Abstract
PurposeThis study aimed to elucidate whether muscle activity (in terms of glucose uptake) between the legs can be considered symmetrical during walking. Furthermore, we aimed to determine whether the [18F]-fluorodeoxyglucose was distributed heterogeneously throughout each muscle, and if so, whether areas of high uptake would be clustered.MethodsTen healthy participants walked on a treadmill at self-selected comfortable walking speed for a total of 90 minutes, 60 minutes before and 30 minutes after intravenous injection of 50 MBq [18F]-fluorodeoxyglucose. Thereafter, a positron emission tomography/computed tomography scan of the lower limb was acquired. Three-dimensional muscle contours of 78 (= 39x2) muscles of the left and right lower limb were semi-automatically determined from magnetic resonance imaging scans. After non-rigid registration, those muscle contours were used to extract [18F]-fluorodeoxyglucose uptake from the positron emission tomography scans.ResultsLarge asymmetries were observed in the lower leg muscles (e.g. median absolute asymmetry index of 42% in the gastrocnemius medialis) and in the gluteus minimus (30% asymmetry) and gluteus medius (15% asymmetry), whereas the uptake in the thighs was relatively symmetrical between the limbs (<6% asymmetry). These were not related to limb-dominance nor to inter-limb differences in muscle volume. The [18F]-fluorodeoxyglucose distribution was not distributed normally; most voxels had a relatively low standardized uptake value, and a minority of voxels had a relatively high standardized uptake value. The voxels with higher [18F]-fluorodeoxyglucose uptake were distributed heterogeneously; they were clustered in virtually all muscles.ConclusionThe findings in this study challenge the common assumption of symmetry in muscle activity between the limbs in healthy subjects. The clustering of voxels with high uptake suggests that even in this prolonged repetitive task, different spatial regions of muscles contribute differently to walking than others.
Highlights
Walking involves a complex integration of muscular contractions with the goal of maintaining stance stability and forward progression
Large asymmetries were observed in the lower leg muscles and in the gluteus minimus (30% asymmetry) and gluteus medius (15% asymmetry), whereas the uptake in the thighs was relatively symmetrical between the limbs (
All of the inter-subject differences disappeared when the results were averaged across subjects, which underlines that averaging individual subject data stemming from both limbs may preclude asymmetries from being noticed or reported
Summary
Walking involves a complex integration of muscular contractions with the goal of maintaining stance stability and forward progression. Walking in healthy persons has traditionally been assumed to be a symmetrical motion, either from assumption or for convenience of data collection and analysis [1]. This assumption may be considered reasonable, as most healthy persons walk without a limp or other visible evidence of asymmetry. In muscle activation (using electromyography), Ounpuu et al found that nine out of ten subjects showed significant differences between the dominant and non-dominant limb in at least three out of seven muscles tested [4]. Asymmetry is an important consideration in validating musculo-skeletal models, which generally assume that healthy persons walk with equal activity levels in muscles in both legs [6, 7]
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