The purpose of this study is to evaluate and compare isometric cervical extension strength (extension strength) of college American football players belonging to different level leagues at the eight cervical positions measured by a cervical extension machine (Medx Inc.) and give some suggestions for preventing neck injuries. The subjects were American football players at N University (n=41) belonging to the first-level league and at G University (n=32) belonging to the third-level league. The findings were as follows.
1. The isometric cervical extension strength/body weight of players at N University was sig-nificantly higher than that of G University at 72°, 54°and 36° (a neutral cervical position) .
2. The extension strength/body weight of the linemen at N University was significantly higher at 108°, 54°and 36°than that of the linemen at G University. While there were no significant differences between universities in the eight cervical positions of back players herein referred to as‘backs’.
In conclusion, it was shown that there were significant differences in the extension strength and the extension strength/body weight between teams of different levels. The differences tended to be larger between the linemen than between the backs. Therefore, it was suggested that the evaluation of neck muscle strength was important for preventing neck injuries especially caused by a mismatch of players at different performance levels.

A study was conducted to evaluate and compare neck muscle strength between two levels of college American football players with the aim of preventing neck injuries. The subjects were American football players at N University (n=52) belonging to the first level league and American football players at G University (n=14) belonging to the third level league. The findings were as follows.
1. The neck muscle strength of freshman players at N University tended to be lower than that of senior players.
2. It was shown that the neck muscle strength/body weight of experienced American football players was 10-30% higher than that of inexperienced players.
3. There was a significant difference in neck muscle strength/body weight between N University and G University in 1997. However, there was no significant difference between them in 1998, because neck muscle strength/body weight of G University players increased by 13-30% after neck muscle training for about nine months. It was suggested that coaching staff must evaluate the neck muscle strength of each player, especially in freshmen who have had no experience of American football, in order to prevent neck injuries because mismatch of performance level may cause catastrophic neck injury.

The purpose of this study was to examine the relationship between isometric cervical extension strength and the cross-sectional area (CSA) of neck extensor muscles in order to obtain fundamental data for conducting neck muscle training to prevent neck injuries. Subjects were 8 males belonging to the judo or American football club in N University. Isometric cervical extension strength was measured at eight positions (0°, 18°, 36°, 54°, 72°, 90°, 108°, maximum flexion) using a cervical extension machine (Medx Inc., USA) . A transaxial magnetic resonance imaging (MRI) image of neck muscles was obtained at the intervertebral disk between C4 and C5 (using an MRI machine made by Hitachi medical Corp., JAPAN) . The findings were as follows.
1. The correlation between isometric cervical extension strength at 36°, 54°and 72°in a neutral position (60.8±1.4°) and the CSA of neck extensor muscles were higher than at other degrees (0°, 18°, 90°, 108°, maximum flexion) . In particular, there was a significant linear relationship at 54° (slightly extended position) .
2. A significant linear relationship was observed between the neck girth and CSA of neck extensor muscles.
In conclusion, there was a closer relationship between isometric cervical extension strengths at certain angles in a neutral position and the CSA of neck extensor muscles. Therefore, it was shown that isometric cervical extension strengths at certain angles in a neutral position mainly represented the CSA of neck extensor muscles.

Purpose: The purpose of this study was to examine the relationship among helmet surface temperature (Thl), head top temperature (Thd) and tympanic temperature (Tty) during American football practice in summer. Methods : The subjects were collegiate American football players. Temperatures were measured in August in 1993 and 1994. Thl, Thd and Tty were measured by infrared tympanic thermometers. Environmental temperatures that were measured were dry-bulb temperature (Td), wet-bulb temperature (Tw), globe temperature (GT) and wet-bulb globe temperature (WBGT) . Results: Significant correlations were observed among all measured temperatures (P<0.0001) . High coefficients of correlation were observed among Thl, Thd and GT. The highest relationships were observed between Thl and Thd (r=.727), and between Tty and Td (r=.766) . The coefficient of correlation between Tty and Thl was higher than that between Tty and Thd. Heat stress of the whole body (F1; heat stress factor: Tty, Td, Tw and WBGT, proportion=71.4%) and head environment factor (F2 ; helmet factor : Thl, Thd and GT, proportion=14.3%) were chosen in factor analysis. A close relationship was observed between both factors (r=.773) . Both GT and Thl showed a high correlation with Thd, which suggests the influence of radiant heat through a helmet on the whole body. Conclusion : The temperature in a helmet is a micro environment temperature surrounding the head. Accordingly, the heat load is reduced by taking the helmet off frequently during football practice.