Physical Training Oct 2007

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Address correspondence to	Dr. Asimenia Gioftsidou,
	Department of Physical Education and Sports Science,
	Democritus University of Thrace,
	Campus
	69100 Komotini, Greece
	tel : +302531039662 fax : +302531039623
	e-mail (agioftsi@phyed.duth.gr).

Soccer is one of the most popular sports and attracts many participants all over the world (Heidt, Sweeterman, Carlona, Traub, Tekulve 2000; Peterson, Chomiak, Graf-Baumann, Dvorak 2000; Soderman, Werner, Pietila, Engstrom, Alfredson 2000). This participation though, leads to a considerable number of musculoskeletal injuries (Inklaar, Bol, Schmikli, Mosterd1996; Dvorak 2000).

Some of the factors responsible for soccer injuries are, contact with an opponent, field conditions, or are training related, e.g., muscle-strength imbalances or deficits (Inklaar 1994; Heidt, Sweeterman, Carlona, Traub, Tekulve 2000). Sahrmann (1989) and, similarly, Caillet (1977) define muscle imbalance as a failure of the agonist-antagonist relationship. The terms balance or imbalance do not refer to equal or unequal torque values, but to the balance between the torque ratios of agonistic and antagonistic muscle groups. Practitioners have often used ipsilateral agonist/antagonist muscle ratios as standards by which to measure the progress of rehabilitation or to assess muscle imbalance (Brown 2000). Concentric hamstring-to-quadriceps torque ratios have been studied extensively (Perrin 1993) with reported averages ranging from 0.5 to 0.75.

Similarly, many researchers report that bilateral differences (>15%) in muscular performance (quadriceps and hamsting) detected with isokinetic measures have been also considered as an important predictor of soccer players´ injuries or a result of previous injury due to an incomplete rehabilitation program (Brown 2000; Leatt, Shephard, Plyley 1987; Croiser, Crielaard 2000). Since soccer frequently involves one-sided activities such as kicking only with one leg, asymmetries in muscle strength between the two legs are possible (Leatt, Shephard, Plyley 1987; Kellis, Gerodimos, Kellis, Manou 2001). However, it is also possible that weaknesses or muscle imbalances identified by isokinetic testing, are almost always associated with a current or previous injury (Wrigley 2000).

The first aim of the present study was to detect the possible imbalances in muscular strength in soccer players. The second aim was to investigate the effect of a specific muscle-training program to restore the normal torque ratios of hamstring and quadriceps, reducing bilateral differences.
Methods

Sample The study was conducted on 68 professional players (age: 24,1 ± 5,7 yr, weight: 76,8 ± 5,7 kg, height: 1,82 ± 0,7 cm) who played in the first Greek National division (two years study). The 1^st year of the study 12 of the 35 players and the 2^nd year 11 from the 33 players were added to this team at the beginning of the preparation period. All the players were healthy without obvious symptoms or pain during soccer training.
Procedure During the preparation period, the 68 players (35 the 1^st year and 33 the 2^nd year) accomplished an isokinetic test of knee flexor and extensor muscle groups (pre-training measure), to detect possible imbalances or bilateral differences in muscular strength. A Cybex Norm (Lumex Corporation, Ronkohoma, NY) dynamometer was used for the isokinetic measurement. Procedures were in accordance with ethical standards of the Committee on Human Experimentation at the Institution at which the work was conducted and with the Helsinki Declaration of 1975.

Prior to undergoing the testing procedure, each subject performed a 10 min warm-up of cycling and a 5 min warm up of stretching exercises. The test was performed in a seated position (hip flexion =110).

A random specifying of starting limb was followed to minimize the effects of learning bias. Three submaximal and one maximal repetition of the trial at each speed were performed before each session to prepare the subject for the testing procedure. Testing was performed at low and moderately high angular velocities, 60° sec^-1 and 180° sec^-1. These angular velocities have been used by many investigators in order to evaluate knee muscular strength of soccer players (Kellis, Gerodimos, Kellis, Manou 2001; Dauty, Poriton-Josse, Rochcongar 2003; Ergun, Islegen, Taskiran 2004). A 30 sec rest was allowed between tests for all subjects (between the two angular velocities). Following the testing of one leg, there was a 3 minute rest and then the testing of the other leg began.

Data were recorded during three maximal repetitions of extension and flexion movements at each speed. The best peak torque value for each angular velocity defined the muscular strength.

These initial measurements detected muscular imbalances or deficits in 15 soccer players the 1^st year, and in 12 players the 2^nd year. These 27 players gave an interview to the rehabilitation trainer of the team about their previous injuries because most of them just had been placed on the team.

The 27 players followed a specific isokinetic training program for 2 months with a frequency of 3 times per week to correct the imbalances and deficits. The isokinetic training program included 10 sets (velocity spectrum exercise). The first 5 sets were executed with maximal effort in both flexor and extensor muscle groups, while the last 5 sets were executed maximally only for the weak muscle group based on the initial measurement (Table 1).

After the completion of the isokinetic training program the 27 involved players and the other 41 healthy soccer players of the team underwent the same isokinetic test (post-training measure).

Analysis

Data were analyzed using the SPSS PC (Version 8.0) program for Windows. Repeated-measures analysis of variance was used to test differences between pre- and post-training measures. Statistical significance was set at p<.05.

Results

According to the pre-training assessments, in 27 of 68 soccer players muscular imbalances or deficits were discovered. Differences between strengths of muscles on different sides of the body were 24% (SD=1) for the knee extensors and 18% (SD=11) for the knee flexors at the angular velocity of 60° sec^-1. At the angular velocity of 180° sec^-1 the differences were 23% (SD=12) for the knee extensors and 20% (SD=13) for the knee flexors. Differences in flexor to extensor strength ratios were also found between agonist and antagonist muscle groups. In 60° sec^-1 angular velocity, the ratio was 52% (SD=21) for the right leg and 51% (SD=22) for the left leg, and at 180° sec^-1 was 57% (SD=18) for the right leg and 59% (SD=20) for the left leg.

These 27 players were interviewed by the rehabilitation trainer about any injuries suffered during the last 3 years. Descriptive statistics showed that 13 players had hamstring strains (13/27), 9 had knee ligament strains (5 Anterior Cruciate Ligament and 4 Medial Collateral Ligament) (9/27), 3 had anterior knee pain (3/27) and 2 had an adductor strain (2/27).
Regarding the effectiveness of the training program applied to the 27 players (training group), repeated measures analysis of variance showed significant differences between pre and post training measures only for the training group. More specifically, the training group appeared to improve in peak torque values; for right knee extensors at the angular velocity of 60° sec^-1 F(1,26)=424,6 p<.05, and 180° sec^-1 F(1,26)=317,8 p<.05 and for right knee flexors at the angular velocity of 60° sec^-1 F(1,26)=411,23 p<.05, and 180° sec^-1 F(1,26)=319,22 p<.05 (Table 2). Similarly results revealed for the for left knee extensors at the angular velocity of 60° sec^-1 F(1,26)=431,5 p<.05, and 180° sec^-1 F(1,26)=339,42 p<.05 and for left knee flexors at the angular velocity of 60° sec^-1 F(1,26)=411,13 p<.05, and 180° sec^-1 F(1,26)=335,45 p<.05 (Table 2).

Angular Velocity
		60° sec-1		180° sec-1
		Right Leg	Left Leg		Right Leg	Left Leg
Knee Extensor Strength (Nm)
Healthy Group
Pre-training	Μ	244.7	243.1		161.4	160.9
	SD	25.3	25.9		21.2	19.8
Post-training	Μ	245.8	242.7		163.1	161.7
	SD	25.3	24.1		20.2	18.9
Training Group
Pre-training	Μ	232.6a	231.8a		156.3a	154.2a
	SD	31.2	31.9		25.3	22.8
Post-training	Μ	246.6 a	243.3a		163.6 a	162.6 a
	SD	21.5	16.8		13.5	16.3
Knee Flexor Strength (Nm)
Healthy Group
Pre-training	Μ	166.1	166.5		123.2	123.6
	SD	20.3	21.6		18.3	16.5
Post-training	Μ	167.8	169.6		126.1	125.9
	SD	18.4	19.6		16.8	16.3
Training Group
Pre-training	Μ	154.9b	156.5 b		112.7 b	111.4 b
	SD	26.1	27.1		19.2	18.4
Post-training	Μ	169.8 b	170.7 b		129.4 b	126.8 b
	SD	19.4	19.2		15.2	15.7

In addition, significant differences in peak torque ratios of flexors to extensors were found between the pre- and post-training measurements. At 60° sec^-1 angular velocity, the ratio changed from 0.52 (±21) to 0.66 (±8) for the right leg, and from 0.51 (±22) to 0.65 (±8) for the left leg. Respectively, at the 180° sec^-1 angular velocity, the ratio changed from 0.57 (±18) to 0.67 (±7) for the right leg, and from 0.59 (±20) to 0.67 (±9) for the left leg.

For the training group, at the post training measures the bilateral differences decreasing significantly (Table 3).

Angular velocity	Bilateral Differences (%)
°sec-1	Knee Extensors				Knee Flexors
	Pre-training		Post-training		Pre-training		Post-training
	M	SD	M	SD	M	SD	M	SD
60°sec-1	24	15	10**	5	15	7	7**	7
180°sec-1	18	12	8**	6	17	5	4**	4

Finally, at the post-training measures there were not significant differences (p>.05) between the training group (27 players) and the rest healthy soccer players (41 players) in peak torque values for knee extensors and flexors at both angular velocities (Table 2).

Discussion
Some coaches, athletes, medical personnel, strength and conditioning staff, believe that injury is often the result of weaknesses in particular muscle groups, which can be detected by isokinetic testing (Wrigley 2000). However, it is still more important that, weaknesses identified by isokinetic testing, are almost always associated with a current or previous injury (Wrigley 2000). More specifically, Schwellnus (2004) reported that many factors are postulated to increase the risk of developing acute muscle injuries, such as previous recent muscle injury (Orchard 2001; Emery, Meeuwisse 2001; McHugh 2004) and past muscle injury (Orchard 2001; McHugh 2004).

Hagglund, Walden and Ekstrand (2005), reported that in Denmark and Sweden 30% and 24% of the injuries in soccer were re-injuries. This is within the range of what has been reported from most studies, where 22-42% were re-injuries (Nielsen, Yde 1989; Engstrom, Forssblad, Johansson, Tornkvist1990; Hawkins, Fuller 1996). Inadequate rehabilitation and premature return to play after injury have been suggested as risk factors for recurrence of injury in previous studies (Hagglund, Walden, Ekstrand, 2005).

In addition, Schwellnus (2004) reported and some other factors who increase the risk of developing acute muscle injuries, such as decreased muscle strength (mainly eccentric muscle strength) (Askling, Karlsson, Thorstensson 2003), and muscle imbalance (decreased eccentric (antagonist) to concentric (agonist) muscle strength) (Dauty, Poriton-Josse, Rochcongar 2003).

The most effective interventions to reduce the incidence of muscle injuries will be those that address known risk factors (McHugh 2004). The best approach is to identify risk factors for the population of interest and then develop interventions specifically for that population. In practice, most teams are interested in applying an intervention without first studying the injury patterns and risk factors in their own athletes. In this situation, one of the most obvious risk factors to address is a history of a previous injury (Emery, Meeuwisse 2001; McHugh 2004). The high rate of recurrence for muscle injuries indicates incomplete recovery and may be attributable to inadequate rehabilitation. Therefore athletes with a history of previous injuries could be placed on a rehabilitation programme to address the involved muscle group (McHugh 2004).

The isokinetic measurement performed in the present study detected many muscle strength imbalances (concentric hamstring-to-quadriceps ratio lower than 0.6) (Heiser, Weber, Sullivan, Clare, Jacobs 2002), and deficits (bilateral differences >15%) (Leatt, Shephard, Plyley 1987; Croiser, Crielaard 2000; Wrigley 2000).

More specifically, in two years, 27 of 68 soccer players on this team appeared to have deficits and/or imbalances in the knee joint muscles, also due to an incomplete rehabilitation program after knee injuries. In accordance with the present study statement is a study performed on sprint runners. Jonhagen and his coworkers (Johnagen, Nemeth, Eriksson 1994) found that sprint runners with a history of very severe hamstring strains were weaker than the uninjured sprinters. Similarly, Ekstrand and Gillquist (1983) studied 180 senior amateur soccer players and mentioned that non-contact knee injuries occurred in knees with pathological ligamentous laxity due to previous knee injuries.

The 27 soccer players followed the specific isokinetic training program for 2 months with a frequency of 3 times per week in order to correct the imbalances and the deficits. As regards the isokinetic training program, it used a method of strength training appropriate to promote an optimal neuromuscular response, a velocity spectrum exercise protocol included 10 sets (Kovaleski, Heitman 2000). The first 5 sets were executed with maximal effort in both flexor and extensor muscle groups, in order to improve the muscle strength, while the last 5 sets were executed maximally only for the weak muscle group (as determined by the initial measurement), in order to correct the muscle imbalances and deficits (Table 1).

The results showed that there were significant differences between the pre- and post-training measures in peak torque values for knee extensors and flexors at both angular velocities. In addition, it is important that these players not only increase their peak torque values but also decrease the variability (standard deviations) of strength measures. Moreover, at the post-training measures there were no significant differences (p>.05) between the training group (27 players) and the healthy soccer players (41 players) in peak torque values for knee extensors and flexors at both angular velocities.

Examining the effectiveness of the training program in correcting strength deficits and/or imbalances, the results showed that a specific isokinetic training program, performed for 2 months with a frequency of 3 times per week, can restore efficiently the muscular performance expressed by peak torque (Table 2). Thus, the application of an isokinetic test is useful in examination of possible muscle strength imbalances or bilateral differences in strength of knee muscle groups, and the performance of a specific isokinetic training program could eliminate these strength deficits and/or imbalances.

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