Researchers have assumed that normal healthy running is a symmetrical process and gait asymmetry is associated with a pathological condition or injury. However, bilateral asymmetry has been found in healthy individuals during walking and running. The causes of asymmetries remain unclear, but could be the result of lateral dominance. A functional asymmetry hypothesis has been proposed in which the dominant limb (D) provides more propulsion and the non-dominant limb (ND) provides more support during walking. If asymmetry is natural and the lower extremities are responsible for different tasks, then there may be functional strength and dynamic balance asymmetries that could reflect the task demands of each leg. PURPOSE: To test the functional asymmetry hypothesis and asymmetry in functional strength and dynamic balance in healthy, recreational runners. METHODS: Participants (male 14, female 14) were healthy runners (mean +/- sd, age 27.39 +/- 6.39 years; mass 67.48 +/- 9.15 kg; weekly training 37.35 +/- 24.51 km; running history 8.88 +/- 6.99 years). Vertical impulse (VI) and propulsive impulse (PI) were used to represent the supportive and propulsive function of the lower extremities. Participants ran across a force plate at 3.5 +/- 5% m/s, in which VI and PI were measured in the D and ND limb. The Star Excursion Balance Test (SEBT) and Triple hop distance test (THD) were used to test dynamic balance and functional strength, respectively. A two-tailed, paired samples t-test with Bonferonni adjustment was calculated to compare the mean scores between the D and ND limbs in each of the measures. An Absolute Asymmetry Index (ASI) was calculated to determine differences in the test measures, regardless of direction. RESULTS: No significant differences were found between the mean scores of the D and ND limbs in any of the tests. ASI revealed that the participants in this study exhibited some level of asymmetry in all of the measures tested (mean +/- sd, SEBT 3.80 +/- 3.30%, THD 4.76 +/- 4.46%, VI 4.61 +/- 4.77%, PI 16.73 +/- 15.13%). CONCLUSIONS: Pooling data can hide asymmetries that exist in healthy individuals. Levels of asymmetry can vary greatly between and within individuals in different tests. Lateral dominance may not be a predictor of asymmetries in healthy individuals. However, asymmetries could be the result of individual compensations or individual differences in lateral dominace in varying tasks. Perfect symmetry should not be expected in healthy individuals, and asymmetry does not necessarily implicate a pathological condition. The high ASI of the propulsive impulse in the participants indicates that a functional asymmetry might exist, but is unique for each individual. PRACTICAL APPLICATIONS: Coaches and trainers can address the specific demands of the individual limbs and/or decrease asymmetries, if desired, by having their athletes perform unilateral training, such as single leg squats, single leg dead lifts, multidirectional lunges and low level single leg plyometric exercises. Consequently, the individual kinetics and kinematics of the limbs will more closely replicate that which is performed while running. Researchers should analyze individual participant data and utilize an asymmetry index when making left/right or dominant/non-dominant comparisons.
(C) 2011 National Strength and Conditioning Association