Association between Slip Severity and Muscle Synergies of Slipping

Abstract

Falls impose significant negative impacts to the US population and economy. A significant number of falls may be prevented via appropriate slip-responses since a strong relation exists between slips and falls. More importantly, as severe slips are more prone to result in a fall, identifying severe slippers along with the responsible factors for their adverse motor control and severe slipping should be the highest priority in fall prevention process. Previous studies have suggested that muscle synergies may be building blocks of the central nervous system in controlling motor tasks. Muscle synergies observed during slipping (‘post-slip-initiation synergies’ or ‘just briefly,’ ‘slipping muscle synergies’), may represent the fundamental blocks of the neural control during slipping. Hence, studying the differences in slipping muscle synergies of mild and severe slippers can potentially reveal the differences in their neural control and subsequently, indicate the responsible factors for the adverse post-slip response in severe slippers. Even though the slipping muscle synergies have been investigated before, it still remains unclear on how the slip severity is associated with the slipping muscle synergies. More importantly, muscle synergies can be interpreted not only as neural blocks but also as physical sub-tasks of the main motor task. Hence, studying the differences of slipping synergies of mild and severe slippers would reveal the discrepancies in sub-tasks of their post-slip response. These discrepancies help pinpoint the malfunctioning sub-function associated with inadequate motor response seen in severe slippers. Twenty healthy subjects were recruited and underwent an unexpected slip (to extract their slipping synergies). Subjects were classified into mild and severe slippers based on their Peak Heel Speed. An independent t-test revealed several significant inter-group differences for muscle synergies of mild and severe slippers indicating differences in their neural control of slipping. A forward dynamic simulation was utilized to reveal the functionality of each synergy. Decomposition of slipping into sub-tasks (synergies), and finding the malfunctioning sub-task in severe slippers is important as it results in a novel targeted motor-rehabilitation technique that only aims to re-establish the impaired sub-task responsible for the adverse motor-response in severe slippers.

Angular momentum regulation may dictate the slip severity in young adults

“…The markers' data were low-passed filtered (at 10 Hz) with a second order Butterworth filter (MATLAB, MathWorks, Natick, MA) [20]. Using the heel marker information after the slip trial, participants were classified to mild and severe slippers based on their peak heel speed (PHS) [13,15] to investigate their inter-group differences. Next, based on heel and toe markers, the heel strike and toe-off were calculated and the corresponding double/single support phase of the gait were measured for each individual.…”

“…Furthermore, the severe slippers experienced a shortened single stance phase following a slip. "Toe-touch" response is a known method to increase the base of support during slipping [15,29]. Toe-touch is responsible to disrupt the gait while slipping to place the swing limb on the ground and is beneficial in reestablishing a wider base of support, providing weight support, and regaining balance.…”

“…Additionally, prior studies have shown that one's risk of fall is affected by both pre-slip control (gait control) and post-slip response (slip control) [14][15][16][17]. In other words, mild slippers possess different control techniques for both walking and slipping compared to severe slippers.…”

Angular momentum regulation may dictate the slip severity in young adults

“…Furthermore, the severe slippers experienced a shortened single stance phase following a slip. “Toe-touch” response is a known way to increase the base of support during slipping (15,29). However, it seems that this strategy is only used in more severe slips, since all mild slippers avoided using this strategy while slipping and continued countering slip on one limb, without a toe-touch.…”

“…The markers’ data were low-passed filtered (at 10 Hz) with a second order Butterworth filter (MATLAB, MathWorks, Natick, MA) (20). Using the heel marker information after the slip trial, subjects were classified to mild and severe slippers based on their Peak Heel Speed (PHS) (13,15) to investigate their inter-group differences. Next, based on heel and toe markers, the heel strike and toe-off were calculated and the corresponding double/single support phase of the gait were measured for each individual.…”