Muscular imbalance is a term frequently used in the fields of rehabilitation and performance sport, describing substantial deviation from normative data or muscle performance differences between limbs (11,28). Handedness, previous injury, or specific sport demands, have been suggested as possible reasons that could result in the development of bilateral muscle strength imbalances among athletes (21,27). Several previous studies have shown side-to-side strength imbalances to be present in well-trained athletes (10,17,21,24,27).
Previous studies have linked bilateral strength imbalance with injury. Knapik et al. (16) found that athletes had a higher injury rate with a knee flexor or hip extensor imbalance of 15% or more on either side of the body. Side-to-side strength imbalance have been suggested as a risk factor for anterior cruciate ligament injury in female athletes (7,14,20), whereas hamstring injuries were shown to be associated with low hamstring muscle side-to-side peak torque ratio at 60°·s−1 (23).
In addition to the higher injury risk associated with muscular imbalances (16,23), athletic performance may also be impaired. However, limited studies have compared the effect of bilateral muscle strength imbalance in terms of performance. Young et al. (33) investigated strength and power determinants of agility performance and found participants who turned faster to one side tended to have a reactive strength dominance in the leg responsible for the push-off action. In addition, Flanagan and Harrison (9) concluded that muscle imbalances, generated by higher leg-spring stiffness for the preferred leg, will result in different explosive jump performances between limbs. Being equally proficient in turning quickly to either sides or using either leg jump or push-off is crucial for an athlete to enhance on-field performance, hence such imbalances are undesirable.
Perhaps one reason for this lack of information is that traditionally, bilateral strength imbalance has been investigated by isokinetic dynamometry (10,17,24,25,27,29). However, this method is often impractical for strength and conditioning coaches and physiotherapists, because of the need for specialized equipment and the time-consuming nature of isokinetic assessment. In addition, isokinetic dynamometry excludes the holistic impact of qualities such as power, reactive strength, or skill performance, which may affect the imbalance measures (6,12). Previous research (21) has suggested that functional laboratory-based tests (bilateral and unilateral vertical jumps and bilateral squats) could all detect significant differences between dominant (D) and nondominant (ND) legs. In addition, it was found that a simple 5-hop field test could also potentially detect significant D to ND imbalances.
Attempts have been made to examine a range of field tests that could help assess lower-limb function (12,21,24,26), to provide practitioners a simple and cost-effective alternative option. However, the findings from these studies present an unclear picture as some contradictions have been reported. Single-leg hop test scores have been found not to be strongly related to isokinetic assessment results (12,24,26); however, its use was suggested (12,26). The triple hop was found to be a good predictor of clinical strength and power performance (24,26), but it has been shown to have low (24) to moderate (26) correlations with isokinetic performance. Finally, other field tests (5-hop, vertical jump, one-leg rising and square hop) were not correlated with isokinetic assessment (21,24,26). In addition to the inconclusive results, Murphy and Wilson (19) stressed the importance of functional tests being able to assess changes in muscular performance after a training or rehabilitation intervention.
Given the aforementioned issues with isokinetic testing and the specificity of functional tests (12), the possibility of field tests being able to assess muscular imbalance warrants further investigation. Other field tests in addition to the ones above must be examined to assist in the creation of a battery of tests. Hence, the aim of this study was twofold: firstly, to investigate whether a range of commonly used unilateral functional field tests can reveal muscle strength imbalance of the lower limbs and secondly, to investigate whether there is any relationship between the various muscle strength qualities assessed in this study. It is hypothesized that functional field tests can detect bilateral lower-limb strength asymmetry as typically found by isokinetic dynamometry.
Experimental Approach to the Problem
The present study used a cross-sectional design to investigate whether a range of standard unilateral functional field tests such as leg press (LP), horizontal hop, vertical and drop jumps (DJs) can reveal muscle strength imbalances of the lower limbs similar to that found by isokinetic dynamometry. Previous studies have used either one test (12) or a combination of unilateral and bilateral tests (21,24,26). It is therefore important to assess the strength qualities of relevant field tests in a purely unilateral way, to examine the possibility of compiling a battery of tests. For the various tests used, the performance of each leg was obtained for subsequent comparison between left and right and strength D (strongest leg) and ND (weakest leg) limbs to examine the imbalance between limbs. Correlational analyses were conducted to determine relationships between imbalance ratios calculated from isokinetic dynamometry results to that calculated from the functional field tests.
Thirteen male university sports participants (mean ± SD: age 21 ± 1.1 years, height 179.9 ± 7.0 cm, body mass 80.8 ± 9.7 kg) took part in the study. The subjects were all experienced, competitive athletes in sports with a high contribution of anaerobic power and where unilateral lower-limb neuromuscular performance was important to successful sports performance. All subjects were free of lower-limb injuries, and they were in the competitive phases of their respective annual plans. The study was approved by the Departmental Ethics Committee and all subjects provided written informed consent to participate.
Testing took place over a 2-day period with (a) seated unilateral LP and horizontal single-leg hop (HOP) for distance, (b) unilateral isokinetic concentric and eccentric knee extensor and flexor strength (CON EXT, ECC EXT, CON FLEX, and ECC FLEX, respectively) and single-leg vertical jump (VJ) and DJ, performed on separate days. The test order for either limb was counterbalanced for all tests to reduce order bias. For all tests except the isokinetic assessment, 2 trials were performed on each limb with the best score used for subsequent analysis.
All subjects were familiarized with the procedures before testing. The subjects had been instructed to refrain from strenuous exercise for 49 hours before testing and to avoid food and caffeine intake for 2 hours preceding the assessments. All subjects completed testing at the same time of the day to avoid any circadian rhythm effects (2). Finally, all equipment used was calibrated according to manufacturers' standardized procedures.
Isoinertial strength describes the phenomenon where force is generated by a muscle or muscle group when accelerating a constant gravitational load (15). Isoinertial strength was assessed using a Concept II dynamometer (Concept II Ltd, Nottingham, United Kingdom) for LP. Initial data in our laboratory from 21 athletic subjects (with similar characteristics to the sample of this study) who performed the LP on 2 separate occasions, yielded an intra-class correlation coefficient [ICC] (3,1) = 0.914. The subjects were seated with their back straight and in contact with the backrest at all times and arms grasping handles attached to the seat. The range of motion was from full knee extension to approximately 90° of knee flexion. Execution form was maintained the same for all subjects and trials. The scores obtained were in kilograms, as the dynamometer calculated force by monitoring the acceleration of the known-resistance flywheel.
Horizontal single-leg hop has been previously used for monitoring rehabilitation of knee-injured athletes (22) with generally high reliability reported; ICC r = 0.96 (1,6), standard error of measurement (SEM) = 4.56 cm (6). The test required the subjects to jump from and land on the same leg and hold the landing position for a further 2 seconds; otherwise, the jump was deemed invalid. The subjects were instructed to jump for maximum distance, and the distance traveled of the valid jumps was recorded to the nearest centimeter.
Single-leg vertical jump and DJ were performed on each leg, while maintaining hands on hips to isolate the contribution from the leg muscles (13). The test-retest reliability of using contact mats to determine VJ variables has previously shown to be reasonable (ICC 2,1 > 0.89) (26). For the DJs, subjects were instructed to step off, rather than jump, from the raised platform to ensure a homogeneous drop distance for each DJ trial. Previous research (32) has shown that instructions on how to perform DJs can have implications on the actual strength quality being measured. Hence, subjects were instructed to perform a ‘bounce’ DJ aiming for maximum height and minimum contact time. The height of the raised platform was 0.2 m. All measures were determined using a Newtest Power timer jump mat (Newtest Oy, Oulu, Finland).
Knee flexor and extensor muscle strength of both limbs was assessed at 60°·s−1 using an isokinetic dynamometer (Contrex, Dubendorf, Switzerland). The subjects were seated with the hip joint at 90° (supine position = 0°). The center of rotation of the knee was aligned with the dynamometer arm rotation axis while extraneous movement was prevented by straps, positioned at the hip, shoulders and tested thigh. Measurements were corrected for gravity and peak torque was obtained from 4 repetitions for both concentric and eccentric contractions, as previously recommended (3). Peak torque was taken from 0° to 90° of knee flexion (full knee extension = 0°). The order of tests was concentric extensor, concentric flexor, eccentric extensor, and eccentric flexor.
Normality of data was examined using the Kolmogorov-Smirnov test and confirmed for all variables with the exception of strength imbalance ratios for HOP and isokinetic eccentric extensor tests. Two-tailed Student's t-tests were used to compare right and left leg and strength D and ND limbs for each strength measure. Imbalance between right and left limbs was calculated for each variable by the formulae (Right leg − Left leg/Right leg × 100). Imbalance between strength D and ND limbs was calculated using (D − ND/D × 100). Pearson's product moment correlation was used to explore relationships amongst strength ratios (D:ND), whereas Spearman's rho was used to investigate relationships with HOP and isokinetic eccentric extensor tests. All statistical analyses were performed in SPSS v 14 (Chicago, IL, USA).
Significant differences were found when comparing D and ND limbs for all strength measures (Table 1-3;p < 0.01) with effects sizes (eta-squared values ranging from 0.41 to 0.79), indicating a true effect (19). No significant differences were observed between right and left limbs (Table 1-3;p > 0.05).
Pearson correlation analysis revealed a significant relationship in D:ND ratio between LP and DJ (r = 0.698, df = 11, p < 0.05) However, there were no significant relationships between isokinetic variables and the functional field tests used in the study (p > 0.05).
The aim of the present study was to investigate the application of various unilateral field tests for the assessment of lower-limb muscle strength imbalance. The study compared a range of functional field-based tests with the traditional isokinetic dynamometry method of assessing lower leg imbalance. Significant differences were found between strength dominant and ND limbs for all methods of assessment, but no significant differences were observed between right and left limbs, substantiating previous research (21). The results suggest the field tests used in the present study can detect lower-limb strength imbalance substantiating the hypotheses of the present study. However, no significant relationship between isokinetic and the field test variables was evident, suggesting that each strength imbalance ratio is independent of each other and it measures distinct strength imbalance ratios.
The results of the present study provide further support to previous findings (10,17,21,27) of the presence of muscular imbalance in athletic individuals. In particular, Newton et al. (21) found significant differences between strength D-ND limbs in peak and average force achieved during bilateral squat, single and bilateral vertical jumps, isokinetic extension and flexion peak torque at 60 and 240°·s−1 and a 5 hop test but not between right and left limbs. The present study produced similar findings in that all tests found significant differences between strength D and ND limbs. Moreover, the percentage imbalance values in Newton et al. (21) ranged from 4 to 16% across the various tests, which compares favorably with the range of 4-12% of the current study.
The present study found no significant differences between right and left limbs for all tests, despite all subjects being right handed and preferring the right limb in throwing and kicking tasks. It is likely that, despite all subjects being ‘right sided,’ some subjects were actually left leg strength dominant, nullifying strength differences when averaged across the groups. Schlumberger et al. (28) suggested against comparing preferred and nonpreferred limb strength, as limb dominance may not actually have sufficient external validity. Indeed, Österberg et al. (24) found no difference in strength when comparing right-left and preferred-nonpreferred sides in female footballers. However, weak-strong leg comparisons yielded a range of side differences between 4 and 16%.
In relation to the above point, another reason for lack of any significant differences between right and left limbs could be the low homogeneity of athletic background of the participants, despite the sample size being similar to relevant previous studies (16,21,29). The subjects in the present study preferred to use the right side for throwing and kicking, and hence, strength asymmetry was not a result of limb preference. However, they were of various sports, and therefore, the development of a strength dominant limb could be the result of the specific training effect of playing the sport on a regular basis (9,21,27); again, nullifying strength differences between right and left limbs and preferred and nonpreferred limbs with different sports participants. Indeed, there was some variation in right to left leg strength dominance across the various tests. If the subject sample was more homogeneous, it is suggested that strength imbalance patterns may have shown a consistent trend toward preferred and nonpreferred sides as has previously been found (10,27). Future research needs to investigate potential imbalance patterns that may develop across sports.
No significant relationships between imbalance ratios were detected from isokinetic dynamometry and the field tests. Our findings largely agree with Newton et al. (21) who found that a 5 hop test imbalance ratio had a moderate, but nonsignificant relationship with isokinetic concentric flexion (r = 0.573, r = 0.381) and extension ratios (r = 0.365, r = −0.147) at speeds of 60 and 240°·s−1, respectively. In addition, they reported a moderate but nonsignificant correlation between average and peak force during a VJ and isokinetic variables.
The lack of any significant relationship between any imbalance ratio suggests that although the field tests can detect imbalances, their magnitude cannot be adequately determined. This is probably because the field tests are indeed more functional involving multiple joints and antagonist co-contractions compared with isokinetic tests, where muscle groups are isolated. Furthermore, the fields tests assess different muscle strength qualities, namely, power (vertical jump and hop tests) or reactive strength (DJs). Hence, subjects may indeed develop strength imbalance toward a particular muscle strength quality depending on how they perform in the sport. This may have implications for screening athletes in that the choice of tests should coincide with the type of muscle strength quality used in their sports action.
Future research is needed to confirm the suggested link between bilateral strength asymmetry with injury and performance, which is as yet inconclusive (5,30). In particular, if imbalance is proven to have negative effects on performance and injury risk, how much imbalance is deemed to be detrimental? Barber et al. (4) using normal subjects and Noyes et al. (22) using anterior cruciate ligament - deficient patients (and functional tests similar to the present study) established an imbalance index of 85% (weaker leg as a % of stronger leg), answering the question with regards to the ‘imbalance threshold’ for injury risk, which substantiates recommendations for isokinetic testing of 15% difference between limbs (8,16,31). However, despite measurements of limb strength in athletic populations (10,34), a similar index for detrimental performance has not been developed.
In summary, the results of the present study support the use of simple field test measures to detect lower-limb strength asymmetry, allowing strength coaches or physiotherapists to quickly determine whether an athlete is in need of a training program that emphasizes unilateral exercises in an effort to isolate each side to improve the bilateral imbalance. These tests maybe useful in monitoring training improvements or recovery after lower-limb injury. However, the current battery of tests did not show correlation with the results obtained from isokinetic dynamometry. Therefore, sports scientists and strength and conditioning coaches should firstly be aware of the need to assess lower-limb strength imbalance and secondly, if sufficient time and equipment is available consider assessing lower-limb strength imbalance by due consideration of the muscle strength qualities that predominate in the sport.
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