Patent Publication Number: US-2013247285-A1

Title: Football helmet

Description:
RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application No. 61/615,445, filed 26 Mar. 2012, the subject matter of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an apparatus and method for use of a football helmet and, more particularly, to a football helmet having specific physical properties. 
     BACKGROUND OF THE INVENTION 
     An estimated 4.2 million adult, adolescent, and youth athletes participate in tackle American-style football yearly in the United States with this type of football having the highest concussion incidence of all sports. Despite apparent advances in protective equipment like football helmets, and increased awareness of safety issues (e.g., leading to changes in rules of competition), head, neck, and spine injuries continue to occur at unacceptably high rates—while a head, neck, and spine injury-reducing football helmet has yet to be produced. So while behavior modification to avoid helmet-to-helmet or helmet-to-ground contact continues, the unfortunate many continue to sustain near-term, and quite possibly long-term, injuries on the football field that are yet to be mitigated as much as possible by next generation helmet design. Clinical consequences of these injuries may include: heightened risk of concussion and other so-called minor traumatic brain injuries (mTBIs), skull fracture, spinal cord injury, osteoligamentous disruption, depression, neurocognitive impairment, impaired motor function, even death, and in the long-term, Alzheimer&#39;s, Parkinson&#39;s, dementia, and Chronic Traumatic Encephalopathy (CTE). 
     Currently, football helmet manufacturers develop helmets that meet the NOCSAE (ND) 001-08 m08b adult helmet drop testing linear acceleration and Gadd Severity Index criteria. This NOCSAE standard emphasizes protection against catastrophic brain injury and skull fracture only, and does not similarly prioritize reducing concussion risk, spine injury risk, or risk of long-term sequelae like Alzheimer&#39;s, Parkinson&#39;s, dementia, or CTE. For adolescent and youth helmets, adult helmets meeting the NOCSAE catastrophic head injury criterion are scaled-down only considering the head anthropometry of these younger players; no specific tailoring of shell, padding or facial and mandibular protector is made to consider youth or adolescent developing neurological systems, developing musculoskeletal systems, or differing injury risks from full-grown adult players. Because younger players are theoretically subjected to lower energy impacts during practice and competition (due to lower effective impact mass and velocity of the colliding players vs. mature athletes), scaled-down adult helmets may not be optimized to mitigate their potential injuries. Adult football helmets are thusly designed solely to mitigate severe skull fracture and catastrophic brain injury; scaled-down adolescent and youth helmets optimized to mitigate adult skull fracture tolerance likely do not address potential risk to the developing skull, brain, neck, and spine of these younger football players. And no helmets have yet been designed to minimize head impact dosage accumulation across a broad range of in-game type impacts (average of ˜25 g, maximum upwards of 50 g to 150 g). Further, there is no existing standard means to quantify these scaled-down adult helmets&#39; influence on youth head, neck and injury risk. 
     Therefore, it may be desirable to develop specific football helmet protective components that minimize head injury risk for adult, adolescent, and youth players. While much football helmet impact testing has been commissioned by the National Football League, NOCSAE, and the National Institutes of Health (NIH), helmet design has remained relatively constant over the past two decades with less of a focus on rigorously proven engineering designs to attenuate energy/momentum transfer to the head, and more attention paid to factors such as industrial design, low-cost manufacturing, and ease of helmet sanitization. Further hamstringing efforts is that currently there is a dearth of objective data correlating intrinsic helmet properties (mass, center of gravity, moments of inertia, force-deflection properties) to football player head, neck, and spine injury risk (risks can be calculated using, for example, Gadd Severity Index, Angular Velocity, Angular Acceleration, spine forces/moments). 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present invention, a football helmet is described. An outer shell is configured to at least partially surround a head of a wearer. An inner liner is located substantially within the outer shell and is configured to contact at least a portion of the head of the wearer. A facial and mandibular protector is attached to the outer shell and is configured to at least partially surround a face of the wearer. The helmet has a center of gravity which, when the helmet is being worn, is substantially the same in three-dimensional location to the center of gravity of the head of the wearer. 
     In an embodiment of the present invention, a football helmet is described. An outer shell is configured to at least partially surround a head of a wearer. An inner liner is located substantially within the outer shell and is configured to contact at least a portion of the head of the wearer. A facial and mandibular protector is attached to the outer shell and is configured to at least partially surround a face of the wearer. The helmet has a helmet moment of inertia which is chosen to reduce a risk of injury to the wearer when force is exerted upon the head of the wearer. 
     In an embodiment of the present invention, a method of protecting at least one body structure of a wearer during athletic competition is described. A football helmet includes an outer shell configured to at least partially surround a head of a wearer. An inner liner is located substantially within the outer shell and is configured to contact at least a portion of the head of the wearer. A facial and mandibular protector is attached to the outer shell and is configured to at least partially surround a face of the wearer. A center of gravity of the helmet is configured to be substantially the same in two-dimensional location to the ventral-dorsal plane of the head of the wearer when the helmet is being worn. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, reference may be made to the accompanying drawings, in which: 
         FIG. 1  is a labeled diagram of a human skull, indicating commonly understood directions and landmarks; 
         FIG. 2  is a side view of a human head showing relative locations of centers of gravity of the head and of currently available helmets during wear; and 
         FIG. 3  is a bar graph giving various helmet weights for a number of different user ages. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In a full-grown midsize male, the head  100 , shown in  FIG. 1 , weighs 4.54 kg (Humanetics ATD, Hybrid III 50 th  Male Specifications). The current range of commercially available adult football helmet masses (also known as “Varsity” helmets) designed for this midsize male head  100  is given below in Table 1: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Varsity helmet mass and percentage of head mass 
               
               
                 Varsity Helmets Meeting NOCSAE Standards 
               
            
           
           
               
               
               
               
            
               
                 MANUFACTURER 
                 MODEL 
                 MASS (kg) 
                 % of Head Mass 
               
               
                   
               
               
                 Adams 
                 A2000 
                 1.40 
                 31% 
               
               
                   
                 A4 
                 1.30 
                 29% 
               
               
                 Riddell 
                 VSR-4 
                 1.90 
                 42% 
               
               
                   
                 Revolution 
                 1.81 
                 40% 
               
               
                   
                 Revolution IQ 
                 1.90 
                 42% 
               
               
                   
                 Revolution Speed 
                 1.87 
                 41% 
               
               
                 Schutt 
                 Air Advantage 
                 1.68 
                 37% 
               
               
                   
                 Air XP 
                 1.77 
                 39% 
               
               
                   
                 DNA Pro+ 
                 1.91 
                 42% 
               
               
                   
                 Ion 4D 
                 1.98 
                 44% 
               
               
                 Xenith 
                 X1 
                 1.97 
                 43% 
               
               
                 Rawlings 
                 Quantum 
                 2.02 
                 44% 
               
               
                 Average 
                 — 
                 1.79 
                 40% 
               
               
                   
               
            
           
         
       
     
     Therefore, as a percentage of head  100  mass, current Varsity football helmets add anywhere from 29% to 44% additional mass to the head  100 . 
     When analyzing youth head  100  sizes and helmet mass, additional data, sorted by age category of the wearer, are presented in Table 2: 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Youth head mass and calculated helmet percentage of head mass 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Average 
                 Calculated 
                 Helmet mass 
               
               
                   
                 Average 
                 Helmet 
                 Head 
                 as % of 
               
               
                   
                 Age (yrs) 
                 Weight (kg) 
                 mass (kg) 
                 head mass 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 7.5 
                 1.74 
                 4.05 
                 43% 
               
               
                   
                 9.1 
                 1.74 
                 4.27 
                 41% 
               
               
                   
                 10.9 
                 1.73 
                 4.37 
                 40% 
               
               
                   
                 15.0 
                 1.89 
                 4.57 
                 41% 
               
               
                   
                 17.0 
                 1.89 
                 4.58 
                 41% 
               
               
                   
                 Mature 
                 1.79 
                 4.54 
                 40% 
               
               
                   
                   
               
            
           
         
       
     
     Thus, when examining helmet mass as a percentage of head  100  mass from real world data, youths aged 7.5 to 17.0 years of age don helmets that weigh 40% to 43% of their total head  100  mass. 
     Current data on head  100  supported mass (HSM), in which helmets weighing 1.59 kg to 3.40 kg were tested in an impact environment, indicates that neck shear and tension loads, as well as extension bending torque and Neck Injury Criterion (Nij), are statistically significantly proportional between increased HSM and these injury risk metrics. Likely, there is also increased axial loading of the spine and similar lateral bending and shear loading injury risk with a heavier helmet. Additional research has shown a linkage between higher neck force, neck torque and Nij, and heavier helmet mass. Finally, studies on neck muscle activation and resulting soreness from inertial loading while wearing helmets showed that higher helmet masses increased neck muscle activation and increased pain for volunteers. These studies pointed to possible injurious consequences with high-g loading. 
     Lighter mass helmets may help with reduce concussion, head  100 , neck and spine injury. Furthermore, even amongst the helmets presented in Table 1, the total mass varies by 0.72 kg which indicates weight can be drastically reduced while maintaining current protection. Ostensibly, all of these helmets offer equivalent protection—they have all passed the same NOCSAE certification criterion—while being constructed of different materials (EPP foam vs. vinyl nitrile vs. air dampers vs. conical plastic, etc). 
     As shown in  FIG. 1 , in a full-grown midsize male, the head  100  center of gravity (CG, shown at  102  in the Figures) is located in the mid-sagittal (XZ) plane in an area 2.6±0.7 cm rostral (in −Z direction) and 0.7±0.6 cm ventral (in +X direction) to the tragion. 
     Current football helmets do not appear to be designed to consider head CG  102  location. As can be clearly seen via the various dots clustered to the left of the CG  102  in  FIG. 2 , all current Varsity helmet CGs (a variety of which are shown generally at  104 ) are located ventral (from ˜2 cm to 7 cm) and rostral (from ˜1 cm to 5 cm) to the head CG  102  (represented by the checkered dot in  FIG. 2 ). The ventral helmet CG  104  location has been shown to have an inversely proportional relationship to neck shear, neck tension, neck flexion, and flexion Nij while having a directly proportional relationship to neck extension and extension Nij. The rostral helmet CG  104  location has been shown to have an inversely proportional relationship to neck tension, neck extension, and extension Nij while having a directly proportional relationship to neck shear and flexion Nij. Data from computer simulations indicate that ventral and rostral helmet CG locations produce the highest risk of neck injury in omnidirectional impacts. Additionally, anecdotal evidence from athlete interviews indicate that this ventral and rostral helmet CG  104  location tends to pull the head  100  ventral and caudal, even causing some athletes to rest their front-heavy helmets against their sternum in between plays. A ventral and caudal posture, such as that encouraged by current Varsity helmet designs, greatly increases risk for head  100 , neck, and spine injury due to improper tackling alignment of the head  100  and neck. Ventral and rostral helmet CG  104  location also exacerbates bending torques for helmet contact locations above the Frankfort plane and could also increase loading for players subjected to inertial loading in a “whiplash” style tackle. Hence, to help reduce head  100 , neck, and spine injury risk or to otherwise protect at least one body structure of the wearer from injury during athletic competition or any other activity, the helmet CG  104  could be placed in line with the ventral-dorsal plane of the head CG  102 , and, optionally, caudal to the head CG  102 . Such helmet CG  104  placement is substantially the same in three-dimensional location as the head CG  102 , and/or is substantially the same in two-dimensional location as the ventral-dorsal plane of the head CG  102  and concurrently caudal to the head CG  102 , may help lend stability to the head/helmet combination, much like the stability of a boat ballasted below the waterline, and may help the head  100  to maintain upright posture as desired. (The phrase “substantially the same” is used herein to indicate that the helmet CG  104  does not need to be precisely identically, but could differ slightly from the “substantially same” head CG, as long as that slight difference has no meaningful effect upon the forces exerted upon the wearer. 
     Moment of inertia (MOI) is a physical property that quantifies an object&#39;s tendency to rotate or not rotate. High MOI is indicative of an object that is more difficult to rotate, but once rotating, carries higher rotational torque. Conversely, an object having low MOI begins rotating more easily but incurs lower rotational torque. A simple example of MOI influence on rotation exists in figure skating. A competitor who is spinning with arms outstretched (highest MOI due to maximum mass distance from body CG) will spin slower than the same competitor with arms tucked tightly (lowest MOI due to minimum mass distance from body CG) at their sides. Football helmet designs have not yet been designed to place mass at predetermined distances from head CG  102 . Hence, selective shell, liner, and facial-mandibular protection mass distribution may positively influence the tendency of the head  100  to rotate. Lower head  100  rotation due to larger mass MOI of a helmet may help lower the risk of mTBI to a player. 
     Following this understanding, it may be desirable for football helmets to have a predetermined rotational MOI about the three XYZ axes to help reduce injury risk. An example of a suitable predetermined MOI maximizes the percentage of helmet mass at the largest available distance from the head CG  102  in all three XYZ axes. Particularly, injury risk due to mTBI in the coronal plane should be primarily addressed through predetermined maximum helmet MOI, as rotation of the head  100  in this plane has been shown to carry higher injury risk with similar impact magnitudes as compared to rotations caused by sagittal and transverse plane directed impacts. 
     Varsity football helmets are designed for the mature Varsity athlete (professional, collegiate, or varsity high school—i.e., any non-youth player). This means that safety testing standards exist only for mature athlete helmets, and not for youth (junior varsity high school, middle school, grade school). Approved mature athlete helmet designs are simply scaled down to accommodate the adolescent or youth player. The result is that youth and adolescent helmets are often nearly identical to Varsity adult helmets in form and function. However, it can be undesirable to treat youth and adolescent football players as “little adults” for helmet design because of the developing neurological, cognitive, and musculoskeletal systems of those younger players. Therefore, scaling of Varsity football helmets down to fit “little adult” heads of younger players might not be appropriate. Table 3 illustrates data from youth helmets currently in use versus adolescent helmets and all commercially available Varsity helmets: 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Age, body weight and helmet weight for Varsity 
               
               
                 (mature) and youth/adolescent football helmets 
               
            
           
           
               
               
               
            
               
                 Average 
                 Average Body 
                 Average Helmet 
               
               
                 Age (yrs) 
                 Weight (kg) 
                 Weight (kg) 
               
               
                   
               
               
                 7.5 (n = 9) 
                 30.1 
                 1.74 
               
               
                  9.1 (n = 27) 
                 39.1 
                 1.74 
               
               
                 10.9 (n = 17) 
                 42.6 
                 1.73 
               
               
                 15.0 (n = 17) 
                 75.8 
                 1.89 
               
               
                 17.0 (n = 22) 
                 82.2 
                 1.89 
               
               
                 Varsity 
                 77.7 
                 1.79 
               
               
                 (Mature, n = 11) 
               
               
                   
               
            
           
         
       
     
     From Table 3, it can be seen that the helmet weights for youths are comparable, and sometimes heavier (15.0 YO and 17.0 YO), than the average of Mature ‘Varsity’ helmets first presented in Table 1. 
     Therefore, youth and adolescent player anthropometry (head  100  and neck size) and neck strength (neck forces and torques) can serve to generate scaling factors to predict desired helmet characteristics. These data are presented in Table 4: 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Head anthropometry from youth/adolescent football players compared 
               
               
                 with ‘Mature’ norms and their respective scale factors 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Head 
                   
                   
                   
                 Head 
                   
                 Head 
                   
               
               
                 Subject 
                 Breadth 
                 Scale 
                 Head 
                 Scale 
                 Circumference 
                 Scale 
                 Calculated 
                 Scale 
               
               
                 (yrs) 
                 (cm) 
                 Factor 
                 Length (cm) 
                 Factor 
                 (cm) 
                 Factor 
                 area (m{circumflex over ( )}2) 
                 Factor 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 7.5 (n = 9) 
                 15.0 
                 0.97 
                 18.1 
                 0.92 
                 53.8 
                 0.94 
                 2.13E−02 
                 0.89 
               
               
                  9.1 (n = 27) 
                 15.2 
                 0.98 
                 18.6 
                 0.95 
                 53.7 
                 0.94 
                 2.21E−02 
                 0.93 
               
               
                 10.9 (n = 17) 
                 15.1 
                 0.97 
                 18.9 
                 0.96 
                 55.0 
                 0.96 
                 2.24E−02 
                 0.94 
               
               
                 15.0 (n = 17) 
                 15.8 
                 1.02 
                 19.9 
                 1.01 
                 57.4 
                 1.01 
                 2.47E−02 
                 1.03 
               
               
                 17.0 (n = 22) 
                 15.6 
                 1.00* 
                 20.0 
                 1.00* 
                 57.7 
                 1.00* 
                 2.45E−02 
                 1.00* 
               
               
                 Mature 
                 15.5 
                 1.00 
                 19.6 
                 1.00 
                 57.1 
                 1.00 
                 2.39E−02 
                 1.00 
               
               
                   
               
               
                 *due to minor data irregularities, scale factors above 1.00 have been rounded down to 1.00 
               
            
           
         
       
     
     Notable in Table 4 is that subjects as young as 7.5 years of age have head  100  anthropometry (breadth, length, circumference) that is very similar (linear scale factors of 92% to 97%) to the corresponding dimensions of mature subjects. When calculating head  100  cross sectional area based on breadth measurements between the left and right temples above each respective ear and length measurements between the forehead and occipital protuberance, the linear scale factor decreases to 89% of the mature subjects, but this difference is still fairly nominal. However, these subjects have different neck anthropometry and neck strength. 
     Table 5 illustrates differences in neck anthropometry: 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Neck anthropometry from youth/adolescent football players compared 
               
               
                 with ‘Mature’ norms and their respective scale factors 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Neck 
                   
                 Neck 
                   
                 Neck 
                   
                 Neck 
                   
               
               
                 Subject 
                 Breadth 
                 Scale 
                 Depth 
                 Scale 
                 Circumference 
                 Scale 
                 Calculated 
                 Scale 
               
               
                 (yrs) 
                 (cm) 
                 Factor 
                 (cm) 
                 Factor 
                 (cm) 
                 Factor 
                 area (m{circumflex over ( )}2) 
                 Factor 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 7.5 (n = 9) 
                 9.3 
                 0.75 
                 8.3 
                 0.75 
                 29.9 
                 0.78 
                 5.99E−03 
                 0.57 
               
               
                  9.1 (n = 27) 
                 9.8 
                 0.80 
                 8.9 
                 0.81 
                 31.4 
                 0.82 
                 6.88E−03 
                 0.65 
               
               
                 10.9 (n = 17) 
                 10.0 
                 0.81 
                 9.1 
                 0.83 
                 31.4 
                 0.82 
                 7.13E−03 
                 0.67 
               
               
                 15.0 (n = 17) 
                 11.9 
                 0.97 
                 10.9 
                 1.00 
                 36.6 
                 0.96 
                 1.02E−02 
                 0.97 
               
               
                 17.0 (n = 22) 
                 12.5 
                 1.00* 
                 11.2 
                 1.00* 
                 38.5 
                 1.00* 
                 1.10E−02 
                 1.00* 
               
               
                 Mature 
                 12.3 
                 1.00 
                 11.0 
                 1.00 
                 38.1 
                 1.00 
                 1.06E−02 
                 1.00 
               
               
                   
               
               
                 *due to minor data irregularities, scale factors above 1.00 have been rounded down to 1.00 
               
            
           
         
       
     
     As seen in Table 5, the anthropometry scale factors (breadth, depth, and circumference) for the youngest subjects (7.5 yrs) ranges from 75% to 78% of the corresponding dimensions for the mature subjects. When examining cross-sectional area, the scale factor drops to 57% of the mature value. This would indicate a significant reduction in cross sectional area, and a concomitant reduction in neck muscle cross-sectional area. The 15.0 and 17.0 year olds had similar neck anthropometry to that of the mature subjects. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Neck Moment of Inertia (MOI) and scaling from youth/adolescent 
               
               
                 football players compared with ‘Mature’ norms 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Neck MOI - 
                   
                   
                   
               
               
                   
                 about 
                   
                 Neck MOI - 
               
               
                   
                 flexion/ 
                   
                 about 
               
               
                 Subject 
                 extension 
                 Scale 
                 lateral 
                 Scale 
               
               
                 (yrs) 
                 axis (Iyy-m{circumflex over ( )}4) 
                 Factor 
                 axis (Ixx-m{circumflex over ( )}4) 
                 Factor 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 7.5 (n = 9) 
                 2.55E−06 
                 0.32 
                 3.20E−06 
                 0.32 
               
               
                  9.1 (n = 27) 
                 3.41E−06 
                 0.43 
                 4.16E−06 
                 0.42 
               
               
                 10.9 (n = 17) 
                 3.68E−06 
                 0.46 
                 4.46E−06 
                 0.45 
               
               
                 15.0 (n = 17) 
                 7.65E−06 
                 0.97 
                 9.07E−06 
                 0.91 
               
               
                 17.0 (n = 22) 
                 8.57E−06 
                 1.00* 
                 1.07E−05 
                 1.00* 
               
               
                 Mature 
                 7.92E−06 
                 1.00 
                 1.00E−05 
                 1.00 
               
               
                   
               
               
                 *due to minor data irregularities, scale factors above 1.00 have been rounded down to 1.00 
               
            
           
         
       
     
     When considering the MOI scaling factors, which are based on neck depth and breadth assuming prismatic beam theory, the MOI about both flexion/extension and lateral bending axes for the 7.5 year-olds is only 32% of the mature subjects. This indicates a significant theoretical drop in resistance to bending torques simply by virtue of the thinner necks on the youth subjects. The 15.0 and 17.0 year-olds had similar MOIs to each other and to the Mature subjects. 
     In order to confirm the design constraints for helmets with respect to subject age, in tandem with anthropometry differences, data on neck strength is collected and analyzed as shown in Table 7: 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Maximum isometric neck forces and scaling developed by 
               
               
                 youth/adolescent football players compared with ‘Mature’ norms 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Ventral 
                   
                 Lateral 
                   
               
               
                   
                 Posterior 
                   
                 Neck 
                   
                 Neck 
               
               
                   
                 Neck 
                 Scale 
                 Force 
                 Scale 
                 Force 
                 Scale 
               
               
                 Subject (yrs) 
                 Force (N) 
                 Factor 
                 (N) 
                 Factor 
                 (N) 
                 Factor 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 7.5 (n = 9) 
                 48.0 
                 0.18 
                 48.8 
                 0.25 
                 52.3 
                 0.28 
               
               
                  9.1 (n = 27) 
                 52.7 
                 0.20 
                 82.2 
                 0.42 
                 74.2 
                 0.39 
               
               
                 10.9 (n = 17) 
                 58.3 
                 0.22 
                 85.1 
                 0.43 
                 72.2 
                 0.38 
               
               
                 15.0 (n = 17) 
                 114.3 
                 0.43 
                 147.5 
                 0.75 
                 149.9 
                 0.79 
               
               
                 17.0 (n = 22) 
                 166.0 
                 0.63 
                 185.7 
                 0.94 
                 186.7 
                 0.98 
               
               
                 Mature 
                 265.2 
                 1.00 
                 196.8 
                 1.00 
                 190 
                 1.00 
               
               
                   
               
            
           
         
       
     
     In Table 7, the youngest subjects (7.5 yrs old) are able to generate isometric neck forces of only 18% to 28% of their mature counterparts. The 17.0 year olds are able to generate similar neck forces in both the ventral and lateral directions. All other subjects generated between 20% and 79% of the mature neck force norms. 
     Table 8 displays the isometric neck torque data for the same groups: 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Maximum isometric neck torques and scaling developed by 
               
               
                 youth/adolescent football players compared with ‘Mature’ norms 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Flexion 
                   
                   
                   
                 Lateral 
                   
               
               
                   
                 Torque 
                   
                 Extension 
                   
                 Torque 
               
               
                   
                 C7 
                 Scale 
                 Torque 
                 Scale 
                 C7 
                 Scale 
               
               
                 Subject (yrs) 
                 (N-m) 
                 Factor 
                 C7 (N-m) 
                 Factor 
                 (N-m) 
                 Factor 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 7.5 (n = 9) 
                 5.0 
                 0.16 
                 5.0 
                 0.10 
                 5.5 
                 0.14 
               
               
                  9.1 (n = 27) 
                 6.5 
                 0.21 
                 10.3 
                 0.20 
                 9.7 
                 0.25 
               
               
                 10.9 (n = 17) 
                 8.0 
                 0.26 
                 11.7 
                 0.23 
                 10.3 
                 0.26 
               
               
                 15.0 (n = 17) 
                 18.8 
                 0.62 
                 24.3 
                 0.47 
                 25.0 
                 0.64 
               
               
                 17.0 (n = 22) 
                 30.6 
                 1.01 
                 34.6 
                 0.67 
                 34.3 
                 0.88 
               
               
                 Mature 
                 30.4 
                 1.00 
                 51.9 
                 1.00 
                 39.1 
                 1.00 
               
               
                   
               
            
           
         
       
     
     In Table 8, as in Table 7, the peak loads are much smaller for the youngest subjects (7.5 yrs old), with torques of only 10% to 16% of the mature norms achievable. It is notable that the oldest subjects (17.0 yrs old) had comparable torque in flexion and lateral bending. All other subjects had neck torques ranging from 20% to 64% of the mature norm values. 
     Based at least partially on the above data and linear scaling methods, and using standard constant-stress scaling equations developed for injury risk, several scaling relationships may be developed. 
     First, constant stress scaling for the force (function of area) and moment (function of circumference) are shown in Table 9: 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Force and moment scale factors based on neck area 
               
            
           
           
               
               
               
            
               
                   
                 λFna Force 
                 λMna Moment 
               
               
                 Subject 
                 scale factor 
                 scale factor 
               
               
                 (yrs) 
                 (neck area) 
                 (neck area) 
               
               
                   
               
               
                 7.5 (n = 9) 
                 0.57 
                 0.43 
               
               
                  9.1 (n = 27) 
                 0.65 
                 0.52 
               
               
                 10.9 (n = 17) 
                 0.67 
                 0.55 
               
               
                 15.0 (n = 17) 
                 0.97 
                 0.95 
               
               
                 17.0 (n = 22) 
                 1.04 
                 1.06 
               
               
                 Mature 
                 1.00 
                 1.00 
               
               
                   
               
            
           
         
       
     
     These scale factors are developed assuming constant failure stress (λ σ =1) and based on injury risk to tearing of ligamentous structures within the neck and spine. This assumption is important to the scale factors, as utilizing peak calculated isometric stresses from data presented in Tables 3 through 8 (calculating normal and shear stress linear scale factors) would substantially alter the results. Notable from Table 9 is that again, the 15.0 and 17.0 year-old subjects have similar force and moment scaling to those of mature subjects, which indicates similar resistances to loading. However, the younger subjects have force scaling of 57% to 67% and moment scaling of 43% to 55% of the mature subjects. This indicates potentially different safe loading characteristics. 
     When applying these scale factors for force and moment to a theoretical helmet mass,  FIG. 3  illustrates the findings with respect to subject age. For each age, the leftmost bar gives current helmet weights, the middle bar shows predicted force scaling helmet weights, and the rightmost bar represents predicted moment scaling helmet weights. 
     Utilizing the previously presented data, theoretical scale factors may also be generated for acceleration, Head Injury Criterion (HIC), and the respective NOCSAE testing linear acceleration and Gadd Severity Index (GSI) limits in Table 10. These data may be helpful in understanding safe testing limits for helmets designed according to  FIG. 3 . 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Acceleration and HIC scaling factors with calculated 
               
               
                 linear acceleration and GSI limits for NOCSAE 
               
            
           
           
               
               
               
               
               
            
               
                   
                 λA Accelera- 
                   
                 Acceleration 
                 GSI 
               
               
                   
                 tion 
                 λHIC HIC 
                 scaling - 
                 scaling - 
               
               
                 Subject 
                 scale factor 
                 scale factor 
                 NOCSAE 
                 NOCSAE 
               
               
                 (yrs) 
                 (head length) 
                 (head length) 
                 test (g) 
                 test 
               
               
                   
               
               
                 7.5 (n = 9) 
                 1.06 
                 1.10 
                 247 
                 1315 
               
               
                  9.1 (n = 27) 
                 1.06 
                 1.10 
                 247 
                 1316 
               
               
                 10.9 (n = 17) 
                 1.04 
                 1.06 
                 241 
                 1270 
               
               
                 15.0 (n = 17) 
                 0.99 
                 0.99 
                 231 
                 1190 
               
               
                 17.0 (n = 22) 
                 0.99 
                 0.99 
                 230 
                 1183 
               
               
                 Mature 
                 1.00 
                 1.00 
                 232 
                 1200 
               
               
                   
               
            
           
         
       
     
     It may be somewhat counterintuitive, but the younger subjects&#39; scaling indicates that they could potentially absorb higher linear acceleration (4% to 6%) as well as HIC (6% to 10%) values. The calculated NOCSAE test values reflect the scaling factors and indicate that future certification tests should allow for higher limits for younger subjects. 
     The combination of Tables 2 through 10 and  FIG. 3  can be analyzed to show that:
         Varsity (Mature) helmet weight is comparable to youth/adolescent helmet weight.   Mature head  100  size is similar to head  100  sizes studied for the 17.0, 15.0, 10.9 and 9.1 year-old groups. The largest difference was seen in the 7.5 year-old group, with head  100  cross-sectional area 89% of the mature norms.   Mature neck size and moments of inertia (MOI) are similar to the 17.0 and 15.0 year-old groups. However, the neck size (57% to 83%) and MOI (32% to 45%) scaling factors drop rapidly for the 10.9, 9.1, and 7.5 year-old groups. This means that the subjects 10.9 years and younger had comparable head  100  size to mature subjects, but much smaller neck size and MOI to resist loading.   Mature isometric neck forces are comparable only for the 17.0 year-old group in ventral and lateral loading. All other groups had appreciably lower forces compared to mature norms, with the youngest subjects capable of generating only 18% to 43% of the mature values in all directions.   Mature isometric neck torque was comparable only for the 17.0 year-old group in flexion and lateral bending. All other groups had appreciably lower torques compared to the mature norms, with the youngest groups capable of generating only 10% to 26% of the mature norm values.   Constant stress scaling indicates that 15.0 and 17.0 year-olds have comparable force and moment scaling to mature subjects. The younger subjects have much different tolerance to force (57% to 67%) and moment (43% to 55%) loading. This lower tolerance must be accounted for in helmet design. Further, if isometric stress scaling were used, as opposed to the assumption of constant stress, these tolerances would decrease further.   When calculating theoretical helmet masses based on force and moment scaling, it can be seen that the largest change could be made to helmets currently worn by subjects 10.9 years-old and younger. The current helmets, weighing 1.74 kg should be modified to 0.86 kg to 1.22 kg.   Finally, although allowable force and moment are reduced for the youngest subjects, the peak allowable linear acceleration, Head Injury Criterion (HIC), and Gadd Severity Index (GSI) is 4% to 10% higher than limits imposed for mature subjects. This is counterintuitive, but is a well-known anomaly in automotive safety scaling of youth versus mature head  100  and neck injury risks. The conservative approach is to leave younger subject tolerance equivalent to the mature subjects until further data are available.       

     In other words, youth and adolescent players have large heads  100  on a slender neck and less strength-to-area for their neck musculature. The youngest subjects not only have a relatively large head  100  on a relatively thin neck, but their neck is also weaker for the same cross-sectional area. Therefore, while youth and adolescent football players have comparable head  100  size to reported mature norm values, their neck size, MOI, isometric force application, and peak resistive torque developed at the C7 vertebra are only a fraction of the corresponding mature norm values. 
     Helmets for youths could be scaled in proportion to their head  100  size as well as neck strength as summarized in  FIG. 3 . This means, for example, that the current 1.74 kg helmet used by 9 year old players should be reduced to 1.22 kg (by force scaling) or 1.00 kg (by moment scaling). For any age youth player, the appropriate scaled helmet mass can be scaled as shown in  FIG. 3 . One caution, however, is that these scaling terms do not necessarily relate to head  100 , neck, or spine injury risk. Hence, lighter helmets should have the same protectivity as illustrated in the acceleration scaling and HIC/GSI scaling in Table 10. For the same 9 year old player, this means the mature acceleration of 232 g should be 247 g and the GSI value of 1200 should be altered to 1315. 
     Because current Varsity (mature) helmet mass comes from 3 areas (about a third each for the facial and mandibular protector, shell, and padding), there is ample room to reduce weight whilst maintaining protectivity. Some optional material design considerations, as well as example materials for a protective helmet, are presented below. 
     An outer shell may be configured to at least partially surround a head  100  of a wearer. One or more inner or intermediate liners may be located substantially within the outer shell and be configured to contact at least a portion of the head  100  of the wearer. A facial and mandibular protector is attached to the outer shell and/or inner/intermediate liner(s) and is configured to at least partially surround a face and mandible of the wearer. 
     Current helmet shells are made from polycarbonate or ABS plastic. These materials are selected mainly for their durability under repetitive impact loading. However, polycarbonate and ABS do not provide maximum energy absorption, as they absorb only 6% to 14% of total work energy during deformation. Furthermore, current shell thickness is a function of the reconditioning process. Much like a hardwood floor, reconditioned football helmets are sanded down after each season and their surfaces refinished. The shells must be of sufficient thickness to allow this process to repeat itself over, for example, the course of the 10-year expected lifespan of a youth helmet. A thinner shell provides more flexural energy absorption. 
     Because durable hard materials like polycarbonate or ABS lack the aforementioned optimized energy absorption, there is an opportunity for the use of durable soft materials like toughened elastomers. This type of “softshell” concept has yet to gain popularity, but data illustrate that the soft shell can attenuate energy (lower peak acceleration with longer contact duration) during a head-to-head contact better than currently used polycarbonate or ABS helmets. Possible suitable shell materials include, but are not limited to:
         ABS (acrylonitrile butadiene styrene)   Aluminum ceramic foams   Ceramic sheeting   Carbon foams   Carbon fiber composites   Closed cell foams   Expanded polystyrene (EPS) or polypropylene (EPP)   Epoxy or thermoplastic composite   Fiberglass   Fluid filled chambers   Magnesium   Nanotubes   Open cell foams   Polycarbonate   Polymer   Polyurethane       

     Another energy absorbing concept is to create a rotary “shell within a shell”. There is no reason for the interior padding to be rigidly attached to both the helmet and to the head  100 . Therefore, energy absorption may be improved by allowing compliance, or some relative motion, between the shell and padding. The padding can maintain contact with the head  100 , the shell can shift and turn about the padding, and the user is protected at all times. This concept is similar to the gliding connections between the skin and skull; if the user&#39;s skin failed to move when the user dons and doffs the helmet, the user&#39;s skin would tear frequently. 
     The helmet may benefit from “Stealth Design”—just as in radar applications with the stealth fighter/bomber wherein the surface geometry deflects radar energy, the same concept can be used to deflect energy in direct contacts, in which non-direct contacts are deflected. This causes the striking object to glance off, thus resulting in less energy absorption by the struck object (eg, helmeted head  100 ) than would have occurred if the blow were direct. 
     Facial and mandibular protectors currently made with carbon steel or titanium are not intended to absorb energy, but instead are designed to prevent facial injuries and remain rigidly in place throughout a collision. Energy absorbing structures, like a piston or static elastomer/foam at the facial and mandibular protector shock mounting points, or a flexural composite or plastic material that provides energy absorption proportional to the force-deflection material properties, may help the helmet to prevent injuries to the user. These facial and mandibular protectors may also be relatively light, thus facilitating the normalization of the CG  104  of the helmet to the CG  102  of the wearer&#39;s head  100 . 
     Facial and mandibular protectors can also move with respect to the shell. Current commercially available designs all utilize rigid facial and mandibular protector-helmet constructs. By making this connection less rigid, more energy is absorbed by the helmet and less energy is therefore transmitted to the head  100 . 
     It may be desirable to reduce the coefficient of friction of the helmet. Plastic shelled helmets have yet to be designed to reduce friction, and hence another opportunity to deflect loading and reduce energy transmission to the head  100  is lost. Low friction can be attained by changing geometry (current helmets flat on the sides could be rounded to deflect blows) as well as material optimization (softshell concepts can create surface tension as low as 20 dyne). 
     There are also several opportunities to save weight with the proposed helmet design. For example, many helmets use carbon steel facial and mandibular protectors. Use of polycarbonate could cut the facial and mandibular protector mass by approximately 80% and total helmet mass by approximately 26%. Using composite facial and mandibular protectors would further reduce mass. 
     Inner padding of helmets has commonly been vinyl nitrile foam, air chambers, high impact expanded polypropylene (EPP), expanded polystyrene (EPS), or the like. These materials are selected for optimal energy absorption at NOCSAE-impact test severities, which are much higher than accelerations experienced on the field. Newer materials, like those listed below, can be used to attenuate energy at a broad range of energies (including NOCSAE-impact tests) to protect against routine and the catastrophic hits. Examples of suitable materials that can be used include:
         Aluminum honeycomb—commonly used by the military and in aerospace applications to resist high strain-rate impacts as well as develop NHTSA crash test barriers.   Aluminum foam—similar to aluminum honeycomb, but with a smaller volume cell construct.   Carbon nanotubes—nanotubes can be constructed in desired orientations to transmit energy pathways around the head  100 . Nanotubes can also be made that will fracture and absorb maximum energy while maintaining strength.   Ceramic plating—used by military as lightweight protection against direct focal contacts. High energy absorption/weight ratio. Would likely be used as intermediate layer (between outer and inner helmet layers) to avoid direct contact with cranium or striking mass which may cause fracture.   Expanded polypropylene (EPP)—current standard helmet material, that can be paired with protective shell constructs.   Expanded polystyrene (EPS)—current standard helmet material that can be paired with protective shell constructs.   Expanded polyurethane (EPU)—material that can be paired with protective shell constructs.   Extruded polystyrene (XPS)—available material that has not been used extensively but has energy absorbing properties.   Fluid filled padding—there is potential for padding incorporating glycerin-type density material to improve impact energy absorption over homogenous padding.   Kevlar (Aramid) honeycomb—may be suitable for impact applications.   Polypropylene honeycomb—similar concept to honeycombs above, different material construction.   Poron XRD—brand name for material available from the Rogers Corporation, of Rogers, CT, and designed to attenuate impacts.   Semi-rigid or flexible polyurethane elastomers and foams—behave similarly to viscoelastic materials. Used in NASCAR and IndyCar SAFER barriers.   Rigid styrene—allows for interstitial cracking, which greatly absorbs energy.   Rigid urethane foam—allows for interstitial cracking, which greatly absorbs energy.   Viscoelastic padding—used to deflect proportional to the impact velocity. Softer against lower g-force impacts, stiffest against highest impact severities. These materials have a complex modulus, which is a function of deflection rate, Young&#39;s modulus, and percent compression. Utilized in automotive air bags and bumper systems and has high potential when paired with concomitant energy absorbing outer layer that minimizes penetration.       

     While aspects of the present invention have been particularly shown and described with reference to the preferred embodiment above, it will be understood by those of ordinary skill in the art that various additional embodiments may be contemplated without departing from the spirit and scope of the present invention. For example, any of the described structures and components could be integrally formed as a single piece or made up of separate sub-components, with either of these formations involving any suitable stock or bespoke components and/or any suitable material or combinations of materials. Though certain components described herein are shown as having specific geometric shapes, all structures of the present invention may have any suitable shapes, sizes, configurations, relative relationships, cross-sectional areas, or any other physical characteristics as desirable for a particular application of the present invention. The helmet may include a plurality of structures cooperatively forming any components thereof and temporarily or permanently attached together in such a manner as to permit relative motion (e.g., pivoting, sliding, or any other motion) therebetween as desired. Any structures or features described with reference to one embodiment or configuration of the present invention could be provided, singly or in combination with other structures or features, to any other embodiment or configuration, as it would be impractical to describe each of the embodiments and configurations discussed herein as having all of the options discussed with respect to all of the other embodiments and configurations. A device or method incorporating any of these features should be understood to fall under the scope of the present invention as determined based upon the claims below and any equivalents thereof. 
     Other aspects, objects, and advantages of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims.