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Abstract

BACKGROUND:

The introduction of high-intensity functional training (HIFT) in sport gave rise to the exploration of determinants of success in sport.

OBJECTIVE:

To analyze whether asymmetry inter-limb is related to reduction in physical performance in amateur HIFT athletes.

METHODS:

Twenty-four HIFT amateur athletes, 11 women and 13 men, participated in the study. All volunteers performed three different jump tests: the squat jump (SJ), the countermovement jump (CMJ), performed unilaterally and bilaterally, and the 20-m sprint test.

RESULTS:

An overall asymmetry inter-limb of 9.5% in women and 9.3% men was not associated with any of the outcome measures. There were no significant differences in both genders between asymmetry, sprint, lower limbs’ muscle power (LLMP), SJ, CMJ and stretch-shortening cycle (SSC) ( $p>$ 0.05).

CONCLUSIONS:

Asymmetry inter-limb in CMJ does not seem to affect the performance of amateur athletes of HIFT. Thus, when the target is to maximise the performance, in the training programme, coaches should prioritise tasks that increase vertical jump capacity at the detriment of the unilateral jump.

Keywords

Inter-limb differences jumping performance reduction sprint muscle power

1. Introduction

High-intensity functional training (HIFT) consists of a physical training program comprising high- intensity functional exercises performed under short periods of recovery or in the absence of recovery intervals [1]. The method was originated in the beginning of the 21st century to increase the physical fitness of its practitioners [2]. Moreover, the modality presents certain benefits for physical conditioning. Smith et al. [3] verified that 10 weeks of HIFT, associated with a paleolithic diet (i.e., long fasting, carbohydrate restriction, etc.) promoted an increase in aerobic capacity and improvement of the body composition of 43 HIFT practitioners. After a few years of its emergence, HIFT gained sporting status [2] and, as a result, it opened space for investigations into the determinants of success in sport [4, 5, 6].

Overall, jumping ability has been evaluated by professionals involved in athletic performance analysis and improvement [7, 8]. This aspect is justified by the specificity presented by this type of movement of sport [9]. In addition, evidence indicates that jumping ability is strongly associated with high-intensity, short-duration running performance [10, 11].

Whether asymmetry inter-limb is a cause of sports injuries is a controversial issue. Tomkinson et al. [12] indicate that asymmetry between limbs may exert negative effects on performance in sport. Kyritsis et al. [13] verified that athletes with an inter-limb difference of $>$ 10% had a probability of re-rupturing the quadrupled anterior cruciate ligament. Regarding physical performance, a systematic review conducted by Bishop et al. [14] revealed that the results in the published literature articles were conflicting. For example, Bishop et al. [15] verified that asymmetry indices of 12.5% in unilateral countermovement jump (CMJ) were related to worse speed performances in professional female soccer players. On the other hand, Lockie et al. [16] found that asymmetry indices of 10.4% in unilateral CMJ did not have any impact on the speed performance in athletes of collective modalities. This ambiguity implies a need for more studies on the relationship of asymmetry between limbs with jump ability and measures of physical performance.

Frequently, asymmetry inter-limb relates to strength which is commonly analyzed using isokinetic dynamometers [17]. However these instruments are very costly and require trained professionals to perform the test, and not replicate sports gestures used in the sport [18]. There is hence a need for a more affordable (human resources and equipment) as well as more functional tests that can use movement patterns similar to the relevant sport, such as a vertical jump.

Therefore, considering the relatively limited research relating to the determinants of success in HIFT, the inconclusive results regarding the relationship between the asymmetry between limb found in CMJ with parameters of physical performance, and the search for testing methods with greater practical applicability, better economics, and sensitivity for the monitoring of the differences between limb, our study aimed to analyse the indices of asymmetries between limb in HIFT amateur athletes and verify the relationship between asymmetries and measures of physical performance. In addition, we analysed the relationship between different performance variables required for HIFT.

2. Methods

2.1 Subjects

A total of 24 amateur athletes (age: 27.9 $\pm$ 5.4 years; body weight: 78.1 $\pm$ 15.2 kg; and height: 170.1 $\pm$ 7.3 cm) 11 women and 13 men volunteered for the study. All participants were informed about the risks and benefits of the research before signing the informed consent form. Inclusion criteria were as follows: (i) age $>$ 18 years; (ii) participation in at least two sessions of strength training and conditioning per week; and (iii) providing consent to participate in all the stages of the study. Participants with osteomioarticular lesions who used supplements or drugs that could impact the results were excluded from the study.

The study was approved by the local ethics and research committee and followed all the ethical standards according to the Helsinki Declaration (CAAE: 02976918.7.0000.5537, N ${}^{\circ}$ . 3.082.357). To meet approval, all HIFT athletes had the option of withdrawing at any point in time, from the study.

2.2 Procedures

This was a cross-sectional study with a quantitative approach. All experimental procedures were performed at the beginning of the week, and athletes were asked not to perform vigorous activities 48 hours before the evaluation. In addition, to mitigate the climatic effects, all tests (i.e., jumps and sprints) were performed indoors at the same hour each day to minimize the impact of circadian rhythm. Athletes were familiar with all the procedures and tests that were routinely performed during their training programs. Before beginning the tests, the athletes underwent a warming protocol proposed by the training centre’s trainer. The athletes initially performed three jumping tests, followed by the sprint tests. This arrangement of tests was defined by the coaches to attenuate fatigue accumulation and, possibly, to potentiate the performance in the tests.

2.3 Physical performance

2.3.1 Speed test (the 20-m sprint)

To analyze the speed, one photocell was positioned 20 m from the starting point (Cefise ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}{\textregistered}}$ , São Paulo, Brazil). The athletes performed the test in duplicate, from standing position. An interval between the 5-minute series was provided and the best score was used for data analysis [19].

2.3.2 Jump assessment

A contact platform (Cefise ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}{\textregistered}}$ , São Paulo, Brazil) connected to the Jump Test Pro 2.10 software was used to measure the jump height. The athletes were familiar and were able to perform the jumps. All attempts were performed with the hands fixed on the hips, and the participants were encouraged to jump as fast as possible [20]. They were free to set the amplitude of the countermovement in the CMJ to prevent changes in the coordination of the jumps. In the unilateral jump, the knee of the leg against the lateral one that was evaluated was maintained flexed at approximately 90 ${}^{\circ}$ from the beginning to the end of the movement. The squat jump (SJ) was performed in accordance with the protocol by Meylan et al. [21]. There were three attempts for each type of jump, with intervals of 15 s between each attempt and 2 min between the different jumps.

To verify the relationship between CMJ and SJ, the eccentric utilization ratio (EUR) was calculated using the following formula: EUR $=$ CMJ (cm)/SJ (cm) [22]. In addition, to verify the asymmetry between the lower limbs, the strong leg versus the weak leg approach was defined as opposed to the dominant and non-dominant leg approach, because it was regarded as a better criterion in asymmetry verification [23]. For this, the following formula, proposed by Impelizzeri et al. [24], was used: [(Strong Leg $-$ Weak Leg)/(Strong Leg)] $\times$ 100.

2.4 Statistical analyses

Normality was tested using the Shapiro-Wilk test and z-score analysis of asymmetry and kurtosis ( $-$ 1.96 to 1.96). Continuous data are reported as means and standard deviation. Pearson’s correlation was used to analyze the correlations of women and men between bilateral and unilateral vertical jump variables and performance tests. For all analyses, a $p$ value of $<$ 0.05 was considered significant and all analyses were performed using SPSS (IBM ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}{\textregistered}}$ , Nova York, USA) version 20.0 software.

3. Results

Data on vertical jump performance characteristics of HIFT amateur female and male athletes are provided in Table 1. Data are reported as average and standard deviations.

Table 1
Descriptive analysis of the performance variables in women and men

Variables	Women		Men
	Mean $\pm$ SD		Mean $\pm$ SD
SPRINT (s)	4.2	$\pm$ 0.51	4.2	$\pm$ 0.53
LLMP (w)	3271	$\pm$ 990	3287	$\pm$ 984.9
LLMP (w/kg)	40.96	$\pm$ 7.37	41.23	$\pm$ 7.39
SJ (cm)	27.28	$\pm$ 7.21	27.54	$\pm$ 7.99
CMJ (cm)	29.18	$\pm$ 8.30	29.50	$\pm$ 8.30
CMJSL (cm)	13.12	$\pm$ 3.81	13.42	$\pm$ 4.30
CMJWL (cm)	11.85	$\pm$ 3.48	12.12	$\pm$ 3.81
DFU	1.27	$\pm$ 0.90	1.29	$\pm$ 0.96
Asymmetry (%)	9.51	$\pm$ 5.86	9.37	$\pm$ 5.66
EUR	1.08	$\pm$ 0.09	1.08	$\pm$ 0.09
SSC	1.90	$\pm$ 267	1.96	$\pm$ 2.62

SD $=$ standard deviation; (s) $=$ seconds; LLMP $=$ lower limbs’ muscle power; (w) $=$ watts; LLMP $=$ lower limbs’ muscle power; (w/kg) $=$ watts/kilogram; SJ $=$ squat jump; (cm) $=$ centimeter; CMJ $=$ countermovement jump; CMJSL $=$ countermovement jump strong leg; CMJWL $=$ countermovement jump weak leg; DFU $=$ difference of unilateral force; EUR $=$ eccentric utilization ratio; SSC $=$ stretch shortening cycle.

There were no significant difference in both genders between asymmetry, sprint, lower limbs’ muscle power (LLMP), SJ, CMJ and stretch-shortening cycle (SSC) ( $p>$ 0.05) (Table 2).

Table 2

Pearson’s linear correlation between asymmetry and performance variables in women and men

Asymmetry	Women		Men
	$r$	$p$	$r$	$p$
Sprint (s)	0.20	0.54	$-$ 0.004	0.99
LLMP (w)	0.17	0.60	$-$ 0.08	0.79
SJ (cm)	0.28	0.40	0.26	0.37
CMJ (cm)	0.16	0.62	0.14	0.63
SSC	$-$ 0.49	0.12	$-$ 0.15	0.60

(s) $=$ seconds; LLMP $=$ lower limbs’ muscle power; (w) $=$ watts; SJ $=$ squat jump; (cm) $=$ centimeter; CMJ $=$ countermovement jump; SSC $=$ stretch shortening cycle; $r=$ correlation.

Women showed strong correlation between strong leg, weak leg and sprint, SJ and CMJ, but there not for muscle power. On the other hand, men showed a strong correlation between strong leg, weak leg and muscular power and SJ (Table 3).

Table 3

Pearson’s linear correlation between strong leg, weak leg, and performance variables in women and men

	Strong leg						Weak leg
	Women			Men			Women			Men
	$r$ ( $r^{2}$ )		$p$	$r$ ( $r^{2}$ )		$p$	$r$ ( $r^{2}$ )		$p$	$r$ ( $r^{2}$ )		$p$
Sprint (s)	$-$ 0.68	(46)	0.01	$-$ 0.50	(25)	0.07	$-$ 0.68	(46)	0.02	0.52	(27)	0.06
LLMP (w)	0.24	(5)	0.46	$-$ 0.55	(30)	0.04	0.19	(3)	0.56	0.56	(31)	0.04
SJ (cm)	0.78	(60)	0.004	0.62	(38)	0.02	0.77	(59)	0.005	0.60	(36)	0.03
CMJ (cm)	0.70	(49)	0.01	0.32	(10)	0.27	0.71	(50)	0.01	0.30	(9)	0.31

(s) $=$ seconds; LLMP $=$ lower limbs’ muscle power; (w) $=$ watts; SJ $=$ squat jump; (cm) $=$ centimeter; CMJ $=$ countermovement jump; $r=$ correlation.

Table 4

Pearson’s linear correlation between SJ, sprint, LLMP, EUR, SSC, and CMJ in women and men

	Sprint		LLMP		EUR		SSC
	$r$	$p$	$r$	$p$	$r$	$p$	$r$	$p$
Women
SJ (cm)	$-$ 0.66	0.02	0.58	0.05	$-$ 0.75	0.007	$-$ 0.54	0.08
CMJ (cm)	$-$ 0.58	0.06	0.58	0.07	$-$ 0.73	0.07	0.34	0.36
Men
SJ (cm)	$-$ 0.37	0.20	$-$ 0.34	0.25	$-$ 0.29	0.32	$-$ 0.20	0.56
CMJ (cm)	$-$ 0.54	0.05	$-$ 0.77	0.80	0.36	0.22	0.45	0.12

SJ $=$ squat jump; (cm) $=$ centimeter; CMJ $=$ countermovement jump; LLMP $=$ lower limbs’ muscle power; EUR $=$ eccentric utilization ratio; SSC $=$ stretch shortening cycle; $r=$ correlation.

In women a strong correlation was found only between sprint and SJ and EUR and SJ. In contrast, no correlation was found between the variables analyzed in men (Table 4).

Figure 1.

Description of the unilateral vertical jump asymmetry of female high-intensity functional training athletes.

Figure 2.

Description of the unilateral vertical jump asymmetry of male high-intensity functional training athletes.

Figure 1 details the asymmetry values of individual female athletes. Of the total of 11 athletes, 4 had asymmetry values of $>$ 15%. In addition, two other athletes presented had an asymmetry of 11%.

Figure 2 details the asymmetry values in men. Of the total of 13 athletes, 3 had asymmetry values of $>$ 15%, while two had an 11% bilateral difference.

4. Discussion

The main findings of this study are as follows: 1) Asymmetry between limbs in CMJ in women and men has no influence on sprint speed of 20 m, SJ, CMJ, power, and SSC; 2) SJ skills performed bilaterally are associated with shorter sprint times of 20 m and higher EUR levels in women only; 3) SJ and CMJ ability in men is not associated with eccentric utilization rate (EUR), stretch-shortening cycle (SSC), sprint of 20 m and muscle power.

Competitive events in HIFT require the athlete to master multiple aspects inherent to fitness in a sporting scenario [5]. Bellar et al. [4] suggested that the presence of short races (up to 800 m) in HIFT competitions supported the idea of the presence of an association between anaerobic performance and success in the modality. In addition, Barbiere et al. [25] identified strong correlations between female athlete classifications and performance of 50- and 400-m sprints ( $r=$ 0.77 and $r=$ 0.69, respectively) in an official HIFT competition. In addition, this training methodology has the capacity to improve athletic performance through improvements in endurance and muscle power of trained athletes [26], and, probably, in the ability to generate power for better velocity performances in rugby [27], and greater heights in SJ activities of baseball, basketball, volleyball, and softball athletes [28].

Since power is the rate at which work is performance or alternatively the multiplication of force by velocity, this physical attribute is a key determinant of success in several types of sport [29, 30, 31]. Therefore, there is a clear need to investigate the factors that may increase the power in sports and reduce the risks of injuries.

Regarding the asymmetry between the limbs in the CMJ, there were no correlations with the abovementioned performance reduction variables, corroborating findings from previous studies [16, 32, 33]. According to McGinnis [34], both the proportion and direction of force and power individually developed by each segment are combined to create only a single force vector. Thus, the existence of inequalities between the dittocould exert a negative effect on jumping skills and sprints [35]. Furthermore, physical performance can be affected by anthropometric asymmetries [35] and strength [36], even though the results of previous studies are conflicting regarding the effect of asymmetries between limbs on jumping ability [14, 37].

Although a few study results [16, 32, 33] indicate that performance is not influenced by the difference between CMJ limbs, asymmetry levels $>$ 15% are associated with injury incidence [24]. In the present study, although we identified averages of asymmetries of 9.5% in women and 9.3% men, we highlight that only seven athletes had values exceeding the physiological limit (i.e., 15%). In contrast, Trzaskoma et al. [38] evaluated a group of healthy women and identified averages of $\sim$ 1% asymmetries, and Yance and Camara [39] identified a mean of 4.55% in a study sample of amateur soccer players. A common methodological aspect in these studies is that both did not compare between strong and weak legs; comparisons were made between dominant and non-dominant legs. Because some authors have shown that certain athletes had lower JS performances in dominant limbs [40, 23], the validity of the measures of asymmetries based on the equations that use comparisons between dominant and non-dominant legs is debatable. However, by using this information, it is possible that the choice of the equation can impact the results of the inter-limb asymmetry and that the use of the equation proposed by Impellizzeri et al. [24] seems to be the most appropriate option.

In our study, we identified strong negative correlations between SJ and sprint ( $r=-$ 0.66, $p=$ 0.02) only in women. The relationship between jumping ability and sprint performance identified in the present study agrees with the findings reported in the published literature [10, 11]. Asadi [41] indicated that these results seemed to be valid because these actions share common biomechanical and bioenergetic aspects. This fact may explain the association between SJ ability and sprint performance, considering that at the moment the participant begins the race, there is acceleration in the frontal direction on the maximum level. In turn, this acceleration depends on the force generated by the muscle against the ground and time, times the time of application [42]. In addition, a common aspect between SJ and sprint is represented by the use of elastic energy provided by SSC [43, 44].

SSC is present in movements composed by eccentric actions followed by concentric actions. In this type of movement, SSC, through the accumulation of elastic energy, potentiates to the force generating capacity with lower metabolic costs [43]. Therefore, Komi and Bosco [45] proposed the use of CMJ to evaluate the use of SSC. However, some authors have questioned the use of this type of movement for SSC evaluation, because this motor action also depends on coordinating factors [46, 47], which could explain the low correlation and absence of a significant difference between CMJ and SSC in women ( $r=-$ 0.34, $p>$ 0.05) and men ( $r=$ 0.45, $p>$ 0.05) in the present study. Therefore, biomechanical factors such as the greater participation of the hip extensor muscles in the countermovement may justify reaching greater heights in the CMJ and not the elastic energy accumulation provided by the SSC [48].

Nevertheless, comparisons between SJ and CMJ represent the most common approach to evaluate the efficiency of SSC [49]. Thus, one of the factors evaluated in our study was EUR, a benchmark of SSC performance, which attains values above 1.00 for trained athletes [22]. In our study, we did not find correlations between CMJ and EUR in women ( $r=-$ 0.73; $p>$ 0.05) and men ( $r=$ 0.36; $p>$ 0.05). Noteworthy, the inability to perform SJ correctly in association with moderate performance of the CMJ may overestimate the EUR [49], probably justifying our results.

Professionals involved with the HIFT prescription should be aware of the factors that can influence the results of their athletes at competitive events, as well as the mitigation of injury risks. However, as a relatively new modality, there are only a few studies that seek to evaluate such variables. Thus, our work is one of the few studies that provides information that can be useful in the evaluation and structuring of HIFT programmes that aim for success in competitions. In addition, the present study demonstrates that the use of vertical jumps, a simple test of low financial cost and movements similar to the one performed in sports, is a better approach to evaluate the presence of asymmetries between limb and as a form of performance analysis.

4.1 Limitations of the study

The current study has some limitations that should be highlighted: i) The cross-sectional design does not allow the establishment of a cause-effect relationship; ii) The evaluated group was formed by amateur athletes, different results may be identified in elite athletes; iii) The evaluation instrument used to measure the performance of vertical jumps was not the gold standard (force platform), however, it is clear that the intention of the study was to provide information based on field tests, which in turn present greater applicability for coaches.

5. Conclusions

We observed that high asymmetry indices are not related to lower performance in speed and power tests. On the other hand, better results in bilateral SJ are associated with shorter sprint time and a lower EUR in female amateur athletes of HIFT. Thus, when the target is to maximize the performance of speed in bilateral activities, in the training programme, coaches should prioritise tasks that increase vertical jump capacity in detriment of the unilateral jump. Furthermore, coaches should be aware of high asymmetry indices, as they might be associated with a greater predisposition to injury.

Footnotes

Acknowledgments

We thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes) for granting a postgraduate scholarship to Rômulo Vasconcelos Teixeira.

Conflict of interest

The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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Asymmetry inter-limb and performance in amateur athletes involved in high intensity functional training

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Subjects

2.2 Procedures

2.3 Physical performance

2.3.1 Speed test (the 20-m sprint)

2.3.2 Jump assessment

2.4 Statistical analyses

3. Results

Table 1 Descriptive analysis of the performance variables in women and men

4.1 Limitations of the study

5. Conclusions

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Descriptive analysis of the performance variables in women and men