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Abstract

OBJECTIVE:

To determine the effects of kicking leg preference on the bilateral leg strength asymmetries of amateur football players.

METHODS

: Thirty-four right-dominant (RD) and twenty-three left-dominant (LD) amateur football players volunteered to participate in the study. Squat and countermovement jump, standing long jump, and isokinetic knee strength (at 60, 180 and 300 ${}^{\circ}$ /s) were tested with respect to inter-limb asymmetry index (ASI).

RESULTS:

There were significant differences in horizontal jump and H/Q ratio at 60 ${}^{\circ}$ /s between DL and NDL in all football players. There were also significant differences in standing long jump, hamstring peak torque at 180 ${}^{\circ}$ /s and H/Q ratio at 60 ${}^{\circ}$ /s in the LD football players ( $p<$ 0.05). Moreover, the LD group had greater percentage of subjects with ASI $>$ 15% for standing long jump, hamstring peak torque at 180 ${}^{\circ}$ /s. Furthermore the percentage of subjects with H/Q Ratio $<$ 0.60 for 60 ${}^{\circ}$ /s was greater in the LD players’ DL.

CONCLUSION:

Bilateral leg strength asymmetries may be affected by a single leg predominantly in the mobilization workouts. Having higher inter-limb asymmetry index, LD players may be screened and trained to minimize leg strength asymmetries.

Keywords

Bilateral strength asymmetry dominant leg leg preference

1. Introduction

In football, mobility workouts (force and direction control in juggling and dribbling, movement timing to trap an approaching ball, and power and accuracy to kick a static or a moving ball) are considered as mastering the tasks [1]. Most of the players predominantly use their dominant leg performing these kinds of mobilization skills (specially kicking to goal) during the matches [2]. A few elite football players use each leg with a similar frequency [2, 3] and this lateral preference in football players is based on the functional advantage of the preferred leg over the non-preferred leg [4]. However, often preferring the dominant leg may cause bilateral asymmetrical strength development [5, 6, 7] because the quadriceps muscles are primarily used concentrically in kicks, jumps, and passes in football [8]. On the other hand, the hamstrings are primarily used eccentrically to stabilize, decelerate, and control the knee, but they are also used concentrically to turn, tackle, and sprint [8, 9, 10]. Therefore, bilateral leg and agonist/antagonist (H/Q) asymmetry may be developed in players who kick and pass the ball using mostly the dominant leg [11, 12].

Several studies [13, 14, 15, 16, 17] investigated both lower limb isokinetic strength and asymmetries in football players. Some studies stated that H/Q ratio less than 0.60 [18, 19, 20] and a bilateral strength asymmetry greater than 15% [8, 13, 14] may increase the risk of knee injuries [8, 13, 14, 21]. Other studies identified risky groups in terms of muscular imbalance related to professional training age [22, 23] and playing positions [23, 24, 25, 26, 27], and differences between sports branches [28, 29] and gender [30]. However, there is limited information about bilateral leg asymmetry concerning leg preference (lateralization) in football players. Indeed, LD players prefer their dominant leg during shooting to goal when compared to RD players [2]. When considered in conjunction with the above, we hypothesize that bilateral strength and H/Q asymmetries may be more developed in the LD players when compared to their RD counterparts. Therefore, the objective of this study was to assess the effects of kicking leg preference on the bilateral leg strength asymmetries in amateur football players.

2. Methods

2.1 Subjects

Fifty seven amateur football players, 34 RD (age: 21.12 $\pm$ 1.85 years; body height: 174.50 $\pm$ 5.18 cm; body weight: 69.42 $\pm$ 6.86 kg) and 23 LD (age: 21.70 $\pm$ 2.03 years; body height: 176.22 $\pm$ 6.27 cm; body weight: 68.73 $\pm$ 5.96 kg) participated in this study. The players did not have any serious injuries for at least six months. The study was approved by the Ethics Committee of Osmangazi University. A written informed consent was obtained from each player in accordance with the ethical standards of the Helsinki Declaration. All players were instructed to avoid demanding exercise for the 48 hours before testing.

2.2 Procedures

Functional jump tests: vertical squat and countermovement jump (SJ and CMJ), horizontal standing long jump (SLJ) and isokinetic knee extensor-flexor strength tests were performed using DL and NDL separately. After the familiarization trials, the main trials were completed within 10 days (functional jump tests in the first day and isokinetic knee strength tests in the-second day). All subjects were encouraged to perform their best in each test. The Renewed Waterloo Foot Preference Survey was used to evaluate each player’s tendency for foot preference.

2.3 Jump tests

Each subject had a 10-min warm-up session including self-paced cycling on a bicycle ergometer, calisthenics, and flexibility exercises. The subjects performed three trials in every jump with each leg (unilateral SJ, CMJ, and SLJ) allowing for 30–45-s recovery within each jump test. The best results in each type of jumps were recorded for statistical analysis. There was a 3-min recovery between the jump tests to minimize the fatigue. Moreover, the subjects’ hands were kept at their hips to prevent any influence of arm movements on each jump.

Vertical jumps were tested with the Newtest Powertimer Testing System (Ele-Products Oy, Finland). The subjects flexed their knees until they felt a comfortable starting position achieved normally at a knee angle of about 85 ${}^{\circ}$ when performing without a preliminary downward movement. The subjects performed CMJ starting from an upright standing position, followed by with a very fast preliminary downward movement by flexing the knee approximately to the same knee angle as the starting position in SJ.

After the SJ and CMJ, the subjects completed the SLJ. The initial position for the SLJ was with one foot from the marker on the floor and completed with the same foot landing on the floor. The distance between the marker to the first point of contact of the toes and the back side of the same foot on the floor was measured with the meter (Stabile, Germany).

2.4 Isokinetic strength test

Each subject had a 10-min warm-up session including self-paced cycling on a bicycle ergometer, calisthenics, and flexibility exercises. Isokinetic concentric unilateral knee extension and flexion torques were tested at 60, 180 and 300 ${}^{\circ}$ /s using the Isomed 2000 (D&R Ferstl GmbH, Germany) dynamometer. Each subject was sat with the hip joint at 90 ${}^{\circ}$ and was instructed to hold handgrips alongside the seat. Pelvis and thigh were stabilized using belts. Additionally, the shoulders were fixed in the ventral-dorsal and cranialcaudal direction by a shoulder rest. The axis of the knee rotation was considered to coincide with the lateral femoral condyle and aligned with that of the actuator. The lower edge of the training pad was placed 2.5 cm over the medial malleolus. The subjects performed three maximum efforts before testing at each angular velocity to warm-up. After a 30-sec rest period, subjects performed five maximal efforts with 60 s rest intervals after each test at the given angular velocities and the highest value of these efforts was considered as the peak torque. The rest time between the two leg tests was 3-min. Subjects were verbally encouraged during all these test processes and provided with concurrent visual feedback in the form of an isokinetic strength curve displayed on the dynamometer monitor.

2.5 Statistical analysis

The analysis was carried out using SPSS 18.0 (Statistical Package for Social Sciences, Chicago, IL, USA). All the obtained data were expressed as mean and standard deviation. Normality of the distribution and homogeneity of variance were calculated with Kolmogorov Smirnov test and Levene test, respectively. The independent t-test was used for comparison of the results of isokinetic knee strength and jump tests. Bilateral leg strength asymmetry percentage differences were calculated using the formula (DL-NDL/DLx100) as previously reported by Newton et al. [31] and Keays et al. [32]. An a priori alpha level of 0.05 was used to determine statistical significance.

Table 1
Jumping ability in amateur football players ( $n=$ 57). Results are group mean $\pm$ SD

Parameters	Football players
	Total ( $n=$ 57)		LD ( $n=$ 23)		RD ( $n=$ 34)
SJ
DL (cm)	20.08	$\pm$ 4.30	19.90	$\pm$ 5.26	20.20	$\pm$ 3.59
NDL (cm)	19.79	$\pm$ 4.29	20.27	$\pm$ 4.86	19.47	$\pm$ 3.89
ASI (%)	8.69	$\pm$ 6.41	13.43	$\pm$ 11.88	8.69	$\pm$ 6.41
ASI $>$ 15%	12 (21.1%)		7 (30.4%)		5 (14.7%)
CMJ
DL (cm)	20.06	$\pm$ 4.17	18.36	$\pm$ 4.73	21.22	$\pm$ 3.34
NDL (cm)	19.58	$\pm$ 4.18	19.03	$\pm$ 4.56	19.97	$\pm$ 3.92
ASI (%)	9.33	$\pm$ 10.41	7.76	$\pm$ 8.16	9.33	$\pm$ 10.42
ASI $>$ 15%	5 (8.8%)		2 (8.7%)		3 (8.8%)
SLJ
DL (cm)	165.31	$\pm$ 13.91	162.13	$\pm$ 12.61	167.47	$\pm$ 14.50
NDL (cm)	169.04	$\pm$ 13.68 ${}^{*}$	166.83	$\pm$ 10.32 ${}^{*}$	170.53	$\pm$ 14.00
ASI (%)	6.23	$\pm$ 4.52	7.56	$\pm$ 5.62	6.24	$\pm$ 4.52
ASI $>$ 15%	7 (12.3%)		5 (13.0%)		2 (5.9%)

SJ: Squat Jump, CMJ: Countermovement Jump, SLJ: Standing Long Jump, DL: Dominant Leg, NDL: Non-dominant Leg, ASI: Inter-limb Asymmetry Index, LD: Left Dominant Leg, RD: Right Dominant Leg, ${}^{*}$ : $p<$ 0.05.

Table 2

Isokinetic knee extensor and flexor peak torque values at 60 ${}^{\circ}$ /s, 180 ${}^{\circ}$ /s and 300 ${}^{\circ}$ /s angular velocities in amateur football players ( $n=$ 57). Results are group mean $\pm$ SD

Parameters	Football players
	Total ( $n=$ 57)		LD ( $n=$ 23)		RD ( $n=$ 34)
60 ${}^{\circ}$ /s QPT
DL (Nm)	169.12	$\pm$ 39.33	176.13	$\pm$ 49.26	164.29	$\pm$ 30.90
NDL (Nm)	163.75	$\pm$ 28.12	166.25	$\pm$ 29.44	162.16	$\pm$ 27.30
ASI (%)	9.63	$\pm$ 9.11	13.02	$\pm$ 9.73	9.64	$\pm$ 9.11
ASI $>$ 15%	15 (26.3%)		7 (30.4%)		8 (23.5%)
180 ${}^{\circ}$ /s QPT
DL (Nm)	133.19	$\pm$ 24.25	130.36	$\pm$ 31.30	135.76	$\pm$ 19.86
NDL (Nm)	133.70	$\pm$ 26.34	135.26	$\pm$ 34.18	132.34	$\pm$ 19.95
ASI (%)	7.46	$\pm$ 5.24	15.33	$\pm$ 11.06	7.46	$\pm$ 5.24
ASI $>$ 15%	10 (17.5%)		8 (34.8%)		2 (5.9%)
300 ${}^{\circ}$ /s QPT
DL (Nm)	105.11	$\pm$ 24.12	101.21	$\pm$ 30.88	108.29	$\pm$ 19.52
NDL (Nm)	103.15	$\pm$ 24.65	103.45	$\pm$ 31.81	104.49	$\pm$ 19.17
ASI (%)	9.17	$\pm$ 7.19	14.89	$\pm$ 17.46	9.18	$\pm$ 7.19
ASI $>$ 15%	15 (26.3%)		8 (34.8%)		7 (20.6%)
60 ${}^{\circ}$ /s HPT
DL (Nm)	110.48	$\pm$ 22.52	107.23	$\pm$ 21.95	112.46	$\pm$ 22.82
NDL (Nm)	114.56	$\pm$ 24.25	113.26	$\pm$ 27.55	114.73	$\pm$ 22.54
ASI (%)	10.95	$\pm$ 8.24	10.33	$\pm$ 11.37	10.95	$\pm$ 9.24
ASI $>$ 15%	16 (28.1%)		7 (30.4%)		9 (26.5%)
180 ${}^{\circ}$ /s HPT
DL (Nm)	93.85	$\pm$ 18.75	89.98	$\pm$ 17.65	95.49	$\pm$ 17.91
NDL (Nm)	97.12	$\pm$ 21.89	95.23	$\pm$ 20.72 ${}^{*}$	97.65	$\pm$ 20.99
ASI (%)	12.14	$\pm$ 12.17	13.06	$\pm$ 12.97	12.14	$\pm$ 12.17
ASI $>$ 15%	19 (33.3%)		10 (43.5%)		9 (26.5%)
300 ${}^{\circ}$ /s HPT
DL (Nm)	79.56	$\pm$ 18.22	71.46	$\pm$ 17.92	83.23	$\pm$ 16.21 ${}^{*}$
NDL (Nm)	75.23	$\pm$ 19.45	72.16	$\pm$ 22.38	77.89	$\pm$ 15.85
ASI (%)	9.26	$\pm$ 8.57	12.86	$\pm$ 26.26	9.26	$\pm$ 8.57
ASI $>$ 15%	16 (28.1%)		9 (39.1%)		7 (20.9%)

QPT: Quadriceps peak torque, HPT: Hamstring peak torque, DL: Dominant Leg, NDL: Non-dominant Leg, ASI: Inter-limb Asymmetry Index, LD: Left Dominant Leg Players, RD: Right Dominant Leg Players, ${}^{*}$ : $p<$ 0.05.

Table 3

Isokinetic knee flexor and extensor (H/Q) ratio at 60 ${}^{\circ}$ /s, 180 ${}^{\circ}$ /s and 300 ${}^{\circ}$ /s angular velocities in amateur football players ( $n=$ 57). Results are group mean $\pm$ SD

Parameters	Football players
	Total ( $n=$ 57)				LD ( $n=$ 23)				RD ( $n=$ 34)
	DL		NDL		DL		NDL		DL		NDL
60 ${}^{\circ}$ /s
H/Q Ratio	0.67	$\pm$ 0.11	0.70	$\pm$ 0.13 ${}^{*}$	0.60	$\pm$ 0.11	0.64	$\pm$ 0.14 ${}^{*}$	0.67	$\pm$ 0.11	0.70	$\pm$ 0.12
H/Q Ratio $<$ 0.60	20 (35.0%)		15 (26.3%)		12 (52.2%)		10 (43.5%)		8 (23.5%)		5 (14.7%)
180 ${}^{\circ}$ /s
H/Q Ratio	0.71	$\pm$ 0.16	0.71	$\pm$ 0.15	0.68	$\pm$ 0.14	0.66	$\pm$ 0.16	0.72	$\pm$ 0.17	0.74	$\pm$ 0.14
H/Q Ratio $<$ 0.60	10 (17.5%)		9 (15.6%)		6 (26.1%)		5 (21.7%)		4 (11.8%)		4 (11.8%)
300 ${}^{\circ}$ /s
H/Q Ratio	0.74	$\pm$ 0.12	0.73	$\pm$ 0.13	0.72	$\pm$ 0.12	0.71	$\pm$ 0.15	0.76	$\pm$ 0.12	0.78	$\pm$ 0.12
H/Q Ratio $<$ 0.60	6 (10.5%)		5 (8.8%)		4 (17.4%)		4 (17.4%)		2 (5.9%)		1 (2.9%)

H/Q ratio: Hamstring/Quadriceps ratio, DL: Dominant Leg, NDL: Non-dominant Leg, LD: Left Dominant Leg Players, RD: Right Dominant Leg Players, ${}^{*}$ : $p<$ 0.05.

3. Results

Table 1 outlines the significant differences between the DL and NDL with regard to SLJ in all the players. When comparing inter-limb differences according to lateral preference a significant difference ( $p<$ 0.05) was detected in the SLJ of the LD players who had greater ASI and percentage of subjects with ASI $>$ 15% than their RD counterparts in all jump tests.

No significant differences were observed between the DL and NDL for all isokinetic strength parameters (Table 2). Significant hamstring PM differences were indicated for 180 ${}^{\circ}$ /s in LD players and also for 300 ${}^{\circ}$ /s in RD players ( $p<$ 0.05). The LD players had greater isokinetic strenght ASI and percentage of subjects with ASI $>$ 15 % for all angular velocities compared to their RD counterparts.

Table 3 outlines the significant differences in isokinetic strength H/Q ratio between the DL and NDL of players at 60 ${}^{\circ}$ /s ( $p<$ 0.05). Similarly, there was a significant difference with regard to the H/Q Ratio at 60 ${}^{\circ}$ /s in LD players ( $p<$ 0.05). Additonally, the percentage of subjects with H/Q Ratio $<$ 0.60 for 60 ${}^{\circ}$ /s was greater in the LD players. Accordingly, the percentage of subjects with H/Q Ratio $<$ 0.60 for 60 ${}^{\circ}$ /s was greater in the LD players’ DL (52.2%) when compared with NDL (43.5%).

4. Discussion

Most of the football players use their DL during, predominantly, kicking, passing, and controlling the ball which may lead to bilateral leg strength asymmetry [11, 12]. Bilateral strength asymmetry greater than 15% [8, 13, 14] and H/Q ratio below than 0.60 [18, 19, 20] may increase the risk of knee injuries. In this context, the primary purpose of the study was to assess the effects of kicking leg preference on the bilateral leg strength asymmetries in amateur football players. In the current study, functional jump tests [33, 34, 35, 36] and isokinetic knee extensor-flexor strength tests [14, 37, 38, 39, 40] were performed to evaluate bilateral lower limbs asymmetries.

In the present study, NDL standing long jump distances were significantly greater than DL. Sannicandro et al. found similar asymetries (6.6%) between legs for standing long jump distances [35], a finding that was challenged by Maulder and Croninbefore [34] and in line with the present findings. Moreover, there were no significant differences in SJ and CMJ between DL and NDL. Evaluating bilateral leg asymmetries between legs using SJ and CMJ in football players may not yield exact results [34]. In football, vertical movements such as SJ and CMJ are not used often. Moreover, footbal players change their jumping leg related to the position in the game. This may be the reason for the absence of assymetries in terms of SJ and CMJ in these players. However, football players mostly use their DL when performing specific movements like kicking, passing, controlling the ball, etc. [1, 2]. These movements take place in the horizontal directions while NDL is used to balance control during these movements. This stuation can lead to stiffness and strenghening of NDL muscles and tendons [41]. Moreover, the standing long jump including multiple joints and antagonist co-contractions is more functional and mimics movements specific to football [42]. Therefore, significantly greater NDL standing long jump distances compared to DL is not surprising.

In the current study, there were no significant isokinetic knee strength differences between the legs. Similarly, Sangnier and Tourny-Chollet did not find any significant HPT difference at between DL and NDL in semi-professional football players [43]. In another study, Zakas reported no isokinetic knee strength asymmetries at 60, 180, and 300 ${}^{\circ}$ /s in players playing in Greek First Football Division League [11]. In contrast to the present study, Rahnama et al. indicated significant isokinetic knee flexion differences at 60 and 180 ${}^{\circ}$ /s between DL and NDL in elite and sub-elite football players [5]. Notably, slow angular velocities do not reflect movement speeds specific to football such as kicking [44]. Furthermore, single-joint, specific muscle groups, and open-chain movements lead to different results from multi-joint, antagonist co-contractions than closed chain movements [31, 37, 40].

The main results of the present study were the bilateral strength asymmetry differences related to leg preference. While significant differences regarding SLJ at 180 ${}^{\circ}$ /s and H/Q ratio at 60 ${}^{\circ}$ /s were observed in the LD subjects, in the RD players the only bilateral difference was at 300 ${}^{\circ}$ /s. Moreover, the LD players had greater percentage of subjects with ASI $>$ 15% for SLJ and hamstring PM at 180 ${}^{\circ}$ /s. Furthermore, the percentage of subjects with H/Q Ratio $<$ 0.60 for 60 ${}^{\circ}$ /s was greater in the LD players’ DL (52.2%) when compared with NDL (43.5%). These results are in agreement with our hypothesis. Supporting our results, Carey et al. demonstrated that LD players preferred their DL more than the RD players (87.2% vs. 74.3%) when kicking to goal in the French 98 World Cup [2]. In another study, Cameron and Adams determined that the LD players had a higher movement discrimination score when compared to the RD players, 0.83 vs. 0.77, respectively. Moreover, LD players had a significantly higher score on their dominant side while right-footers had no left-right side difference [45]. This points out to a bilateral leg preference among RD players; it has been speculated that using one leg dominantly caused the occurrence of asymmetrical strength development [5, 6, 7]. Due to the fact that the quadriceps is primarily used concentrically in passing, kicking, and jumping in football [8], the hamstrings muscles are primarily used eccentrically in controlling, decelerating, and stabilizing the knee, but also used concentrically in sprinting, turning, and tackling [8, 9, 10]. Therefore, bilateral leg and agonist/antagonist (H/Q) asymmetry may be more developed in the LD players who prefer dominant leg predominantly to kick and pass the ball.

5. Conclusion

To summarize only standing long jump and H/Q ratio asymmetries were found between the DL and NDL of football players. Similar to SLJ and H/Q ratio, hamstring strength asymmetries at 180 ${}^{\circ}$ /s were found in the LD football players. Moreover, the LD group had greater percentage of subjects with ASI $>$ 15% for standing long jump, hamstring strength at 180 ${}^{\circ}$ /s and lower H/Q Ratio $<$ 0.60 compared with the RD players whereas the only significant bilateral asymmetries between legs at 300 ${}^{\circ}$ /s were detected in RD players. Thus, it is suggested that the standing long jump may be more suitable to determine bilateral leg strength asymmetry while isokinetic knee strength measurement will be more suitable to determine agonist/antagonist (H/Q ratio) differences. The results may help strength coaches to determine bilateral asymmetries between legs using these tests that may be also useful information in monitoring strength development or recovery process after lower-limb injury. In particular, football coaches should pay special attention to LD football players in order to avoid lower extremity injuries because of vulnerability to leg strength asymmetries. We also suggest that LD players who have higher inter-limb asymmetry index may be screened and trained to minimize leg strength asymmetries.

Footnotes

Acknowledgments

This study was supported by grants from Anadolu University.

Conflict of interest

None to report.

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The effects of kicking leg preference on the bilateral leg strength asymmetries of amateur football players

Abstract

OBJECTIVE:

METHODS

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Methods

2.1 Subjects

2.2 Procedures

2.3 Jump tests

2.4 Isokinetic strength test

2.5 Statistical analysis

Table 1 Jumping ability in amateur football players ( n = 57). Results are group mean ± SD

4. Discussion

5. Conclusion

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Jumping ability in amateur football players ( $n=$ 57). Results are group mean $\pm$ SD