Head protection in football STEPHEN E. REID, MD, HERBERT M. EPSTEIN, MD, THOMAS J. O’DEA, MA, MICHAEL W. LOUIS AND STEPHEN E. REID, JR.
The modern helmet
is
the result of labo-
ratory testing using hypothetical impacts in a rigid environment Helmets that are capable of withstanding these impacts may not necessarily cushion the bram sufficiently to avoid mjury The purpose of this study is to develop a method for measuring the stress the helmet is subjected to in regulation football games and to determme bram tolerance to the re-
peated impacts The highest of the multiple peaks of acceleration resulting from a single impact and the duration of all the peaks are the measurements reported m this study. Of 650 impacts the highest peaks ranged between 40 and 530 Gs while the durations were from 20-420 milliseconds. Twelve high intensity impacts produced accelerations between 180 and 400 Gs and had durations of 300-400 milliseconds This could be m the range of bram tolerance to impact because a concussion followed one of these
impacts.
From the Department of Surgery, Evanston Hospital, Evanston, Illinois and the Northwestern University Medical School, Chicago, Illinois
Stephen E. Reid is Associated Professor of Surgery, Northwestern University Medical School, Chicago, Illinois; Senior Attending Surgeon, Evanston Hospital, Evanston, Illinois, Team Physician, Northwestern University FootDr.
ball Team.
This
Study
was
supported by
Michael W
Louis, The Sports Foundation, Inc. and the W. Grainger Foundation, Inc. 86
W
Attention
has been drawn to knee injuries in football but these injuries can only end the careers of famous football players while head injuries can be fatal. This paper will point to the problem and include the development of the present day helmet, information needed to improve head protection, how to acquire this knowledge, the data on six hundred and fifty impacts to the head of a football player obtained by telemetry and an explanation of the discrepancies between this data and that which has been measured in the laboratory. The commonly quoted fatality statistics on head injunes in football give no indication of the true incidence of brain trauma. Although concussion refers to some degree of loss of consciousness after impact with little if any gross evidence of brain damage and likely to be temporary, it will be used in this paper to indicate any degree of brain injury that is accompanied by loss of consciousness. The injury statistics quoted by Robey (1) imply that fifty-four thousand concussions occur annually in high school football while others report that one out of eight intercollegiate players suffers a concussion. The football helmet was introduced at Rutgers University in 1896 but head protection was not mandatory until 1940. This legislation caused manufacturers to become involved in improving protection and in 1945 Dye (2) began work on the engineering phase of the head protection problem. He concluded that head blows received in football are of short duration and that energy no greater than eighty-three foot-pounds could be expected. This engineering thesis prevails today because the modern plastic helmet which made its appearance in 1947 has
maximum
which
energy
absorbing capabilities
rarely exceed seventy-five foot-
pounds. The method for testing helmets has not the past twenty-five years and consists of delivering an impact to a helmet that contains a head form fitted with an accelerometer and calculated to have a mass equivalent to that of a severed human head The impact is developed by simply dropping the weighted helmet four to six feet onto an unyielding surface or impacting it with a pendulum striker. The helmet is graded by the maximum accelerations in gravitational units (G) that the helmet can accept before failure occurs. The Northwestern University telemetry studies on brain tolerance to impact showed that these criteria for helmet standards give no indication that the human brain can tolerate even the acceptable accelerations. Engineers then developed a Severity Index for brain tolerance to impact from an acceleration-time tolerance curve which was produced in the laboratory with human cadaveric heads (3) This Severity Index is based on the assumption that the force required to produce a linear skull fracture in a cadaveric head is equivalent to the force that will produce a concussion in a football player. This assumption is not valid because skull fracture in football is extremely rare even in fatal head injuries Furthermore, concussions in football continue to occur in players while wearing the helmets that satisfy the Severity Index both before and after these concussion producing impacts. The football helmet nevertheless has been constantly improving to keep pace with the changing requirements of the mod-
changed during
ern
game. Man’s knowledge of the living human brain includes its structural and physiological characteristics as well as its tolerance to thermal changes, drugs, deprivation of oxygen, water, minerals, glucose and sleep. A blow to the head caused the skull to deform and even vibrate and the contained brain to glide about in the protective hydraulic system provided by the cerebrospinal fluid but brain tolerance to impact is unknown. Methods of measuring these forces have challenged the ingenuity of researchers in the field because man is reluctant to submit
his head to possibly lethal blows. Extensive research has been done using experimental models, animals and human cadavers but the extrapolation of such data to the living human brain has been troublesome. Additional information is necessary to validate helmet testing and this must include brain tolerance to impact and the stress requirements of the helmet under actual playing conditions. With the use of modern electronics it is now feasible to obtain impact data from the football field by means of radiotelemetry. This pioneering study was undertaken in 1961 because the AMA was concerned with the incidence of head injuries in football. This was the first time telemetry had been applied in such a vigorous environment and nine years were required to perfect the technique and to determine the type and placement of transducers to record the necessary data What data is essential in order to make meaningful performance tests for a football helmet? The magnitude of impact that the head can safely tolerate must be known and the parameters of force that will convey this information must be measured. The force encountered on the field is important but it need not be measured because it is the response to this force rather than the force itself that causes head injuried. For example, a man who jumps from a burning building is not injured by the fall but by the sudden stop. If the deceleration were prolonged as occurs when the man is caught in a life net no injury would occur. As an example in sports, a boxer may deliver two blows of equal intensity and one may produce a concussion in his opponent but the other punch may cause no injury at all depending upon how the head reacts to the blows. No single parameter of motion is able to assess completely the effect of impact to the head of an active football player because head injury is dependent not only upon the magnitude and direction of impact and the structural features and physical reactions of the skull but also upon the state of the head while receiving the impact. The head form used in the laboratory is a freely movable body with a constant mass and its responses to blows are of very short duration while on the football field the head of a living player is not a free body but is 87
fixed to the torso
TABLE I
by a strong muscular neck.
Thus, the effective
of the head which is its resistance to change of motion when struck by some external force will vary greatly depending upon the state of readiness of the head-neck-torso system and its responses to blows are of relatively long duration From experience with the type of impacts encountered on the football field the magnitude and duration of accelerations that result from these impacts are reliable measurements of the response of the head to blows. The placement of accelerometers on the head of a player is critical since they cannot be screwed to the skull as is done in cadaveric experimentation or placed at the center of gravity as in the head form used to test helmets. These transducers are held firmly on the head by the snug suspension system in the helmet The physiologic effect of impacts are monitored with a telemetered electroencephalograph (EEG) from each side of the brain so that an end point can be determined before concussion occurs. Two channels of EEG are not sufficient to diagnose brain damage but can elicit gross changes when a disturbance in the basic mass
background rhythm is observed. The impact data, EEG and video action are recorded on tape Since the fall of 1970, six hundred and fifty measured impacts to the head of a player were telemetered during the games on the Northwestern University football schedule. The vector quantity of the highest of the peaks of acceleration recorded on the frontally placed accelerometer and the highest of the peaks recorded of the two accelerometers placed on either side of the head ranged between 40 and 530 G’s (Table I) The duration of response of these complicated accelerations of multiple peaks that result from a single complicated impact is recorded as the time during which the entire response occurs (Figure I). The duration of these six hundred and fifty impacts ranged between 20 and 420 milliseconds. The values of fifty of these impacts ranged between 150 and 450 G’s with durations between 200 and 400 milliseconds but twelve of these fifty had durations of 300 400 milliseconds and produced acceleration peaks between 180 and 400 G’s (Figure 2). The greatest peak of 88
Figure
these
1-Two impacts occurring
in rapid succession The tracing of the second impact shows typical multiple peaks of acceleration.
accelerations encountered by the measured 530 G’s but the entire duration of the response was only 60 milliseconds. Durmg the response to an impact that lasted 420 milliseconds there were also many peaks of acceleration but the greatest peak measured only 120 G’s. During the 1972 season the durations of response were somewhat shorter than those measured during the previous two football seasons but the peak amplitudes were somewhat greater (Table II). This difference can possibly be explained by the fact that a less experienced player was instrumented during the 1972 season. What is the significance of this data and how does it relate to brain tolerance to impact? It can be concluded that under these conditions of physical fitness that brain tolerance to impact must fall within the range of 180 400 G’s with durations of 300-400 milliseconds (Table III) because
player
occurred as the result of one of these impacts. The telemetered data of the response to this impact showed the typical graph with a duration of 310 milliseconds and during this time multiple peaks of acceleration were recorded with the highest of the peaks measuring 188 G’s (4). On one other occasion although there was no clinical evidence of concussion, there was a disturbance in the basic background rhythm of the telemetered EEG (Figure 3). This occurred after a series of impacts of intermediate intensities and following this series of impacts the instrumented player stated that he &dquo;felt fuzzy&dquo; It was surprising to note that the concussion producing impact caused peaks of acceleration which were of less magnitude than others that caused no clinical evidence of brain injury and that the duration of this impact was not relatively significant. Several other effects of the impact which were not measured could be responsible for this apparent inconsistency of the telemetered data 1. The whiplash that was seen in video action could cause stretching of the cervical cord which results in some effect on the activity of the reticular core of the brain stem. 2 The direction of the blow to the left frontotemporal area of the head could produce an unmeasured amount of rotational acceleration of the brain. 3. The flattening of cortical activity on the side opposite the site of the blow, as shown in the telemetered EEG demonstrates the contrecoup effect. 4. The fact that a similar high intensity blow was encountered at an area on the head directly opposite the site of im-
pingement of the concussion producing blow just two plays previously could
concussion
again demonstrate fect of impacts.
the cumulative ef-
5. Video action showed that the head of the instrumented player spinning into the path of the knee of the oncoming ball carrier resulted in an increase in the involved mass of the head which resulted in the absorption of more kinetic energy than occurred in other impacts of seemingly greater intensity. The complex graphs of telemetered accelerations were compared with the graphs produced in laboratory testing of helmets similar to the helmet worn by the instrumented player and were found to be very different in magnitude, duration and configuration. The laboratory graph is very simple and consists of a single smooth line that rises uniformly to a single peak of acceleration and gradually falls to the baseline producing measurements of about 250 G’s and with a duration of only 2-10 milliseconds (Figure 4). It would seem that the telemetered data is faulty because the duration of response to impact on the football field is about forty times greater than the maximum duration of response that the helmet is capable of accepting before failure occurs. The explanation of this discrepancy is that conditions that prevail on the field are entirely different from those set up in the laboratory. The duration of response to impact depends not only upon the magnitude and type of force applied but also the characteristics of the target. The force may be delivered in a very short time such as a blow from a hammer or a pendulum striker or the force may be sustained and complicated such as is delm-
Figure 2-High-intensity impacts followed by
two
lesser impacts. 89
ered in football in blocking or tackling (5). If the target is freely movable it will have a constant mass and its duration of response to a blow will be very short as is the case with a golf ball resting on its tee. On the other hand, if the motion of the target is restricted it will have a variable mass and the duration of response to impact will be prolonged as occurs with the head of a football player attached to the body by a muscular neck. This physiologic response to impact missing in laboratory testing of helmets is highly developed in the well conditioned football player with quick reflexes in an environment where man is competing with man. The innocent spectator on the side lines at a football game realizes his own lack of physiologic response when a play runs into him. The reaction of a football player undergoing test conditions of an impact to the side of his helmet demonstrates how he receives the blow. He tenses all the muscles in his body and braces himself for the impact. He watches out of the corner of his eye so that he will know when to react. He seems to feel for the assault and if the magnitude of the force is so great that injury is likely, he causes his head to catch the blow much as a baseball player softens the blow by causing his gloved hand to move in the direction of the ball. These protective reflexes of the athlete in his state of readiness provide three lines of defense to an impact. First, the whole body of the athlete reacts to a blow directed at his head in such a way that its efficiency is reduced by causing the blow to be of a glancing type and allowing the smooth, hard exterior of the shell of the helmet to deflect the blow. This allows the head to receive only a percentage of the intensity of the impact. The second line of defense is demonstrated by applying NewM x A). The ton’s Law of Motion (F response of the head to an impact is restricted when the tone of the muscles of the neck is increased because this adds more mass to the head. The same force (F) applied to this greater mass (M) will result in less acceleration (A). Furthermore, if the head were allowed to move as a free body whiplash or even greater injury to the neck would occur. The third line of defense is =
90
TABLE II
TABLE III
demonstrated with another application of Newton’s Law of Motion (F MAV/T) The force (F) of an impact is less when the speed of the blow is reduced IV) over a longer period of time (T). For example, the boxer avoids a knock-out when he &dquo;rolls with the punch&dquo; because it takes a longer time for the fist to come to a halt on his chin. The goal in head protection is to give the brain a smooth ride through a football game. The shock absorber effect of the helmet must be adjusted to the load placed on it. The physiologic response of the athlete to impact changes the character and magnitude of the force that reaches the helmet. This is the load the helmet must absorb and it can only be obtained on the football field with telemetry. This method has already indicated the level of brain tolerance to impact but, in addition, has shown that the =
head of
a
previous movable
studies, does
football with
player contrary not react as a
to
all
freely
constant mass. This the response to impact to be prolonged; the character of the recorded accelerations to be complex; and the change in velocity of the mass to be an inaccurate measurement of brain tolerance. The physiologic response of the conditioned athlete is responsible for the great difference in the data telemetered from the football field and that produced in the laboratory. The fact that the duration of impacts encountered on the field is at least forty times greater than those produced in the laboratory does not seem inconsistent because the kinetic energy developed by the knee in running is also more than forty times greater than the maximum energy absorbing capabilities of the helmet.
body
single finding
a
causes:
SUMMARY
Head
Figure 3-Telemetered sultingfrom
EEG tracings showing moderate impacts
changes
Figure 4-Single peak of acceleration
in
re-
injuries in football continue to be a problem. The efforts of engineers to develop criteria for head protection have been ham-
simple graph of laboratory-tested helmet. 91
pered by the lack of information about brain tolerance and the physiologic response to impacts. This information is now available with the application of radiotelemetry to the football field. Now the shock absorber effect of the helmet can be adjusted to a realistic load, that results from the physiologic response of the athlete. References 1
Robey JM, Blyth CS,
92
Mueller FO Athletic
Injuries-Application of Epidemiologic Methods JAMA 217 184 189, 1971 2 Dye ER Engineering Research on Protective Headgear Am J Surg 98 368 72, 1959 3. Gurdjian ES, Roberts VL, Thomas LM Tolerance curves of acceleration and intracranial pressure and protective index in experimental head injuries J Trauma 6 600-4, 1966 4 Reid SE, Tarkington JA, Epstein HM, et al Brain tolerance to impact in football. Surg Gynecol and Obstet 133 929-36, 1971 5. Reid SE, Epstein HM, O’Dea TJ Impacts in football Selling Sporting Goods 25 50-2, 1972
The modern helmet
is
the result of labo-
ratory testing using hypothetical impacts in a rigid environment Helmets that are capable of withstanding these impacts may not necessarily cushion the bram sufficiently to avoid mjury The purpose of this study is to develop a method for measuring the stress the helmet is subjected to in regulation football games and to determme bram tolerance to the re-
peated impacts The highest of the multiple peaks of acceleration resulting from a single impact and the duration of all the peaks are the measurements reported m this study. Of 650 impacts the highest peaks ranged between 40 and 530 Gs while the durations were from 20-420 milliseconds. Twelve high intensity impacts produced accelerations between 180 and 400 Gs and had durations of 300-400 milliseconds This could be m the range of bram tolerance to impact because a concussion followed one of these
impacts.
From the Department of Surgery, Evanston Hospital, Evanston, Illinois and the Northwestern University Medical School, Chicago, Illinois
Stephen E. Reid is Associated Professor of Surgery, Northwestern University Medical School, Chicago, Illinois; Senior Attending Surgeon, Evanston Hospital, Evanston, Illinois, Team Physician, Northwestern University FootDr.
ball Team.
This
Study
was
supported by
Michael W
Louis, The Sports Foundation, Inc. and the W. Grainger Foundation, Inc. 86
W
Attention
has been drawn to knee injuries in football but these injuries can only end the careers of famous football players while head injuries can be fatal. This paper will point to the problem and include the development of the present day helmet, information needed to improve head protection, how to acquire this knowledge, the data on six hundred and fifty impacts to the head of a football player obtained by telemetry and an explanation of the discrepancies between this data and that which has been measured in the laboratory. The commonly quoted fatality statistics on head injunes in football give no indication of the true incidence of brain trauma. Although concussion refers to some degree of loss of consciousness after impact with little if any gross evidence of brain damage and likely to be temporary, it will be used in this paper to indicate any degree of brain injury that is accompanied by loss of consciousness. The injury statistics quoted by Robey (1) imply that fifty-four thousand concussions occur annually in high school football while others report that one out of eight intercollegiate players suffers a concussion. The football helmet was introduced at Rutgers University in 1896 but head protection was not mandatory until 1940. This legislation caused manufacturers to become involved in improving protection and in 1945 Dye (2) began work on the engineering phase of the head protection problem. He concluded that head blows received in football are of short duration and that energy no greater than eighty-three foot-pounds could be expected. This engineering thesis prevails today because the modern plastic helmet which made its appearance in 1947 has
maximum
which
energy
absorbing capabilities
rarely exceed seventy-five foot-
pounds. The method for testing helmets has not the past twenty-five years and consists of delivering an impact to a helmet that contains a head form fitted with an accelerometer and calculated to have a mass equivalent to that of a severed human head The impact is developed by simply dropping the weighted helmet four to six feet onto an unyielding surface or impacting it with a pendulum striker. The helmet is graded by the maximum accelerations in gravitational units (G) that the helmet can accept before failure occurs. The Northwestern University telemetry studies on brain tolerance to impact showed that these criteria for helmet standards give no indication that the human brain can tolerate even the acceptable accelerations. Engineers then developed a Severity Index for brain tolerance to impact from an acceleration-time tolerance curve which was produced in the laboratory with human cadaveric heads (3) This Severity Index is based on the assumption that the force required to produce a linear skull fracture in a cadaveric head is equivalent to the force that will produce a concussion in a football player. This assumption is not valid because skull fracture in football is extremely rare even in fatal head injuries Furthermore, concussions in football continue to occur in players while wearing the helmets that satisfy the Severity Index both before and after these concussion producing impacts. The football helmet nevertheless has been constantly improving to keep pace with the changing requirements of the mod-
changed during
ern
game. Man’s knowledge of the living human brain includes its structural and physiological characteristics as well as its tolerance to thermal changes, drugs, deprivation of oxygen, water, minerals, glucose and sleep. A blow to the head caused the skull to deform and even vibrate and the contained brain to glide about in the protective hydraulic system provided by the cerebrospinal fluid but brain tolerance to impact is unknown. Methods of measuring these forces have challenged the ingenuity of researchers in the field because man is reluctant to submit
his head to possibly lethal blows. Extensive research has been done using experimental models, animals and human cadavers but the extrapolation of such data to the living human brain has been troublesome. Additional information is necessary to validate helmet testing and this must include brain tolerance to impact and the stress requirements of the helmet under actual playing conditions. With the use of modern electronics it is now feasible to obtain impact data from the football field by means of radiotelemetry. This pioneering study was undertaken in 1961 because the AMA was concerned with the incidence of head injuries in football. This was the first time telemetry had been applied in such a vigorous environment and nine years were required to perfect the technique and to determine the type and placement of transducers to record the necessary data What data is essential in order to make meaningful performance tests for a football helmet? The magnitude of impact that the head can safely tolerate must be known and the parameters of force that will convey this information must be measured. The force encountered on the field is important but it need not be measured because it is the response to this force rather than the force itself that causes head injuried. For example, a man who jumps from a burning building is not injured by the fall but by the sudden stop. If the deceleration were prolonged as occurs when the man is caught in a life net no injury would occur. As an example in sports, a boxer may deliver two blows of equal intensity and one may produce a concussion in his opponent but the other punch may cause no injury at all depending upon how the head reacts to the blows. No single parameter of motion is able to assess completely the effect of impact to the head of an active football player because head injury is dependent not only upon the magnitude and direction of impact and the structural features and physical reactions of the skull but also upon the state of the head while receiving the impact. The head form used in the laboratory is a freely movable body with a constant mass and its responses to blows are of very short duration while on the football field the head of a living player is not a free body but is 87
fixed to the torso
TABLE I
by a strong muscular neck.
Thus, the effective
of the head which is its resistance to change of motion when struck by some external force will vary greatly depending upon the state of readiness of the head-neck-torso system and its responses to blows are of relatively long duration From experience with the type of impacts encountered on the football field the magnitude and duration of accelerations that result from these impacts are reliable measurements of the response of the head to blows. The placement of accelerometers on the head of a player is critical since they cannot be screwed to the skull as is done in cadaveric experimentation or placed at the center of gravity as in the head form used to test helmets. These transducers are held firmly on the head by the snug suspension system in the helmet The physiologic effect of impacts are monitored with a telemetered electroencephalograph (EEG) from each side of the brain so that an end point can be determined before concussion occurs. Two channels of EEG are not sufficient to diagnose brain damage but can elicit gross changes when a disturbance in the basic mass
background rhythm is observed. The impact data, EEG and video action are recorded on tape Since the fall of 1970, six hundred and fifty measured impacts to the head of a player were telemetered during the games on the Northwestern University football schedule. The vector quantity of the highest of the peaks of acceleration recorded on the frontally placed accelerometer and the highest of the peaks recorded of the two accelerometers placed on either side of the head ranged between 40 and 530 G’s (Table I) The duration of response of these complicated accelerations of multiple peaks that result from a single complicated impact is recorded as the time during which the entire response occurs (Figure I). The duration of these six hundred and fifty impacts ranged between 20 and 420 milliseconds. The values of fifty of these impacts ranged between 150 and 450 G’s with durations between 200 and 400 milliseconds but twelve of these fifty had durations of 300 400 milliseconds and produced acceleration peaks between 180 and 400 G’s (Figure 2). The greatest peak of 88
Figure
these
1-Two impacts occurring
in rapid succession The tracing of the second impact shows typical multiple peaks of acceleration.
accelerations encountered by the measured 530 G’s but the entire duration of the response was only 60 milliseconds. Durmg the response to an impact that lasted 420 milliseconds there were also many peaks of acceleration but the greatest peak measured only 120 G’s. During the 1972 season the durations of response were somewhat shorter than those measured during the previous two football seasons but the peak amplitudes were somewhat greater (Table II). This difference can possibly be explained by the fact that a less experienced player was instrumented during the 1972 season. What is the significance of this data and how does it relate to brain tolerance to impact? It can be concluded that under these conditions of physical fitness that brain tolerance to impact must fall within the range of 180 400 G’s with durations of 300-400 milliseconds (Table III) because
player
occurred as the result of one of these impacts. The telemetered data of the response to this impact showed the typical graph with a duration of 310 milliseconds and during this time multiple peaks of acceleration were recorded with the highest of the peaks measuring 188 G’s (4). On one other occasion although there was no clinical evidence of concussion, there was a disturbance in the basic background rhythm of the telemetered EEG (Figure 3). This occurred after a series of impacts of intermediate intensities and following this series of impacts the instrumented player stated that he &dquo;felt fuzzy&dquo; It was surprising to note that the concussion producing impact caused peaks of acceleration which were of less magnitude than others that caused no clinical evidence of brain injury and that the duration of this impact was not relatively significant. Several other effects of the impact which were not measured could be responsible for this apparent inconsistency of the telemetered data 1. The whiplash that was seen in video action could cause stretching of the cervical cord which results in some effect on the activity of the reticular core of the brain stem. 2 The direction of the blow to the left frontotemporal area of the head could produce an unmeasured amount of rotational acceleration of the brain. 3. The flattening of cortical activity on the side opposite the site of the blow, as shown in the telemetered EEG demonstrates the contrecoup effect. 4. The fact that a similar high intensity blow was encountered at an area on the head directly opposite the site of im-
pingement of the concussion producing blow just two plays previously could
concussion
again demonstrate fect of impacts.
the cumulative ef-
5. Video action showed that the head of the instrumented player spinning into the path of the knee of the oncoming ball carrier resulted in an increase in the involved mass of the head which resulted in the absorption of more kinetic energy than occurred in other impacts of seemingly greater intensity. The complex graphs of telemetered accelerations were compared with the graphs produced in laboratory testing of helmets similar to the helmet worn by the instrumented player and were found to be very different in magnitude, duration and configuration. The laboratory graph is very simple and consists of a single smooth line that rises uniformly to a single peak of acceleration and gradually falls to the baseline producing measurements of about 250 G’s and with a duration of only 2-10 milliseconds (Figure 4). It would seem that the telemetered data is faulty because the duration of response to impact on the football field is about forty times greater than the maximum duration of response that the helmet is capable of accepting before failure occurs. The explanation of this discrepancy is that conditions that prevail on the field are entirely different from those set up in the laboratory. The duration of response to impact depends not only upon the magnitude and type of force applied but also the characteristics of the target. The force may be delivered in a very short time such as a blow from a hammer or a pendulum striker or the force may be sustained and complicated such as is delm-
Figure 2-High-intensity impacts followed by
two
lesser impacts. 89
ered in football in blocking or tackling (5). If the target is freely movable it will have a constant mass and its duration of response to a blow will be very short as is the case with a golf ball resting on its tee. On the other hand, if the motion of the target is restricted it will have a variable mass and the duration of response to impact will be prolonged as occurs with the head of a football player attached to the body by a muscular neck. This physiologic response to impact missing in laboratory testing of helmets is highly developed in the well conditioned football player with quick reflexes in an environment where man is competing with man. The innocent spectator on the side lines at a football game realizes his own lack of physiologic response when a play runs into him. The reaction of a football player undergoing test conditions of an impact to the side of his helmet demonstrates how he receives the blow. He tenses all the muscles in his body and braces himself for the impact. He watches out of the corner of his eye so that he will know when to react. He seems to feel for the assault and if the magnitude of the force is so great that injury is likely, he causes his head to catch the blow much as a baseball player softens the blow by causing his gloved hand to move in the direction of the ball. These protective reflexes of the athlete in his state of readiness provide three lines of defense to an impact. First, the whole body of the athlete reacts to a blow directed at his head in such a way that its efficiency is reduced by causing the blow to be of a glancing type and allowing the smooth, hard exterior of the shell of the helmet to deflect the blow. This allows the head to receive only a percentage of the intensity of the impact. The second line of defense is demonstrated by applying NewM x A). The ton’s Law of Motion (F response of the head to an impact is restricted when the tone of the muscles of the neck is increased because this adds more mass to the head. The same force (F) applied to this greater mass (M) will result in less acceleration (A). Furthermore, if the head were allowed to move as a free body whiplash or even greater injury to the neck would occur. The third line of defense is =
90
TABLE II
TABLE III
demonstrated with another application of Newton’s Law of Motion (F MAV/T) The force (F) of an impact is less when the speed of the blow is reduced IV) over a longer period of time (T). For example, the boxer avoids a knock-out when he &dquo;rolls with the punch&dquo; because it takes a longer time for the fist to come to a halt on his chin. The goal in head protection is to give the brain a smooth ride through a football game. The shock absorber effect of the helmet must be adjusted to the load placed on it. The physiologic response of the athlete to impact changes the character and magnitude of the force that reaches the helmet. This is the load the helmet must absorb and it can only be obtained on the football field with telemetry. This method has already indicated the level of brain tolerance to impact but, in addition, has shown that the =
head of
a
previous movable
studies, does
football with
player contrary not react as a
to
all
freely
constant mass. This the response to impact to be prolonged; the character of the recorded accelerations to be complex; and the change in velocity of the mass to be an inaccurate measurement of brain tolerance. The physiologic response of the conditioned athlete is responsible for the great difference in the data telemetered from the football field and that produced in the laboratory. The fact that the duration of impacts encountered on the field is at least forty times greater than those produced in the laboratory does not seem inconsistent because the kinetic energy developed by the knee in running is also more than forty times greater than the maximum energy absorbing capabilities of the helmet.
body
single finding
a
causes:
SUMMARY
Head
Figure 3-Telemetered sultingfrom
EEG tracings showing moderate impacts
changes
Figure 4-Single peak of acceleration
in
re-
injuries in football continue to be a problem. The efforts of engineers to develop criteria for head protection have been ham-
simple graph of laboratory-tested helmet. 91
pered by the lack of information about brain tolerance and the physiologic response to impacts. This information is now available with the application of radiotelemetry to the football field. Now the shock absorber effect of the helmet can be adjusted to a realistic load, that results from the physiologic response of the athlete. References 1
Robey JM, Blyth CS,
92
Mueller FO Athletic
Injuries-Application of Epidemiologic Methods JAMA 217 184 189, 1971 2 Dye ER Engineering Research on Protective Headgear Am J Surg 98 368 72, 1959 3. Gurdjian ES, Roberts VL, Thomas LM Tolerance curves of acceleration and intracranial pressure and protective index in experimental head injuries J Trauma 6 600-4, 1966 4 Reid SE, Tarkington JA, Epstein HM, et al Brain tolerance to impact in football. Surg Gynecol and Obstet 133 929-36, 1971 5. Reid SE, Epstein HM, O’Dea TJ Impacts in football Selling Sporting Goods 25 50-2, 1972
