Discussion
hallux rigidus, windlass, Midtarsal
HALLUX RIGIDUS, SECONDARY, CONTRACTION
The purpose of this study was to explore the extent of coupling within the foot and to investigate potential contributing factors through systematic manipulation of the MTP joint during walking. From previous work, we expected that when the MTP joint was locked (either in a neutral or extended position) the midtarsal joint would exhibit less ROM and less positive work generation concomitant with the decrease in MTP ROM and negative work absorption. Overall, the data collected in this study support our foot coupling hypotheses; when MTP mechanics are altered the midtarsal joint responds both kinematically and kinetically.When the MTP joint was prevented from fully extending, the midtarsal joint was also proportionally affected, matching previous research showing kinematic coupling between these two foot joints. The decrease in ROM at the MTP joint induced by our locked brace conditions is comparable to other studies where MTP motion was altered through various mechanisms (e.g. hallux rigidus [Our results also confirm that coupling extends to the kinetics of the foot. During the locked brace conditions, the energy generated by the midtarsal joint and absorbed by the MTP joint were both reduced compared to unconstrained conditions. These results are congruent with previous research that showed changes in midtarsal kinetics secondary to changes in MTP alterations due to task variations [The magnitude and timing of the kinetic and kinematic coupling between the MTP and midtarsal joints can be visualized when plotting the angle and power profiles of both joints together (Fig. Midtarsal and MTP angle and power curves during stance phase (top row). Range of Motion (ROM; left bottom) and joint work (right bottom) values are presented as bar graphsWe anticipated that a deeper look into MTP joint power components would provide additional insight into kinetic foot coupling. MTP power is the product of the joint moment and angular velocity, with the moment further subdivided into the GRF and its moment arm, represented by the COP. We noted that the hallux COP for the locked brace conditions was further anterior (Fig. Ground reaction forces under the forefoot and hallux segments (left). Center of pressure location relative to the MTP joint center with standard deviation error bars (right)Our results provide some preliminary insights into midtarsal power generation in late stance and its associated coupling with the MTP joint; however, specific tissue contributions are still unclear. Likely midtarsal contributors include active muscle contraction or passive energy storage and return from tendons, ligaments, and fascia that cross the midfoot region. In our study, the induced changes seen at the midtarsal joint are likely primarily due to those tissues that span both the midfoot and toes, shortening this list. For instance, the arch supporting ligaments (plantar calcaneonavicular ligament and long and short plantar ligaments) and larger extrinsic muscles (tibialis posterior and fibularis longus/brevis) would primarily affect only the midtarsal joint if altered by the MTP restrictions induced in this study. Thus, main coupling contributors might be narrowed to the plantar aponeurosis and MTP flexors (flexor hallucis/digitorum longus/brevis, adductor hallucis, and quadratus plantae). While we do not rule out a role for the plantar aponeurosis and associated passive energy transfer through the windlass mechanism, previous research has suggested that its contribution to midtarsal kinematics is likely small [There are two potential overlapping mechanisms for the role of the MTP flexors in MTP-midtarsal joint coupling: 1- altered muscle activation, and 2- altered joint stiffness. The brace may have inhibited MTP flexor activation or increased co-contraction, resulting in decreased muscle forces and muscle-based moments across both joints. Alternately (or in addition), the braced conditions may have altered the foot’s posture just enough to constrain the motion of both the MTP and midtarsal joints. By inhibiting forward progression, the foot may be in a disadvantageous position for generating propulsive power at the midtarsal joint; thus, similar toe flexor muscle activations would have a reduced effect on both MTP and midtarsal motion (and thus power) by increasing joint stiffness. While we did not calculate a stiffness metric, this is apparent in the similar moments but altered motion at both joints. This altered stiffness could be due to altered muscle lengths/velocities, muscle–tendon strains, or joint congruency. Future studies employing electromyography, dynamic imaging, and musculoskeletal modeling may help tease out specific mechanisms and tissue contributions.The strong energetic coupling seen in this study suggests that interventions aimed at modulating MTP work should not ignore the midtarsal joint. As a primary example, research in footwear toe springs has suggested that reductions in MTP negative work could reflect an increase in running energetic efficiency [Our methods had a few additional limitations that should be mentioned. As mentioned, our locked brace conditions did not fully lock the MTP joint, in either neutral or extended positions; instead, wearing the brace resulted only in slightly less peak motion for the MTP joint (~ 7 degrees). However, the decrease in peak MTP motion resulted in less midtarsal ROM (1.3 degrees) and provided the necessary stimulus to probe distal foot coupling. A fully locked brace would likely have been too extreme, resulting in large compensations elsewhere. It is also possible that the straps used to secure the brace to the foot, that wrap around and under the midfoot, could have altered pressure slightly. For this reason, we used both a no brace (CON) and unlocked brace conditions (BR_UN). Comparing the two, the brace appeared to constrain the foot slightly, with most mean curves slightly offset in the direction of the two constrained conditions. Most of these were not significant, however, and the four conditions served to provide a progression from most to least constrained, confirming the directionality and proportionality of the changes. Additionally, we only analyzed coupling within the sagittal plane. Axes within the foot are not orthogonal and investigation into other planes may offer additional insight into foot coupling [