A Physiological Review of American Football

(reprinted with the permission of the author)

Danny M . Pincivero and Tudor 0. Bompa

• Sports Medicine, University of Pittsburgh, Pittsburgh, Pennyslvania, USA
• Department of Kinesiology, York University, Toronto, Ontario, Canada

 

Contents

Summary            . . . . . . . . . . . . . . .

1. Energy Systems .  .  .  .  .  .  .  .  .  . .
2. Energy Systems Utilised in Football
3. Performance Measures
4. Size and Body Composition . . 5. Strength     . . . . . . . . .

6. Speed and Anaerobic Power
7. Cardiovascular Endurance  .
8. Positional Requirements .
9. Performance Improvement  .
10. Conclusion .  .  .  .  . .

 

Summary                        

American  football  has been  one of the most  popular  sports in  North America within the past century and has recently received support and increased partici­ pation from European nations. Two of the biggest concerns regarding  participa­  tion in American football are the high incidence of injury and the physical demand for preparation. A basic u nderstanding of the physiological systems utilised in the sport of football is necessary in order to develop optimal  training  programmes geared specifically for preparation as well as the requirements of individual field positions. Previously, it has been assumed that football relies primarily on an anaerobic source of energy for adenosine triphosphate (ATP) resynthesis with approximately 90% coming from the  phosphocreatine  (PCr)  energy  system.  In lieu of research conducted specifically with football players, it appears that the energy contribution from the anaerobic glycolytic pathway in this sport has been underestimated.   The  elevated  blood  lactate  levels  observed  i n  football  players following game participation cast doubt on this hypothesis. Identifying posi­tion specific characteristics may also enhance the development of training pro­ grammes based on the requirements of the different positions. It appears that offensive and defensive linemen are generally larger, have higher levels of percent body fat and have greater absolute strength scores than all other positions. Offens­ ive backs, defensive backs and wide recei vers tend to display the lowest percent­ ages of body fat, lower absolute strength scores, fastest times over 5, 10, 40 and 300m and the highest relative V02max values. Linebackers appeared to represent a tran­ sition group mid way between the backs and linemen for size, body composition, strength, speed and endurance as well as positional duties. Findings within the literature suggest that a lack of cardiovascular development of university and professional football players may prove to be a hindrance to performance with specific regards to thermal regulation. Additional aerobic conditioning as well as the reduction of percent body fat would not only enhance performance, but might play a key role in preventing injuries and allowing a smoother transition into life after football.

_____________________________________________________________________

To the novice observer American football may appear to be a game dependent upon brute force and the ability to overpower opponents. However, this game heavily taxes the physiological systems of those who take part. American football, in fact, has been reported to combine the physical qualities of nearly all other sports – size, strength, power, speed, agility and endurance) 1 1 The psychological capabilities of an individual as well as the strate­ gies and tactics of a team are of vital importance to success. Physical preparation, however, is  the prime concern in developing athletes into football players. American football is unlike other sports in that the potential and incidence of injury are very high and participants must be optimally developed physically in order to withstand the constant phys­ ical contact and the physiological demands of the game.

The physical demands of the game include strength, speed, power, agility, flexibility, as well as aerobic and anaerobic endurance necessary. The objective of this review is to analyse the game of American football with regard to the different physical parameters previously stated. An analysis of the literature yields some interesting facts con­ cerning the development of college and profes­ sional football players. It appears that throughout the literature, a trend concerning the lack of cardio­ vascular development in American football players is quite evident.r 1 -41 Furthermore, a relationship ap­ pears to exist between body composition (i.e. per­ cent body fat) and its relation to physical performance in tests of strength, speed, and endurance.13, 5-81

The physical requirements of American football will be discussed from 2 different viewpoints. Firstly, the basic characteristics of the sport and its elements will be examined with respect to the con­ tribution of different biomotor abilities as well as the systems in which energy resynthesis takes place. Secondly, an analysis and grouping of the different positions into distinct categories will be addressed with respect to objective measures such as strength, speed, body composition and endur­ ance. With this information established, the selec­ tion of certain physical characteristics that are most important to the development of football players can take place. Furthermore, certain physical char­ acteristics that appear to have been neglected or have the potential for performance improvement can be determined.

It is through this in-depth review and compila­ tion of the existing research that a systematic and scientific approach to football can be taken, which may enhance the performance of individuals and teams. By means of this, proper training parame­ ters can be set according to the specific needs of the sport, which may consequently alter traditional methods used in the past.

1. Energy Systems

For muscular work the high energy compound ATP is required. When this chemical compound is broken down into its 2 constituent subunits, aden­ osine diphosphate (ADP) and an inorganic phos­ phate (Pi) molecule, energy is released and muscle contraction occurs.f91 ln order to contin ue muscular work, these 2 molecules must be resynthesised so as to be broken down again for continued energy release. But this resynthesis requires energy in it­ self which is generally provided by 3 different metabolic pathways: the PCr system; anaerobic glycolysis; and the aerobic system.191

During the onset of high intensity exercise, ATP resynthesis occurs via the PCr system with the aid of the enzyme creatine kinase. This enzyme func­ tions as a catalyst to the reaction between ADP and PCr resulting in the formation of ATP.f 1°1 PCr is catalysed so that the liberated phosphate ion is do­ nated to ADP to form ATP.llO, I I J This is the most rapid method of supplying ATP to the muscles and is the primary system used for ATP resynthesis at the onset of exercise and during short term and high intensity work as seen in American foot­ ball) 1 2• 1 31 However, PCr is stored within the mus­ cle fibre in limited amounts and the duration of maximal intensity exercise utilising this energy
system has been reported to last approximately I to 10 seconds.191

The second anaerobic method by which ATP formation takes place during short term, high in­ tensity exercise  is  through  anaerobic  glycoly­ sis.l9,1 1 ·1 3l This system, along with the PCr system, resynthesises ATP for muscular contraction in the absence of oxygen. Also referred to as fast glycol­ ysis, this system functions by breaking down blood glucose  or  muscle  glycogen  to  form  pyruvic acid, and eventually its reduced form,  lactic  acid,  with the  net  formation  of  2  or  3  ATP  molecules.I 14 1 Throughout this process, hydrogen ions (H+) are liberated into solution and bind with the  molecule  nicotinamide adenine dinucleotide (NAD) to  form the  reducing  equivalent  NADH  and  H+.191 In the case of anaerobic glycolysis where oxygen is not adequately supplied to the working muscles, pyru­ vic acid accepts the hydrogen ions donated by NADH, thereby liberating that molecule so that further binding  with  H+ may  occur. Pyruvic acid then forms lactic acid and allows the continuation of glycolysis.191 However, during intense  exercise, large amounts of lactic acid are produced, which immediately dissociate and release H+ that can ad­ versely affect exercise  performance.  This  increase in intracellular H+ concentration  reduces  the  abil­ ity of muscle cells to resynthesise ATP and also hinders the contractile process of muscle contrac­ tion.19· 1 1 l This metabolic system is the princi pal one used  in  those  events  lasting  from  “’30 sec to 2 min, such as a 400m race)9, 1 1 l There is evidence that the anaerobic glycolytic pathway is stimulated much earlier following the onset of high intensity exercise.l 1 3·1 5·1 6l

In the presence of oxygen, the glycolytic path­ way is still active in the form of ‘slow’ or aerobic glycolysis.l 14l In this case, the hydrogen ions that are liberated in solution are bound to NAD and flavin adenine dinucleotide (FAD) and transported to   the  mitochondria   (in  the  electron   transport chain) in order to resynthesise ATP. It appears that this system operates in conjunction with the aero­ bic or oxidative  system,  in which  numerous ATP molecules  are  generated  by  the  mitochondria.l 1 1 1

Exercise activities that exceed “‘4 min duration, or those of a lower exercise intensity, tend to use the aerobic system as the primary source of ATP pro­duction.191 It should be noted that the transfer be­ tween energy systems is a gradual one that occurs in a continuum and is a function of oxygen avail­ ability, exercise intensity and duration.

2. Energy Systems Utilised in Football

 American football requires many physical qual­ ities that appear to be independent of playing posi­ tion. Such abilities include rapid acceleration, high running speed, good  jumping  ability,  explosive force of muscles, speed endurance, strength endur­ ance, power of the throw and deceleration.1 1 1 The body must have certain qualities to perform these necessary abilities. Football players must have an aerobic capacity to provide power throughout a prolonged, intermittent duration and to recover quickly in short  pauses.  Maxi mu m  anaerobic power is also necessary to perform powerful move­ ments and quick start acceleration. Groups of mus­ cles must have the ability to provide stabilisation thereby contributing to maximal muscle force and explosive   strength.   Local   muscular   endurance is necessary to provide consistently repeated  ru nning at high  speeds for long distances. I 1 1

American  football can be classified as an   acy­clic sport that is composed of integral functions performed in one action.I 1 7 1 For example, such functions    including    backpedalling,    cutting   and tackling are often performed in one integrated ac­ tion during a given play. On the other hand, running is classified as a cyclic sport since the motor acts of running comprise cyclic, repetitive move­ ments.[ l ?J

With respect to the primary energy systems used in football, it appears that the PCr and lactic acid systems (anaerobic glycolysis) provide the bulk of energy productionJ2· 1 2·1 81 It has been suggested by Fox and Matthewsl 1 2l that the PCr system  provides 90% of energy production in football whereas the lactic acid system contributes the remaining 10%. This contribution of the energy systems may hold true for football since it is a sport in which very short, highly intense bursts of energy (2 to 5 sec) are required, followed by brief periods of recovery lasting from 25 to 40 sec.l 191 However, this ratio of the energy system contribution may be slightly el­ evated in light of more current  research.

In a study conducted  at the Canadian   Football League (CFL) all-star game in 1979, Zapiec and Taylor[ 201 noted that athletes’ playing time varied from 5 minutes and 42 seconds to 9 minutes and 48 seconds during a  2-hour  and  19-minute  game.  In an earlier study carried out by  Craigl 1 91 that   exam­ined exposure time in professional football it was determined that maximal partici pation was 13.5 minutes. The shorts bursts of activity over such a prolonged period of time are indicative of a  maxi­mal or near maximal  intermittent  work regimen.l 2 1

Lactate levels in the players examined by Zapiec and  Taylor1201  increased  3-  to  5-fold  following completion  of  the  game,  although, blood glucose levels were not significantly affected. Similar find­ ings were also reported by Smith and  Jackson121 I who observed significantly higher blood lactate levels in college football players. Their results showed elevated blood lactate levels from a pre­ game value of  1.67 ml/L, to 4.39 and 5.08 ml/L at the half-time and postgame periods, respec­ tively.l21 i

It was also observed that the professional play­ ers in the Zapiec and Taylor120l study had a higher relative area of fast twitch (FT) muscle fibres to slow twitch (ST) muscle fibres. An important point to note from this study is that the postgame muscle glycogen stains showed a reduction in content pri­ marily  in the  FT fibres.l 20l FT fibres are rich   in glycogen and glycolytic enzymes thereby enabling these fibres to have a large anaerobic   capacity.l9l

Since FT fibres contract rapidly and develop more force than ST muscle fibres, it appears that the re­ sults reported by Zapiec and Taylor[ZOJ support the notion that football  is a strenuous  intermittent  type of exercise that relies on the PCr and lactic acid system for energy production.l9.I I  J

It has been established that PCr is restored rap­ idly following  vigorous  activity.l 1 2,22.231 However, since the recovery period during football is very short (25 to 40 seconds), the amount of PCr replen­ ishment that occurs may be less than optimal. In fact, it has been demonstrated that the time required to replenish intramuscular CP stores in recreational individuals following isolated quadriceps  exercise may  range  from  55  to  90  seconds.l 22 1  Repeated bouts of high intensity exercise incorporating brief periods of recovery  suggest that football may rely more on the glycolytic pathway for its energy pro­ duction that the I 0% previously cited.l9l This find­ing is reinforced by the elevated lactate levels mea­ sured by Zapiec and Taylor.1201 However, it  should be stressed that the PCr system may still be the primary system for ATP resynthesis. Based on the research cited, the 9/ I PCr to  lactic  acid  system ratio may appear slightly  elevated. Over the course  of a football game, ATP resynthesis via anaerobic glycolysis may  become  a more  important  factor as a means for providing energy for work under con­ ditions of PCr depletion  and fatigue_l2, 9l

3. Performance Measures

When an individual commences participation in football he must first acquire the basic skills nec­ essary in order to be competitive. These skills in­ clude catching and throwing, backpedalling and changing direction, blocking and tackling. As the individual progresses from the high school level to university or professional football, he will very likely specialise at a particular position. Playing different   positions   in  the   sport  places  varying biomechanical and physiological demands on these individuals. The objective of the following section is to compare and contrast these physio­ logical and biomechanical attributes and based on these findings, develop distinct categories of play­ ers. Such variations include body size, body com­ position, strength, speed and cardiovascular endur­ ance. Since consistent data on quarterbacks and kickers is limited, these 2 positions will not be in­ cluded in this analysis.

 4. Size and Body Composition

Traditionally, coaches have used the criteria of size and maximum strength as the most important variables in selecting top potential football players. However, the element of size can be a very mis­ leading factor when selecting players. With respect to football or other types of athletic skills, body composition plays a large role in successful perfor­ mance. Body composition has been shown to be related  to strength,  speed  and cardiovascular  en­durance in many  studies conducted  on university and professional football players.17,8·24·251 In order to  develop  distinct  groups  of  players  by   position based on objective data, size and body composition appear to have been some of the most obvious and consistent measures utilised.

Various studies conducted on football players have yielded from 2 to 5 categories of  posi­ tions.17·8·1 8·25·261 One of the consistent findings among these studies was that offensive and defen­ sive linemen had statistically  similar  sizes  as  did the offensive backs, receivers and defensive backs. The linemen were taller and heavier  than  the backs in each of these studies. It appears that the linemen and backs represent opposite ends of the spectrum with  linebackers midway  between, thus yielding   at least 3 categories of players.12,7·8·27 1 With reference to body composition in percent fat and lean body­ weight, similar results were also found. The line­ men  exhibited  higher  values  for percent  fat and lean bodyweight than the backs, who statistically displayed  significantly  lower values.12,5,7,8,24,271

Linebackers once again appeared to display values midway between these 2 groups, although in some of these studies, linebackers showed more similar­ ities to backs while in other studies they had more in common with linemen (table I). This trend indicates the uniqueness of the linebacker position and therefore establishes these players as a distinct group.1281  In  fact, Wilmore  et al.f 8l  observed that professional linebackers were similar in size and body composition to world-class discus throwers as reported by Fahey et al.1291 Based on previous findings, it may be suggested that the  recom­ mended optimal percent body fat for college  foot­ ball players is as follows: defensive backs IO, offensive backs 12, linebackers 13, defensive line­ men  15, and offensive linemen 17%.161

5. Strength

 

Strength training has been the cornerstone of football player development within the last 20 to 30 years. Strength can be defined as the maximum force that can be generated by  a muscle or   group of muscles  against a resistance.I 1 1 , 1 7 1  Very  often when evaluating football players, the 1 repetition maximum ( 1 RM) is used. This method refers to the maximum amount of weight that can be   lifted during 1 complete dynamic repetition of a particular movement.191 The most commonly utilised ex­ ercises are the bench press, the squat and the power clean, with the bench press being a test to evaluate upper body strength, the squat used to assess lower body  strength  and the power clean to test  overall strength   and  explosive  power.1281   Studies   that utilised dynamic methods have produced strength norms and averages for the National Collegiate Athletic Association (NCAA) division I, II, and III university football players. It has been found in these  studies  that  linemen  (offensive  and defensive) have significantly higher absolute strength values  than  backs.128·30·31 1  These  findings also demonstrate a tendency for linebackers to fall mid­ way between the linemen and backs, which is con­ sistent with respective size and body composition values. In a study that examined a number of  performance variables in division I -AA football play­ ers, Barker et aJ.132 1 found that offensive and defensive  backs  had  statistically   lower  values  for   a I -RM lift in the squat exercise than  defensive  line-   differences  existed  between  any  of  the  positions.  men. However, when strength was evaluated as the Black and Roundyl 331 demonstrated  the consistent  number of repetitions performed  at a percentage of   trend with respect to absolute strength as division  I -A each  player’s  respective   I -RM  (70  and  90%),  no         defensive  linemen  and  offensive  guards displayed higher mean values for the bench press and back squat as compared with cornerbacks and wide re­ ceivers. As depicted in table II, there also appears to be a difference in strength levels between NCAA division I and division II college football players. Division I football players displayed statistically greater values for bench press and power clean strength than both division II and III players. Divi­ sion II players were also found to have statistically greater bench press strength values than division III playersJ 31 I

With respect to relative strength, which is the ratio between an athlete’s absolute strength and his bodyweight, division II backs exhibited higher val­ ues that those of the linemen.13°1 Similar differ­ ences in relative strength in division I-AA were also observed by Barker et aL,(321 who found that offensive backs and linebackers displayed higher values than offensive linemen. Contrary to these findings, Olson and Hunter[ 281 have shown that in division I football players, linemen exhibited the highest val ues for relative strength. The greater rel­ ative strength values obtained by the linemen in the Olson and Hunter[ 281 study have been attributed to differences in the intensity and type of training reg­ imen, variable emphasis on strength training or to genetic selection. Furthermore, because human muscle has been shown to generate a constant (spe­cific) tension of approximately 30 N/cm 2 of muscle cross-section, the muscular mass of the linemen is likely to be higher than that of players in other positions.[ 34-3s1

In a study conducted on National Football League (NFL) players by Wilmore et aL,[81 a sim­ ilar trend appeared between backs and linemen with respect to absolute strength. Offensive and defensive linemen were stronger than offensive backs, receivers, and defensive backs with respect to the standing press, the curl and the bench press.181 Linebackers in this study, however, had similar strength val ues to those of the linemen and in fact, exhibited a higher average value for the bench press than did either offensive or defensive linemen. The linebackers also demonstrated higher average strength val ues for the standing press than the offensive linemen. Higher values of percent body fat observed in the linemen as compared with the linebackers may have played a significant and detrimental role in dynamic strength perfor­ mance.181 Although this explanation is purely spec­ ulative, a lower level of percent body fat may have the potential for improving the efficiency of move­ ment ultimately enhancing muscular activation and, therefore, strength performance.

Another method that is used when assessing strength levels in football players is through isokinetic dynamometry, which evaluates muscle force against accommodating resistance at a prede­ termined angular velocity.1391 A study that exam­ ined division  II football players found similar  re­sults to the previous findings of Mayhew et al.f26.30J

Using a Cybex II dynamometer,  distinct  differ­ ences were found between linemen and backs in regards to quadriceps and hamstring peak  torque with linemen tending to be stronger than backsJ 26 1 However, when peak torque was determined per kilogram of  bodyweight,  backs  demonstrated higher values than  the  linemen.1261

Although it seems evident that a greater magni­ tude of strength is required to play the position of offensive or defensive lineman, the literature ap­ pears to support the notion that higher levels of body fat represent a factor limiting performance as shown by the lack of positional differences in rel­ ative strength. Although linemen seem to display higher values for lean body mass (table I), the spe­ cific tension generated by skeletal muscle suggests that these players should demonstrate greater rela­ tive strength scores than backs. However, this does not appear to be the case. Further support for the adverse effects of excessive body fat are presented later in this review. Football is not just a game of size and strength but also of speed and power. The examination of speed and the role of cardiovascu­ lar endurance may help to delineate this observed trend in grouping football players.

Table I. Body composition values and sizes of university and professional American football players

Study	Type	n	Height (cm)	Weight (kg)	Body fat (%)	Lean body mass (kg)
Wickkiser & Kellyl61	University
	DB	15	178.3	77.3	11.5	68.4
	OB & WR	15	179.7	79.8	12.4	69.6
	LB	7	180.1	87.2	13.4	75.4
	OL & TE	13	186	99.2	19.1	79.8
	DL	15	186.6	97.8	18.5	79.3
Wilmore & Haske1117l	Professional
	DB	4	184.4	85	7.7	78.4
	OB & WR	10	184.2	91.8	8.3	84.1
	LB	6	189.7	107.6	18.5	87.7
	OL& TE	12	193.5	113.2	15.5	95.4
	DL	12	192.2	120.6	18.7	97.7
Smith & Byrd13l	University
	DB	4	183.7	80.4	9.6	72.7
	OB	5	181.5	183.1	13.8	71.6
	OL	11	189.2	97.9	14.6	83.5
	DL & LB	7	188.8	99.9	14.3	85.6
Wilmore et al.18l	Professional
	DB	26	182.5	84.8	9.6	76.5
	OB & WR	40	183.8	90.7	9.4	81.9
	LB	28	188.6	102.2	14	87.6
	OL & TE	38	193	112.6	15.6	94.7
	DL	32	192.4	117.1	18.2	95.8
Burke et al.124J	College
	Backs	20	181.4	85.5	13	74.4
	Linemen	33	187.3	101.6	21.8	79.5
White et al.15l	College
	DB	8	178.9	77.6	7.3	72
	OB	17	179.5	81.8	11.5	72.2
	LB	6	181.8	90.2	11.6	79.6
	OL	13	185.9	99.7	14.8	84.7
	DL	14	183.1	96.6	13.2	83.5
Gleim12l	Professional	51
	DB & WR		173.4	83.6	5.7
	OB		183	90.7	9.6
	LB & TE		189.2	103.8	12.5
	OL & DL		191.2	117.6	17
Housh et al.1251	University
	DB	14	172	83
	OB & WR	14	172.4	81.2
	OL	13	174.8	110.2
	DL	14	173.8	100.1
Seiler et al.l27J	University
	Backs	17		83.6	9.7
	Linebackers	11		99.3	12.5
	Linemen	13		117	16.5

Abbreviations: DB= defensive back; DL= defensive lineman; LB = linebacker; n = sample size; OB= offensive back; OL = offensive lineman; TE = tight-end;  WR = wide receiver.

 

6. Speed and Anaerobic Power

Speed can be defined as the capacity to travel or move quickly from one point to another,   whereas power can be defined as mechanical work per­ formed per unit of time.l9,,,,4o1 Since the predominant energy pathways utilised in football are the PCr and anaerobic glycolytic systems, the 40yd (36m) dash has been used as the standard test of football speed. However, the applicability of this test has been questioned since there are few times a player actu­ ally runs 40yd during a game.l 25lA study by Crews and Meadorsl 25l on 48 university football players revealed high positive relationships  between  5 (4.5m) and 40yd run times and between 15 ( 13.Sm) and 40yd run times. Therefore, it  was  concluded  that performance in a 40yd run is representative of how fast a player can move at 5 and I Syd and was therefore  deemed  to be  an  appropriate  test  of football speed. However, Seiler et al.r27 1 found low to moderate relationships between the mean running velocity after Syd and the mean running velocity in the final 35yd of a 40yd dash in division I college football players. These results subsequently led to the conclusion that the 40yd run may not be an accurate predictor of initial running velocity.1271 The confounding results in these studies indicate the necessity of utilising sport-specific as well as position-specific field tests that are more accu­ rately related to football performance.

Studies carried out of 40yd dash times of pro­ fessional and university football players have yielded results that have grouped players into the categories previously suggested (table IIl).12,28,30,331

When examining division I college football play­ ers, wide receivers, offensive backs and defensive backs  displayed  the fastest  times  over 40yd with defensive and offensive linemen having the slowest times.l28,331 Linebackers, once again, tended  to fall midway between these 2 groups, This trend was also evident from an evaluation of 51  professional  NFL  players  by  Gliem,l 21 who  found  that defensive backs and wide receivers had the fastest times  followed  in order by  offensive backs, linebackers and tight ends, and finally offensive and defensive linemen. Mayhew et a1.t 3o1 also observed this trend as division II backs demonstrated faster times over 40yd than linemen. More recently, how­ ever, Barker et aJ.l32 l utilised a number of different running tests to evaluate positional group differ­ ences: the 5, I O and 300yd shuttle ru n tests, Their results revealed that offensive linemen displayed slower times over 5 and l Oyd as well as the 300yd shuttle run than offensive backs, defensive  backs and linebackers. Defensive linemen also displayed slower times during the 300yd shuttle run than the offensive   backs   but   did   not   display statistically different  times  than  the  backs  for the  5 and  I Oyd backs as opposed to offensive linemen.l321 Further­ more, offensive linemen were found to have lower val ues for vertical displacement than lineback­ ers.1321 These findings appear to be consistent  with those  of  Black  and  Roundyl 33 1   who  also  found higher vertical jump values  for  defensive  backs  and wide receivers  as opposed  to  linemen.  Barker et aJ.1321 attempted  to explain  the observed  differences in vertical jump power between the offensive linemen and backs based on the following factors:

1. the offensive linemen had the highest percent body fat of any other position thereby reducing jumping efficiency;
2. the offensive linemen demonstrated the lowest relative strength scores.

Results obtained by Mayhew et aJ.141 I lend support to these notions as significant negative correlations were observed between percent body fat and anaer­ obic  power  corrected  for  bodyweight  in college shuttle runs.1321 Black and Roundyl 331 also found this trend with respect to the 40yd dash, as offens­ ive and defensive linemen displayed slower times than cornerbacks and wide receivers. Comparisons in 40yd  run  times  have demonstrated differences between division I, II and III college football play­ ers.1311 Fry and Kraemer131 I reported that division I and II football players displayed faster ru n times over 40yd than their division III counterparts. Sim­ ilar to the highest strength values observed in divi­ sion I players over division II and III players, it appears that a continuum from division I to III ex­ ists regarding athletic fitness and performance. Such a trend may be attributed to superior player selection and recruitment as well as variable advancements in training.

Table III. Mean 40yd (36.6m) dash times (sec) for university and professional American football players

Professional

DB, WR                     4.58 ± 0.12

OB                            4.81 ± 0.21

LB, TE                       4.93 ± 0.14

OL, DL                      5.08 ± 0.21

University, Division 1

WR                           4.60

OB                            4.63

DB                            4.64

LB                             4.78

OL                            4.98

DL                            5.08

University, Division II

Backs                        4.91 ± 0.22

Linemen                     5.22 ± 0.26 University  division 1A

Cornerback                 4.48 ± 0.12

WR                           4.46 ± 0.11

DL                             4.99 ± 0.19

OL                             5.08 ± 0.15

University

Division I                    4.88 ± 0.27

Division II                   4.92 ± 0.26

Division Ill                  4.96 ± 0.27

Abbreviations: DB = defensive back; DL = defensive lineman; LB = linebacker; OB = offensive back; OL = offensive lineman; TE = tight-end; WR = wide receiver.

The assessment of power in American football players has often been accomplished by the  stand ing vertical jump  test or the use of a power index. The ability to jump higher has been observed  to  be significantly greater for offensive and defensive players. Furthermore, significant positive correla­ tion  coefficients  were also demonstrated between percent body fat and agility, 10 and  40yd  dash times. l41 1  Seiler et al.l27 1 continued  to observe this trend as backs generally had greater values for an­ aerobic power corrected for bodyweight, as ob­ tained through the Wingate and  Margaria-Kalamen tests, than linebackers and linemen. Although the linemen observed by Seiler et al.l 27 1  generated   the highest absolute power indices of the 3 groups of players, their values appeared consistently lower than backs and linebackers when corrected for bodyweight. Since power production over repeated plays during the duration of a football game may be the key factor for successful performance on the offensive line, drastic improvements for this vari­ able may be possible through changes in body com­ position.

 

7. Cardiovascular Endurance

 

Endurance can be generally defined as the capacity to perform a type of activity which involves many muscle groups and systems for a   prolonged period  of  time.1 17 1  The most  common  method of measuring cardiorespiratory function  in  athletes is an incremental treadmill test that measures the rate of oxygen consumption (v’02) This measure can be defined as the functional capacity of the cardiorespiratory system to deliver blood to the working   muscles   during   maximal   and   supramaximal (> 100% v’02max) work while maintaining mean arterial blood  pressure.191 It appears that football players generally do not have a well developed cardiovascular system as compared  to  athletes  in  other  sports.17,81   In fact, with specific reference to v’02max, university and professional football players demonstrate similar values to those of age-matched controls.12,1 8,281 Since football is an anaerobic-type sport, the role  of cardiovascular development has not been emphasised in training programmes for these players. It may be for this reason that there does not appear to be a distinct trend between groups of players as far as v’02max is concerned although backs  tended  to have  higher  relative  values than linemen (table IV). These differences in v’02max may explain similar findings by Barker et aI.1321 who demonstrated that offensive  and  defensive backs had faster times over a 1.5 mile (3.2km) run  test  than  offensive and defensive  linemen,  and linebackers. These observations reflect differences in the aerobic capacity between  the different groups of players as well as demonstrating a trend among the linemen who displayed higher values of percent body fat. However, in addition to aerobic cardiorespiratory function, physiological fitness as measured through blood lactate analyses has lacked documentation within the literature and may there­ fore be an important factor in demonstrating adaptive characteristics in American football as well as examining relationships with performance. Sub­ sequent sections of this review will attempt to associate findings regarding size, body composition, strength, speed and endurance with performance requirements of different positions.

 

8. Positional Requirements

 

Based on measurements of size, body composition, strength, speed and endurance, 3 distinct groups of football players may be established: (i) offensive and defensive linemen; (ii) defensive backs, offensive backs and wide receivers; and (iii) line­ backers and tight-ends. The position of tight-end may also represent a transitional  group along with linebackers, although they have been grouped with either the offensive backs, the offensive linemen or the linebackers. Since the position of tight-end re­ quires the player to make blocks on defensive line­ men as well as to run down field to catch passes as a receiver, it would seem justifiable to place these players in a category with the linebackers as a transitional group between backs and  linemen.

The results of the different physical parameters previously cited appear to be representative of the requirements for different positions. For example, success while playing the offensive and defensive line  depends  on  the ability  to execute  the movements of charging, blocking and tackling with greater force and greater speed of execution.142.431

Moreover, linemen must have a high degree of  instantaneous strength and, in the case of the defensive lineman, be able to move quickly, hit the offensive opponent with considerable impact and then be able to move away quickly to the point of action.(7.431 These facts clearly demonstrate that playing football, especially at the line position, is dependent upon power which is related to maxi­ mum strength. Since maximum strength is one component that should be developed in all football players, it should be combined with the element of speed in order to produce a greater degree of power, particularly  in the case of the lineman.

The positions of offensive back, defensive back, and wide receiver have been characterised as those having the lowest values for size and strength, a low percentage of body fat, yet demonstrating the fastest times over 40yd. With respect to the position of defensive back, quick, agile movements are required as well as a great deal of manouverability and speed in order to cover wide receivers _[ 3•71 These necessary qualities of the defensive  back may offer an explanation as to why these players have one of the lowest percentages of body fat, since excessive levels of body fat may be detrimental to playing this position. Receivers and offensive backs also require a great deal of speed and finesse. As  a result,  strength  development  may  not have been emphasised for these players. However, the lower relative absolute strength scores that appear to be characteristic of defensive backs and receivers suggest that this is one area of development that may need to be improved, since strength enhancement in the proper manner can increase power as long as a relatively low percentage of body fat is maintained.

The position of linebacker has been  described as the core position of the defence aimed primarily at tackling the ball-carrier should they get past the line of scrimmage. Linebackers are also responsi­ ble for covering tight-ends and offensive backs running downfield to receive passes. Therefore, it appears evident as to why those who play the line­ backer position have size, strength and speed val­ ues that fall midway between those of the backs and the linemen. These players are required to per­ form duties similar to those of the defensive line­ men and defensive backs. As a result, an equal de­ velopment of power, strength, speed, endurance and agility is optimal for linebackers since their duties range from contacting offensive linemen and tackling running backs to running downfield  to cover receivers and tight ends.

 

The 3 distinct groups of players that have been established in this review can be used to develop a training programme for a football team keeping in mind the predominant energy systems and the characteristics of each position. Although more differences exist between specialty positions (e.g. a wide receiver spends more time catching footballs then a defensive back), dividing players into the 3 groups may be beneficial when the objectives are to develop strength, speed and endurance. How­ ever, when designing such a programme based on these findings, 2 factors that should receive more consideration are body composition and cardiovascular fitness. In the past, these 2 aspects of training have been somewhat neglected. What appears to be uncertain at the present time, however, is whether improvement in these 2 areas can facilitate better performance.

 

 

 

 

Table IV. Endurance capacity (V02max) of university and professional American football players

Study	Type	N	Umin	ml/kg/min
Wilmore & Haskelll71	Professional
	DB	2	4.5	54.5
	OB, WR	2	5.1	52.4
	LB	3	5.4	51.1
	OL, TE	4	6.2	52.6
	DL	4	5.6	43.5
Wilmore et al.181	Professional
	DB	25	4.5 ± 0.4	53.1 ± 6.2
	OB. WR	39	4.7 ± 0.5	52.2 ± 5.0
	LB	28	5.3 ± 0.6	52.1 ± 4.9
	OL, TE	35	5.6 ± 0.8	49.9 ± 6.6
	DL	27	5.3 ± 0.6	44.9 ± 5.4
Smith & Byrdl31	University
	DB	4	4.77 ± 0.30	59.3 ± 1.00
	OB	5	5.00 ± 0.37	60.2 ± 4.27
	OL	11	5.44 ± 0.60	55.9 ± 7.41
	DL, LB	7	5.28 ± 0.68	53.2 ± 7.32

Abbreviations: DB = defensive back; DL = defensive lineman; LB = linebacker; OB = offensive back; OL = offensive lineman; TE = tight end; WR = wide receiver.

 

9. Performance Improvement

 

The utilisation of tests designed to assess per­ cent body fat and physiological  fitness have been a necessary tool for many football coaches as a means of monitoring the physical state of an ath­ lete. For example, 3 300lb (135kg) offensive line­ men for a professional team reported early to sum­ mer camp in 1988 specifically for a weight loss and conditioning programme developed by the team’s management.( 44 1  University  football  coaches, also should be aware of the negative effects of a high percent body fat. Excessive body fat has been associated with a reduction in speed, power and en­ durance.1451 Significant negative correlations have been found between percent body fat and 40yd ru n times.17- 251 Crews and Meadors125 1 observed that if players’ weights were higher than their optimal playing weight, they tended to display slower reaction times and slower run times at 5, 15, and 40yd. Further evidence of this finding can also be based on individual case information as well. Wilmore and HaskeJJ!7l examined 2 cases: a 27 llb (22.5% fat) defensive tackle and a 235lb ( 18.8% fat) running back who played for a professional team. After these 2 players were convinced to lower their playing weights, the defensive tackle dropped his weight to 2581b (13.9% fat) and the running back dropped to 2 I 61b ( 1 1.5% fat). Each player had what he and his coaches felt was his best year in professional football while playing at this new reduced weight.171

Another factor that may enhance the performance of football players is the development of cardiovascular fitness. It has been demonstrated that the majority of game injuries occur in the second and fourth quarters, which are the latter portions  of  the  2 halves.144.46.471 Players that demonstrate a reduced ability to utilise oxygen during recovery may increase the likelihood of fatigue toward the later stages of a game and therefore in­ crease their risk of injury.181 Therefore, with a better developed cardiovascular system, players may be better able to maintain a higher performance level throughout a game with relatively less effort, resulting in better play.181 More specifically, endurance training has been shown to increase the capac­ ity of the muscle to extract oxygen,  which  is believed to be primarily due to an increase in cap­ illary density and secondarily to the increase in the myoglobin concentration and  mitochondria  num­ ber. l 1 1 .481 Subsequently the body has a better ability to utilise oxygen to carry out muscular work which results  in  less  PCr  depletion  and  less  lactate  and hydrogen   ion  formation.191  Further  evidence presented by Takahashi et al.1221 found that endurance trained  runners  had  a  significantly  greater   ability to resynthesise intramuscular PCr following severe and exhausting quadriceps exercise than untrained individuals. The combination of improved substrate resynthesis and lactic acidosis buffering would allow the football player to more efficiently utilise PCr as a rapid and immediate source for ATP production during game situations. Furthermore, during practice, less accumulation of lactic acid during submaximal drills would  occur  as a result of a faster rise in oxygen uptake.l 9l Another benefit that a well developed cardiovascular system may provide occurs during summer football practices where thermal load can potentially burden a play­ ers’ physiological system. Since the principal role of aerobic conditioning on the heart is to augment its ability to function as a volume pump, a resulting increase in cutaneous blood flow enhances the   removal of internal heat which may reduce the chance of acute heat stress.f2A91 As a result of these adaptations, incorporating aerobic training may help prevent injuries, improve performance, de­ crease the chance of fatal nontraumatic collapse, and condition the athlete to lead a healthier life after retirement. [21

 

10. Conclusion

 

Research into the literature has yielded some very interesting and useful information regarding American football. Firstly, American football can be classified as an intermittent type sport that primarily utilises the PCr system for its energy supply with secondary involvement of anaerobic glycolysis. In addition to this, 3 basic groups of players exist on a typical football team: (i) offensive and defensive linemen; (ii) offensive backs, defensive backs and wide receivers; and (iii) linebackers and tight ends. Based on parameters such as size, body composition, strength, speed and endurance while excluding some position-specific variables such as catching or throwing, an optimal training programme can be developed for each group of players in order to improve their performance. However, more attention should be directed towards the elements of body composition and cardiovascular endurance since development in these 2 areas may improve performance for the reasons previously cited. In addition, the improvement of body com­ position with a decline of excessive weight may reduce the risk of coronary artery disease, hyper­ tension, stroke and diabetes.l501 Aerobic conditioning assists with the problem of excessive weight since cardiovascular training increases the utilisation of free fatty acids for energy production.[ 91 Not only may an increased emphasis in these areas help to improve performance, but it may also assist one to live a more healthy  lifestyle after football.

Acknowledgements

 This paper is dedicated to the memory of Edgar R. Nowalkoski, MSc, ATC, CAT (C).

Original article: American Football Physiology by Dott. Tudor Bompa

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