We investigated the existence of a midtarsal joint locking mechanism using cadaveric simulations of normal gait. Previous descriptions of this phenomenon led us to hypothesize that non-coupled rotations of the calcaneocuboid and talonavicular (i.e., midtarsal) joints and cubonavicular and talocalcaneal joints occur at heel strike and during weight acceptance, after which joint rotations cease with all bone-to-bone orientations remaining constant during the latter portions of stance phase. Three-dimensional kinematics of the talus, calcaneus, cuboid, and navicular were recorded along with muscle and ground reaction forces. Finite helical axis parameters and joint angles of directly articulating bones were subsequently derived and examined. During weight acceptance, the midtarsal joints everted with obvious changes in the relative orientation of their helical axes. The relative non-parallel orientation of these axes then remained constant until late in stance when these joints inverted and dorsiflexed toward their original pre-stance orientation. The cubonavicular and talocalcaneal joints demonstrated complimentary behavior. Contrary to our hypothesis, the midtarsal joints remained compliant during foot flat and even more so during push-off, despite divergent joint axes. Joint rotations were present after weight acceptance, thereby challenging the concept that midtarsal joint locking produces a rigid lever during push-off. © 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:110–115, 2014.
The foot displays bimodal behavior during the stance phase of walking. During weight acceptance, the forefoot is reasonably flexible enabling it to conform to terrain; upon heel rise and thereafter it stiffens to propel the body forward. This changeover from a flexible to a rigid construct is frequently attributed to the so-called “midtarsal joint locking mechanism,” and the most logical explanation for its existence is changes in the relative orientations of calcaneocuboid and talonavicular joint axes.
Blackwood and colleagues measured range of motion of the midtarsal joints across static positions of cadaveric feet in an attempt to quantify the midtarsal joint locking mechanism. Other studies calculated joint angles between the tarsal bones during normal gait in vivo and in vitro,[5, 6] but the midtarsal joint locking mechanism was only briefly mentioned, without specific attempts to rigorously define the underlying kinematics responsible for this behavior. The purpose of the current study was to search for clear evidence of a midtarsal joint locking mechanism and better define its mechanistic attributes using a broad set of kinematic measurement techniques. The relative motions of the four bones (talus, calcaneus, cuboid, and navicular) comprising the midtarsal joints were measured during dynamic cadaver simulations of gait. We hypothesized that non-coupled joint rotations of the calcaneocuboid and talonavicular (i.e., midtarsal) joints and cubonavicular and talocalcaneal joints occur during weight acceptance, after which joint rotations cease, with all bone-to-bone orientations remaining constant during the latter portion of stance phase. Characterization of joint motion was achieved by deriving Cardan angles and finite helical axis parameters of directly articulating bones of the midfoot and hindfoot during the stance phase of normal walking.
MATERIALS AND METHODS
Ten normal fresh frozen donated cadaver extremities (5M/5F, 59.2 ± 14.2 years) were evaluated. Actual body weights at death were unavailable; experiments were therefore based on purposely conservative estimates of body mass, ranging from 35 to 50 kg, to prevent specimen overload. Dynamic simulations of the stance phase of gait were conducted at 1/20th the velocity of typical walking using a robotic dynamic activity simulator.[7, 8] The specimens were prepared by removing all soft tissues 5 cm superior to the malleoli while preserving skin and retinaculi about the ankle. The entire length of the tendons of six muscles were preserved for attachment to linear actuators via freeze clamps: tibialis anterior (TA), tibialis posterior (TP), peroneus longus (PER), flexor hallucis longus (FHL), flexor digitorum longus (FDL), and triceps surae (TS). The tibia and fibula were transected ∼23 cm superior to the sole of the foot, and PMMA along with an intramedullary tibial rod were used to cement the proximal ends to a metal coupling device that was subsequently connected to kinematic actuators of the apparatus (Fig. 1).
Input data for shank kinematics and target ground reaction forces were selected from an in-house library derived from live subjects with matching foot size and sex. Input data for muscle force profiles were constructed from rectified EMG profiles with adjustments for force-length and force-velocity properties. The estimated body mass of each donor was used to normalize the peak contractile abilities of the muscles. During the simulations, sagittal plane tibia kinematics and six muscle actuations were adjusted until target vertical ground reaction force profiles were attained. Data collection was repeated three times for each specimen.
Tibia, talus, calcaneus, navicular, and cuboid were instrumented with marker clusters (Fig. 1) composed of four retro-reflective markers (6 mm diameter) connected by carbon fiber rods (0.16 mm diameter). Marker trajectories were recorded at 100 Hz using a 7-camera passive 3D photogrammetry system (EVaRT, Motion Analysis Corp, Santa Rosa, CA) with a typical 3D reconstruction residual of 0.3 mm. Muscle forces and ground reaction forces were recorded at 100 Hz by using inline load cells (A.L. Design, Buffalo, NY) and a multi-component force platform (OR-6, AMTI, Newton, MA), respectively.
The recorded marker data were smoothed using a dual-pass 4th-order Butterworth filter at 2 Hz cutoff frequency, and homogeneous coordinate transformation matrices for each bone were obtained by a least squares method. Only 90% of the stance phase was analyzed due to marker occlusion and dropout towards toe off. Finite helical axis parameters and bone-to-bone rotations (joint angles) were computed for the four directly articulating midtarsal bones: calcaneocuboid (cuboid with respect to calcaneus), talonavicular (navicular with respect to talus), cubonavicular (navicular with respect to cuboid), and talocalcaneal (calcaneus with respect to talus) joints.
Bone-to-bone joint angles were obtained by taking the ZYX Cardan decomposition of the relative transformation of the distal bone with respect to the proximal bone. The neutral pose was defined from a frame of data taken at the instant in a trial when the tibia was vertically oriented. The anatomical coordinate systems for the proximal bones were defined such that the inversion/eversion, internal/external rotation, and dorsiflexion/plantarflexion axes were aligned with the X, Y, and Z axes of the global coordinate system at the neutral pose, respectively. Ensemble averages of joint angle profiles over three trials were used to calculate across-subject mean and standard deviation profiles.
Finite helical axis parameters of each joint (n, unit vector along the helical axis, results not shown, and ϕ, angle of rotation about the helical axis) were obtained using the relative transformation of the distal bone with respect to the proximal bone. Included angles (θ) formed by the unit vectors of two helical axis pairs, calcalaneocuboid-talonavicular (midtarsal) joints, and cubonavicular-talocalcaneal joints, were then calculated. Profiles of ϕ and θ were averaged over the three repeated trials. Across-specimen ensemble mean profiles of ϕ and θ with standard deviations were calculated.
During data collection, we observed qualitative behavior suggestive of the previously described “locking mechanism,” with a visually compliant structure during weight acceptance, transforming to an apparently more rigid lever during push-off. Ground reaction forces and muscle forces recorded from three trials were consistent (Fig. 2). As per normal behavior, onset of muscle activation for the TS and first peak of TP occurred by design at ∼20% of stance, and all plantar flexor muscle forces peaked, along with vertical ground reaction forces, at ∼80% of stance. Foot flat was observed between ∼20 and 70% of stance.
Three phases of behavior were identified in all time series plots (Figs. 2-4). During weight acceptance (0–20% of stance phase), the midtarsal joints everted (Fig. 3a), rotating (ϕ) in similar counterclockwise directions and at similar rates about their respective helical axes, but with obvious changes in the relative orientations (θ) of those axes (Fig. 4a). The relative orientation of the two axes then remained constant (θ = 65°) during foot flat (20–70% of stance phase) while rotating about the helical axis of each joint clockwise at similar rates. As propulsive force increased during heel-rise and push-off (70–90% of stance phase), the midtarsal joints inverted, while the relative orientation of their helical axes returned toward its original pre-stance value (θ = 45°). Angular displacement (ϕ) about the helical axis of the calcaneocuboid joint continued in the same direction and at a similar rate as during the foot flat phase, while talonavicular joint rotated about its axis at a higher rate. The cubonavicular and talocalcaneal joints demonstrated similar complementary behavior (Figs. 3b and 4b). Contrary to expectations, all four joints displayed continuous rotations during foot flat and during push-off. Motions occurred primarily in the coronal and sagittal planes with less motion observed in the transverse plane (Fig. 3).
Mann stated that the inverted talocalcaneal joint causes divergence of the rotation axes of the calcaneocuboid and talonavicular joints, thus geometrically limiting the range of motion in both joints during the later portion of stance. Our data suggest an alternative mechanism that might still be viewed as midtarsal joint locking, but not necessarily with accompanying increases in joint rigidity. We noted relatively rapid changes in the included angles (θ) between the helical axes of the calcaneocuboid-talonavicular (Fig. 4a) and cubonavicular-talocalcaneal (Fig. 4b) joint pair during weight acceptance (0–20% of stance). Joint rotations (ϕ) about each respective helical axis also demonstrated faster rotations during weight acceptance than during foot flat (Fig. 4a and b). These responses are consistent with seating and accommodation of opposing articular surfaces as they are brought together under substantial load at initial contact.
Consistent with the concept of divergent axes, at ∼20% of stance the orientations (θ) of these axes “locked” into relative position at a calcaneocuboid-to-talonavicular included angle of ∼65° and a cubonavicular-to-talocalcaneal included angle of ∼90° until the end of foot flat (70% of stance), but this did not prevent further rotations. Instead, all joints continued to rotate, but in a more systematic and progressive manner indicative of smooth sliding at the tightly apposed articular surfaces of each joint pair under gradually increasing muscle load.
Rotations about the helical axes of the talonavicular (Fig. 4a) and talocalcaneal (Fig. 4b) joints again became more rapid as muscle and ground reaction forces reached their peaks during the final phase of push-off (70% to 90% of stance). Contrarily, their paired calcaneocuboid and cubonavicular joints maintained slower steady rotations similar to those measured during foot flat. The orientations of the calcaneocuboid-talonavicular joint axes then appeared to “lock” into a new, perhaps more advantageous, relative position, characterized by an included angle of ∼45° for final propulsion. Our data suggest that this orientation is likely maintained over the swing phase until the heel again strikes the ground. Thus, the helical axes of the midtarsal joints hold two constant relative orientations over the entire gait cycle, one corresponding to hindfoot inversion at push-off, during swing, and at weight acceptance, and another corresponding to hindfoot eversion during foot flat. The transition between these orientations at the onset of foot flat is characterized by changes in the slopes of angular displacement (ϕ) about each respective helical axis (Fig. 4).
The relative orientations (θ) between the helical axes of the calcaneocuboid-talonavicular joints (Fig. 4a) had a standard deviation of nearly 45° during weight acceptance that was gradually reduced to ∼25° during push off. The opposite trend was found in the rotation angle (ϕ) about the helical axes of these joint pairs (i.e., increased variability during push off). The cubonavicular-talocalcaneal joint pair (Fig. 4b) demonstrated less variability distributed more uniformly throughout stance. Considerable within (trial-to-trial) and across-specimen variability occurred under the same input conditions, leading one to conclude that the orientations of the helical axes are extremely sensitive to slight alterations in bone position, which demonstrated comparatively low variability (Fig. 3). This was particularly true prior to push off when the joints were less constrained. Increased constraints during push off likely decreased the variability of the relative orientation of the helical axes (θ), while increased constraints and external loading increased variability of the angular rotations (ϕ).
Despite internal joint motion, externally the foot appears to be acting as a rigid lever during heel-rise and push-off. We postulate this to be attributable to the additional constraints imposed on the midtarsal joints by increasing plantar flexor muscle forces and ground reaction forces, kinetics that produced a clear transition at heel-rise in all kinematic variables examined (Figs. 3 and 4) as the entire midfoot appeared to function as a single column under the imposed load. We also recorded steady calcaneal inversion with respect to the talus after 25% of the stance phase, in concert with the onset of the plantarflexors at and their increasing force output leading to heel-rise (Fig. 3b). Indeed, the high forces transmitted by the Achilles tendon (2.2X to 2.5X body weight) place the foot into a more supinated position ideally suited for force transmission through the extremity. These forces contribute to the reduced ranges of motion measured in the metatarsals during calcaneal inversion, compared to eversion. Regardless of these efficiencies, our findings refute the notion that divergent joint axes serve to stiffen the foot after weight acceptance.
An important strength of this study was the use of both finite helical axis parameters and Cardan joint angles to characterize internal joint kinematics of the calcalaneocuboid-talonavicular and cubonavicular-talocalcaneal joints pairs during representations of normal gait, thereby capitalizing on the merits of each analysis to develop a more complete conceptual model of actual behavior. The Cardan angles recorded here define the motions of each joint within each plane and generally agree with prior measurements of in vivo and in vitro internal kinematics. For the helical axis approach, it was not the orientations of the axes (data not shown), but the geometric relationship between axis pairs (θ) that was most illustrative. Plots of θ clearly delineated periods in early and late stance where the relative orientations of the two midtarsal joint axes were changing, from a period during mid-stance where the relationship remained fixed; while complementary plots of ϕ served to demonstrate continuous rotations irrespective of timing and axis orientations.
These data provide interesting and reasonably convincing new perspectives on the midtarsal locking mechanism; nevertheless, they are the product of an in vitro model and should be confirmed in vivo. Though sophisticated, the model fails to capture reality in several respects. Simulations were conducted at 1/20 of normal velocities; higher speeds might alter the measured kinematics. The body weights employed were estimates and certainly somewhat less than the actual weights of the donors, which impacted not just the ground reaction forces, but also the muscle forces employed in these experiments. In addition, the intrinsic muscles of foot could influence kinematic behaviors and these were not represented in the model.
In conclusion, our data shed new light on the function of the midtarsal joints of the foot during stance and redefine the “joint locking mechanism” and its mechanical consequences. We conclude that the midtarsal joints undergo coupled rotations during weight acceptance and transition to more organized and unified rotations during foot flat, which are then followed by fast and forceful rotations during push-off that reestablish their original pre-stance orientations. Contrary to our hypothesis, the midtarsal joints remained compliant during foot flat and even more so during push off, despite divergent joint axes, demonstrating gradual dorsiflexion and a gradual return to an inverted position. These findings challenge the widely accepted theory of bimodal behavior whereby midtarsal joint locking transitions the foot from a relatively flexible structure during weight acceptance to a rigid structure during push off.
This research was supported by a grant from the NIH (5R03HD050532-02). The authors appreciate the efforts of Ms. Kelly Twomey and Mr. Brad Hendershot for post-processing of the motion analysis data. We also thank Dr. Andrew Hoskins and Mr. Joseph Strella for their invaluable technical expertise. There were no conflicts of interest in the performance of this research.