This review summarizes the effects of bilateral asymmetry on running and cycling performance and risk of injury in healthy subjects and the influence of leg preference. We define the term leg preference derived from lateral preference as representative of the choice for one side of the body to perform a motor action. Useful information is provided for biomechanical and physiological research and coaching with relevance to an understanding regarding the occurrence of lower limb asymmetry.To provide a synopsis of what is known about bilateral asymmetry in human running and cycling and its relationship to limb preference, especially in the context of competitive sport performance and risk of injury.Structured narrative review.The relationship between asymmetry and lower limb preference was reviewed using Medline(®), Sciencedirect(®), and Scopus(®) search engines considering studies published in English until June 2009.The environment characteristics may influence running asymmetries, which are more frequent in angular parameters. Environment characteristics are related to ground irregularities requiring compensatory movements changing the mechanical workload on joints and bones, which may influence asymmetries in biomechanical parameters between lower limbs. The relationship between asymmetry and injury risk should be assessed with caution since running speed may influence asymmetry in injured and non-injured subjects who often show similar asymmetry levels. Symmetry can be improved with increasing running speed. In addition to running speed, coaches and athletes interested in minimizing lateral difference should consider a training regime aimed at correcting asymmetry which may negatively affect running technique by influencing the compensatory movements that an athlete usually performs. During cycling, bilateral differences are frequently found and vary with the competitive situation, pedaling cadence, exercise intensity and exercise duration. Regardless of the variability of asymmetry index between subjects, few suggestions are available to overcome lateral differences. Most of the research suggests that bilateral pedaling asymmetries decrease as the workload increases, however the relationship to injury risk was not clearly addressed. For both running and cycling, few investigations examined the central mechanisms of neuromuscular control, and no study addressed the effect of asymmetry on performance.Collectively, the volume of studies supporting symmetry is small and to a large extent research considered unilateral assessment. Preferred limb performance can differ from the contralateral limb. In the context of biomechanical and physiological investigations, we believe that further studies should address the role of lower limb symmetry on human motor performance and injury risk focusing on the energetic cost, muscle efficiency and the neuromuscular aspects such as muscle activation and motor units firing rate.