Full length articleAsymmetry between lower limbs during rested and fatigued state running gait in healthy individuals☆
Introduction
Normal gait is often considered to be symmetrical, which has influenced data collection methodology. Bilateral data collection may complicate the interpretation of results, especially when pooling limbs since combining data from right and left limbs will magnify effect size but negate the assumption of independent observations [1]. To satisfy this assumption, the researcher is required to choose one limb to be representative of both limbs [1]. However, symmetry of gait, the perfect congruence between right and left measures, may not occur in healthy populations [2]. Statistically significant differences in kinematic and kinetic measures between limbs have been observed, contradicting the assumption of gait symmetry [3], [4], [5]. Previous findings indicate that separate limbs are used by individuals more heavily for stabilization, propulsion, or braking during walking [3], [4], [5]. While differences could be attributed to limb dominance, as is established within the upper extremity, lower extremity (LE) differences have been reported despite controlling for dominance [3], [6].
Upper extremity dominance is identified as the limb preferred for writing tasks, a learned neuromuscular skill; LE dominance has been categorized based on the limb’s role in either stabilization or mobilization. For example, the leg used for kicking a ball is the mobilization limb and the stance leg is the stabilization limb. Arguments have been made for both mobilization and stabilization limbs to be considered dominant [4]. Both functions require precise neuromuscular adaption and modulation for successful task execution. Although definitions of LE dominance are not standardized, asymmetry appears to be present in normal gait [3], [4], [5]. Evaluations of the neuromuscular modulation of the LE may help quantify asymmetry relative to gait.
Measures of stiffness can be used to assess neuromuscular modulation of the entire LE [7]. The leg acts as a spring absorbing shock during braking and returning energy during propulsion according to the spring-mass model [7]. The nervous system can modulate the stiffness of the leg spring, via musculoskeletal changes in joint excursions, to maintain ideal center of mass excursion despite changes in running surface [7] and velocity [8]. Vertical stiffness (Kvert), the ratio of maximum vertical ground reaction force (GRF) over center of mass displacement [7], provides a measure to evaluate neuromuscular modulation symmetry.
Research evaluating asymmetry has largely examined walking [3], [4], [5], [6]. Imbalances in propulsion measures have been shown to increase as walking velocity increases [5], indicating a potential for greater asymmetry in running. Biomechanical asymmetry not detrimental during walking could potentially become pathological or injurious in running due to the increased kinetic demands placed upon the musculoskeletal system. Populations affected by LE overuse injuries have demonstrated significant differences between the injured and non-injured limbs [9], but symmetry angles quantifying variation between limbs were not significantly different than controls [9], [10]. Researchers have suggested that evaluations following fatiguing exercise are necessary, due to the potential for asymmetry becoming more pronounced with fatigue [10]. As a growing body of research appears to contradict the assumption of gait symmetry, there remains a need to evaluate differences between limbs of healthy individuals in conditions more physically demanding than walking [2], [3], [4], [6], [9], [10], [11]. Therefore, the purpose of this study was to evaluate asymmetry of kinetic and kinematic variables in both rested and fatigued state running. We hypothesized that there would be significant differences between limbs at rested and fatigued states. We also hypothesized that the effects of fatigue will enhance the magnitude of asymmetry.
Section snippets
Material and methods
Twenty healthy participants (14 Males, 6 Females), without previous LE surgeries, volunteered to participate (Table 1). Prior to data collection, participants completed an informed consent form approved by the University Institutional Review Board and a brief medical history questionnaire evaluated by a Certified Athletic Trainer (ATC). Only participants placed in the low risk group according to American College of Sports Medicine (ACSM) Risk Stratification Categories were included [12].
Results
During the exhaustive protocol, all participants met criteria for VO2max attainment [17]. Mean time from exhaustive protocol completion to fatigued state trials was 6:32 ± 2:31 min. Participants reported significantly (p < 0.05) higher RPE scores at fatigued state (gait) trial onset (RPE 9.95 ± 2.50)compared to onset (RPE 6.24 ± 0.63) and completion of rested state (gait) trials (RPE 8.29 ± 1.71). RPE scores significantly (p < 0.05) increased from onset to completion of the fatigued state trials (RPE 11.33 ±
Discussion
The current study found significant differences in kinematic and kinetic measures between limbs in a healthy population, at both rested and fatigued states. These findings support both our initial hypothesis and previous research reporting differences between limbs [3], [4], [6]. Gait asymmetry in healthy individuals has been previously referred to as functional asymmetry, in which limbs are asymmetrical based upon differentiation of primary function, particularly stabilization or propulsion [4]
Conflict of interest
The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
Acknowledgement
This research was partially funded by a grant from the Far West Athletic Trainers’ Association.
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The authors would like to thank the Far West Athletic Trainers’ Association for grant funding to support the data collection of this project.
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Ms. Putnam is now at Henry Ford Health System Orthopaedics & Sports Medicine, Detroit, MI, USA.