Neuromechanical adaptations of foot function to changes in surface stiffness during hopping

Abstract

Humans choose work-minimizing movement strategies when interacting with compliant surfaces. Our ankles are credited with stiffening our lower limbs and maintaining the excursion of our body's center of mass on a range of surface stiffnesses. We may also be able to stiffen our feet through an active contribution from our plantar intrinsic muscles (PIMs) on such surfaces. However, traditional modelling of the ankle joint has masked this contribution. We compared foot and ankle mechanics and muscle activation on Low, Medium and High stiffness surfaces during bilateral hopping using a traditional and anatomical ankle model. The traditional ankle model overestimated work and underestimated quasi-stiffness compared to the anatomical model. Hopping on a low stiffness surface resulted in less longitudinal arch compression with respect to the high stiffness surface. However, because midfoot torque was also reduced, midfoot quasi-stiffness remained unchanged. We observed lower activation of the PIMs, soleus and tibialis anterior on the low and medium stiffness conditions, which paralleled the pattern we saw in the work performed by the foot and ankle. Rather than performing unnecessary work, participants altered their landing posture to harness the energy stored by the sprung surface in the low and medium conditions. These findings highlight our preference to minimize mechanical work when transitioning to compliant surfaces and highlight the importance of considering the foot as an active, multi-articular, part of the human leg.

Neuromechanical adaptations of foot function when hopping on a damped surface

“…While the ankle contributed much of the increase in positive work performed in the Damped condition, the intrinsic foot muscles were able to contribute significant portion of this work (25% of that performed at the ankle). That the foot can contribute to replace energy dissipated by a damped surface is a novel finding and when considered with the other recent work from our group (13,14,17,18,23,25), further highlights the versatility of our feet in the control of movement.…”

“…However, we uncovered in prior work that running in cushioned shoes with a viscoelastic midsole leads to an increase in intrinsic foot muscle activation, compared with barefoot running ( 19 ). When considered with our recent findings that footwork and muscle activation decrease when hopping on an elastic surface ( 14 ), we propose that feet may also play an important role in how humans adjust to operating on surfaces that dissipate energy.…”

“…However, this research used passive rigid-foot models that ignore the dynamic structures of the foot and can lead to considerable inaccuracies in measures of work and quasi-stiffness about the ankle joint ( 12 , 13 ). We have previously highlighted the significance of considering the foot as an active, multiarticular part of the lower limb when humans adapt to different surfaces ( 14 ). It is therefore important to understand the contribution of the human foot in tuning lower limb mechanics when interacting with a damped surface.…”

Foot form and function: Variable and versatile, yet sufficiently related to predict one from the other

“…Through the combined efforts of its passive and active components, the foot is capable of transitioning between compliant spring, energy producing motor and energy absorbing damper during tasks with different work requirements at the centre of mass (Birch et al, 2021;Riddick et al, 2019;Smith et al, 2021). To determine how feet with different morphologies behave during such different tasks, participants were asked to walk and run (Froude number = 0.25 and 1.00, respectively) at three inclinations each (walking: -20, 0, 20%; running: -10, 0, 10%) on a force-sensing, fore-aft, tandem treadmill (AMTI, USA).…”

“…As Lauder (1995) argues, function is not defined by musculoskeletal structure itself, but by how it is used, i.e., "the motor programs in the central nervous system". There is ample evidence for this argument in relation to the foot, as various investigations have shown that the neuromuscular control of intrinsic and extrinsic foot muscles plays an important role in the mechanical function and versatility of the foot (Birch et al, 2021;Kessler et al, 2020;Riddick et al, 2019;Smith et al, 2021). Thus, future investigations examining the link between foot form and function might benefit from also examining the potential link to the neuromuscular control of relevant muscles during various locomotor tasks.…”