The effect of carbon fiber insole stiffness on margin of stability during normal treadmill walking

Introduction: Older adults utilize their foot and ankle differently than younger adults during functional activities such as walking. This difference could be a key factor contributing not only to mobility decline but also to a greater falls risk in our rapidly aging population. Despite conventional efforts that are by design time-consuming and which have limited compliance without supervision (e.g., strengthening or balance training), one-third of people over age 65 fall annually and 20-30% of these falls lead to moderate to severe injury. Compared to younger adults, we have shown that older adults walk with reduced foot stiffness, equivalent to walking on ‘flat tires.’ Requisite foot stiffness and the mechanical leverage it conveys play a critical first line of defense in the successful deployment of neuromuscular strategies to mitigate instability during walking and thereby an unexplored but promising target for low-cost, scalable, and widespread intervention. Carbon fiber insoles designed to augment foot stiffness also have the potential to enhance the mechanical leverage needed to effectively deploy neuromuscular strategies to mitigate the instability elicited by walking balance perturbations. We hypothesized that augmenting shoe stiffness via carbon fiber insoles would increase margins of stability (MoS) – an important biomechanical feature of walking balance – in younger and older adults.

Materials and

Methods: 10 younger and 8 older adult subjects have thus far completed the experiment. Participants were fitted with a safety harness and walked in a standardized neutral shoe (New Balance 880) on a force-sensing treadmill (Bertec, Corp.) for 5 minutes to allow their movement patterns to stabilize. Subjects then completed three 2-min walking trials at their preferred speed in randomized order: (i) no carbon fiber insole, (ii) a 1/16” carbon fiber insole, and (iii) a 1/8” carbon fiber insole. Carbon fiber insoles were custom-fabricated in a range of sizes using a water jet. A 16-camera motion capture system (Motion Analysis, Corp., Santa Rosa, CA) recorded the 3D positions of a whole-body marker set in synchrony with 3D ground reaction force data. We calculated MoS-ML using previously published methods. Specifically, we first used the center of a polygon created by the anterior and posterior iliac spine marker positions as a surrogate for the body’s center of mass (CoM). We then calculated the extrapolated CoM (xCoM), corrected for treadmill velocity, and normalized to leg length. Finally, we calculated MoS at the instant of heel-strike as distance between the xCoM position and the edge of the base of support (BoS), defined as 5th metatarsal marker for mediolateral MoS (MoS-ML) and the 1st metatarsal marker for anteroposterior MoS (MoS-AP).

Results, Conclusions, and Discussions: Data thus far from 5 analyzed younger adults (4F/1M, 27.2±5.0 yrs, 72.7±10.3 kg, 1.7±0.1 m) show that, in 4/5 subjects, MoS-ML increased progressively with increasing insole stiffness (Fig. 1). Conversely, no between-condition difference is yet apparent in MoS-AP. We interpret these larger MoS-ML values, which indicate that the xCoM is further from the lateral BoS, to suggest that increasing foot stiffness via carbon fiber insoles and the mechanical leverage it conveys precipitate a more “stable” walking strategy. This is highly relevant; mediolateral balance in walking is under much more active control than anterior-posterior balance and is disproportionately affected by perturbations. We have no reason to expect a different biomechanical response to carbon fiber insoles among our older adult participants. To the contrary, we anticipate even larger benefits for older adults who, compared to younger adults, lack sufficient foot stiffness and exhibit higher gait instability. We envision our data across these cohorts of younger and older adults pointing to low-cost interventions to augment foot mechanics and thereby a scalable means to address falls as a public health concern in our aging population.

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