b These figures were not presented in the original article, but were calculated based on data provided in the article.

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The rate of reinjury to football players appears to be a neglected area of research in the literature. This area warrants further research because it appears that prior injury may be a risk factor for further injury. What little information is available comes from three of the studies reviewed here. In a 4-year study Blyth and Mueller (10) found that 60% of high school players with a prior injury had another injury, whereas only 40% of players without any prior injury had a new injury The NATA study (64) documented that over 10% of the injuries reported in the 1986 study of high school players were reinjuries to a previously hurt area and that 4% of those injuries were directly related to the prior injuries. Although this study indicated a rate of 65 injuries per 100 players, it was noted that only 37% of the players in the study were injured at least once, implying that a considerable proportion of the players were injured more than once, though not necessarily to the same body part. At the collegiate level, Zemper (104, 105) noted that although there was an injury rate of 45 per 100 players, only 33.2 players out of 100 incurred at least one injury, implying that the remaining injuries were recurrent injuries or new injuries to a different body part in a player who had been injured previously.

The authors of many studies discuss whether practices or games are more dangerous or carry a higher risk. However, most studies make comparisons based on a percentage breakdown of injuries occurring in practices and games, and conclude that because more injuries (or a greater percentage) occur in practices, they are riskier for a player than games. For instance, the 1986 NATA high school study (64) states that 62% of the recorded injuries occurred in practices and only 38% in games. When considering this type of data, it must be remembered that a percentage breakdown of total injuries is not a rate and therefore has no relation to estimating risk. Because a team has at least five or six times as many practices as games, and usually more players participate in practice than games, it would be expected that the total number (or percentage) of injuries would be higher in practices. This type of information has its importance; as the NATA summaries correctly point out, more injuries occur in practices in high school, where there is less likely to be adequate medical coverage. This is a situation that needs to be addressed. But this does not mean that an individual player is at a greater risk of injury in practices. Among the few studies that correctly address this issue are the collegiate studies done by the NCAA (67) and by Zemper (104, 105), in which injury rates are calculated separately for practices and for games, and a relative risk can therefore be calculated based on these rates. The NCAA study (67) provides a practice injury rate of 3.99 per 1,000 A-E and a game injury rate of 35.45 per 1,000 A-E, from which a relative risk of 8.9 can be calculated. This indicates that an individual player is 8.9 times as likely to be injured in a game as he is in a practice, even though the NCAA data also show that the majority (57%) of the injuries occur in practice. The data from Zemper (105) show a practice rate of 3.79 per 1,000 A-E and a game rate of 32.11 per 1,000 A-E, so the game injury rate in this study is 8.5 times higher than the practice rate.

3.1 Injury Onset

Given that football is a collision sport, it would be expected that most football injuries are acute, as opposed to overuse or gradual onset injuries. None of the studies reviewed here directly addressed this issue, so no conclusions or comparisons can be drawn. The only hint along these lines comes from some unpublished data from Zemper on college football, which indicates that over a 4-year period only 5% of 4,559 reported injuries (that kept a player out for one day or more) were overuse or gradual onset injuries.

3.2 Injury Type

A review of the literature summarized in Table 4.3 indicates that at both the high school and college levels the three most commonly occurring types of injuries in the game of football are sprains (ligaments), strains (muscles), and contusions. This result was fairly consistent across all the studies, including the only two college level studies that provided a breakdown of injuries by type. The great discrepancies seen in the percentages of sprains and strains reported by Lackland (52) raise the question of how these two injury types were defined in this particular study, but also may reflect other differences between this and other studies, such as definitions of a reportable injury and methods of reporting and collecting data.

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Information about injury type and location is important because it allows all parties involved in the game of football to pay particular attention to these areas so that effective training, conditioning, preventive, and rehabilitative measures can be developed and employed.

3.3 Injury Location

The head (skull and brain) is particularly susceptible to injury because it cannot be conditioned to accept trauma, and once injured is further susceptible to injury (5, 22, 35, 50). Cantu (22) describes three principles to understand how biomechanical forces produce head injury: (a) Coup injuries occur when the head in a resting state is struck by another object producing maximal brain injury beneath the point of cranial impact; (b) contra-coup injuries occur when the head collides with a nonmoving object producing maximal brain injury opposite the side of cranial impact; and (c) direct injuries to the brain tissue may occur in the presence of a skull fracture when the bone displaces at the moment of impact.

Cantu (22) also lists several other types of head injuries that may occur as a result of football. These include intracranial hemorrhage: (a) epidural hematoma, (b) subdural hematoma, (c) intracerebral hematoma, and (d) subarachnoid hematoma; and malignant brain edema syndrome.

In both coup and contra-coup injuries, shearing forces (a force applied parallel to a surface) appear to be the major type of stress generated by an applied force. Two less severe types of stress are also generated by an applied force. These include compressive and tensile (negative pressure) stresses (22).

Head injuries also may occur as the result of an indirect (glancing) blow to the head resulting in impulsive loading in which the head is set into motion (22). Concussions may occur when "the skull is put into motion before the contained brain" (74).

Many definitions of a concussion exist in the literature, but there does seem to be some consensus that concussions are either transient or protracted and are the result of traumatically induced alterations, impairments, and dysfunction of mental (neural) function with manifested signs and symptoms including one or more of the following: loss of awareness, amnesia, dizziness, headaches, blurred vision, double vision, loss of equilibrium, feelings of deja vu, and visual and auditory hallucinations (14, 22, 35, 50).

A review of the literature indicates that cerebral concussions are the most frequently occurring types of head injuries in the game of football at the high school and college levels (3, 13, 22, 54, 104, 106). As shown in Table 4.3, concussions account for at least 5% of all reported football injuries, or at least one out of every 20 injuries.

The high incidence of cerebral concussion in the game of football is compounded by the fact that athletes appear to be susceptible to more severe head injury following an initial concussive event in which symptoms from the prior injury persist. In the literature this has been labeled as the second impact syndrome (22, 81). Furthermore, studies have shown that players suffering an initial concussion are four times more likely to sustain a second concussion compared with a player with no prior concussion (22, 35). The study of high school players by Gerberich et al. (35) involved only loss of consciousness injuries in arriving at their finding of four times greater risk for a subsequent injury. A recent study of college football players by Zemper (109) showed that players with a history of cerebral concussion (of any severity) any time during the previous 5 years were six times as likely to suffer a new concussion as those with no history. Albright et al. (3) documented that 26 of 78 players with an initial head injury had a subsequent head injury during their collegiate career, and these were more severe than the initial injury. In the same vein, Cantu (22) stressed that athletes are less likely to suffer subsequent concussions if they have fully recovered from the initial cerebral concussion.

There appears to be some controversy in the literature concerning the mechanisms involved in cervical neck/spine injuries occurring in football. Hyperflexion, hyperextension, and axial loading have all been proposed as the primary mechanism contributing to cervical injuries.

Injuries to the cervical spine traditionally have been attributed to hyperflexion and/or hyperextension mechanisms (23, 32, 33, 79, 82). Hyperflexion, in which the head is driven downward (as seen in head butting), may be the more dangerous and frequent mechanism involved in cervical spine injuries (33, 55). In a retrospective analysis conducted by Maroon (55) seven neck injuries involved hyperflexion as the primary mechanism of injury. Five of the seven players injured were involved in the process of tackling.

Schneider et al. (82) suggested that cervical spine injuries documented in their study resulted from hyperextension (shear forces) when the face mask was impacted, driving the posterior rim of the helmet against the back of the neck and providing a "guillotine" effect. This suggestion led Schneider et al. (82) to recommend that the rim of the helmet be cut higher to prevent its impact on the neck. However, this suggestion has been refuted, since more dangerous hyperextension occurs to the upper cervical spine when the helmet is cut high. A reduction in force is seen when the posterior rim of the helmet is allowed to make contact with the back of the neck and even further loading conditions are evident when the posterior rim of the helmet makes contact with the shoulder pads (23).

Axial loading occurs when the neck is slightly flexed, cervical lordosis is straightened, and the spine is converted into a segmented column. When a load is applied to the top of the helmet, as in spearing, force is transmitted down the straightened cervical spine, and an axial load is created and transmitted to the spinal structures. This results in the spine being compressed and crushed, and fractures and/or dislocations will occur (96).

Injuries to the cervical spine can be divided into four categories: (a) sprains, (b) strains, (c) fractures, and (d) dislocations (7). Torg (95) further categorized spinal injuries into five groups according to their severity. Group I is made up of mild sprain and strain injuries that improve with or without treatment, and includes burners or stingers (pinched cervical nerve syndrome). Group II is made up of moderate injuries and includes compression fractures, degenerative spine changes, and brachial plexus axonotmesis. Group III is made up of more severe neck injuries and includes unstable lesions with fractures, dislocations, and/or subluxation without neurologic deficit. Group IV is made up of very severe injuries including fracture-dislocations with neurologic deficit. Group V is made up of catastrophic disabling injuries, which include permanent quadriplegia or death. We will discuss the latter group in section 4.2--Catastrophic Injuries.

Lateral flexion (stretching, bending, or traction of the cervical nerve roots) has been identified as a mechanism of injury in cervical neck/spine injuries. This mechanism results in brachial plexus injuries, commonly called burners or stingers. These occur when a player receives a forceful blow to the head from the side with simultaneous depression of the opposite shoulder. Injury also may result from head extension or by depression of the shoulder while the neck and head are fixed.

A review of the literature indicates that brachial plexus injuries associated with a prickly burning sensation in the shoulders, and pain and weakness in the neck, arms, and hands, are the most commonly observed neck injuries (33, 54, 87). Robertson et al. (77) in a study of collegiate football players indicated that approximately 50% of the players on the team experienced at least one and sometimes more injuries of the plexus during a season. Furthermore, players with repeated incidence of this type of injury were at higher risk of developing further weakness and neurological dysfunction. Dolan et al. (32) report that a collegiate football team may experience an average of six stingers during a single game with as many as five or six major neck injuries, including fractures, disk or ligament disruptions, or persistent neuropathies, during one football season. Albright et al. (3) indicated that the majority (55%) of the neck injuries suffered by collegiate players were of a soft tissue or skeletal nature with sprains being the most common type of injury (64% with a mean time loss of 5.5 days). When reviewing high school incidence rates, Albright et al. (2) found that 32% of college freshman players demonstrated evidence of prior neck injury sustained during their high school football careers.

Injuries to the upper extremities commonly occur in the game of football. The most frequently reported cases appear to be the shoulders, hands, and fingers. The frequent occurrence of these injuries is not surprising considering that the hands and fingers are minimally protected by padding and are under constant trauma through blocking and tackling, and although the shoulder is protected by pads, it is often the initial contact point in tackling and blocking. The hand may be particularly vulnerable to injury because it is used often in direct contact against the headgear, the face mask, the shoulder pads, and the opponent's body. Injuries to the fingers also may occur as players grasp the opponent's belts, jerseys, and pads while tackling and blocking.

Football, a violent collision and contact sport, puts some of the greatest demands on the lower extremity because it is the major weight-bearing area of the body and is usually fixed to the ground. Additionally, football players are required to make severe running, cutting, and jumping moves, all adding to the stress placed on the lower body. The knee is particularly vulnerable to injury because it is minimally protected by padding and is often in an exposed position (20, 73, 90).

Because knee motion is limited to internal and external rotation as the leg straightens (extension) and bends (flexion), injury may occur when the joint is stressed into awkward and unusual positions, usually as the result of direct contact from other players. Noncontact injuries often involve unnatural twisting motions and drills that prevent the natural rotation of the knee joint. Such injuries are commonly seen in cutting maneuvers when the foot is fixed to the ground, causing damage to the anterior cruciate ligaments, the collateral ligaments, menisci, and patella (30, 90, 99).

A review of Tables 4.4 and 4.5 demonstrates that, at the high school and college levels, the knee is the most frequently injured body part in the lower extremity. Among the most common knee injuries in football are ligament injuries (grade I, grade II, and grade III sprains), of which the medial collateral ligament is the most commonly sprained (31). The frequent occurrence of knee sprains is a concern because a significant percentage of these injuries may require surgical intervention and result in significant time loss from active participation (31, 47). Additionally, injury to the medial collateral ligament often increases the likelihood of further injury to the supporting structures of the knee, particularly damage to the anterior cruciate ligament (70). Other common knee injuries include medial femoral condyle fractures, epiphyseal fractures, anterior and posterior cruciate damage, meniscus injuries, and patellofemoral arthritis (8, 45, 58, 63, 100).

A review of Tables 4.4 and 4.5 shows that injuries to the lower extremities also commonly occur in the ankle, toe, hip, quadriceps, and hamstring. According to Rishel et al. (76), most ankle injuries are inversion sprains that result in tears to the lateral ligament structures. Damage to these ligaments leads to ankle instability, making the athlete more susceptible to recurrent ankle injuries.

Injuries to the toe, particularly the metatarsal-phalangeal joint (bunion joint of the big toe), also are significant lower extremity injuries commonly seen in football players. This injury is referred to as turf-toe (100).

A 5-year retrospective study conducted by Heiser et al. (40) demonstrated that injuries to the hamstring may also be common among collegiate football players, highlighting the need for an evaluation and rehabilitation program to prevent and lessen the frequency and severity of hamstring strains. The study revealed that, prior to a strengthening and rehabilitation program, 7.7% of the players reported hamstring strains, with 31.7% sustaining one or more recurrent injuries. Players utilizing a Cybex II program reported only a 1.1% hamstring injury rate with no recurrent injuries being reported.

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