This study examined age-related changes in dynamic balance (DB) ability, and the relationship between DB ability and isometric knee extension strength (IKES). Subjects were 100 females who regularly performed some light gymnastic exercises at a gymnastics club once or twice a week. Subject ages ranged from 20 to 85 years old. The measured items were height, body weight (BW), IKES, and DB ability. Results were as follows: 1. The average DB ability tended to gradually decrease as the subjects got older. There were some significant differences in the average DB ability between the 20 to 29 and 60 to 69 age groups, and between the 20 to 29 and over 70 age groups. 2. It was shown that there was a significant negative correlation between age and DB ability (r=0.471, p<0.001). 3. There was a significant negative correlation between age and IKES/BW (r=0.579, p<0.001). 4. It was shown that there was a significant positive correlation between IKES/BW and DB ability (r=0.368, p<0.001). 5. There was a significant negative correlation between BMI and DB ability (r=0.370, p<0.001). This study showed that DB ability rapidly decreased over 60 years old, and also the value tended to be higher in persons with a higher knee extension strength and lower BMI. Therefore, it was suggested that it is important to increase the knee extension strength and maintain an appropriate BMI in order to maintain DB ability.

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 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.

The purpose of this study was to examine the effect of two kinds of neck muscle training on the isometric cervical extension strength (ICES) and cross-sectional area (CSA) of the neck extensor muscles.The subjects which were examined consisted of 22 male college judo athletes. Each was assigned to one of three groups: shrug and upright rowing training (SU); shrug, upright rowing and dynamic neck muscle training (COM); and control (CONT). The SU and COM groups trained 3 days per week for 9 weeks. The ICES and CSA of the neck muscles were measured before and after muscle training.The ICES of the COM group showed a significant increase after training. For the CSA, although a significant increase was only found in a superficial area of the neck extensor muscles in the SU group, the COM group showed significant increases in all areas.This study determined that combined neck muscle training is effective for developing the neck extensor muscles.

The purpose of this study was to examine the effects of dynamic neck muscle training using a cervical extension machine (CEM) on isometric cervical extension strength (ICES) and a cross-sectional area of neck extensor muscles.Subjects were 18 male college judo athletes divided into a control group (n=10) and training group (n=8), respectively. In the training group, dynamic neck muscle training was performed for a 6 week training period, followed by a 10 week training period. There was a detraining period of 12 weeks between the first training period and the second. The ICES was measured at eight angles using a CEM, and the neck muscle cross-sectional area was determined using magnetic resonance imaging.The ICES and cross-sectional area of neck extensor muscles in the training group showed significant increases after the second training period. In particular, the increase in the cross-sectional area was greater in the deepest layer of the neck extensor muscles (rotator, multifidus and semispinaris cervicis muscles) than in the superficial layer (trapezius muscle). In the control group, no significant changes in ICES or cross-sectional area were observed.In conclusion, it was shown that dynamic neck muscle training using a CEM was effective in developing both ICES and the cross-sectional area of neck extensor muscles, especially in the deepest layer.

This study examined peculiarities in both muscle strength and cross-sectional area (CSA) among soccer players in comparison with those of archers as contrasting athletes. Subjects were 15 male soccer players and 9 male archers at N University. Measurement items were height, body weight (BW), isokinetic muscle strength (knee extension, knee flexion, hip extension, hip flexion) and CSA (psoas muscles). Isokinetic muscle strength (30, 120, 240°/sec.) was measured by Cybex6000 (Lumex Co.), and the psoas muscle CSA was determined using magnetic resonance imaging (Hitachi, Japan). Results were as follows : 1. There was no significant difference in isokinetic knee extension and flexion strength/BW at all angular velocities between soccer players and archers. 2. On isokinetic hip flexion strength/BW, the average values of soccer players were significantly higher at all angular velocities than those of archers. However, there was no significant difference in the average values of isokinetic hip extension strength at all angular velocities between the two groups. 3. The average value for the psoas muscle CSA in soccer players was significantly higher than that of archers. In this study, the biggest difference in muscle strength between soccer players and archers was hip flexion strength, and the CSA of the psoas muscle in soccer players, which is the main component of hip flexion, was significantly larger than that of archers. These findings showed the peculiarity of soccer players due to the constant demands of movements involved in ball kicking and running during practice and competition.

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.