Children, at the age of 6–18 years can run with FP though seemingly only in about 90% of the strides ( Rozumalski et al., 2015). Infants without independent walking experience toddling on a treadmill but with body-weight support show a shift from DS to FP at speeds of around 0.75 m/s ( Vasudevan et al., 2016). Running may be defined as having a flight phase (FP) in contrast to walking where there is a double support phase (DS).
Is it generally true that an immature locomotor pattern is represented by fewer muscle synergies and less focal activation peaks? Here, we ask whether there is similar change in the number of synergies and the shape of their waveforms during the development of running. The shape of the synergies’ waveforms evolves from wide, sinusoidal shapes to more focal ones with shorter activation duration from toddlers, to preschoolers and adults ( Dominici et al., 2011). In neonates, two muscle synergies are present resembling the reflexive stepping pattern seen at birth, while in toddlers two additional are present, i.e., a total of four synergies can be observed that persist to and during adulthood ( Dominici et al., 2011 Sylos-Labini et al., 2020). When children develop walking skills, the number of muscle synergies that accompany the cyclic movement of the lower extremities increases ( Dominici et al., 2011). The central nervous system can activate large groups of muscles by small sets of descending neural signals at specific moments during the gait cycle ( d’Avella et al., 2003 Ting and Macpherson, 2005 Bizzi and Cheung, 2013). Muscle synergies reflect a common neural input to multiple muscles easing the control of complex motor tasks like locomotion ( Bernstein, 1967 Bizzi and Cheung, 2013). The ability to run with a FP is clearly associated with an increase in the number of muscle synergies. We believe that the increase in the number of muscle synergies in older children relates to motor learning and exploration. Against our expectations, the age groups did not differ significantly in the timing or duration of synergies. The number of synergies tended to be smaller in toddlers than it did in preschoolers and school-age children but the adults had the lowest number for both conditions. The number of synergies across groups and conditions needed to cover sufficient data variation ranged between four and eight. In particular the activity duration of the medial gastrocnemius muscle was weakly correlated with age. We found that toddlers (2–3.5 years) and preschoolers (3.5–6.5 years) utilize a “walk-run strategy” when learning to run: they managed the fastest speeds on the treadmill by combining double support (DS) and flight phases (FPs). Muscle activity of 15 bilateral leg, trunk, and arm muscles, ground reaction forces, and kinematics were recorded during comfortable treadmill walking and running, followed by a muscle synergy analysis. We investigated the development of locomotion in 23 children aged 2–9 years of age and compared them with seven young adults. We hence hypothesized that in children learning to run the number of accompanying synergies increases and that some of the synergies’ activities display a temporal shift related to a reduced stance phase as observed in adults. When these locomotor patterns emerge, it is likely that synergies emerge as well. Steering small sets of synergies is commonly believed to simplify the control of complex motor tasks like walking and running. Muscle synergies reflect the presence of a common neural input to multiple muscles. Department of Human Movement Sciences, Faculty of Behavioural and Movement Sciences, Amsterdam Movement Sciences & Institute for Brain and Behavior Amsterdam, Vrije Universiteit Amsterdam, Amsterdam, Netherlands.
Bach, Andreas Daffertshofer and Nadia Dominici *
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