J Physiol 588.22 (2010) p 4341 4341
Changing our thinking aboutwalking
Jonathan NortonDivisions of Pediatric Surgery andNeurosurgery, Department of Surgery &Centre for Neuroscience, University ofAlberta, Edmonton, Alberta, Canada
The act of walking seems so simple when weperform it; we just put one leg in front of theother, and most of us are able to do otherthings at the same time. Watching childrenwho are learning to walk, however, providesus with some insights into how complicatedthe whole process is and the tremendouslevel of sensori-motor integration requiredfor safe walking.For a number of years we have known that
infants can step on a moving treadmill beltbefore they can walk independently (Yang& Gorassini, 2006). Adults with completespinal cord injuries can also be trainedto step on a moving treadmill belt (Yang& Gorassini, 2006) and this has providedsome of the strongest evidence to datefor the existence of human spinal centralpattern generators (Dietz, 2003). However,for over-ground walking a spinal patterngenerator does not appear to be sufficient.Supraspinal control is needed to provideboth the drive for locomotion as well asthe coordination to negotiate a complexenvironment.In this issue of The Journal of Physiology,
Petersen et al. (2010) describe a series ofrecordings made on children while theywalk on a treadmill at a self-selectedpace and while they perform a staticankle dorsiflexion. Using the techniqueof intramuscular coherence they examinedchanges in common drive from the motor
cortex to the tibialis anterior muscle. Thismethod is an elegant approach to studyingnervous system function. Surface EMGrecordings that are entirely non-invasive canbe used to obtain information concerningthe neural drive that produces an action.Most commonly, recordings for coherenceanalysis have been made from pairs ofmuscles, such as in our study of incompletespinal cord-injured subjects (Norton &Gorassini, 2006). Recordings from two sitesof the same muscle, as used in this studyof children, are more suited to this analysisthan pairs of muscles. Neural drive to twoportions of a muscle is likely to be higherthan to two independent muscles, even ifthey act synergistically. Care must be takento avoid cross-talk between the electrodepairs but this group have previously showntechniques that avoid this problem (Hansenet al. 2005).Although many techniques exist for
assessing the neural control of movement,such as reflex studies and motor-evokedpotentials, a big advantage of the coherenceapproach is that it does not perturb thesystem. This method assesses the control ofthe movement as it happens, rather thanthe prior state or readiness of the system(Nielsen, 2002). There are shortcomings,however; in particular we are left to wonderabout the remaining non-coherent activity.How much is lost as an artifact of theanalysis technique andhowmuch representsnon-coherent neural drive is uncertain. Wedo not know the true maximum coherenceif all drive came from a single cortico-spinal origin. For instance, at 24 Hz thehighest level of coherence is well under 0.5and in many instances and frequencies thecoherence is below 0.1, potentially leavingup to 90% of the drive at that frequency ofunknown origin.What is remarkable in the studybyPetersen
et al. (2010) is the relationship betweenthe age of the subject and the coherence
in the -band during static contractionsand -band during walking. These clearage-related differences indicate that theneural drive to the movement changes withage and could be considered as a marker forskill level in these relatively young children.By kinematic measures, these childrenappeared tohave increased their skill level, asevidenced by reducedmovement variability.Previous studies have shown changes incoherence with visuo-motor skill learningfor this muscle (Perez et al. 2006) and others(Semmler et al. 2004). Changes in motorunit synchrony during development havealso been reported (James et al. 2008) butthis is the first study to examine the changesduring a functional, lower-limb task suchas walking without overt motor training.It is yet to be determined whether thedevelopmental increase in coherence relatesto a maturation of functional coordinationwithin the corticospinal tract or this neuraldrive displacing non-cortical drive to themuscle.
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C 2010 The Author. Journal compilation C 2010 The Physiological Society DOI: 10.1113/jphysiol.2010.200204