7 to 0.89), moderate (0.5 to 0.69), low (0.3 to 0.49), and null (< 0.3) (Mukaka 2012). A p value <0.05 was considered significant. IBM SPSS Macintosh software, version 24.0 (IBM Corp., Armonk, NY, USA) was used for data analysis.

Results

Flexibility

At baseline, the median total score on the flexindex was 46.5 (RIQ 40.75 - 52) points, with a minimum and maximum of 36 and 56 points, respectively (Figure 4). After the training program, the median flexindex score was 54 (RIQ 50.75 - 58.25) points.

Figure 4. Box plot for the variable flexindex pre- and post-training.

When determining the changes between the beginning and end of the program, a mean difference of 7.9 (SD 5.28) points was observed, with a minimum of 1 and a maximum of 15 points. These differences were statistically significant (p=0.001). Figure 5 shows the individual scores on the flexindex variable pre- and post-program.

Figure 5. Individual scores in the flexindex variable pre- and post-training.

Note: the sport of the participants is represented by solid (Boxing) and dotted (Muay Thai) lines.

Table 3 presents the pre- and post-program comparisons of the flexibility of the different movements evaluated.

Table 3

Comparison of pre- and post-training flexibility

Variables	Pre	Post	p-value
Ankle dorsiflexion	2 (1.75 - 2)	2 (2 - 3)	0.014
Ankle plantar flexion	2 (1 - 2)	3 (2 - 3)	0.008
Knee flexion	3 (3 - 4)	3 (3 - 4)	0.56
Knee extension	2 (1.75 - 2)	2 (2 - 2)	0.32
Hip flexion	3 (2 - 3)	2.5 (2 - 3)	0.56
Hip extension	3 (2 - 3)	3.5 (2 - 4)	0.014
Hip adduction	3 (3 - 3.25)	4 (3 - 4)	0.18
Hip abduction	3 (2.75 - 3)	3 (3 - 3)	0.32
Trunk flexion	3 (2 - 3)	3 (3 - 3)	0.046
Trunk extension	2.5 (2 - 3.25)	3 (3 - 3.25)	0.034
Lateral trunk flexion	3 (2 - 4)	3 (2.75 - 4)	0.48
Wrist flexion	2 (2 - 2)	2 (2 - 3)	0.083
Wrist extension	2 (2 - 2)	2 (2 - 2.25)	0.32
Elbow flexion	3 (2 - 4)	3 (3 - 3.25)	0.41
Elbow extension	2 (2 - 2)	2 (2 - 2)	0.99
Posterior adduction of the shoulder from  abduction to 180º	2.5 (1.75 - 3)	3 (2 - 4)	0.034
Posterior adduction or shoulder extension	1 (1 - 2)	2 (1 - 2.25)	0.059
Posterior shoulder extension	1 (0.75 - 2)	1 (1 - 2)	0.18
Lateral rotation of the shoulder with 90° abduction  and 90° elbow flexion	1.5 (1 - 2)	2.5 (1.75 - 3)	0.024
Medial shoulder rotation with 90° abduction  and 90° elbow flexion	2.5 (2 - 3.25)	4 (3 - 4)	0.014

 

Note: n=10, all numerical values are expressed as median and interquartile range (IQR).

Range of Motion

Table 4 presents the pre- and post-flexibility program comparisons for the ROM of the different joint groups evaluated. Statistically significant differences were only observed when comparing the values in the ankle and shoulder joints (p=0.006 and p=0.005, respectively). 

Table 4

Comparison of pre- and post-training range of motion.

Variables	Pre	Post	p-value
Ankle	1.75 (1.5 - 2)	2.5 (2.5 - 3)	0.006
Knee	2.5 (2 - 3)	2.5 (2.5 - 3)	0.41
Hip	2.88 (2.31 - 3)	3.13 (2.69 - 3.5)	0.12
Trunk	2.5 (2 - 3.33)	3 (3 - 3.33)	0.06
Wrist	2 (2 - 2.38)	2 (2 - 2.63)	0.32
Elbow	2.5 (2 - 3)	2.5 (2.5 - 2.63)	0.41
Shoulder	1.7 (1.2 - 2.5)	2.4 (1.95 - 2.85)	0.005

Note: n=10, all numerical values are expressed as median and interquartile range (IQR).

Figure 6 shows the medians of each joint group evaluated pre- and post-flexibility training program.

Figure 6. Range of motion pre- and post-training.

Note: Bar graph of the range of motion variable for the different joint groups evaluated pre- and post-training program.

Speed Production

Table 5 presents the comparisons in the Speed pre- and post-Flexibility program. No statistically significant differences were observed in any of the evaluated gestures. 

Table 5

Comparison of Speed pre- and post-Flexibility program.

Variables	Pre	Post	Average of the differences	p-value
Jab (m/s)
1	4.69 (0.7)	4.54 (0.6)	-0.15 (0.60)	0.45
1.2	4.13 (0.73)	4.32 (0.62)	0.19 (0.89)	0.52
1,2,3,4	5.43 (1.3)	5.88 (1.4)	0.46 (1.01)	0.18
Direct (m/s)
1	5.41 (0.68)	5.82 (0.85)	0.41 (0.84)	0.16
1.2	4.48 (0.62)	4.67 (0.82)	0.19 (0.80)	0.47
1,2,3,4	5.06 (1.66)	5.12 (1.32)	0.06 (1.08)	0.86

Note: all numerical values are expressed as mean and standard deviation (SD).

Correlation between changes in Flexibility and Speed

To correlate the change in the Flexibility variable (flexindex) and the changes in the Velocity production of the different straight stroke gestures (Jab and Direct, 1, 1,2 and 1,2,3,4), Spearman's rho correlation coefficient was used (See Table 6). The strength of the association was low when correlating Flexibility and Speed production of Jab 1, Direct 1, and Direct 1,2 gestures. The strength of the association was null when correlating Flexibility and Speed production in Jab 1,2, Jab 1,2,3,4, and Direct 1,2,3,4 gestures (Table 6).

Table 6

Correlation between changes in flexindex and Speed in Jab and Direct

	Spearman's rho	p-value
Jab (m/s)
1	-0.426	0.22
1.2	-0.257	0.47
1,2,3,4	-0.120	0.74
Direct (m/s)
1	-0.363	0.30
1.2	-0.309	0.38
1,2,3,4	-0.171	0.64

Note: all numerical values are expressed as mean and standard deviation (SD).

Discussion and conclusions

Discussion

Several researches have mentioned the benefits in the performance of different gestures after the implementation of programs for the improvement of Flexibility (Del Río Valdivia et al., 2015; Hunter & Marshall, 2001; Kokkonen et al., 2007 and 2010), but in none of the cases specific parameters of punching gestures in combat sports have been evaluated. For this reason, in the present study, we investigated the relationship between improvements in Flexibility and Speed productions of straight punching gestures.

Despite the fact that in the development of the intervention program Flexibility exercises were performed under the FNP method, in contrast to the results evidenced in the research work of Rees et al. (2007), this would not seem to have had a direct impact on the production of Strength of the evaluated gestures. Although it has been considered that the speed of execution of fist strikes influences the Force and Power developed (Mack et al., 2010; Tiwari et al., 2020), the use of a device that specifically measured the latter variables, such as the one employed by Dunn et al. (2019) in their research which had a load cell to measure the applied Force, would have been very useful in order to obtain a greater amount of data on the performance of the specific gestures evaluated.

The analysis of the results of the present research showed a considerable improvement in the flexindex levels. Since this is an indicator of the Flexibility of the subjects at a global level (Araujo, 2008), a positive impact on this capacity can be glimpsed. At the moment that it has not been possible to evidence an improvement in the productions of straight stroke speed, this prevents the possibility of proposing that there is any positive impact on the SSC from the increase in the levels of Flexibility as proposed by some authors (Gleim & McHugh, 1997; Kim, 2006; Medeiros & Lima, 2017; Rodríguez Casallas & Gracia Díaz, 2015; Weineck, 2005).

Taking into account that the 6-week Periodization program for flexibility training applied on Boxing and Muay Thai athletes, showed statistically significant improvements on the ROM of the shoulder and ankle joints of the participants, this could serve as a reference for use in similar populations of athletes with whom we seek to optimize this capacity. In addition, this Periodization is aligned with the proposals of researchers such as Lima et al. (2019), who mentioned the importance of systematization to obtain positive results.

It should be noted that although the periodization of the flexibility training only contemplated the work on the shoulder, wrist and ankle areas, the comparison of the passive ROM pre- and post-training also showed improvements on the trunk and hip areas, and this could be attributed to the effect of the interconnection of the muscles through the kinetic chains of movement (McAtee & Charland, 2010). This could promote a more efficient functioning of the different body segments during the performance of sporting gestures, so it would be appropriate to consider the analysis of its relationship with the incidence of injuries.

Although it has not been possible to establish a relationship between improvements in the levels of flexibility and speed in specific gestures, the findings of research that have positively related these two variables make it necessary to further investigate this possible association in hitting athletes. This requires abandoning the stereotypes unfounded by the traditionality generated after thousands of years of history in these sports (Balmaseda, 2009; Trial & Wu, 2013), and openness on the part of coaches to encourage this type of research work.

Conclusions

In light of the results of the present research, it has been possible to investigate the effect of the implementation of a flexibility program on joint ROM and speed of execution of straight punches in a sample of ten Boxing and Muay Thai athletes.

After the evaluation of the Flexibility from the Flexitest method in 20 passive joint movements, and having evidenced the lowest levels of this capacity in shoulders, wrists, and ankles, a 6-week periodization program was carried out for its improvement.

When determining the changes between the beginning and end of the program, a statistically significant difference was observed in the levels of global flexibility (flexindex) of the participants.

With respect to the joint groups in which changes in ROM were determined, statistically significant differences were only observed when comparing the values in the ankle and shoulder joints, demonstrating that the wrists did not follow the same trajectory.

Finally, no statistically significant differences were observed in Speed productions in any of the evaluated gestures. The strength of the association between the changes in the levels of Flexibility and the Speed productions of the striking gestures was low when correlating the isolated Jab (1) and Direct (2) strikes, and the Direct throwing 1,2. The strength of the association was null when correlating Flexibility and Speed production in the Jab throwing 1,2 and 1,2,3,4 gestures and the Direct hit by throwing 1,2,3,4. These results prevent the generation of any association between the improvements obtained in Flexibility and Speed production of straight fist strikes.

Because the sample of participants was small, this has generated two limitations to the study: the impossibility of having a control group and its respective randomization.

On the other hand, working with athletes who are in the competitive calendar generates a difficulty for the longitudinal projection of a research, because in many cases the training loads must be modified when a competition is approaching, interfering with the development of the work.

Future research should consider the evaluation of other performance variables in these athletes, such as the application of punching strength, agility, and endurance, as well as other strikes (cross, uppercut, and kicks in the case of Muay Thai). In addition, working with a control group and a randomized sample would allow a more robust conclusion on the evidence of the results obtained in the research.

References

Apostopoulos, N., Metsios, G. S., Flouris, A. D., Koutedakis, Y., & Wyon, M. A. (2015). The relevance of stretch Intensity and position – a systematic review. Frontiers in Psychology, 6(1128). https://doi.org/10.3389/fpsyg.2015.01128

Araujo, C. G. (2005). Flexitest. El método de evaluación de la flexibilidad. Paidotribo.

Araujo, C. G. (2008). Flexibility assessment: Normative values for flexitest from 5 to 91 years of age. Arquivos brasileiros de cardiologia, 90, 257-263. https://doi.org/10.1590/s0066-782x2008000400008

Balmaseda, M. (2009). Escuela Cubana de Boxeo. Su enseñanza y preparación técnica. Wanceulen.

Basar, S., Duzgun, I., Guzel, N. A., Cicioglu, I., & Celik, B. (2014). Differences in Strength, flexibility and stability in freestyle and Greco-Roman wrestlers. Journal of back and musculoskeletal rehabilitation, 27(3), 321-330. https://doi.org/10.3233/BMR-130451 

Braganca de Viana, M. M., Bastos de Andrade, A., Salguero del Valle, A., & González Boto, R. (2008). Flexibilidad: conceptos y generalidades. EFDeportes Revista Digital. https://www.efdeportes.com/efd116/flexibilidad-conceptos-y-generalidades.htm 

Chaabene, H, Behm, D. G., Negra, Y., & Granacher, U. (2019). Acute effects of static stretching on muscle strength and power: an attempt to clarify previous caveats. Frontiers in Physiology, 10(1468). https://doi.org/10.3389/fphys.2019.01468 

Cheraghi, M., Alinejad, H. A., Archi, A. R., & Shirzad, E. (2014). Kinematics of Straight Right Punch in Boxing. Annals of Applied Sport Science, 2(2), 39-50. https://doi.org/10.18869/ACADPUB.AASSJOURNAL.2.2.39

Copoví Lanusse, R. (2015). Análisis del volumen de entrenamiento pliométrico para la mejora del salto. Apunts. Educación Física y Deportes, 120, 43,51. https://doi.org/10.5672/apunts.2014-0983.es.(2015/2).120.06 

Del Río Valdivia, J. E., Flores Moreno, P. J., González, J. B., Barajas Pineda, L. T., Medina Valencia, R. T., & Gómez Gómez, E. (2015). Efectos de un programa de flexibilidad en el desarrollo de la fuerza muscular en jugadoras de futbol femenil. Educación Física y Ciencia, 17(2). http://www.efyc.fahce.unlp.edu.ar/article/view/EFyCv17n02a06 

Dunn, E. C., Humberstone, C. E., Iredale, K. F., & Blazevich, A. J. (2019). A damaging punch: Assessment and application of a method to quantify punch performance. Translational Sports Medicine, 2, 146– 152. https://doi.org/10.1002/tsm2.71

El-Ashker, S. (2018). The impact of a boxing training program on physical fitness and technical performance effectiveness. Journal of Physical Education and Sport, 18(2), 926-932. https://doi.org/10.7752/jpes.2018.02137

Farinatti, P, Rubini, E., Silva, E., & Vanfraechem, J. (2014). Flexibility of the Elderly after One-Year Practice of Yoga y Calistenia. International Journal of Yoga Therapy, 24(1), 71-77. https://doi.org/10.17761/ijyt.24.1.5003007856u32q52

Franchini, E., & Herrera-Valenzuela, T. (2021). Developing flexibility for combat sports athletes. Revista de Artes Marciales Asiáticas, 16(1). https://doi.org/10.18002/rama.v16i1s.7005 

Gleim, G. W., & McHugh, M. P. (1997). Flexibility and Its Effects on Sports Injury and Performance. Sports Medicine, 24(59, 289-299. https://doi.org/10.2165/00007256-199724050-00001 

Hohmann, A., Lames, M., & Letzeier, M. (2005). Introducción a la Ciencia del Entrenamiento. Paidotribo.

Hunter, J. P., & Marshall, R. N. (2002). Effects of power and flexibility training on vertical jump technique. Medicine and Science in Sports and Exercise, 34(3), 478-86. https://doi.org/10.1097/00005768-200203000-00015. 

Kokkonen, J., Nelson, A. G., Eldredge, C., & Winchester, J. B. (2007). Chronic Static Stretching Improves Exercise Performance. Medicine and Science in Sports and Exercise, 39(10), 1825-1831. https://doi.org/10.1249/mss.0b013e3181238a2b. 

Kokkonen, J., Nelson, A. G., Tarawhiti, T., Buckingham, P., & Jason, B. (2010). Early-Phase Resistance Training Strength Gains in Novice Lifters Are Enhanced by Doing Static Stretching. Journal of Strength and Conditioning Research, 24(2), 502-506. https://doi.org/10.1519/JSC.0b013e3181c06ca0 

Leite, T. B., Costa, P. B., Leite, R. D., Novaes, J. S., Fleck, S. J., & Simao, R. (2017). Effects of Different Number of Sets of Resistance Training on Flexibility. International Journal of Exercise Science, 10(3), 354-364. https://pubmed.ncbi.nlm.nih.gov/28966703

Lenka, P. K., & Shah, V. B. (2019). A Comparative study of Agility, Flexibility and Explosive power of National level Player of Karate, Boxing and Taekwondo. International Journal of Health, Physical Education and Computer Science in Sports, 35(1), 152-154. https://www.journalofsports.com/archives/2019/vol4/issue1/4-1-54

Lima, C. D., Brown, L. E., Li, Y., Herat, N., & Behm, D. (2019). Periodized versus Non-periodized Stretch Training on Gymnasts Flexibility and Performance. International Journal of Sports Medicine, 40, 779-788. https://doi.org/10.1055/a-0942-7571 

Lima, P.O., Lima A. A., Coelho, A. C., Lima, Y. L., Almeida, G. P., Bezerra, M. A., & de Oliveira, R. R. (2017). Biomechanical Differences in Brazilian Jiu-Jitsu Athletes: The Role of Combat Style. International Journal of Sports Physical Therapy, 12(1), 67-74. https://pubmed.ncbi.nlm.nih.gov/28217417

López, I., Cirer-Sastre, R., Los Arcos López de Pariza, J., Corbi, F., Calleja Gonzalez, J. y Sitko, S. (2020). Concurrent validity and reliability of an accelerometer to assess punching velocity in boxers. Gazzetta medica italiana. https://www.researchgate.net/publication/342231155_Concurrent_validity_and_reliability_of_an_accelerometer_to_assess_punching_velocity_in_boxers

Mack, J., Stojsih, S., Sherman, D., Dau, N., & Bir, C. (2010). Amateur boxer biomechanics and punch force. ISBS - Conference Proceedings Archive. https://ojs.ub.uni-konstanz.de/cpa/article/view/4491 

Marinho, B. F., Del Vecchio, F. B., & Franchini, E. (2011). Condición Fïsica y Perfil Antropométrico de Atletas de Artes Marciales Mixtas. Revista de Artes Marciales Asiáticas, 6(2), 7-18. https://doi.org/10.18002/rama.v6i2.4

McAtee R., & Charland, J. (2010). Estiramientos Facilitados. Panamericana.

Medeiros, D. M., & Lima, C. S. (2017). Influence of chronic stretching on muscle performance: systematic review. Human Movement Science, 54, 220-229. https://doi.org/10.1016/j.humov.2017.05.006

Montealegre Suárez, D. P., & Vidarte Claros, J. A. (2019). Perfil antropométrico, somatotipo y condición física de niños patinadores de Neiva. Acciónmotriz, 22, 43-50. https://accionmotriz.com/index.php/accionmotriz/article/view/129

Omcirk, D., Vetrovsky, T., Padecky, J., Vanbell, S., Malecek, J., & Tufano, J. J. (2021). Punch trackers: correct recognition depends on punch type and training experience. Sensors, 21(2968). https://doi.org/10.3390/ s21092968 

Peck, E., Chomko, G., Gaz, D. V., & Farrell, A. M. (2014). The Effects of Stretching on Performance. Current. Sports Medicine Reports, 13(3), 179-85. https://doi.org/10.1249/JSR.0000000000000052 

Piorkowski, B. A., Lees, A., & Barton, G. (2011). Single maximal versus combination punch kinematics. Sports Biomechanics, 10(1), 1-11.  https://doi.org/10.1080/14763141.2010.547590 

Rees, S. S., Murphy, A. J., Watsford, M. L., McLachlan, K. A., & Couts, A. J. (2007). Effects of Proprioceptive Neuromuscular Facilitation Stretching on Stiffness and Force-Producing Characteristics of the Ankle in Active Women. Journal of Strength and Conditioning Research, 21(2), 572-577. https://doi.org/10.1519/R-20175.1

Roa López, I. B. (2009). Índice de Flexibilidad en Deportistas de Rendimiento de la Ciudad de Bogotá. Umbral Científico, 15, 34-39. https://www.redalyc.org/articulo.oa?id=30415144005 

Rodríguez Casallas, J. I., & Gracia Díaz, A. J. (2015). Evaluación del método Flexitest en los niños y niñas de la escuela de ciclismo de Cajicá – categoría pre infantil e infantil. Actividad Física y Deporte, 1(2). https://revistas.udca.edu.co/index.php/rdafd/article/view/309 

Sánchez-Sánchez, J., Pérez, A., Boada, P., García, M., Moreno, C. y Carretero, M. (2014). Estudio de la flexibilidad de luchadores de kickboxing de nivel internacional. Archivos de Medicina del Deporte, 31(2), 85-91. https://api.semanticscholar.org/CorpusID:159902475

Saraiva, A. R., Reis, V. M., Costa, P. B., Bentes, C. M., Costa e Silva, G. V., & Novaes, J. S. (2014). Chronic effects of different resistance training exercise orders on flexibility in elite judo athletes. Journal of Human Kinetics, 40, 129-137. https://doi.org/10.2478/hukin-2014-0015 

Schwartz, J., Takito, M. Y., Del Vecchio, F. B., Antonietti, L. S., & Franchini, E. (2015). Health-related physical fitness in martial arts and combat sports practitioners. Sport Sciences for Health, 11, 171-180. https://doi.org/10.1007/s11332-015-0220-6 

Shrier, I. (2004). Does stretching improve performance? A systematic and critical review of the literature. Clinical Journal of Spors Medicine, 14(5). https://doi.org/10.1097/00042752-200409000-00004 

Slimani, M., Chaabene, H., Davis, P., Franchini, E., Cheouer, F., & Chamari, K. (2016). Performance Aspects and Physiological Responses in Male Amateur Boxing Competitions: A Brief Review. Journal of Strength and Conditioning, 31(4), 1132-1141. https://api.semanticscholar.org/CorpusID:19859722

Tiwari, A. K., Pandey, A. S., Dhillon, M. S., & Badhyal, S. (2020). Design and Development of a Device for Performance Analysis and Injury Prevention in Boxing. Journal of Postgraduate Medicine Education and Research, 54(4), 231–235. https://www.jpmer.com/doi/JPMER/pdf/10.5005/jp-journals-10028-1401

Trial W., & Wu, T. (2013). A kinematic analysis of the Thai boxing clinch. Advances in Biomechanics and Applications, 1(1), 057-066. https://doi.org/10.12989/aba.2013.1.1.057

Weineck, J. (2005). Entrenamiento Total. Paidotribo. 

Wongputthichai, P., &  Ketchatturat,J. (2017). The Effect of Applied Plyometric Training Program on Anthropometry, Muscle Strength Test and Flexibility Test in Male Thai Boxing Athletes. KKU Research Journal, 17(3). https://api.semanticscholar.org/CorpusID:114097352