Anticipatory muscle activations to coordinate balance and movement during motor transitions: A narrative review

Abstract

Background: Maintaining balance while moving isvital forday-to-day activities. Akeychallenge inthe comprehension ofhuman movement istodetermine howmuscles contribute tobalance-movement coordination. Motor transitions, defined asmovements executed between twosteady balance states, areparticularly interesting phases tostudy balance-movement coordination because alarge, discrete change inwhole-body momentum may disturb balance. During voluntarily-initiated motor transitions, anticipatory muscle patterns provide the biomechanical conditions thatarefavourable toboth maintaining balance andexecuting themovement. Research question: What arethemechanical consequences ofanticipatory muscle activations forbalancemovement coordination during voluntarily-initiated motor transitions? Methods: Wereview thebiomechanical contributions oftheanticipatory muscle activations identified inthe literature during four types ofvoluntarily-initiated motor transitions, through theprism ofthree balance mechanisms (‘moving thecentre ofpressure (CoP)’,‘counter-rotating segments’,and‘applying new external force(s)’).Inparticular, weinvestigate howanticipatory muscle activations modulate whole-body centre ofmass acceleration. Results: Weshow thatthemechanical consequences ofanticipatory muscle activations have been extensively described, butmainly using the‘moving theCoP ’ mechanism. Unlike their roleduring steady balance states, both ‘moving theCoP ’ and‘applying new external force(s)’ mechanisms create arequired mechanical instability during theanticipatory phase ofmotor transitions. The‘counter-rotating ’ mechanism may actasastabiliser during motor transitions, butadditional research isneeded toclarify thisassumption. Significance: Thisreview establishes thatmuscle activation processes have different mechanical consequences for balance-movement coordination during theanticipatory phases ofmotor transitions, compared tosteady balance states. Because themechanical instability thatiscreated canleadtofalls, abetter understanding ofthemechanisms underlying motor transitions isneeded toenable thedesign ofmore effective fallprevention programs and/or devices forpopulation with balance deficits.