SYSTEMS AND METHODS FOR ATTENUATING ROTATIONAL ACCELERATION OF THE HEAD

[0001] This application claims priority to co-pending U.S. Provisional Application Ser. No. 61/482,967, filed May 5, 2011, which is hereby incorporated by reference herein in its entirety.

[0002] Sports concussion and traumatic brain injury have become important issues in both the athletic and medical communities. As an example, in recent years there has been much attention focused on the mild traumatic brain injuries (concussions) sustained by professional and amateur football players, as well as the long-term effects of such injuries. It is currently believed that repeated brain injuries such as concussions may lead to diseases later in life, such as depression, chronic traumatic encephalophathy (CTE), and amyotrophic lateral sclerosis (ALS).

[0003] Protective headgear, such as helmets, is used in many sports to reduce the likelihood of brain injury. Current helmet certification standards are based on testing parameters that were developed in the 1960s, which focus on the attenuation of linear impact and prevention of skull fracture. An example of a linear impact is a football player taking a direct hit to his helmet from a direction normal to the center of his helmet or head. Although the focus of headgear design has always been on attenuating such linear impact, multiple lines of research in both animal models and biomechanics suggest that both linear impact and rotational acceleration play important roles in the pathophysiology of brain injury. Although nearly every head impact has both a linear component and a rotational component, rotational acceleration is greatest when a tangential blow is sustained. In some cases, the rotational acceleration from such blows can be substantial. For instance, a football player's facemask can act like a lever arm when impacted from the side, and can therefore apply large torsional forces to the head, which can easily result in brain trauma.

[0004] Although the conventional wisdom is that the components of modern protective headgear that are designed to attenuate linear impact inherently attenuate rotational acceleration, the reality is that such components are not designed for that purpose and therefore do a relatively poor job of attenuating rotational acceleration. It therefore can be appreciated that it would be desirable to have a system and method for attenuating not only linear impact to but also rotational acceleration of the head, so as to reduce the likelihood of brain injury.

[0031] As described above, current protective headgear is primarily designed to attenuate linear impact. However, it has been determined that both linear impact and rotational acceleration from torsional forces contribute to brain injury, such as concussion. Disclosed herein are systems and methods for attenuating rotational acceleration that results from impacts to the head. The systems can take the form of protective headgear, such as helmets, that comprise an outer shell and an inner liner that enable the shell to rotate relative to the head to reduce rotational acceleration of the head and brain that can occur from impacts. In some embodiments, the liner or a portion of the liner can move relative to the shell to decouple the shell from the liner and the wearer's head. In other embodiments, the liner includes material that is specifically designed to yield to tangential forces and therefore enables the shell to rotate relative to the wearer's head. In both cases, rotational forces applied to the helmet from impacts are not directly transmitted to the head. Instead, those forces are dissipated over time to reduce brain shear.

[0032] In the following disclosure, various embodiments are described. It is to be understood that those embodiments are example implementations of the disclosed inventions and that alternative embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure.

[0033] Described in the following disclosure are solutions to the problem of rotational acceleration of the brain that results from impact to the head. More particularly, disclosed are dynamic head suspension systems that directly address tangential forces that cause the highest shear strains on the brain and the brain stem. Two general solutions are described below. In a first solution, a liner provided within a helmet shell can slide within the shell so as to enable relative rotational motion between the shell and the liner. In addition, one or more elements are provided within the shell that absorb the rotational force so that it is not directly transmitted to the head. In a second solution, an engineered material with desirable shear properties is used within the helmet shell. The engineered material can form part of the liner and is specifically designed to attenuate and dissipate rotational energy from impacts to the head by undergoing controlled lateral and/or rotational shear, thus absorbing rotational energy and dissipating it over time before transmission to the head and the cerebrum. In both solutions, the dynamic head suspension system can be modular in design, enabling adaptability for use in a wide range of helmets and other protective headgear. By optimizing protection from both linear impacts and rotational acceleration, both solutions decrease the transmission of shear force to the brain and therefore lower the incidence of brain injury, such as concussion.

[0034] FIGS. 1-8 illustrate a first embodiment of a protective helmet 10 that is designed to attenuate both linear impact and rotational acceleration. The helmet 10 is an example embodiment of the first solution and therefore comprises a liner that can slide relative to the shell.

[0035] Beginning with FIGS. 1-5, the helmet 10 is generally configured as an American football helmet. Although that particular configuration is shown in FIGS. 1-5 and other figures of this disclosure, it is to be understood that a football helmet is shown for purposes of example only and that the shell design shown in the figures is merely representational of an example football helmet shell. The helmet need not be limited to use in football and furthermore can take alternative forms. For example, the helmet 10 can be formed as another type of sports helmet or a military helmet.

[0036] With continued reference to FIGS. 1-5, the helmet 10 generally includes an outer shell 12 and an inner liner 14. In the illustrated embodiment, the shell 12 is shaped and configured to surround the wearer's head with the exception of the face. Accordingly, the shell 12, when worn, extends from a point near the base of the wearer's skull to a point near the wearer's brow, and extends from a point near the rear of one side of the wearer's jaw to a point near the rear of the other side of the wearer's jaw. In some embodiments, the shell 12 is unitarily formed from a rigid material, such as a polymer or metal material. By way of example, the shell 12 can be a molded acrylonitrile butadiene styrene (ABS) or polycarbonate alloy shell.

[0037] Irrespective of the material used to construct the shell 12, the shell includes an outer surface 16 and an inner surface 18. In some embodiments, the shell 12 can further include one or more ear openings 20 that extend through the shell from the outer surface 16 to the inner surface 18. The ear openings 20 are provided on each side of the shell 12 in a position in which they align with the wearer's ears when the helmet 10 is donned. Notably, the shell 12 can include other openings (not shown) that serve one or more purposes, such as providing airflow to the wearer's head.

[0038] As is further shown in FIGS. 1-5, a facemask 22 can be secured to the front of the helmet 10 to protect the face of the wearer. In some embodiments, the facemask 22 can comprise one or more rod-like segments that together form a protective lattice or screen. When used, the facemask 22 can, for example, be attached to the helmet 10 at points that align with the forehead and jaw of the wearer when the helmet is worn. The facemask 22 can be attached to the helmet 10 using screws (not shown) that thread into the shell 12 or into fastening elements (not shown) that are attached to the helmet. Although a particular facemask configuration is shown in the figures, alternative configurations are possible. Moreover, the facemask 22 can be replaced with a face shield or other protective element, if desired.

[0039] The liner 14 generally comprises one or more pads that sit between the shell 12 and the wearer's head when the helmet 10 is worn. In the illustrated embodiment, those pads include a top pad 24, opposed lateral pads 26, a rear pad 28, and opposed forward pads 30. The top pad 24 is adapted to protect the top of the wearer's head. In the illustrated embodiment, the top pad 24 is elongated in a direction that extends along the sagittal plane of the wearer so as to extend from a rear top portion of the head to a front top portion of the head. The top pad 24 is further curved to generally follow the curvature of the wearer's head. Accordingly, the top pad 24 forms a concave inner surface 32 that is adapted to contact the wearer's head.

[0040] The lateral pads 26 are adapted to protect the sides of the wearer's head. The lateral pads 26 are generally rectangular and extend from the edges of the wearer's face to points behind (and above) the user's ears. Each lateral pad 26 includes a void 34 that provides space for an ear of the wearer. Like the top pad 24, the lateral pads 26 are curved to follow the curvature of the wearer's head. Accordingly, the lateral pads 26 form concave inner surfaces 36 that are adapted to contact the wearer's head.

[0041] The rear pad 28 is adapted to protect the rear of the wearer's head and, like the lateral pads 26, is generally rectangular. Also like the top pad 24 and the lateral pads 26, the rear pad 28 is curved to follow the curvature of the wearer's head and forms a concave inner surface 36 that is adapted to contact the wearer's head. As is apparent from the figures, the lateral edges of the rear pad 28 comprise compression regions 40 that are adapted to compress when the pad abuts one of the lateral pads 26 to dissipate rotational force. For this reason, the rear pad can be considered to function as a force dissipation pad. In the illustrated embodiment, the compression regions 40 comprise vertical grooves 42 that are formed in the rear pad 28 to reduce the amount of material near the lateral edges of the pad to enable those edges to more easily compress inward toward the center of the pad. The purpose behind this functionality is described below in the discussion of the use of the helmet 10 in relation to FIGS. 7 and 8. The formation of the grooves 42 results in the creation of vertical ridges 43 that extend from the top of the rear pad 28 to the bottom of the rear pad.

[0042] The forward pads 30 are positioned within the shell 12 so as to protect the sides of the wearer's face, for example the cheek and jaw region of the face. In the illustrated embodiment, the forward pads 30 are generally oval and, like the other pads, are curved to follow the curvature of the wearer's head. The forward pads 30 therefore form concave inner surfaces 44 that are adapted to contact the wearer.

[0043] Each of the above-described pads of the liner 14 can be formed of a dense, resilient material that absorbs linear forces. In some embodiments, the pads are made of a high-density foam material such as polyurethane, ethylene-vinyl acetate (EVA), or expanded polypropylene. In further embodiments, the foam can have be variable density foam. Regardless, the top pad 24, rear pad 28, and the forward pads 30 can be securely affixed to the inner surface 18 of the shell 12 so that they will not move relative to the shell when the helmet is used. In some embodiments, those pads are secured to the shell 12 using a suitable adhesive such as glue, or suitable fastening elements such as snap or hook-and-loop fasteners. Unlike those pads, however, the lateral pads 26 are free to move relative to the shell 12 to dissipate certain rotational forces that act on the shell. Such movement is facilitated by a generally horizontal raceway 46 formed on each side of the shell 12. One such raceway is illustrated in FIG. 6, which shows a cross-section of the helmet 10 with the liner 14 removed. In the illustrated embodiment, each raceway 46 is defined by parallel, generally horizontal upper (first) and lower (second) ribs 48 and 50 that extend inward from the inner surface 18 of the shell 12. In the illustrated embodiment, the upper rib 48 is positioned near the top of the shell 12 and extends from a point near the front of the shell to a point near the back of the shell, while the lower rib 50 is positioned near the bottom of the shell below the ear openings 20 and likewise extends from a point near the front of the shell to a point near the back of the shell. By way of example, the ribs 48, 50 are unitarily formed with the shell 12 during a molding process and extend inward from the shell inner surface 18 by a distance of approximately 3 to 30 millimeters (mm).

[0044] The lateral pads 26 are sized and configured to fit within the raceway 46 between the upper and lower ribs 48 and 50. More particularly, the lateral pads 26 are sized and configured to be bound along their top and bottom edges by the upper and lower ribs 48 and 50, respectively, as shown in FIG. 4. Because the lateral pads 26 are not secured to the inner surface 18 of the shell, they can slide forward and rearward (horizontally) along the raceways 46, at least until they abut another pad of the liner, such as the rear pad 28. Free sliding of the lateral pads 26 can be facilitated by providing one or both of the inner surface 52 of the raceways and the outer surfaces 54 of the lateral pads 26 with a low-friction surface. Such a surface can comprise a coating or a layer of a low-friction material that is applied to one or both of the raceway inner surfaces 52 and the pad outer surfaces 54. By way of example, the low-friction material can comprise polytetrafluoroethylene (PTFE). In addition or exception, a lubricant, such as graphite powder, can be provided between the raceway inner surfaces 52 and the lateral pad outer surfaces 54 to encourage relative motion. In still other embodiments, friction between the lateral pads 26 and the raceways 46 can be reduced by incorporating friction-reducing elements, such as rollers or ball bearings (not shown), between the lateral pads 26 and the raceways 46.

[0045] FIG. 7A illustrates the helmet 10 as worn by a wearer, such as a football player. As is apparent from the figure, the helmet 10 is centered on the wearer's head so that the center of the facemask 22 aligns with the center of the wearer's face. FIG. 8A is a cross-sectional top view of the helmet orientation illustrated in FIG. 7A. These figures can be considered to show the orientation of the helmet 10 on the wearer's head before a rotational force is applied to the helmet.

[0046] FIGS. 7B and 8B show the orientation of the helmet 10 on the wearer's head after a rotational force has been applied to the helmet. The rotational force can, for example, be the result of a blow that is sustained by the shell 12 or the facemask 22 that causes the shell to rotate about an imaginary vertical axis, designated herein as the z-axis, that extends through the center of the wearer's head from bottom to top. Of course, the rotational force may be just a component of the impact to the helmet 10 and may be accompanied by a linear force that is imparted to the helmet. With a conventional helmet that is adapted to fit tightly to the wearer's head, such a rotational force would result in immediate rotation of the head and would result in a substantial rotational acceleration being applied to the brain. However, because the lateral pads 26 are free to slide along the raceways 46, the shell 12 of the helmet 10 can rotate without concomitant rotation of the wearer's head. In particular, the wearer's head and the lateral pads 26 can remain relatively stationary, at least in terms of rotation, while the shell 12 rotates relative to the head and the lateral pads about the z-axis.

[0047] Such relative motion is illustrated in FIGS. 7B and 8B. In the example of FIGS. 7B and 8B, the helmet 10 has been impacted such that the shell 12 shifts to the right about the z-axis from the perspective of the wearer. As can be appreciated from FIG. 8B when compared with FIG. 8A, the lateral pads 26 remain in their original positions on the head despite the shifting of the shell 12 because they can slide along their raceways 46. Because the other pads of the liner 14 are secured to the inner surface 18 of the shell 12, however, the other pads have shifted relative to the lateral pads 26 and the head. As is shown in FIG. 8B, the rear pad 28 has shifted to the extent that it has abutted against the left-side lateral pad 26. In particular, as is further shown in FIG. 8B, the left side compression region 40 of the rear pad 28 has compressed due to its collision with the left-side lateral pad 26. This compression of the compression region 40 absorbs at least some of the rotational force that has been applied to the shell 12 and dissipates that force over time (albeit a short period of time) so that less rotational acceleration is applied to the brain. This results in less likelihood of the wearer sustaining a brain injury, such as a concussion. Depending upon the material and dimensions of the rear pad 28 and the compression regions 40, the rear pad may also serve to urge the shell 12 back toward its original orientation on the head as would a compression spring.

[0048] FIG. 9 illustrates a second embodiment of a protective helmet 10′ that comprises a liner that can slide relative to the shell. The helmet 10′ is similar in many ways to the helmet 10 illustrated in FIGS. 1-8 and described above (the top pad 24 and the forward pads 30 are not shown for clarity). However, the helmet 10′ comprises alternative lateral pads 26′. Like the lateral pads 26 of the helmet 10, the lateral pads 26′ are generally rectangular and are adapted to slide along a raceway 46 defined by upper and lower ribs 48 and 50. In addition, the lateral pads 26 can be made of a foam material such as polyurethane, ethylene-vinyl acetate (EVA), or expanded polypropylene. Unlike the lateral pads 26, however, the lateral pads 26′ comprises multiple narrow, elongated vertical ribs 56 that are separated by multiple narrow, elongated vertical troughs 58. With such a configuration, the lateral pads 26′ are adapted not only to slide relative to the shell 12 but also to laterally deflect in the horizontal direction due to the narrow widths of the vertical ribs 56 to further dissipate rotational energy from impacts to the head. In embodiments in which the vertical ribs 56 alone provide adequate rotational energy dissipation, the lateral pads 26′ can be securely affixed to the inner surface 18 of the shell 12 such that the pads will not slide relative to the shell.

[0049] In the embodiments of FIGS. 1-9, the lateral pads 26, 26′ slide along raceways 46 and are confined by upper and lower ribs 48 and 50. In alternative embodiments, the lateral pads 26, 26′ can be enabled to slide relative to the shell 12 using rails. FIGS. 10 and 11 illustrate two example rail systems that can be incorporated into a helmet, such as the helmet 10. Beginning with FIG. 10, a first rail system 60 comprises at least one rail 62 that is provided on the inner surface 18 of the shell 12. If relative movement of the shell 12 around the z-axis is desired, the rail 62 can be a horizontal rail. As is further shown in FIG. 9, the lateral pad 26, 26′ is provided with a groove 64 that is sized and configured to receive the rail 62. In some embodiments, the groove 64 can be reinforced with a rigid material, such as a polymer material (not shown). Regardless, the groove 64 can slide along the rail 62 to enable relative movement of the lateral pad 26, 26′ and the shell 12. As with the raceway embodiments, free sliding of the lateral pad 26, 26′ can be facilitated through use of low-friction materials and/or lubricants.

[0050] FIG. 11 illustrates a second rail system 66 that is the inverse arrangement of that shown in FIG. 10. The second rail system 66 comprises at least one groove 68 that is provided on the inner surface 18 of the shell 12. If relative movement of the shell 12 around the z-axis is desired, the groove 68 can be a horizontal groove. As is further shown in FIG. 11, the lateral pad 26, 26′ is provided with a rail 70 that is sized and configured to be received within the groove 68. In some embodiments, the rail 70 can be reinforced with a rigid material, such as a polymer material (not shown). Regardless, the rail 70 can slide along the groove 68 to enable relative movement of the lateral pad 26, 26′ and the shell 12. As with the raceway embodiment, free sliding of the lateral pad 26, 26′ can be facilitated through use of low-friction materials and/or lubricants.

[0051] In the above-described helmet embodiments, movement of the liner relative to the shell is constrained to one direction. For example, when horizontal raceways or rails are used, the lateral pads of the liner can only slide horizontally relative to the helmet. In such a case, only the rotational forces about the z-axis can be attenuated. It is noted, however, that all rotational forces can be attenuated when the helmet includes means that enable the pads to slide in any direction relative to the helmet. FIG. 12 illustrates an example of such means. More particularly, FIG. 12 illustrates an isolation bushing 80 that can be used to decouple a liner pad 82 from a shell 84 of a helmet.

[0052] As is shown in FIG. 12, the pad 82, which can comprise a foam material, is supported by a substrate 86, which can comprise a rigid polymer material. The isolation bushing 80 comprises compression springs 88 that are confined by stops 90 that extend out from the inner surface 92 of the shell 84. Each spring 88 contacts a stop 90 at one end and a rib 94 that extends out from the pad substrate 86 at the other end. With such a configuration, the pad 82 can move from side to side (as indicated by the double-sided arrow 96) relative to the shell 84 and the springs 88 will both absorb the force (i.e., a rotational force) causing the relative motion and urge the pad back toward its original position shown in FIG. 12. Notably, further springs (not shown) can be provided in a plane parallel to the springs 88 but in a direction normal to the springs 88 to likewise absorb forces that cause the pad 82 to move relative to the shell 84 in a direction normal to the arrow 96 (i.e., into an out of the page). With such an arrangement, the pad 82 can slide in any direction that is substantially parallel to the shell 84 and can therefore absorb any rotational force applied to the shell irrespective of its direction. Accordingly, the pad 82 can provide omnidirectional rotational force absorption.

[0053] Omnidirectional rotational force absorption can be provided with other means. FIGS. 13-23 illustrate examples of such means. More particularly, those figures illustrate an example of the second solution to attenuating rotational acceleration in the form of a protective helmet that comprises a liner that incorporates one or more engineered materials that are specifically designed to attenuate linear impact and dissipate rotational energy from impacts to the head by undergoing controlled lateral and/or rotational shear.

[0054] Illustrated in FIGS. 13-17 is a third embodiment of a protective helmet 100. The helmet 100 is similar to the helmet 10 and therefore similar components will only briefly be described. The helmet 100 generally includes an outer shell 102 and an inner liner 104. The shell 102 is shaped and configured to surround the wearer's head with the exception of the face and can be unitarily formed from a rigid material such as ABS or polycarbonate alloy.

[0055] Irrespective of the material used to construct the shell 102, the shell includes an outer surface 106 and an inner surface 108. In some embodiments, the shell 102 can further include one or more ear openings 110 that extend through the shell from the outer surface 106 to the inner surface 108. A facemask 112 can be secured to the front of the helmet 100 to protect the face of the wearer and can be attached to the helmet 100 using screws (not shown) that thread into the shell 102 or into fastening elements (not shown) that are attached to the helmet.

[0056] The liner 104 generally comprises one or more pads that sit between the shell 102 and the wearer's head when the helmet 100 is worn. In the illustrated embodiment, those pads include a top pad 114, a rear pad 116, a front pad 118, rear lateral pads 120, upper lateral pads 122, and lower lateral pads 124. In the illustrated embodiment, each of the pads has an inner component or layer and one or more outer components or layers, with the inner layers being adapted to contact the wearer's head and the outer layers being adapted to attach to the inner surface 108 of the shell 102. As in the previous embodiments, each pad can be curved to adapt to the curvature of the wearer's head. Therefore, the inner layer of each pad can have a concave inner surface.

[0057] The nature of the inner and outer layers of each pad of the liner 104 can be selected to achieve whatever characteristics that are desired. In one embodiment, the inner layers are composed of a foam material to absorb linear forces and the outer layers are composed of a three-dimensional spacer fabric that comprises no cell-based foam and that is adapted to absorb both linear impact and lateral and/or rotational shear forces. FIG. 18 illustrates an example of one such configuration of pad 130. Such a configuration can be used, for example, for the top pad 114, the rear lateral pads 120, the upper lateral pads 122, and the lower lateral pads 124 of the helmet 100. As is shown in FIG. 18, the pad 130 comprises a relatively thin inner layer 132 and a relatively thick outer layer 134. By way of example, the outer layer 134 can be approximately 15 to 30 mm thick and the inner layer 132 can be approximately 3 to 15 mm thick.

[0058] As mentioned above, the inner layer 132 can be made of a foam material and the outer layer 134 can be made of a three-dimensional spacer fabric. An example of a suitable three-dimensional spacer fabric is illustrated in FIG. 19. As is shown in that figure, the three-dimensional spacer fabric 140 comprises a top layer of material 142 that is separated from a parallel bottom layer of material 144. The two layers 142, 144 can each be a woven fabric that comprises a plurality of glass or polymer fibers aligned in both the warp and the weft directions of the fabric. In some embodiments, the fibers are combined to form multiple yarns that are woven together to form the fabric. By way of example, the three-dimensional spacer fabric 140 is approximately 8 to 20 mm thick and the layers 142, 144 comprise yarns that have approximately 400 to 600 individual filaments and that range from approximately 65 to 300 tex. Each filament is approximately 8 to 13 microns (μm) in diameter. In some embodiments, the layers 142, 144 have approximately 60 to 80 pick ends and approximately 60 to 70 warp ends per square 10 mm.

[0059] Extending between the two layers of material 142, 144 in a direction generally perpendicular to the layers are multiple glass or polymer fibers 146 that maintain the separation between the two layers and absorb lateral and rotational shear forces. In some embodiments, the fibers 146 are combined to form multiple yarns that extend between the two layers. By way of example, each fiber or yarn has similar characteristics to those used to form the layers 142, 144. Regardless, the fibers or yarns 146 are coupled to the layers 142, 144. In some embodiments, the fibers or yarns 146 are alternately threaded through the top and bottom layers 142, 144 in a continuous fashion so that each fiber or yarn can have multiple lengths that extend between the two layers. As can be appreciated from FIG. 19, those lengths can be curved. More particularly, the lengths can form S-shapes and inverted S-shapes that, when viewed together from an end of the fabric 140, form a repeating figure-8 pattern (see FIG. 18).

[0060] In some embodiments, the fibers used to construct the three-dimensional spacer fabric are fiberglass or aramid fibers. One commercial example of such a three-dimensional spacer fabric 140 is Parabeam™ material available from Parabeam b.v. in The Netherlands. Before the three-dimensional spacer fabric 140 is used to form a pad, the top and bottom layers 142, 144 are separated and fabric is impregnated with a polymeric resin, such as thermoplastic polyurethane, poly caprolactum (nylon), or epoxy resin, so as to coat the fibers and threads in resin. The resin can be applied using a vacuum infusion process and then cured to govern the rigidity of the end material, from very flexible to very rigid. This structural integrity or rigidity provided by the cured resin is what enables the three-dimensional spacer fabric 140 to absorb both linear impact and shear forces. In some embodiments, the finished three-dimensional spacer fabric 140 has a shear strength of approximately 15 to 25 pounds per square inch (psi) and a shear modulus of approximately 250 to 350 psi. In one example embodiment, an infusible low 800 to 1,000 centipoise thermoplastic polyurethane resin can be used to produce a three-dimensional spacer fabric having compression and shear characteristics that are approximately equivalent to a 40 to 100 A durometer shore hardness material. Such a fabric possesses substantially instantaneous spring-back characteristics following compression or shear deformation.

[0061] Referring back to FIG. 18, the combination of the foam inner layer 132 and the three-dimensional spacer fabric outer layer 134 results in a pad 130 that is both well suited to absorb linear and rotational forces, thereby greatly reducing the opportunity for brain injury. Unlike the raceway and rail embodiments discussed above, helmets that incorporate pads such as the pad 130 are adapted to absorb all rotational forces imposed upon the shell and not just those about the z-axis of the head. Therefore, helmets such as the helmet 100 can provide even greater protection against harmful rotational accelerations.

[0062] FIG. 20 illustrates another example of a pad 150 that can be incorporated into a helmet such as the helmet 100. Such a configuration can be used, for example, for the rear pad 116 and the front pad 118 of the helmet 100. As is shown in FIG. 20, the pad 150 comprises a relatively thick inner layer 152 and two outer layers 154. The inner layer 152 can be made of a foam material and the outer layers 154 can be made of a three-dimensional spacer fabric. In the embodiment of FIG. 20, the inner layer 152 is thicker to better absorb linear impacts, which may be received with greater frequency from the front and the rear of the helmet. The outer layers 154 can be relatively narrow (see also FIGS. 16 and 17) such that they provide greater rotational force attenuation in the horizontal direction than in the vertical direction.

[0063] FIG. 21 illustrates a pad 160 that is a variation on the pad 150. The pad 160 comprises a relatively thick inner layer 162 and a single outer layer 164. The inner layer 162 can be made of a foam material and the outer layer 164 can be made of a three-dimensional spacer fabric. In the embodiment of FIG. 21, the outer layer 164 has a width that is smaller than that of the inner layer 162. This arrangement illustrates that one can alter the amount of three-dimensional spacer fabric used in a pad to provide the desired characteristics of energy attenuation.

[0064] FIG. 22 illustrates a further pad 170 that can be used in a helmet like the helmet 100. The pad 170 is similar to the pad 130 of FIG. 18 and therefore comprises a foam inner layer 172 and a three-dimensional spacer fabric outer layer 174. In the embodiment of FIG. 22, however, the inner layer 172 comprises two distinct foam layers 176 and 178. By way of example, the inner foam layer 176 can have a lower density than the outer foam layer 178 so that the inner layer 172 provides the comfort the wearer desires as well as the force absorption that is required to protect his brain from injury.

[0065] FIG. 23 illustrates a further variation on the pad 130. Like the pad 130, the pad 180 comprises an inner layer 182 and an outer layer 184. In this embodiment, however, the inner layer 182 is a further layer of three-dimensional spacer fabric like the outer layer 184. In some embodiments the inner layer 182 is the same as the outer layer 184 but is less thick, which changes its compression and/or shear properties. In other embodiments, the inner layer 182 can have different fibers, yarns, resin, or other aspects that alter its compression and/or shear properties.

[0066] In the foregoing disclosure, various embodiments have been described. As was noted above, alternative embodiments are possible. As an example, although multiple embodiments have been described as having liners comprising multiple discrete pads, in alternative embodiments one or more of the pads can be combined. For instance, one or more of the pads that surround the sides and back of the head can be combined and the combined pad can slide as a whole relative to the shell. As another example, various discrete aspects of the disclosed embodiments can be combined to form other embodiments. For instance, the lateral pads of the embodiments of FIGS. 1-9 can include the three-dimensional spacer fabric of the embodiment of FIGS. 13-17 to provide a further means of dissipating rotational force. All such alternative embodiments are deemed to fall within the scope of this disclosure.