COMPARISON OF LATERAL ABDOMINAL MUSCLE THICKNESS IN YOUNG MALE SOCCER PLAYERS WITH AND WITHOUT LOW BACK PAIN

Abstract

Background

While researchers have investigated low back pain (LBP) and its association with the thickness of trunk muscles in the general population, few articles have studied this relationship in athletes.

Hypothesis/Purpose

To compare the lateral abdominal muscle thickness and other possible functional risk factors in young soccer players with and without LBP.

Study Design

Cross-sectional study

Methods

Thirty young male soccer players, with and without LBP, from the Premier League participated in this study. The thicknesses of the external oblique, internal oblique and transversus abdominis muscles were measured via musculoskeletal ultrasound imaging, bilaterally. In addition, hamstring flexibility, lumbar spine flexion range of motion, and trunk extensor muscle endurance were measured and were compared in those with and without the history of LBP.

Results

The mean age of the subjects was 17.4 (+/- 1.1) years. There was no statistically significant difference between groups (p > 0.05). Subjects with a history of LBP during their lifetime of sports participation (sports life), within the prior year, and within the prior month had statistically significant lower external oblique muscle thickness bilaterally (p<0.05). Subjects with a sports life history of LBP had lower internal oblique muscle thickness on both sides (p<0.05). Moreover, those with a sports life history of LBP had significantly less hamstring flexibility than the non-LBP group on the dominant limb (p < 0.05).

Conclusion

In this sample group of young soccer players, abdominal muscle ultrasound measurements were different between players with and without LBP. Further longitudinal studies are needed to evaluate the role of these muscles as LBP risk factor for soccer players.

Levels of Evidence

3a

INTRODUCTION

Low back pain (LBP) has become a common complaint amongst athletes.¹ It is reported that the prevalence of LBP is between 1.3% and 6.5% in elite male soccer players,² and reported LBP odds ratio is about 1.6 (CI: 1.3-2.2) in soccer players.³ Several risk factors such as height, weight, high levels of physical activity, muscle endurance, and flexibility have been proposed as the risk factors for LBP in young athletes.⁴ However, there may be other unknown risk factors.

Researchers have suggested that unilateral or bilateral abdominal muscles’ with altered motor control, including internal oblique (IO), external oblique (EO), and transverse abdominis (TrA) muscles, may have a role in LBP.^5,6 In addition, alteration in the ability of these muscles during drawing in maneuver has been found in subjects with LBP in comparison with normal subjects.^7,8 Regarding the role of these muscles in the athletes’ LBP, Hides et al. showed that cricketers with LBP had less activity of TrA muscle in the draw in maneuver during MRI assessment,⁹ and elite Australian Football League players with LBP showed lower recruitment during abdominal drawing in maneuver also measured via MRI.⁷ Additionally, Rostami et al. revealed that off-road cyclists who suffered from LBP had thinner TrA and smaller cross-sectional area of lumbar multifidi muscles in comparison with cyclists without LBP, when assessed using musculoskeletal (MSK) ultrasound.¹⁰ It could be hypothesized that changes in EO, IO, TrA thickness and their activation might have a role in soccer players’ LBP, and that stabilizing exercises that focus on coordination and strengthening of these muscles ¹¹ could help reduce the risk of spinal injury and may be useful in the treatment of LBP in soccer players.

No study has investigated the status of EO, IO, and TrA muscles thickness in young soccer players with and without LBP, while there are several studies on this topic for other sports.^7,9,12 The purpose of this study was to compare the lateral abdominal muscle thickness, and other possible functional risk factors (including hamstring flexibility,⁴ trunk extensor muscles endurance,¹³ leg length discrepancy¹⁴ and lumbar spine flexion range of motion¹⁵) in young soccer players with and without LBP.

METHODS

Study population

A cross-sectional study was conducted in the Sports Medicine Research Center. This study was in accordance with the principles of the Declaration of Helsinki and the study design and protocol were approved by the institutional review board and ethics committee of Tehran University of Medical Sciences.

Fifty-five young male players between the ages of 16-20 years were enrolled in the study. They were playing at the youth soccer league in three soccer clubs (all members of clubs). A written informed consent was obtained from all subjects and/or their parents. A trained interviewer attended the training camp and asked the players to complete the questionnaire (Appendix 1). LBP was defined as “a pain between the last rib and lower gluteal fold as you can see in the following mannequin (labeled with a gray area on the questionnaire), which is bad enough to limit or change an athletes’ daily routine or sports activities for more than one day” according to a previous study by Noormohammadpour et al.¹⁶ Exclusion criteria were considered as those with history of direct trauma to the lumbar area, those who had leg pain or paresthesia in addition to back pain, musculoskeletal deformity (i.e. scoliosis or kyphosis), history of lumbar or abdominal surgery, systemic disease which may have influence on abdominal muscle thickness, and those subjects who had participated in exercises which dominantly activate EO, IO, and TrA muscles during the prior six months. Subjects were divided into different subgroups according to their history of LBP: lifetime LBP (experience of LBP at any time at their life), sports life LBP (experience of LBP at any time after beginning sports participation), prior year LBP (experience of LBP in the past year), prior month LBP (experience of LBP in the past month), recent LBP (experience of LBP over the past 48 hours), and no history of LBP (no experience of LBP in any time at their life).

Measurements

Subjects were asked to visit the Sports Medicine Research Center for all measurements and examinations. In addition, age, LBP VAS (visual analog scale from 0 to 10) during last episode, training sessions or competition absence, and care-seeking behaviors due to LBP were asked in the questionnaire (Appendix 1).

Height: Height of the subjects was measured by asking the subjects to stand straight without shoes, place their heels together, look straight ahead and take a deep breath and hold it.¹⁷

Weight: Weight was measured by asking the subjects to wear light sportswear (a T-shirt and shorts) and stand on a digital scale with the accuracy of 0.1 kilograms.¹⁷

Muscle thickness: A SonoSite sonography device (FUJIFILM SonoSite Inc., Bothell, WA, USA) with a 6 to 13 MHz linear transducer was used to measure the thickness of the abdominal muscles (EO, IO, and TrA) at rest in the B-mode format, bilaterally. Because of known effect of food consumption on abdominal muscle thickness,^18,19 subjects were asked to attend without having consumed breakfast. A point 25 mm anteromedial to the midpoint between the inferior rib and the iliac crest to the mid-axillary line was used as the standardized position to measure EO, IO, and TrA thickness where their fascial margins were parallel.²⁰ Subjects assumed a hook-lying position and the measurement of muscle thickness was performed by the transducer in transverse plane position at the center point of the image using the caliper feature of the device. The subjects could not view the process and the measurements were taken when they were at the end of the normal exhalation. Adequate gel was applied between the transducer and the skin to increase the contact area and reduce the need for the excess pressure of probe on the skin (which could lead to measurement errors). For more precise measurement, the depth of the image was manipulated so that the EO, IO, and TrA thickness filled approximately 40–50% of the ultrasound display.²¹ The distance between the top of the inferior fascial layer and bottom of the superior fascial layer of each muscle was considered as EO, IO, and TrA thickness, respectively.²⁰ The EO, IO, and TrA thickness bilaterally was measured twice in a one-hour interval by the same assessor for the assessment of reliability and mean of the two measurements for each muscle on each side was used for analysis.

Trunk extensor muscle endurance: In order to assess the trunk extensor muscles endurance, the Sorensen test was used. Subjects laid prone on the examination table, with the upper edge of the iliac crest at the edge of the table. To fix subjects’ lower limbs on the examination table, three straps were used at hip, knee, and ankles. Subjects were asked to cross their hands across the front of their chest and hold their upper trunk in an isometric horizontal position, while the amount of time that the subject could maintain this position was recorded.²² An inclinometer was applied gently between the two scapulae, and when the trunk was tilted down more than 5 to 10 ° the test was stopped.

Lumbar spine flexion range of motion (ROM): The amount of forward flexion of the subjects was assessed by using a tape measure to record the linear distance between the spinous processes of the 12^th thoracic and the 1^st sacral vertebrae in standing, at rest position and also while performing as much active forward flexion as possible.²³ A SonoSite sonography device with a 2 to 5 MHz curved transducer was used to find spinous processes. Difference between the two measurements was used for analysis.

Leg length measurement: Previous studies have shown that difference in leg length can be a risk factor for LBP in school children and young athletes;^14,24 therefore, leg length was measured with the subject in supine position. The distance between the anterior superior iliac spine and medial malleolus was recorded by using a tape measure to find the length of both limbs.²⁵

Hamstring muscle flexibility: Subjects were asked to lie down in the supine position and flex their hip joint to 90 degrees. The examiner then passively extended the knee, while preventing rotation and abduction of the subjects’ knee until pain or tightness of muscles limited the extension of the knee joint. The center of a standard goniometer was placed on the surface of lateral epicondyle of the femur, then its proximal arm was placed on the femur towards the greater trochanter and its distal arm was placed on the fibula toward lateral malleolus. The difference between measured data at this angle and 180 degrees was used for analysis.²⁶

Statistical Analysis

Data analysis was performed using Statistical Package for Social Sciences (SPSS, version 16, Chicago, Inc., US) and p < 0.05 was considered statistically significant. Quantitative and categorical variables were described as mean (SD) and number (percent), respectively. The Kolmogorov-Smirnov test was applied to assess the normal distribution of the data. To evaluate the association between LBP presence, and EO, IO, TrA thickness and other variables, a multiple linear regression model was used with adjustment for age and body mass(weight) of the participants as the potential confounders of outcomes.²⁷ Intraclass correlation coefficient (ICC) and standard error of measurement (SEM) were calculated to evaluate within-subject reliability.

RESULTS

Fifty-five young male players between 16-20 years old were recruited to participate in the study from three soccer clubs. However, twenty-five were excluded because seven subjects had the history of radicular leg pain symptoms, and eighteen subjects participated in a routine core stability program. Therefore, thirty subjects participated in the study. Table 1 displays the demographic characteristics of the study subjects. Mean (SD) age, and BMI of the subjects who reported sports life LBP were 17.2(1.1) years, and 20.7(2.7), respectively. Subjects participated in their first competition at a mean age of 13.4(2.3) years and their mean training time was 9.8(1.4) hours per week. The LBP features of the subjects are summarized in Table 2. The mean VAS was 4.9(1.4) out of 10. Demographic and clinical measurements for the subjects with and without the history of LBP are displayed in Table 3. There were no statistically significant differences regarding age, BMI, weekly training hours, and age of starting to compete between the different groups in the study population (p > 0.05).

Table 1.

The demographic characteristics of the study subjects.

Variable	Min	Max	Mean	SD
Age (years)	16.0	20.0	17.4	1.1
Weight (Kg)	49	100	65.6	11.3
Height (cm)	164	192	176.2	6.2
BMI (Kg/m2)	17.0	26.3	21.1	2.3
Training/week (hours)	5.0	15.0	9.8	1.7
Age of starting to compete (years)	9.0	17.0	13.4	2.4

SD: Standard deviation, BMI: body mass index

Table 2.

The characteristics of the population under study.

Variable	%(N)
LBP
Lifetime prevalence	50.0(15)
Sports life prevalence	50.0(15)
Last year prevalence	30.0(9)
Last month prevalence	16.7(5)
Point (recent) prevalence	10.0(3)
Care-seeking behaviors
Visit to LBP specialist	10.0(3)
Use of medication	16.7(5)
Plain radiography	3.3(1)
MRI	3.3(1)
Absent due to LBP
From training session	43.3(13)
From competition	16.7(5)
Dominant leg
Right	66.7(20)
Left	33.3(10)

N: Number

LBP, Low back pain; MRI, Magnetic resonance imaging

Table 3.

Demographics and clinical measurements between the subjects with and without history of LBP.

Variables			Sports life History of LBP		Last year History of LBP		Last month History of LBP
		Mean(SD)	p value	Mean(SD)	p value	Mean(SD)	p value
Age		LBP +	17.2(1.1)	0.45	17.2(1.1)	0.62	17.0(1.0)	0.42
		LBP -	17.6(1.0)		17.5(1.1)		17.5(1.1)
BMI		LBP +	20.7(2.7)	0.29	21.6(2.9)	0.63	20.1(1.9)	0.36
		LBP -	21.5(1.9)		20.9(2.0)		21.3(2.4)
VAS		LBP +	4.9(1.4)	0.001*	4.7(1.8)	0.001*	5.6(1.2)	0.001*
		LBP -	0.0(0.0)		0.0(0.0)		0.0(0.0)
Training/week (hours)		LBP +	9.8(1.4)	0.77	9.7(1.6)	0.93	9.6(2.2)	0.78
		LBP -	9.8(1.9)		9.9(1.8)		9.8(1.6)
Age of starting to compete (years)		LBP +	13.5(2.3)	0.67	13.2(2.6)	0.98	11.8(2.5)	0.19
		LBP -	13.2(2.5)		13.4(2.4)		13.7(2.3)
Sorensen test (seconds)		LBP +	124.9(39.2)	0.63	128.3(47.8)	0.78	139.8(46.3)	0.30
		LBP -	116.9(32.3)		117.7(29.8)		117.1(32.8)
Forward flexibility of lumbar spine (cm)		LBP +	7.0(1.2)	0.38	6.7(1.0)	0.13	6.4(0.9)	0.15
		LBP -	7.6(1.7)		7.6(1.6)		7.5(1.5)
Right hamstring flexibility (degrees)		LBP +	7.4(5.7)	0.13	8.1(6.9)	0.27	5.2(4.5)	0.82
		LBP -	4.8(4.9)		5.2(4.5)		6.3(5.6)
Left hamstring flexibility (degrees)		LBP +	9.1(8.6)	0.35	9.3(10.5)	0.79	5.0(2.2)	0.37
		LBP -	6.2(5.6)		6.9(5.6)		8.2(7.8)
Leg length difference (cm)		LBP +	0.07(0.3)	0.29	0.1(0.3)	0.82	0.0(0.0)	0.35
		LBP -	0.2(0.4)		0.1(0.4)		0.2(0.4)
EO(mm)	Right	LBP +	7.2(1.7)	0.001*	7.3(1.8)	0.03*	6.6(0.6)	0.03*
		LBP -	10.9(2.3)		9.8(2.7)		9.5(2.7)
	Left	LBP +	7.5(1.6)	0.001*	7.7(1.7)	0.03*	7.2(0.6)	0.04*
		LBP -	11.1(2.2)		9.9(2.7)		9.7(2.7)
IO (mm)	Right	LBP +	9.3(2.3)	0.042*	9.8(2.4)	0.57	9.1(2.1)	0.32
		LBP -	11.0(2.4)		10.3(2.5)		10.4(2.5)
	Left	LBP +	9.0(1.3)	0.040*	9.7(0.8)	0.99	9.7(0.9)	0.91
		LBP -	10.7(2.1)		9.9(2.2)		9.9(2.0)
TrA (mm)	Right	LBP +	3.6(0.8)	0.29	3.8(0.9)	0.63	3.3(0.1)	0.36
		LBP -	3.9(1.0)		3.7(0.9)		3.9(0.9)
	Left	LBP +	3.6(0.9)	0.15	3.9(1.1)	0.84	3.2(0.8)	0.08
		LBP -	4.2(1.2)		3.9(1.2)		4.0(1.1)

LBP+: had history of low back pain; LBP-: did not have history of low back pain; BMI: body mass index; VAS: Visual Analog Scale for pain; Sorensen test: For measuring back muscle endurance; EO: external oblique; IO: internal oblique; TrA: Transversus Abdominis; SD: Standard deviation; Right: Right side of the body; Left: Left side of the body

*Statistically Significantly different: (p-value<0.05)

In comparison with subjects with no history of LBP, the forward flexibility of lumbar spine was not different in subjects with any of the groups with LBP (p > 0.05). Subjects with sports life and prior year history of LBP had less right and left hamstring muscle flexibility in comparison with those without a history of LBP, although the differences were not statistically significant.

Subjects with sports life, prior year, and prior month history of LBP had significantly lower EO muscle thickness bilaterally (p<0.05). In addition, subjects with sports life history of LBP had significantly lower IO muscle thickness bilaterally (p<0.05). Subjects without history of LBP had higher TrA muscle thickness but the results were not statistically significantly different. The ranges of ICC and SEM were from 0.75 to 0.89 and from 0.07 to 0.6, respectively; which indicate within-subject reliability of ultrasound measurements.

In total, 66.7% (n=20) of subjects were right leg dominant. Comparison of the clinical measurements between the subjects with and without the history of LBP according to the dominant or non-dominant leg is shown in Table 4. Subjects with sports life history of LBP had statistically significant lower hamstring flexibility in their dominant limb. In addition, they had significantly lower IO and EO muscles thickness on both dominant and non-dominant sides (p<0.05).

Table 4.

Clinical measurements between the subjects with and without history of LBP according to dominant or non-dominant foot, presented as mean(SD).

Variables			Sports life History of LBP	p-value	Last year History of LBP	p-value	Last month History of LBP	p-value
hamstring flexibility (degrees)	Dominant	LBP +	8.2(6.2)	0.04*	8.2(6.6)	0.17	5.4(3.5)	0.93
		LBP -	4.0(4.2)		5.2(5.0)		6.2(5.9)
	Non- dominant	LBP +	8.3(8.4)	0.75	9.2(10.7)	0.89	4.8(3.6)	0.32
		LBP -	7.0(5.8)		6.9(5.1)		8.2(7.5)
EO (mm)	Dominant	LBP +	7.2(1.7)	0.001*	7.3(1.8)	0.04*	6.6(0.4)	0.04*
		LBP -	10.7(2.4)		9.8(2.8)		9.5(2.8)
	Non- dominant	LBP +	7.5(1.5)	0.001*	7.7(1.7)	0.04*	7.2(0.7)	0.04*
		LBP -	11.1(2.1)		9.9(2.6)		9.7(2.6)
IO (mm)	Dominant	LBP +	9.3(2.0)	0.03*	9.8(2.4)	0.72	9.1(2.1)	0.37
		LBP -	10.9(2.3)		10.2(2.2)		10.3(2.3)
	Non- dominant	LBP +	8.9(1.7)	0.03*	9.7(0.8)	0.84	9.7(0.9)	0.80
		LBP -	10.8(2.2)		10.0(2.5)		9.9(2.3)
TrA (mm)	Dominant	LBP +	3.4(0.8)	0.34	3.8(0.9)	0.63	3.3(0.1)	0.37
		LBP -	4.0(1.2)		3.8(1.1)		3.9(1.1)
	Non- dominant	LBP +	3.6(0.9)	0.15	3.9(1.1)	0.82	3.2(0.8)	0.08
		LBP -	4.1(1.0)		3.8(0.9)		4.0(1.0)

SD: Standard deviation; LBP+: had history of low back pain; LBP-: did not have history of low back pain;

EO: External Oblique, IO: Internal Oblique, TrA: Transversus Abdominis.

*Statistically Significant: (p-value<0.05)

DISCUSSION

The results of the current study indicate that young male soccer players with sports life LBP had thinner EO and IO muscles in comparison with subjects without LBP. Subjects with sports life and prior month history of LBP had thinner TrA muscles in comparison with subjects without the history of LBP but this difference was not statistically significant. This is the first study that shows such a difference in this population. In addition, these players had significantly lower hamstring flexibility in their dominant leg.

There are several studies in the literature indicating that the TrA muscle plays an essential role in the biomechanical stability of the lumbar spine; however, regarding athletes with or without LBP, there is a controversy among different studies which have investigated lateral abdominal muscle thickness.^8,10,12 Similarly, Rostami et al. found that the thickness of lateral abdominal muscles was lower in adult male off-road cyclists with LBP in the cycling position.¹⁰ Gray et al. demonstrated that the total thickness of lateral abdominal muscles is greater in cricket fast bowlers without LBP on the non-dominant side, and the IO thickness was less in bowlers with LBP than those without LBP (p=0.02).¹² Gildea et al. showed that thickness of IO and TrA at rest and during abdominal drawing-in maneuver did not differ between ballet dancers with and without LBP via MRI.⁸ Additionally, Gill et al. found no significant difference in relative thickness of the TrA based on the history of back pain in collegiate single sided rowers (p: 0.075).²⁹ The controversy between studies could be explained by various sports physical demands and activation of these muscles in different sport-specific positions which could not be detected in resting measurements.

Previous researchers have shown the role of lateral abdominal muscles contributors to core stability and strength of trunk, which can indirectly impact athletes’ performance and allow them to train with fewer injuries.^30-32 These muscles’ role in stability is attributed to their ability to produce concordant forward flexion, lateral flexion, and rotational movements. Additionally, they can control external forces which are imparted in the opposite direction of these movements. Also, they are responsible for maintaining posture and distribution of muscle forces in the trunk for rapid movements and power generation.^30-32 Soccer players have shown shorter reflex latencies for trunk muscles in comparison with non-players against sagittal plane perturbations.³³ It could be that in soccer players, the activation of EO, IO, and TrA during cutting maneuvers which require trunk side flexion and rotation, play a role in the spinal injury prevention. Therefore, investigation of EO, IO, and TrA muscles’ thickness and their recruitment in functional and sports-related positions is important in soccer players and other athletes.

Differences in these muscles’ activation in sports-related positions and movements may explain the controversy in the results of different studies.^{8,10,12,28,29} As Rostami et al. showed the difference between groups in the cycling position, applying other sports specific positions could be helpful for future assessments instead of resting position/status. The role of trunk muscles in the different sports and dominant side of the body during sports may affect the interpretation of findings, also different LBP definitions^8,10,12 in studies make it difficult to draw firm conclusions. Therefore, further studies with a unique definition of LBP and functional measurements of lateral abdominal muscles may be helpful in this field.

The results of the current study did not show any association between LBP and the number of training hours or LBP and the age when players started to compete. This finding is in accordance with Tunas et al. who did not find any association between history of LBP in the previous year and number of seasons playing soccer at the top level and also annual training volume.³⁴ In a study by van Hilst et al. no relation between LBP and years of experience was reported for young soccer players.³⁵

These results did not show a significant association between the difference in length of the limbs and LBP or VAS score. This finding is in accordance with a previous study by Biering-Sorensen that reported leg length discrepancies could not predict LBP in subjects during the prior year.³⁶ Similar to Biering-Sorensen's findings this study found a significant association between lower hamstring muscles flexibility of the dominant limb and history of sports life LBP,³⁶ while Stutchfield et al. showed that there was no association between LBP and hamstring flexibility in young male rowers (mean age was 20 years).³⁷ Although Biering-Sorensen³⁶ reported that increased endurance of back muscles can prevent LBP, the current findings did not show a significant association between trunk extensor muscles endurance and LBP. This finding could be due to differences between LBP definitions.

Study Limitations

The first limitation is the retrospective nature of the study in which subjects were asked about their history of LBP in past 12 months or throughout their lifetime of sports participation which is prone to recall bias and memory lapses. Second, this study is cross-sectional, thus findings could not evaluate cause and effect relationship between muscle thickness or other factors and LBP. Another limitation is that muscle thickness was measured at rest; while there are reports that indicate muscles thickness measured during drawing-in maneuver (ADIM) or active straight leg raise test may be more precise for assessing lateral abdominal muscular recruitment. Finally, the sample size was not large. Authors suggest further studies with a uniform protocol for these muscles’ contraction such as active straight leg raise test.

CONCLUSIONS

The results of the current study indicate that young soccer players with the history of sports life LBP had thinner EO and IO muscles and less hamstring flexibility in comparison with players without LBP. There was no significant difference in the TrA thickness between groups. No significant association was detected between LBP and age, height, weight, weekly training hours, the age of starting to compete and endurance of trunk extensor muscles in young athletes. Further studies with assessments conducted in sports specific positions and during functional tasks could be helpful to evaluate the association between LBP and thickness of the TrA muscle.

Appendix 1.