Muscular strength is an important component of success in athletic performance. Athletic performance requires different motor demands depending on the required movement pattern. In many sports, movement patterns are comprised of either bilateral movements of the lower extremities (powerlifters [PL] and rowers) or unilateral movements (jumpers [J] and basketball players). However, reliance on these movement demands may cause muscle imbalances within the body. Muscle imbalance, or bilateral asymmetry, can be interpreted as the imbalance in strength between 2 halves of the body (2). Multiple factors, including specific sport demands (11) and over dependence on one side of the body (6), may contribute to the development of strength asymmetries within the muscular system and may have an effect on an athlete’s performance or risk for injury.
Muscle strength imbalance has been shown to occur in athletes and nonathletes alike. Jacobs et al. (6) investigated the peak hip abduction torque production of 42 healthy subjects; the average bilateral strength differential was 11.6 ± 8.31%. In another study, the investigators studied the contralateral limb differences in competitive swimmers and observed that the imbalance in strength increased when swimmers approached exhaustion (14). These findings identify that the strength imbalances are naturally present at rest and that they can be exacerbated with prolonged activity. In addition, a study conducted on female collegiate athletes showed that a bilateral strength imbalance of 15% or more in hip extension or knee flexion was associated with a higher risk for injury (7).
The effects of sport-specific demands on strength asymmetries have been investigated in the upper extremities because they are often identified as dominant (D) or nondominant (ND) (6). According to Gabbard and Hart (3), the development of one side as accuracy dominant and one side as strength dominant is natural. However, limb dominance might promote unequal task performance and lead to poor development of motor skills bilaterally (19). Repeated performance of tasks by one limb can cause neuromuscular adaptations like neural innervations and muscle activation (1) and tissue changes like bone developments and muscle hypertrophy (5,10) that enhance or decrease the limbs physical performance. It has been postulated that the weaker or more imbalanced an extremity muscle group is, the more prone it is to injury (8,18), although others have questioned whether there is a link between muscle imbalance and injury (4). A retrospective study, of 27 patients who sustained a quadriceps tendon rupture, found that the ND leg was twice as likely to be injured as the D leg (12). Although studies have debated the relationship between strength asymmetry and injury, there does seem to be some correlation between them.
A relationship between strength imbalance and injury (12) increases the need to examine how different sport-specific demands would lead to the disproportional use of each limb. Results from different studies indicated that the limb symmetry was highly dictated by the major movement mechanic of the sports (13,15). These 2 studies illustrated different symmetry patterns in rowers and soccer players during an open chain kinetic exercise. Parkin et al. (13) looked at the symmetry pattern in 19 male rowers. The results showed that there was no significant difference between the left and right legs for both isometric and isokinetic strength tests. This study suggested that the movement mechanics of rowing would not lead to an asymmetrical pattern in the lower extremities. In contrast, a study of 41 English Premier League club soccer players found bilateral strength differences in preferred and nonpreferred legs, using isokinetic testing (15). This study reinforced association between sport-specific demands and the unique dominance found between limbs.
Studies have identified limb symmetry patterns among athletes of either bilateral or unilateral predominant contraction sports but have not compared them with each other with the same protocol. Additionally, many researchers have tested strength imbalance using isokinetic testing, which is dissimilar to typical weight-bearing athletic movements. The purposes of this study were: first, to determine if a force imbalance exists in D and ND legs between unilateral predominant and bilateral predominant sports and second, to investigate the source of the force imbalance. Power lifters (bilateral predominant) and jumpers (unilateral predominant) were recruited and were assessed using a vertical jump because this is a closed kinetic chain exercise necessary in land-based athletic performance.
Experimental Approach to the Problem
A randomized, between-group, repeated-measure design was used for this investigation. Both powerlifters (PL) and field jumpers (J) were tested in 5 countermovement vertical jump conditions looking at force, velocity, and power output of each leg, which is a similar protocol to Newton et al. (11). The jump conditions included (a) double-leg jump (2L), (b) dominant leg–specified double-leg jump (D2L), (c) nondominant leg–specified double-leg jump (ND2L), (d) dominant leg–specified single-leg jump (DSL), and (e) nondominant leg–specified single-leg jump (NDSL). The subjects were tested on 2 different days with at least 24 hours in between. Subjects were asked to replicate their diet and fluid intake during the test. Each testing day began with body weight measurement, followed by a standardized dynamic warm-up protocol and warm-up jumps with a minute rest in between each jump. A 5-minute rest was given between the warm-up jumps and 3 baseline jumps. The testing jump proceeded 1 minute after the baseline jump.
Eleven men (age: 25 ± 3.29 years, height: 175.14 ± 8.89 cm, body mass: 85.08 ± 16.25 kg) who participated in competitive powerlifting were recruited to the bilateral sport group and 8 men (age: 19.38 ± 1.41 years, height: 179.04 ± 6.23 cm, body mass: 73.95 ± 6.32 kg) who were Division III collegiate field jumpers were recruited to the unilateral sport group. Subjects who participated in both teams were excluded from the study. All subjects participated in their sport for at least 1 year and did not report any injury during the 6 months before the study. The J and PL were participating in their in-season training at the time of the study. The 2 groups’ strength training programs were designed by a certified strength and conditioning specialist. Subjects’ total body water and fat-free mass were measured by the Tanita Body Composition Analyzer BC-418. All subjects understood the risks and benefits of the study and then signed the informed consent to agree to participation. The study was approved by the Springfield College Institutional Review Board for use on human subjects in research.
Subjects reported to the laboratory for testing on 2 separate days, each day at the same time, between the hours of 11 AM and 3 PM. On the first day, participants filled out the training history questionnaire before being weighed on the Tanita Body Composition Analyzer BC-418. The participants then performed a standardized warm-up (dynamic warm-up and warm-up jumps). Three of the 5 jumps (2L, D2L, ND2L, DSL, and NDSL) were randomly picked for each participant to perform; subjects performed 3 warm-up jumps for each jump, with a 1-minute rest in between each jump. A 5-minute rest was given between the warm-up jumps and the beginning of testing. During testing, each jump was performed in 3 separate trials, with a 1-minute rest in between. At the second visit, the same procedure as day 1 was followed, except for administering the training history questionnaire. The jumps performed on day 2 were the remaining 2 jumps plus 1 jump that was performed on day 1. A minimum of 24-hour rest between the 2 days was given. Participants were asked to replicate the diet and fluid intake throughout the testing period.
Countermovement Vertical Jump
For each test, the subjects performed 3 separate, maximal-effort jumps on the forceplate (Advanced Medical Technologies, Watertown, MA, USA; data analyzed with DartPower 2.0). The subjects were asked to keep hands on the hips throughout the jumps to eliminate the confounding variables of upper body movement. During 2L jumps, the subjects stood with both feet, shoulder width apart on the forceplate. During D2L and ND2L jumps, the tested limb stood on the forceplate and the nontested limb stood on the platform, which was placed next to the forceplate. For the DSL and NDSL jumps, subjects were told to hold the knee of the nontested limb flexed at 90° and not to move it throughout the jumps to minimize the effect of additional propulsion.
Training History Questionnaire
Subjects were asked to list the lower extremity exercises they performed during their typical weight training work out. The differences between the bilateral (squat, deadlift) and unilateral (unilateral squat, RDL) exercises were analyzed by using a ratio.
Limb Symmetry Index
Muscle force imbalance was operationally defined as the Limb Symmetry Index score (LSI). If LSI = 0, a balance in force production is shown between 2 legs. LSI is calculated using following equation (16):
All data sets were tested for assumptions for linear statistics, including normal distribution and homogeneity of variance; the above variables met all the criteria for use of linear statistics. Test-retest reliability of the dependent variables showed intraclass correlation coefficients of R ≥ 0.947. Mixed factorial ANOVAs (2 × 5) (group × jump) were then used to analyze the force, power, and velocity. Independent t-test was used to examine the LSI between PL and J. Significance in the study was set at p ≤ 0.05.
Cohen’s d was used to determine the magnitude of difference between PL and J in force, power, and velocity by using effect size (ES). The equation for ES is the absolute values of PL mean − J mean/average of PL and J SD. The magnitude of the effect was classified as small, moderate, and large when the ES is 0.2, 0.5, and 0.8, respectively.
No significant difference was found for the lower limb fat-free mass between D and ND legs for both groups (p = 0.44). Baseline jump heights (day 1: 38.37 ± 5.12 cm; day 2: 37.98 ± 5.32 cm; p = 0.64) did not show a difference between the 2 testing days. As shown in Figure 1, the LSI was significantly higher (p = 0.04) in the unilateral group (6.73 ± 1.84%) than in the bilateral group (2.74 ± 0.74%). Figures 2 and 3 show the difference in force production depending on limb dominance for each subject in SL and 2L jumps, respectively.
There was no significant interaction effect in force, power, and velocity between PL and J among all jumps. However, the SL jump showed a significantly (p < 0.05) higher force and power and a significantly lower velocity than the double-leg jump for both D and ND legs. Finally, the D leg produced a higher force and velocity (p < 0.05) than the ND leg during any type of double-leg jump.
Table 1 shows the ES, mean, and SD of the average maximum relative force, power, and peak velocity among different jumps for both PL and J. PL produced a greater force production in 2L, D2L, and ND2L jumps and a lower force production in DSL and NDSL jumps than the J.
To the best of our knowledge, studies have examined symmetry patterns in either unilateral or bilateral contraction sports, and this is the first study examining the two side by side, using closed kinetic chain protocols. The primary finding of this investigation was that the J had a greater LSI compared with PL. In other words, the difference in force production between D and ND limbs is greater in J than in PL, which is in agreement with other studies (13,15). The other finding was that the PL produced a higher force during 2L, D2L, and ND2L jumps than J, whereas the J produced a higher force during DSL and NDSL compared with PL.
In our work, we have shown that chronic training and different sport-specific demands contributed to a significant difference in LSI between 2 groups (LSI: J = 6.73%; PL = 2.75%); however, no significant difference was seen when comparing D and ND limbs. Our findings are similar to other works on jumpers, runners (17), and basketball players (20), yet other researchers have disagreed (7,9,11). The disagreement between our work and that of others that show an asymmetrical pattern between limbs might be explained by the differences in the testing protocol (7,9,11). These groups of investigators observed asymmetrical force production within sets, instead of between sets.
Differences in strength asymmetry may be influenced by disruptions to the body systems caused by physical activity. The nature of sports and specific training protocols induce upstream stimuli on numerous body systems (cardiovascular, skeletal, neuromuscular, etc.) and cause a downstream signaling event to restore equilibrium within the body (adaptation). Upstream stimuli, such as movement patterns, velocity, and biomechanics of motion, create mechanical stresses to help execute the same motion repeatedly (11,15). Stimuli initiated by sports participation will then trigger a series of events that will cause musculoskeletal and neuromuscular adaptations in the body (5,10,11). Mechanical and neuromuscular stress will ultimately lead to changes in the kinematic and kinesthetic performance. Table 1 shows the different neuromuscular adaptations between 2 groups. PL had a greater force and power production in double-leg situations, whereas J had a greater force and power production in single-leg situations. We speculated that differences in muscle mass (results showed no differences between limbs) and rate coding might exist because of the nature of the sports and may have influenced performance (16).
Athletes of various sports have shown differences in the limb symmetry and these differences have been attributed to behaviors specific to their required activity. Muscular and neuromuscular adaptation to the sport-specific demands may be responsible for a difference in force production between PL and J. Limb preference has also been associated with injury risk in athletes. Strength testing is not a typical screen for athletes, but neglecting the existence of limb asymmetry may predispose athletes to poorer performance or injury.
In our work and the work of others, limb symmetry patterns are observed among different sports (jumpers, powerlifters, soccer players, and rowers). It is important for the coach to acknowledge the existence of the force imbalance between limbs, especially for the unilateral predominant contraction sports. Because the cause of the imbalance could be due an inappropriate strength training program or chronic sports training, failure to recognize the force imbalance between limbs may predispose the players to injury. Coaches should implement a force imbalance assessment pre and post season and develop a strength program that can target the athlete’s strength disparities.
No external financial support was received for this study.
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