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Abstract

BACKGROUND:

Vertical jumping capacity is important in modern soccer to gain advantage over opponent during the defensive and offensive activities. Predictors of jumping capacity in soccer players are not well studied.

OBJECTIVE:

The main aim of our study was to evaluate how isokinetic strength of the thigh muscles is related to jump performance.

METHODS:

Twenty-five ( $N=$ 25) elite soccer players volunteered to participate in the study. Thigh muscle strength (concentric for hamstrings and quadriceps, and eccentric for hamstrings only) was evaluated at 60 ${}^{\circ}$ /sec using an isokinetic dynamometer, while jump capacity was evaluated using a bilateral force plate to measure the jump height of squat (SJ) and countermovement (CMJ) jump. Linear regression model was used to evaluate how well can isokinetic strength parameters (independent predictors) predict jumping capacity in soccer players (dependent variable).

RESULTS:

In a linear regression model concentric quadriceps strength has significantly explained the vertical jump height, while other parameters (hamstring concentric or eccentric strength) did not reach the significance level ( $p>$ 0.05). When left and right concentric quadriceps strength was used the model was statistically significant ( $F=$ 4.76, $p=$ 0.02; R square 0.31), but only right concentric quadriceps strength was significant predictor ( $t=$ 2.62, $p=$ 0.016). Strength asymmetry did not significantly influence the jump height, but was associated with lower ratio of CMJ/SJ height indicating that players without asymmetry have better jumping capacity in the terms of utilizing the stretch shortening cycle.

CONCLUSIONS:

Coaches should consider quadriceps strength as an important predictor of jumping capacity in top-level soccer players which is again related to sprint performance in soccer. Future studies are needed to evaluate the contributing impact of hamstring strength in this regard.

Keywords

Isokinetics strength asymmetry leg power vertical jump performance

1. Introduction

The physical demands in modern soccer are steadily increasing due to new tactical requirements of the game. Tactical measures as for example “pressing” and “counterpressing” in terms of “immediate ball recovery” are requiring more high-intensity exercise periods during the match compared to match activities and tactical roles some decades before [1]. In this context, the amount of high-speed running during the match separates top-class professional soccer players from moderate professional soccer players [2]. More high-intensity performance periods during the match [1] and consequently a higher level of intensity during exercise/training sessions may provoke numerous muscle injuries. For instance, current research is focusing on strength imbalances and prevention of hamstring muscles injuries in soccer as well as in other sports [3, 4, 5, 6, 7].

However, high speed running or sprinting is a typical concentric-eccentric movement, which soccer players have to perform straight-line or with changes of direction (COD) on a nonlinear trajectory [8, 9]. While playing soccer, muscles have to generate and absorb high forces during acceleration and deceleration [10]. In this context, the assessment of lower extremity muscle strength through isokinetic testing is very advisable in soccer due to the complexity of muscle activities during the match [11, 12]. Particularly the result of isokinetic bilateral strength measurement of agonist and antagonist muscle activities seems to be a valid method in soccer [10, 13]. Furthermore the results of isokinetic dynamometry strength measurement (concentric as well as the eccentric) could help prevent muscle injuries through strength exercises, which could reduce muscular imbalances [5, 14, 15].

Previous papers have confirmed a relationship between isokinetic knee strength and single-sprint performance as well as repeated-sprint ability in soccer and rugby players [16]. These mentioned conditional abilities of a soccer player are vital for good match performance. The diagnostics of the explosive power of lower extremities by vertical jump and force platform is an important method regarding the investigation of closed kinetic chain activities [17, 18]. Furthermore, the results of vertical jump performance (countermovement jump and squat jump) can provide useful performance data, which explains the functionality of the neuromuscular system in lower extremities of athletes [19, 20]. Power production in lower extremities seems necessary for a good athletic performance in soccer in terms of acceleration, maximal velocity and agility [21, 22].

As indicated, there is a large body of scientific research regarding isokinetic strength measurements and the explosive power of lower extremities. However, currently little attention has been paid to the possible relationship between isokinetic performance and vertical jumping performance [23, 24, 25]. The main aim of our research was to evaluate that relationship in top-level soccer players [23]. In particular, we explored the extent to which strength parameters can predict vertical jumping capacity.

2. Material and methods

2.1 Participants

Twenty-five male soccer players (age 22.5 $\pm$ 3.4; height 182.7 $\pm$ 6.9 cm; weight 77.9 $\pm$ 5.7 kg) of a first Slovenian national soccer League club were evaluated. Prior to participation, all subjects were informed, understood procedures and purpose of the study and gave their written consent to participation in this study which complied with the Code of Ethics of the Declaration of Helsinki.

2.2 Procedures

Body height was measured by GPM (Swiss) anthropometer while body composition was measured by octopolar bioimpedance InBody 720 (Biospace, Seoul, Korea) which was validated in different studies [26, 27]. The latter instrument enables the analysis of five basic body parts independently: the left and right upper limb, trunk, and left and right lower limb, using frequencies of 1, 5, 50, 250, 500 and 1000 kHz [28].

The isokinetic testing was performed by an experienced examiner in the Laboratory for isokinetic testing at the Faculty of Sport in Ljubljana, Slovenia. The laboratory was air-conditioned, and room temperature was held between 22–24 ${}^{\circ}$ C. Testing was performed between 10 AM and 4 PM over a period of one week. Each testing session started with a warm up consisting of cycling for 6 minutes at moderate pace (50–100 W), followed by a 15 second stretch of quadriceps and hamstrings.

Concentric of the quadriceps and hamstrings and eccentric testing for the hamstring only were conducted using iMoment, SMM isokinetic dynamometer (SMM, Maribor, Slovenia). The use of this dynamometer has been reported [29, 30]. This is a self constructed dynamometer made in Slovenia. The performance on this dynamometer was previously tested for absolute agreement on a group of physical education students using intraclass correlation coefficient (ICC) in a test-retest manner, but the results were not published (Table 1) and authors are aware of the fact that this represents an important limitation of current study.

Table 1
Intraclass correlation coefficients for self-constructed isokinetic dynamometer used in the study

Parameter	N	ICC	95% confidence
			interval for ICC
Quadriceps concentric	365	0.936	0.921–0.948
Hamstring concentric	276	0.906	0.881–0.926
Hamstring eccentric	128	0.943	0.920–0.960

The participants were tested in sitting position. Forward sliding on the seat was prevented with the use of proper 4-point belts that were pushing pelvis downward and backward, but were not uncomfortable for the participants and were also controlling the trunk movement. The thigh of the tested leg was secured using a special additional belt. The subjects were instructed to hold the side handles of the chair during testing as stabilization with hand-grip can enhance knee extension maximal torque [31]. The axis of rotation of knee joint was identified through the lateral femoral condyle and aligned with the motor axis. A range of motion (RoM) of 60 ${}^{\circ}$ was set from 90 ${}^{\circ}$ to 30 ${}^{\circ}$ knee flexion (full extension considered 0 ${}^{\circ}$ ) as previously used in the literature [32, 33], as the short RoM does not significantly influences changes in peak torque values for both quadriceps and hamstrings [33, 34]. Testing was performed at 60 ${}^{\circ}$ /s for both hamstring (H) and quadriceps (Q) in concentric mode and for H in eccentric contraction mode. Gravity error torque was recorded for every subject. Prior to testing each participant performed a series of 3 submaximal repetitions at a given velocity while the device was in a continuous passive mode (CPM). After the initial CPM set each participant performed 5 maximal contractions in the following order: (1) five consecutive concentric Q and H contractions followed by a 60 s pause, (2) five eccentric H contractions. When testing of one side was completed, a 3-minutes’ break followed during which the machine setting was changed to accommodate for the opposite leg. The first tested leg was assigned randomly for each subject. There was no verbal encouragement during testing repetitions.

Jumping capacity was evaluated using a bilateral force plate (S2P, Ljubljana, Slovenia) by measuring the jump height which was calculated from the flight time of squat (SJ) and countermovement (CMJ) jump. The size of each plate was 300 $\times$ 600 mm. The signals were acquired at 1,000 Hz and transferred to a personal computer via the USB interface. The ARS software (Analysis & Reporting Software; S2P Ltd., Ljubljana, Slovenia) was used for acquisition and treatment of the selected jump parameters.

2.3 Statistical analysis

Analyses were conducted using SPSS for Windows (Version 21.0; SPSS, Inc., Chicago, USA). Data were presented using descriptive statistics (Means $\pm$ SD). Furthermore, we performed a linear regression model in order to evaluate how well can isokinetic strength parameters predict jumping capacity in soccer players. Three separate regression analysis were performed for each jump type where quadriceps concentric, hamstring concentric and hamstring eccentric strength was used as a predictor of vertical jump height (squat or countermovement). In addition, a one-way ANOVA was used in order to compare jump height data of players with and without important strength asymmetries. Asymmetry cutoff of 10% was used in calculations. Statistical significance was set at $p\leqslant$ 0.05.

3. Results

In a linear regression model (Table 2) only concentric quadriceps strength has significantly explained the vertical jump height, while other parameters (hamstring concentric or eccentric strength) did not reach the significance level ( $p>$ 0.05).

Table 2
Countermovement and squat jump height prediction using strength parameters – a linear regression model

Jump type	Model	Predictors	Unstandardized		Standardized	$t$	Sig.	Collinearity
			coefficients		coefficients			statistics
			B	Std. error	Beta			Tolerance	VIF
Countermovement	1 ( $F=$ 4.76, $P=$ 0.02)	(Constant)	0.228	0.067		3.408	0.003
jump (CJM)		QLcon	0.000	0.000	$-$ 0.198	$-$ 0.757	0.458	0.481	2.08
		QRcon	0.001	0.000	0.684	2.618	0.016	0.481	2.08
	2 ( $F=$ 2.18, $P=$ 0.11)	(Constant)	0.213	0.089		2.411	0.026
		QLcon	0.000	0.000	$-$ 0.219	$-$ 0.631	0.536	0.299	3.34
		QRcon	0.001	0.000	0.632	1.826	0.084	0.301	3.32
		HLcon	0.000	0.001	0.011	0.034	0.973	0.363	2.75
		HRcon	0.000	0.001	0.079	0.238	0.814	0.327	3.06
	3 ( $F=$ 1.50, $P=$ 0.24)	(Constant)	0.198	0.095		2.091	0.052
		QLcon	0.000	0.000	$-$ 0.199	$-$ 0.535	0.600	0.278	3.60
		QRcon	0.001	0.000	0.719	1.916	0.072	0.273	3.66
		HLcon	0.000	0.001	0.150	0.347	0.733	0.205	4.88
		HRcon	0.000	0.001	$-$ 0.060	$-$ 0.160	0.875	0.271	3.69
		HLecc	0.000	0.000	$-$ 0.286	$-$ 0.854	0.405	0.344	2.91
		HRecc	0.000	0.000	0.168	0.621	0.543	0.523	1.91
Squat jump (SJ)	1 ( $F=$ 3.01, $P=$ 0.07)	(Constant)	0.227	0.073		3.126	0.005
		QLcon	0.000	0.000	$-$ 0.102	$-$ 0.376	0.711	0.484	2.07
		QRcon	0.001	0.000	0.532	1.958	0.063	0.484	2.07
	2 ( $F=$ 1.93, $P=$ 0.14)	(Constant)	0.176	0.092		1.900	0.072
		QLcon	0.000	0.000	$-$ 0.055	$-$ 0.161	0.873	0.312	3.21
		QRcon	0.000	0.000	0.267	0.787	0.441	0.313	3.19
		HLcon	0.000	0.001	$-$ 0.178	$-$ 0.577	0.570	0.379	2.64
		HRcon	0.001	0.001	0.432	1.325	0.200	0.340	2.94
	3 ( $F=$ 1.22, $P=$ 0.34)	(Constant)	0.181	0.100		1.806	0.088
		QLcon	0.000	0.000	$-$ 0.063	$-$ 0.172	0.865	0.292	3.42
		QRcon	0.000	0.000	0.220	0.599	0.556	0.293	3.41
		HLcon	$-$ 0.001	0.001	$-$ 0.276	$-$ 0.663	0.515	0.227	4.40
		HRcon	0.001	0.001	0.505	1.369	0.188	0.290	3.45
		HLecc	0.000	0.001	0.173	0.525	0.606	0.362	2.76
		HRecc	0.000	0.001	$-$ 0.072	$-$ 0.261	0.797	0.523	1.91

When left and right concentric quadriceps strength was used the model was statistically significant ( $F=$ 4.76. $p=$ 0.02; R square 0.31), but only the right concentric quadriceps strength served as a predictor ( $t=$ 2.62. $p=$ 0.016). Right quadriceps concentric strength was a significant predictor of vertical jump height ( $t=$ 3.02. $p=$ 0.006) in a univariate model as well, where only right quadriceps strength was used in a model ( $F=$ 9.12. $p=$ 0.006. R square $=$ 0.29). It is interesting to note that significant relationship was found only for CMJ, while the same approach for the SJ did not reach the significance level ( $p=$ 0.07).

As the unilateral quadriceps concentric strength was an important predictor of the jump height we have also analyzed how the possible strength asymmetry of quadriceps influence the jumping capacity. One-way ANOVA was used in order to compare player with important strength asymmetry ( $>$ 10%) with players with no strength asymmetry ( $<$ 10% difference). The cut-off value of 10% was based on our previous review [38] where we have showed that in healthy (uninjured) soccer players the differences between left and right side (or dominant vs. non-dominant leg) are below 10%. The results have shown that there was general trend for players that had no strength asymmetry to perform better in jumping tasks, but there were no statistically significant differences among the groups.

However, the only difference that we have noticed was the ratio between CMJ and SJ height where players with asymmetry had lower ratio (1.01; 95% CI 0.97–1.06) in comparison with players without asymmetry (1.09; 95% CI 1.04–1.13). This finding could indicate that players without asymmetry may have had better jumping capacity in the terms of utilizing the stretch shortening cycle, but as we did not control for this factor, it can be rather speculative.

4. Discussion

The primary finding of our study indicates that quadriceps concentric strength significantly explains up to 30% of the vertical jump height in soccer players and that this finding is true only for CMJ [23]. We can hypothesize that the stretch shortening cycle (SSC) during CMJ could explain that only CMJ was significantly related to quadriceps concentric strength, as it employs both the storage of elastic energy and the stimulation of the muscle spindle stretch reflex [35].

Some previous studies [18, 22] are in line with our main finding i.e. there is a conspicuous link between concentric quadriceps strength and CMJ height. Furthermore, there is also a link between good jumping capacity and better sprint performance in soccer players indicating possible common explanation via SSC [21, 22]. Our finding that strength asymmetry is not related necessarily to impaired vertical jump performance in soccer players is also in agreement with previous studies [36].

Moreover, we have also shown that players without asymmetry in quadriceps strength perform better in the terms of CM/SJ ratio. The last finding contradicts the results of a computer simulation study [33], which has claimed that the effect of bilateral asymmetry of $>$ 10% muscle strength did not affect the height of a CMJ due to strength compensation processes between weak and strong leg during the jump.

Finally the results of our study indicate that strength capacity and strength ratio (Table 3) of the current sample are within the previously reported normal range for healthy soccer players with low injury risk [3, 4, 5, 6, 7, 11, 34].

Table 3
Jumping capacity and isokinetic knee strength at 60 ${}^{\circ}$ /s

				Strength asymmetry
		Mean	Std. dev.	Mean	Std. dev.
Jumping capacity	Squat jump (SJ)
	Height (m)	0.37	0.05	NA
	Power (W/kg)	55.44	6.12	NA
	Left	27.74	3.25	$-$ 0.05	4.28
	Right	27.72	2.97
	Countermovement jump (CMJ)
	Height (m)	0.39	0.05	NA
	Power (W/kg)	53.02	5.35	NA
	Left	26.24	2.84	0.97	4.60
	Right	26.50	2.60
	Ratio CMJ/SJ	1.05	0.08	NA
Strength capacity	Strength (Nm/kg)
	Concentric quadriceps (Qc)
	Left	3.80	0.46	$-$ 9.28	14.01
	Right	3.51	0.46
	Concentric hamstrings (Hc)
	Left	2.27	0.24	$-$ 3.69	10.91
	Right	2.20	0.22
	Eccentric hamstrings (He)
	Left	2.18	0.37	5.94	15.00
	Right	2.33	0.32
	Strength ratios
	HQR(Hc/Qc)
	Left	0.60	0.07	NA
	Right	0.63	0.06
	DFR (He/Qc)
	Left	0.58	0.11	NA
	Right	0.67	0.14
	He/Hc ratio (He/Hc)
	Right	0.95	0.13	NA
	Left	1.06	0.17

The possible limitation of this study relate to a single velocity isokinetic test apart from the already mentioned issue of the choice of dynamometer. In this connection, we should mention former studies [23, 39] that recommended higher isokinetic velocity tests in order to show better relationships between isokinetic extensions with vertical jump heights. In addition, a moderate sample size needs to be considered as a limiting factor for even firmer conclusions. For further studies, we could recommend also including counter movement jump with arm swing as it increases jump performance duo to its inclusion of hip joint muscles [40] and represent more complete muscle chain, which is involved in vertical jumping. Finally, the difference observed players with asymmetry compared to those without asymmetry in relation to the ratio between CMJ and SJ height where (1.09; 95% CI 1.04–1.13) could indicate that the latter may have had better jumping capacity in the terms of utilizing the stretch shortening cycle, but as we did not control for this factor, it can be rather speculative.

5. Conclusion

To conclude, there is a significant relationship between concentric quadriceps strength and vertical jump performance in top-level soccer players where stronger quadriceps is related with better jumping capacity.

Footnotes

Conflict of interest

All authors declare no conflict of interest.

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Predictors of vertical jumping capacity in soccer players

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Material and methods

2.1 Participants

2.2 Procedures

Table 1 Intraclass correlation coefficients for self-constructed isokinetic dynamometer used in the study

3. Results

Table 2 Countermovement and squat jump height prediction using strength parameters – a linear regression model

Table 3 Jumping capacity and isokinetic knee strength at 60 ∘ /s

Footnotes

Conflict of interest

References

Table 1
Intraclass correlation coefficients for self-constructed isokinetic dynamometer used in the study

Table 2
Countermovement and squat jump height prediction using strength parameters – a linear regression model

Table 3
Jumping capacity and isokinetic knee strength at 60 ${}^{\circ}$ /s