1. Toe box crush strength (fig 1a,b): The resistance to a deforming dorsoplantar force applied to the toebox of each boot was measured. A hydraulic ram with a rounded end was placed over the center of the toebox of each type of boot. The force versus deformation was then plotted for each boot.
   
   

   
   Figure 1a - Toebox Crush Strength Test Design

2. Toe box dorsiflexion stiffness (fig 2a,b): Each boot was placed over a steel support with a rigid last placed inside the boot to firmly clamp the mid and hind part of the boot. A dorsiflexion plate was constructed such that the moment applied to create the dorsiflexion force to the toebox via the outsole could be reliably measured. A protractor was used to measure the rotational movement associated with each load level in all tests except for the valgus loading, in which the angles were measured radiographically (see below).
   
   
   Figure 2a - Toebox Dorsiflexion Stiffness Test Design              Figure 2b - Toebox Dorsiflexion Stiffness Testing Apparatus
   
   
3. Ankle dorsiflexion stiffness (fig 3a,b): In an effort to assess the flexibility and protection against extreme applied dorsiflexion loads, each boot was placed in a jig and a dorsiflexion moment was applied to the plantar outsole of the fore part of the boot at the metatarsal head level. A wooden "leg" with an articulated ankle (fig. 12ab) was constructed to support the calf portion of the upper boot and allow relative movement of the foot and leg to simulate the function of the human ankle and subtalar joints. Ankle dorsiflexion moments were applied and the angle of deformation for each load was then measured.
   
   
         Figure 3a - Ankle Dorsiflexion Stiffness Test Design               Figure 3b - Ankle Dorsiflexion Stiffness Testing Apparatus
   
   
   

   Figure 12a & 12b - Articulated Wooden Simulated Ankle/Subtalar Joint
   
   

4. Ankle plantarflexion stiffness (fig 4a,b): For each boot tested a series of extrinsic loads were applied in a downward direction at the metatarsal head level to create an plantarflexion moment about the ankle. The applied moment versus the angular deformation was measured with a similar jig system as for dorsiflexion.
   
   

   
   Figure 4b - Ankle Plantarflexion Testing Apparatus

5. Valgus ("lateral") stiffness (fig 5a,b): Three-point bending loads were applied to each boot in a valgus direction, using the above simulated leg/ankle joint construct. The angular deformation of the simulated wooden ankle joint was imaged via posteroanterior radiographs taken at each load level through the boot. The angle was imaged via a metal rod placed axially within the center of the simulated leg and "heel," providing an image of the angular deflection of the ankle/subtalar joint model within the boot as it displaced from the applied load (Fig. 13ab).
   
   
                   Figure 5a - Valgus 'Lateral' Stiffness Test Design               Figure 5b - Valgus Testing Apparatus
   
   

Toe box crush strength: The force vs deformation relationship was plotted on a graph (fig 6). All of the modern boots exhibited considerably greater resistance to a downward deforming load than the older style boot. In all the modern boots tested, the toe boxes demonstrated a linear deformation to applied load until the toe box depressed completely, contacting the insole at approximately 30mm of deformation which showed a flattening of the curve as the load was increased. The boots with the stiffer toe boxes (AXO, SIDI SRS, SIDI, Alpinestar Vector) deformed approximately half as much at a given load as the other models.

Figure 6 - Toebox Crush Strength

Figure 7 - Toebox Dorsiflexion Stiffness

Figure 8 - Toebox "kink"

Figure 9 - Ankle Dorsiflexion

Figure 10 - Ankle Plantarflexion Stiffness

Figure 11 - Ankle Stiffness to Applied Valgus Load

Off-road motorcycle protective footgear has evolved considerably over the last three decades with the development of sport-specific protective and functional features such as a rigid toebox, impact/abrasion/puncture resistance, stiff sole, inversion/eversion ankle constraint and modern fastening systems. The loss of proprioception and restriction of motion of the ankle, foot and toes presents a potential compromise to the efficient movements associated with competition riding activity.

The purpose of this paper was to examine the mechanical properties of the boots and attempt to describe how relevant design features may prevent skeletal injury while maintaining physiologic motion, proprioception and pressure sensation necessary for competitive off road riding. Detailed information is scarce in the scientific literature regarding incidence and severity of foot and leg injuries in off road motorcycling. Anecdotal reports of the authors and other orthopaedic surgeons suggest a broad spectrum of injuries including stubbing type injuries to the great toe, metatarsal fractures, midfoot fracture dislocations, fractures of the talar neck, ankle, tibial pilon and tibial shaft, essentially the entire spectrum of known injury types. No published information is available regarding the severity or frequency of these injuries. We examined the mechanical properties of modern boot designs with special reference to the known mechanisms of skeletal injury to the foot, ankle and leg.

Toebox Crush Strength: All of the modern boots studied had considerable reinforcement to the toebox periphery, but the dorsal toebox shell was in some models more easily deformable and none had a steel reinforced toe, a common feature to work boots, possibly because of the tendency of steel to dent permanently, whereas plastic materials can dissipate the energy of an impact by initially deforming, then returning to their original shape. All the boots studied had strong plastic material which resisted a dorsoplantarly directed crush force relatively well in comparison to the all leather vintage boot, however some models had notably softer dorsal toebox material which may aid in the ability to sense the shift lever during gear changes but could potentially offer less protection from a dorsoplantar blow to the foot.

Toebox Dorsiflexion Stiffness: An important feature of protective footwear is the ability of the boot to resist dorsiflexion loads that frequently occur during stubbing type injuries. Several mechanisms are responsible for toe/forefoot injury while riding off road motorcycles, such as a stubbing injury from an anteroposteriorly directed impact, (fig 14), a forceful dorsoplantarly directed force tending to compress the toebox and plantarflex the toes at the metatarsophalangeal joints (fig15) as well as a similarly injurious force directed upward (from plantar to dorsal) which tends to force the toes into extreme dorsiflexion at the metatarsophalangeal joints (fig 16). Theoretically, boots that create greater resistance to dorsiflexion at the metatarsophalangeal joints would be more protective against dorsiflexion type stubbing injuries of the forefoot (fig 16). Though flexibility of the forefoot or "break" of a shoe is necessary for normal walking it is unknown whether motion at the shoe break of a riding boot is necessary to competitively pilot an off road motorcycle. In contrast, downhill ski boot design, for example has evolved to an infinitely rigid sole designed to securely attach to the ski. The results of this study suggest that a rigid sole may be an acceptable compromise to yield a stronger boot structure and not significantly compromise function, as several of the boots had a very stiff, essentially non existent shoe break similar to a downhill ski boot.

Figure 16 - Mechanism of Forefoot Dorsiflexion Injury

Ankle Plantarflexion: All modern boots have features in common to allow some degree of ankle motion and provide protection in critical areas. Boot manufacturers have gone about it differently, however, with most (Alpinestars, Fox, AXO) relying exclusively on controlled deformation of energy absorbing material and placement of softer materials in crease areas (fig 18), while others (SIDI) attempt to enhance protection by incorporating an exoskeletal structure, using a distinct articulation to allow ankle motion (fig19).

The ability to plantarflex the ankle appears to be a helpful movement when entering a soft corner during competition (fig 17). While the minimum necessary physiologic plantarflexion motion required during riding activity is not clear, the features of the exoskeletal design to prevent motion past ten to fifteen degrees of plantarflexion but maintain a linear progressive resistance to motion exceeding this range may afford more protection from hyperplantarflexion injuries. The other designs appear to offer less resistance as the deflexion progressed past this limit.

Ankle Dorsiflexion: It is not apparent from current boot designs how much ankle dorsiflexion is necessary for satisfactory boot performance. The study showed that in this regard all the modern boots were very similar, resisting ankle dorsiflexion forces in a linear, gradual manner at dorsiflexion angles of approximately less than thirty degrees. The need for ankle dorsiflexion movement in off road motorcycle competition is unclear. "Upshifting" the gearbox requires upward movement of the forefoot against the gearshift lever, which can be accomplished by raising the leg via flexion of the hip and knee. A small amount of additional active ankle dorsiflexion may be necessary for optimal performance, possibly explaining why all the modern boots functioned similar. The ability of the boot to decisively limit extreme ankle dorsiflexion (such as when a supercross rider becomes separated from the machine at significant height off a jump and lands in the crouched position.may be an important safety feature which is the rule in downhill ski boots, for example, where the rigid endpoint to ankle dorsiflexion allows weight transfer to the anterior tibial shaft. However, the potential for transfer of loads proximally, causing injury to the knee is a known result of the evolution to a rigid ski boot design.

Valgus Stiffness: There appears to be a distinct advantage to the exoskeletal design with regard to resistance to valgus deformation about the ankle. The more rigid shell resisted valgus deforming forces better than the other designs with the exception of the AXO boot with a very rigid, nonarticulated shell and stiff ankle resisted valgus loads equally well as the SIDI exoskeletal designs, but achieved this strength at the expense of ankle motion in all planes. The exoskeletal design (SIDI) type capitalizes on the superior resistance to bending forces about the ankle with a rigid outer shell while theoretically preserving physiologic dorsi and plantarflexion movement with an articulated ankle. Interestingly, using the articulated wooden leg-ankle model, the moment required to produce ankle dorsiflexion was similar in both exoskeletal and bellows based designs because of the shearing resistance produced by the sliding material about the anterior aspect of the SIDI boot and was highly dependent upon the tightness of the fastener buckle at the anterior ankle level. In vivo, the user may naturally fasten this buckle more loosely than we did using the experimental wooden mockup.

A common feature observed with the Alpinestars, Fox and AXO brands is the presence of time-dependent deformation or "creep" secondary to the thicker foam bellows structure about the ankle. Slower movements about the ankle are met with less resistance however the rate of creep is exceedingly slow and does not appear to enhance motion at physiologic riding speeds. This bellows type foam ankle crease structure of these boots may contribute to the "breaking in" feeling as the material becomes more flexible with use.

CONCLUSION

The widely held belief that the evolution of off-road motorcycle boot technology has enhanced the safety of the sport is broadly supported by the results of this study, based on in vitro testing. Two distinct approaches to boot design have addressed the need for protection and motion of the foot and ankle. The exoskeletal designs tested provide less resistance to ankle plantarflexion which may be a performance advantage over other boot designs. An exoskeletal design may afford superior protection to impact injury near the ankle anteriorly and posteriorly but the in vivo significance of this is unknown.

Unlike injuries associated with skiing, there is little scientific understanding of the mechanism of off road motorcycle associated lower extremity injuries. Despite the lack of scientific data, boot design continues to evolve with seemingly better protective features. Future research efforts to study in vivo injury patterns and their association with boot design such as with a lower extremity injury registry among orthopaedic surgeons who treat these injuries will help us to better understand the relationship between lower extremity protective devices and injury prevention.