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Abstract

BACKGROUND:

Lower limb muscular asymmetry is not well studied and may have a negative impact on performance.

OBJECTIVE:

To estimate how muscular strength and strength asymmetry affect jumping performance in soccer players.

METHODS:

Twenty-eight male professional soccer players took part in the study. The countermovement jump (CMJ) without arm swing was used to determine jumping height. Muscle strength was measured concentrically at 60 and 300 ${}^{\circ}$ /s.

RESULTS:

The peak moment of the knee extensors was positively and significantly correlated with the CMJ; $r=$ 0.608 at 300 ${}^{\circ}$ /s and $r=$ 0.489 at 60 ${}^{\circ}$ /s. The asymmetry of the knee flexors between the stronger and weaker leg was negatively and moderately correlated with the CMJ at 300 ${}^{\circ}$ /s ( $r=-$ 0.396). The regression model ( $R^{2}=$ 0.474) showed that an increase of 0.18-Nm/kg in the relative strength of the knee extensors at 300 ${}^{\circ}$ /s (by one SD) was related to an increase of 3-cm in the CMJ. Reducing the asymmetry of the knee flexors by 6.8 percentage points (by one SD) was related to a rise of 1.7-cm in the CMJ.

CONCLUSIONS:

Greater strength in the knee extensors, preferably tested at higher velocity, and reduced asymmetry in the strength of the lower hamstring muscles have a statistically significant effect on the CMJ.

Keywords

Local muscle strength isokinetic testing countermovement jump

1. Introduction

Soccer is a demanding game. Players need the endurance to cover distances of more than 10 km during a match [1], but they also need the explosive power to perform short, sharp activities like sprints and vertical jumps [2, 3, 4].

Vertical jumping is a frequent and important element of soccer. Players typically make on average 15 headers during the match [4]. The height of the vertical jump is important for soccer players for two reasons. First, the height of vertical jump is an expression of power, which is an important determinant of performance [5], and second, jumping is the second most important action for scoring or assisting [6].

The height of the vertical jump depends on muscular strength. The countermovement jump (CMJ) is the most common test for measuring vertical jump performance. It has been found that 31% of the height of the vertical jump can be explained by the strength of the knee extensors and flexors [7], and an even greater 38% of the height of vertical jump can be explained by the strength of the knee and hip extensors and the ankle plantar flexors [8]. There are several studies that indicated a higher peak moment in the knee extensors and flexors to be related to greater height in vertical jumping [9, 10, 11].

Muscular asymmetry is related to increased risk of injury and reduced performance [12, 13, 14, 15]. The two main types of muscular balance are the inter-limb deficit and the Hamstring-Quadriceps ratio (H/Q ratio). If the inter-limb muscular strength difference becomes too large or the Hamstring-Quadriceps ratio becomes too small, muscular asymmetry results. It is suggested that a bilateral deficit of 10% or 15% can lead to an increase in injuries [16, 17, 18]. The studies that focus on how muscular asymmetry affects performance use a bilateral deficit limit of 10% [7, 19].

A difference in the strength of the knee extensors in the right and left legs and a low H/Q ratio is a common problem in soccer players [10, 13, 16, 20, 21]. One reason for this is that actions where only one leg is used dominantly occur frequently in soccer [22]. The effect of muscular strength asymmetry on lower limb injuries in soccer players has been widely studied [23, 24, 25, 26, 27], but the effect on performance has not received as much attention.

There is evidence that asymmetry in the muscle strength of the knee extensors and flexors has a negative effect on multi-directional running speed [9, 28]. However, studies on the effect on directional running speed do not find any statistically significant relationship with asymmetry [10], or find that the effect only arises for asymmetry of the hamstring muscles and not for asymmetry of the extensors [9]. The effect of inter-limb muscular asymmetry and the H/Q ratio on jumping height has been found to be statistically insignificant. Only a weak relationship has been found between asymmetry in the strength of the knee extensors and flexors and jumping height [9, 10, 29]. It has been reported that in volleyball players inter-limb muscular strength asymmetry did not seem to be associated with the vertical jump performance [29]. On the other hand, a bilateral deficit limit of 10% did not seem to have a significant effect on jumping performance [19].

Only two studies have tested the relationship between strength asymmetry and vertical jump performance in elite soccer players [9, 10]. These studies find only weak or statistically insignificant relationships between the bilateral deficit and vertical jump performance or between the H/Q ratio and vertical jump performance.

The purpose of this study is to estimate the effect of muscular strength and strength asymmetry on jumping performance in soccer players. Our study contributes to the limited literature on the effect of asymmetry in muscular strength on jumping performance in soccer players.

2. Methods

2.1 Participants

The study looks at 28 male soccer players (10 defenders, 10 midfielders and 8 forwards) playing in Estonian Premium League teams (age 23.4 $\pm$ 4.5-y, height 181.5 $\pm$ 7.7-cm, mass 77.6 $\pm$ 8.8-kg), who participated in the study on a voluntary basis. The code of Ethics of the World Medical Association was followed. Written informed consent was obtained from all participants, and permission was received from the Tallinn Medical Research Ethics Committee to conduct tests with humans for this study (Decision number 2241, application number 1817, issued on 19 February 2018).

2.2 Procedures

The soccer players rode a veloergometer for 10-m, and performed light dynamic stretching exercises and three to five warm-up jumps before taking the jumping and strength tests. This follows the approach taken by [30]. A countermovement jump test without arm swing was used in the study. The athletes were instructed about the testing procedures before jumping. They were instructed to stand with their arms on their hips. The lowest point of knee flexion should be approximately 90 ${}^{\circ}$ during the jump. First, permission to jump was given and when the athlete was ready he jumped. The countermovement jump was performed on two force plates of type 9286A Kistler. The Kistler Mars 3.03 testing software using the flight time method was used to measure the height of the jump. Previous studies have found that the Kistler force platform and Mars software are reliable tools for measuring CMJ height without arm swing [31, 32, 33]. A resting period of at least 30-s was given between jumps. The best result from the three jump attempts was used for the analysis [10].

Muscle strength was measured with an isokinetic dynamometer Humac Norm (USA), a reliable tool for measuring the parameters of isokinetic strength [34, 35]. Concentric actions were measured at angular velocity of 60 and 300 ${}^{\circ}$ /s. Both velocities have been validated and used in the related literature [17, 35, 36, 37, 38, 39, 40, 41]. To avoid artefacts, the acceleration and deceleration phases were eliminated [42].

The dynamometer was set in accordance with the manufacturer’s instructions and was used in a sitting position where the angle of the back was 85 ${}^{\circ}$ . The athletes completed two tests. There was a trial run before each test so that the athletes could be familiarized with the movement. There were 2-min for resting between the trial and the test, and a 3-min rest between the two tests. The pause between the sides of the body was 5-min while the order was randomised. The athletes made five concentric repetitions at a velocity of 300 ${}^{\circ}$ /s [40] and three concentric repetitions at a velocity of 60 ${}^{\circ}$ /s [16, 36, 38, 39] during the trial run and the test.

2.3 Parameters measured

The CMJ value was used as a parameter for performance [43]. The best peak moment (PM in Nm) of the repetitions performed at both the testing velocities and for the right and left legs separately was used as a measure of the local muscular strength of the knee extensors (EX) and flexor (FL). The PM values were normalised to body mass and the values for the right and left legs were averaged for the measure of strength (Nm/kg). The unilateral muscle strength balance, noted as the Hamstring-Quadriceps ratio (H/Q ratio), was calculated for both of the testing velocities using the formula: H/Q ratio $=$ PM FL/PM EX $\times$ 100 (%). The contralateral strength and H/Q ratio asymmetries (ASY) were calculated using the formula: ASY $=$ (stronger limb $-$ weaker limb)/stronger limb $\times$ 100 (%) [44].

The following muscle strength and asymmetry parameters were registered for the analysis:

•
PM EX 60 ${}^{\circ}$ /s – relative peak moment of knee extensors at angular velocity of 60 ${}^{\circ}$ /s (Nm/kg)
•
PM EX 300 ${}^{\circ}$ /s – relative peak moment of knee extensors at angular velocity of 300 ${}^{\circ}$ /s (Nm/kg)
•
PM FL 60 ${}^{\circ}$ /s – relative peak moment of knee flexors at angular velocity of 60 ${}^{\circ}$ /s (Nm/kg)
•
PM FL 300 ${}^{\circ}$ /s – relative peak moment of knee flexors at angular velocity of 300 ${}^{\circ}$ /s (Nm/kg)
•
H/Q Ratio 60 ${}^{\circ}$ /s – H/Q ratio at angular velocity of 60 ${}^{\circ}$ /s (%)
•
H/Q Ratio 300 ${}^{\circ}$ /s – H/Q ratio at angular velocity of 300 ${}^{\circ}$ /s (%)
•
ASY EX 60 ${}^{\circ}$ /s – contralateral asymmetry of knee EX PM at angular velocity of 60 ${}^{\circ}$ /s (%)
•
ASY EX 300 ${}^{\circ}$ /s – contralateral asymmetry of knee EX PM at angular velocity of 300 ${}^{\circ}$ /s
•
ASY FL 60 ${}^{\circ}$ /s – contralateral asymmetry of knee FL PM at angular velocity of 60 ${}^{\circ}$ /s (%)
•
ASY FL 300 ${}^{\circ}$ /s – contralateral asymmetry of knee FL PM at angular velocity of 300 ${}^{\circ}$ /s (%)
•
PM H/Q Ratio 60 ${}^{\circ}$ /s – contralateral asymmetry of H/Q ratio at angular velocity of 60 ${}^{\circ}$ /s (%)
•
PM H/Q Ratio 300 ${}^{\circ}$ /s – contralateral asymmetry of H/Q ratio at angular velocity of 300 ${}^{\circ}$ /s (%)

2.4 Statistical analysis

The statistical computations were performed using the statistical software IBM SPSS for Windows 23.0 (IBM, Chicago, USA). The normality of the data distribution was controlled for with the Shapiro-Wilk test before the parametrical statistical tools were applied. The descriptive statistics were computed as mean, standard deviation and coefficient of variation (VAR $=$ SD/mean $\times$ 100) for CMJ height, peak moment of muscle groups, H/Q ratio, and asymmetry characteristics. The associations between local muscle strength characteristics and CMJ performance was verified by the Pearson correlation test. The strength of the correlation was evaluated qualitatively using the scale proposed by [45]: $r<|0.3|$ small or weak, $|0.3|<r<|0.5|$ moderate, $|0.5|<r<|0.7|$ large or strong, $r>|0.9|$ very large or very strong. Stepwise linear regressions were used to evaluate the linear effect on CMJ height of multiple local muscle strength factors. The significance level was set at the 95% confidence level ( $p<$ 0.05) for all the statistical tests.

3. Results

The average value of the countermovement jump test without arm swing was 35.2 $\pm$ 5.2-cm (Table 1) which outlines also the absolute PM, the normalized PM and strength asymmetry values of the knee extensor and flexor muscle groups at both speeds.

Table 1
Descriptive statistics of the countermovement jump; the absolute peak moment and peak moment relative to body weight of the knee extensors and flexors, the H/Q ratio and the asymmetry parameters

$n=$ 28	Mean	SD	Var (%)
Countermovement jump without arm swing (cm)	35.2	5.2	15
Peak moment (Nm)
EX 60 ${}^{\circ}$ /s	231.29	31.05	13
EX 300 ${}^{\circ}$ /s	121.08	16.58	14
FL 60 ${}^{\circ}$ /s	145.74	22.26	15
FL 300 ${}^{\circ}$ /s	83.00	16.69	20
Peak moment (Nm/kg)
EX 60 ${}^{\circ}$ /s	3.01	0.32	11
EX 300 ${}^{\circ}$ /s	1.58	0.18	11
FL 60 ${}^{\circ}$ /s	1.90	0.23	12
FL 300 ${}^{\circ}$ /s	1.08	0.17	15
H/Q ratio 60 ${}^{\circ}$ /s	63.50	8.40	13
H/Q ratio 300 ${}^{\circ}$ /s	66.90	8.10	12
Asymmetry (%)
EX 60 ${}^{\circ}$ /s	8.5	5.6	66
EX 300 ${}^{\circ}$ /s	6.0	4.2	70
FL 60 ${}^{\circ}$ /s	10.4	6.2	59
FL 300 ${}^{\circ}$ /s	7.7	6.8	89
H/Q ratio 60 ${}^{\circ}$ /s	9.3	5.8	63
H/Q ratio 300 ${}^{\circ}$ /s	9.2	7.4	80

The PM was highest for the knee extensors at 60 ${}^{\circ}$ /s. The indicators of muscular asymmetry show a large degree of variability between the subjects, especially for the ASY values of the knee extensors and flexors and the difference in the H/Q ratio between the stronger and weaker legs at an angular speed of 300 ${}^{\circ}$ /s.

Table 2 outlines the results of the Pearson correlation analysis. The PM values of knee EX were positively related with the CMJ height; a strong relationship was found between KN EX strength at 300 ${}^{\circ}$ /s and the CMJ height, and a moderate relationship was found between KN EX strength at 60 ${}^{\circ}$ /s and the CMJ height. No statistically significant correlation was found between jumping height and the knee FL PM or the H/Q ratio measures at any testing velocity. The muscle strength asymmetry of the knee FL at a velocity of 300 ${}^{\circ}$ /s was moderately and negatively related to the CMJ performance. Other ASY characteristics of local muscle strength were not related to the CMJ height.

Table 2

Correlation between the CMJ height and peak moment, the H/Q ratio and asymmetry values

	$n=$ 28	CMJ height				$n=$ 28	CMJ height
		$r$	$R^{2}$	$p$ value			$r$	$R^{2}$	$p$ value
Peak moment (Nm/kg)	EX 60 ${}^{\circ}$ /s	0.489 ${}^{\ast\ast}$	0.239	0.008	Asymmetry (%)	EX 60 ${}^{\circ}$ /s	$-$ 0.123	0.015	0.532
	EX 300 ${}^{\circ}$ /s	0.608 ${}^{\ast\ast}$	0.370	0.001		EX 300 ${}^{\circ}$ /s	0.178	0.032	0.366
	FL 60 ${}^{\circ}$ /s	0.351	0.123	0.067		FL 60 ${}^{\circ}$ /s	0.164	0.027	0.405
	FL 300 ${}^{\circ}$ /s	0.344	0.118	0.073		FL 300 ${}^{\circ}$ /s	$-$ 0.396 ${}^{\ast}$	0.157	0.037
	H/Q ratio 60 ${}^{\circ}$ /s	$-$ 0.152	0.023	0.44		H/Q ratio 60 ${}^{\circ}$ /s	0.086	0.007	0.664
	H/Q ratio 300 ${}^{\circ}$ /s	$-$ 0.287	0.082	0.139		H/Q ratio 300 ${}^{\circ}$ /s	$-$ 0.229	0.052	0.241

Table 3

Linear regression model for predicting the CMJ height

	Unstandardised coefficients		Standardised coefficients
	Beta	Std. Error	Beta	$t$	Sig.
(Constant)	10.859	6.961		1.56	0.131
PM EX 300 ${}^{\circ}$ /s	16.599	4.278	0.567	3.88	0.001
ASY FL 300 ${}^{\circ}$ /s	$-$ 0.249	0.112	$-$ 0.326	$-$ 2.227	0.035
$R^{2}=$ 0.474, $p>$ 0.01

The countermovement jump height without arm swing is statistically significantly positively related with relative peak moment of the knee extensors at an angular speed of 60 ${}^{\circ}$ /s and 300 ${}^{\circ}$ /s, and the ASY of the knee flexors between the stronger and weaker legs at an angular speed of 300 ${}^{\circ}$ /s.

Table 3 presents the results of the linear regression analysis, where the dependent variable is the countermovement jump height without arm swing and the independent variables are the muscular strength parameters of the knee extensors and flexors. The descriptive power of the knee EX strength at a velocity of 300 ${}^{\circ}$ /s for predicting the CMJ height was approximately 37%, but when the asymmetry of the knee FL at 300 ${}^{\circ}$ /s was entered into the model, the descriptive power rose by 10 percentage points. Entering the other local strength characteristics that were measured did not improve the model significantly. The regression model thus indicates that increasing the strength of the relative knee extensors at 300 ${}^{\circ}$ /s by 0.18-Nm/kg (i.e. by one standard deviation) increases the CMJ height by 3-cm on average, and reducing the asymmetry of the knee FL by 6.8% (i.e. by one standard deviation) increases the jump height by 1.7-cm on average.

4. Discussion

The height of the CMJ in this study was about the same as in the previous study [46]. The relative PM of the knee extensors and flexors in our study is at a similar level to those found in other studies and the variation of the peak moment is slightly less than in other studies [10, 38].

The unilateral strength balance measured as the H/Q ratio was over 60% on average at both testing velocities. This value is at a level that is common among athletes [47, 48]. The balance values that we measured for knee flexors and extensors were also similar to those for athletes from other sports in other studies [29, 49, 50]. We also found the same tendency for the H/Q ratio to increase as the testing velocity increased [16, 29, 51, 52]. The asymmetry in contralateral strength was lower than 10% on average, except for the flexors ASY at 60 ${}^{\circ}$ /s. This is defined as the biologically normal bilateral difference [18, 21]. However, the variability in the asymmetry values was remarkably large in our study.

One of the main findings of our study was that the CMJ performance is positively related to the muscle strength of the knee extensors, but not to the strength of the knee flexors. This is in line with previous studies that also find that stronger knee extensors were related to a greater height for the countermovement jump [7, 8, 11, 52, 53]. It is also in line with the inverse-dynamically computed joint work distribution in the CMJ, where 49% of the work is done by the muscles acting at the knee joint [54]. It is notable that the knee EX PM measured at an angular speed of 300 ${}^{\circ}$ /s is a stronger predictor of CMJ height than the PM of the same muscle group at an angular speed of 60 ${}^{\circ}$ /s. The tendency for the relationships between the strength values and the CMJ performance to be stronger at higher testing speeds has also been presented in some previous studies [8, 29, 53]. This indicates that isokinetic strength tests at higher angular velocities can provide more useful information for evaluating athletic performance.

Like the strength of the flexors, the H/Q ratio of the flexors did not correlate with the CMJ height. This is in line with some previous studies [52] but in variance with findings from a population of volleyball players [29].

The strongest predictor of the CMJ height was the knee EX strength at 300 ${}^{\circ}$ /s, which explained 37% of the total variability in the CMJ height. Adding the asymmetry of the knee flexors at 300 ${}^{\circ}$ /s to the model increased the explanatory power of the model to 47% ( $R^{2}=$ 0.47). The R square values of similar models from other studies were lower and range between 0.31 and 0.38 [7, 8]. Earlier studies did not include strength asymmetry in the model. If we compare models with only the strength of the knee extensors at 60 ${}^{\circ}$ /s, then our results of $R^{2}=$ 0.24 are comparable with those of the [7] model, where $R^{2}=$ 0.31. That study used the values of the left and right legs separately in the model, but our study used the average value of both legs. Another study [8] included the isokinetic strength characteristics of all three major lower limb joint crossing muscle groups in their model and also found that the strongest predictor of the CMJ height is the strength of the knee extensors at a higher testing velocity of 180 ${}^{\circ}$ /s. In the linear combination of the PM values of the ankle plantar flexors and hip extensors though, that model explained only 38% of the variability in the CMJ height. These results are also in line with the suggestion [55] that training the knee extensors of all the lower extremity muscles is the most effective way to improve vertical jump height. Indeed, Tsiokanos et al. [8] used absolute peak moment values, but when the CMJ height was normalised with body mass, then the local strength performance of all three of the leg muscle groups mentioned described 75% of the external mechanical work during jumping. This indicates that in sports where performance is related to moving the body mass of athletes fast, normalising local muscle performance characteristics with body mass is more informative. All the muscle performance characteristics were normalised with body mass in this study. The unstandardized coefficient of the relative peak moment of the knee extensors at 300 ${}^{\circ}$ /s was 16.6 in our study. This means that when the relative peak moment of the knee extensors increases by one standard deviation or by 0.18 Nm/kg, the jumping height increases by approximately 3-cm, or 8.5%.

The most important result of our study was that the CMJ height is inversely related to the difference in the knee flexors between the stronger and weaker legs at a high testing velocity of 300 ${}^{\circ}$ /s. This relationship was presented in the results of the correlation analysis ( $r=-$ 0.396), and in the results of the regression model, where the value of the unstandardised coefficient was $-$ 0.249. This means that when the asymmetry of the knee flexors increases by one standard deviation or by 6.8 percentage points, jumping height falls by 1.7-cm or 4.8% on average. Previous studies have not confirmed the relationship between the asymmetry of the knee extensors and flexors and vertical jumping height, or else the effect of the asymmetry on jump performance has been very small [7, 9, 10, 19]. Yoshika and co-authors [19] used a computer simulation model and concluded that a bilateral difference of 10% in the strength of the leg muscles has a very small effect because of the compensatory work of the stronger leg. However, they accounted only for the contractile and segmental mass properties in the computational model as they were searching for the best solution for jump height, and so these results may not be applicable in the real world.

There is evidence that a bilaterally different distribution of lean mass between the muscles of the leg and the pelvis region is related to asymmetry of power during CMJ, and asymmetry of more than 10% in contralateral power during jumping is strongly related to lower performance in the jump [56]. It may be true that the leg extensors of the stronger side of the body can generate compensatory force during propulsive force generation [19], but when the stabilising muscle groups have a bilaterally uneven capacity to produce force, the ability to stabilise joints, direct forces and maintain balance during a two-legged action may be compromised. Our study confirms that a bilateral difference in the strength of the knee extensors was not related to a lower height in the jump, but asymmetry in the knee flexors was related to the height of the jump. The knee flexors are known to be the main providers of dynamic knee joint stability [37, 51] and it may be expected that the contralateral asymmetry is negatively related to the CMJ height. Our findings are partly in line with the results of [49], who found a moderate negative correlation between the CMJ height and the peak force asymmetry measured by force plates during an isometric mid-thigh pull action. This action needs a large contribution from the strength of the hamstring muscles, as well as from the hip extensors [57] and knee stabilisers [37, 51, 7] also find support for there being an indirect relationship between the asymmetry of the thigh muscles and the jump performance. They show that soccer players without asymmetry were able to use more effectively the stretch-shortening cycle, which is an important component in CMJ performance.

A limitation of our study was that it did not include the bilateral force application characteristics from the force plates in the analysis. In addition, future research needs to incorporate the jump kinematics, or body position, and kinetics characteristics like joint moment and power, and strength and mobility measurements for all the main lower limb muscle groups in its analysis. This will allow mechanism-level explanations for explaining the relationships between local muscle state characteristics and jumping ability.

5. Conclusions

The results of our study show that the best predictors of the CMJ performance are the isokinetic local muscle strength characteristics measured at higher testing velocities. Increased asymmetry in the strength of the knee extensors and lower asymmetry in the strength of the hamstring muscles have a statistically significant positive effect on the height of the countermovement jump.

Author contributions

CONCEPTION: Mikola Misjuk.

PERFORMANCE OF WORK: Mikola Misjuk and Indrek Rannama.

INTERPRETATION OR ANALYSIS OF DATA: Mikola Misjuk and Indrek Rannama.

PREPARATION OF THE MANUSCRIPT: Mikola Misjuk.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Mikola Misjuk and Indrek Rannama.

SUPERVISION: Mikola Misjuk and Indrek Rannama.

Ethical considerations

The code of Ethics of the World Medical Association was followed. Written informed consent was obtained from all participants, and permission was received from the Tallinn Medical Research Ethics Committee to conduct tests with humans for this study (Decision number 2241, application number 1817, issued on 19 February 2018).

Funding

The authors report no funding.

Footnotes

Acknowledgments

The authors have no acknowledgments.

Conflict of interest

The authors have no conflicts of interest to report.

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The effect of muscular strength and strength asymmetry on jumping height in soccer players

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.2 Procedures

2.3 Parameters measured

3. Results

Table 1 Descriptive statistics of the countermovement jump; the absolute peak moment and peak moment relative to body weight of the knee extensors and flexors, the H/Q ratio and the asymmetry parameters

5. Conclusions

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Descriptive statistics of the countermovement jump; the absolute peak moment and peak moment relative to body weight of the knee extensors and flexors, the H/Q ratio and the asymmetry parameters