The aim of the present study was to investigate height change of high school football players during a single game. Ten high school football players served as participants. The participants were selected according to position and expected playing time. The chosen positions experience the repetitive longitudinal loading of the spine that may lead to a creep response in the vertebral disk. Height was measured using a standard physician beam scale with height rod. A practicing certified athletic trainer served as the tester for all measures (pre – post). A paired samples T-test was performed to determine significance between height before and after the game. A significant difference was shown in height magnitude (Mpre = 176.56±6.9cm, Mpost = 175.81±6.94cm, p = .032). The results indicate that high school football players’ height decreases during the course of a game. This process is likely due to the creep response caused by intermittent high impact compressive loading of the spinal column, as well as low impact continuous compressive forces from equipment weight.
**Key words:** American football, compression, spinal shrinkage, creep response
American football (football) places many physical demands on its participants due to the aggressive nature of the sport. External forces from running, blocking and tackling can cause much stress on the human body. Even with protective equipment such as helmets and pads, these forces are inevitable. During the course of a game, football players may experience substantial longitudinal loading of vertebral column from the compressive forces of running and tackling as well as the continuous load due to equipment mass. This loading of the spine may accelerate the creep response which could result in a decrease in height after a game.
Spinal creep is a process by which continual loading or compressive forces placed upon the spinal column cause a reduction in the vertical size of the intervertebral discs. This creep response is due to the viscoelastic properties of the intervertebral discs of the spinal column, and is also referred to as spinal shrinkage. When compressive loading of the spine exceeds the interstitial osmotic pressure of the discal tissue, water is expelled from the intervertebral discs. This results in a loss of disc height which is reflected as a loss in stature (11). Since the spinal column composes about 40% of total body length, and the intervertebral discs account for roughly one-third of the length of the spinal column (Reilly, 2002), fluid loss from the discs can potentially cause substantial change in stature.
Studies of the intervertebral discs have shown that by narrowing in response to compressive forces, the discs also stiffen, which alters the dynamic response characteristics of the intervertebral disc complex (7). Once the disc has been narrowed and stiffened, its ability to absorb sudden direct and indirect changes in force is reduced, and thus the disc is therefore more susceptible to injury (9), and is often suggested to be a major causal factor of back pain (8). Some of the sports that have the highest risk of these injuries are football, ice hockey, and rugby (1). Within the sport of football it is believed that there is an increase in risk factors associated with spinal creep that may cause many athletes to develop low back pain (5). Because specific spine injuries like fracture, disc herniation, and spondylolysis are more frequent in football players (5), the occurrence of spinal shrinkage during a football game may be greater than other activities.
Studies have investigated spinal shrinkage in various activities ranging from running (4), weight lifting (3) and circuit training (6), but currently there exists a gap in the literature surrounding spinal creep and American football. The compressive loads that can affect the vertebral column include gravity, changes in motion, truncal muscle activity, external forces and external work (13) all factors that can be involved in football. These factors may lead to an accelerated creep response which could result in a decrease in height after a game. In a sport such as football, any minute decrease in stature may mean the difference between blocking a last second field goal, or making a game winning catch. Chronic exposure to these factors may also lead to back pain or injuries to the spine or discs. Therefore, the purpose of this study was to investigate the amount of shrinkage due to spinal loading during a high school football game.
Ten high school football players took part in the study. Mean values for height and weight were 176.6±6.9cm and 86.4± 9.5kg, respectively. All players were high school seniors aged 18 years and were selected according to position and expected playing time. The positions chosen were ones that experience the repetitive longitudinal loading of the spine that may lead to a creep response in the vertebral discs. This information was determined after interviewing the coach for the team and from observations made at other similar games. Based on these criteria, eligible (18yr old) players were recruited who started at the following positions: linebackers, running backs, and linemen. Players were also selected who would be likely to play the entire game with very few rest breaks.
A standard physician beam scale with height rod was used in this study for measuring changes in stature before and after participation in the game. All measurements were collected by a practicing certified athletic trainer. The apparatus was accurate to within 0.01 inches and all measurements were converted to millimeters.
The football game used for this experiment was an evening high school football game, which took place after a regular day of school. An evening game was selected to ensure that any shrinkage occurring from normal daily activities would not affect the results of the study. Participants were measured barefoot while standing and wore t-shirt and shorts for both pre-game and post-game measurements. Pre-game measurements were taken prior to warm ups to ensure that starting heights reflected absolutely no football activity. Post-game measurements were taken immediately after completion of the game. Three consecutive measurements were taken each time by the certified athletic trainer to ensure that the apparatus was reliable.
#### Data Analysis
The effects of playing football on changes in stature were analyzed using a paired sample T-test. Post hoc power calculations were performed following any statistically significant finding. Comparisons were made between the pre- and post-game height measurements. All statistical analyses were performed with the use of a modern computer software package (SPSS 17.0 for Macintosh, G*Power 3). Statistical significance was set a priori at an alpha level > 0.05.
The mean and standard deviation for the pre-game height measurements was 176.6 ± 6.9 cm. Post-game measurements yielded a mean and standard deviation of 175.8 ± 6.9 cm. The results show that there was a significant increase in spinal shrinkage due to participation in a high school football game (p =0.032, power = 0.674). The average height loss for the ten participants was 7.62 (±SD = 9.25) mm.
The present study showed that participation in a high school football game causes measurable height differences before and after the game, the demonstrated mean loss of stature was 7.62mm. It can be assumed that the decrease in height is due to the increased external forces and equipment weight that are involved in the sport. These potentially lead to a rise in the intradiscal pressure and fluid to be expelled, resulting in a reduction in disc height. Though it is logical that loss of intervertebral disc height is responsible for all variations in height, it is also possible that the cartilage in joints and the soft tissue covering the scalp and soles of the feet may have been compressed. However, the total height of the intrajoint cartilage is small and the degree of compression is thought to be negligible (6). The soft tissue covering the scalp is also thin and the height rod of the scale used for measurement would compress the tissue to an insignificant level. The tissue covering the soles of the feet might also be compressed upon standing but it is likely that equilibrium was quickly reached (6). As a result, the measured changes in stature can be considered to reflect only the changes in disc height.
The spinal shrinkage recorded during a football game was greater than what was observed in previous research of other activities. The 7.62 mm decrease in stature in this study was greater than the 3.25 mm decrease during a 6 km run (6), 5.4 mm decrease during circuit-weight training (6), 3.6 mm decrease during weight training (3), and 1.81 mm during a drop jump regimen (2). Although shrinkage during participation in football was greater than other activities, it is not the greatest recorded occurrence of spinal shrinkage. The results of this study are comparable to the 7.8 mm loss in height during a 19 km run (6), and much less than the recorded loss of 11.2 mm during static loading with a 40 kg barbell (14).
A study that examined spinal recovery in pregnant women showed that women with lower back pain were unable to recover from spinal shrinkage to the same extent as women with no lower back pain (12). These findings suggest that lower back pain may be related to the diminished ability to recover, rather than the magnitude of the spinal shrinkage imposed during the task. Since there is believed to be a relationship between football and the development of lower back pain (5), this could suggest that football players may have a diminished ability to recover from spinal compression. This may be provoked by the magnitude and frequency of spinal loading that a football player is subjected to.
The inability of the spine to recover may also lead to serious acute and chronic injuries to the spine and discs. Football is considered to be one of the sports with the highest risks for the occurrence of spinal injuries (1). Many of the spinal injuries that are common in football include fractures, disc herniation, and spondylolysis (5). There may also be a positive correlation between the years of involvement in football and the chances of developing degenerative disc disease (5).
Based on prior research, it can be assumed that more spinal shrinkage occurs during participation in a football game as compared to other less impactful activities because of a greater spinal load. Football players experience this load on the spine not only from running, but also from the static load from the weight of equipment and from direct impact forces caused by collisions with other players. Both these components, running (6) and static loading of the spine (14), have been found to cause accelerated loss in stature. This combination, along with the collisions during a football game, may be the reason for greater spinal shrinkage.
Although the present study was conducted on high school players, the results should be also consistent with higher levels of play. A previous study was conducted to compare the response to spinal loading between different age groups of males (10). When comparing younger males (18-25 years of age) and older males (47-60 years of age), it was found that regardless of age the pattern of spinal shrinkage between the two groups was similar. Based on this research, high school, college, and professional football players should experience a similar response to spinal loading during a game.
### Applications In Sport
In a game such as football, winning and losing can be a matter of inches. If a player decreases in height at the end of a game, the extra length could be the difference in catching a football, blocking a kick, or batting down a pass. Thus this height difference might be the difference between winning and losing. The degree of hydration may play a role in the extent of the creep effect and should not be overlooked. It may be beneficial to conduct future research on the effects of height decrease on athletic performance. Future research may also investigate if frequent practice of spinal unloading throughout a player’s career can prevent or reduce spinal injuries and back pain.
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#### Figure 1
Percent change in height pre- to post-game among high school athletes participating in American football.
### Corresponding Author
Brian J. Campbell, PhD, ATC
Department of Kinesiology
University of Louisiana at Lafayette
225 Cajundome Blvd.
Lafayette, LA 70506
Brian J. Campbell is the Curriculum Coordinator of Exercise Science at the University of Louisiana at Lafayette. Dave Bellar, PhD is the Exercise Physiology Lab Director at the University of Louisiana at Lafayette. Kristina Estis is a Certified Athletic Trainer for Champion Sports Medicine at St. Vincent’s Birmingham. Tori Guidry is an undergraduate student of Exercise Science at the University of Louisiana at Lafayette. Matt Lopez is a DPT student at the University of South Alabama.