Sports Medicine 14 (5): 277-288, 1992 0112-1642/92/0011-0277/$06.00/0 © Adis International Limited. All rights reserved. SP01180
Strength, Flexibility and Athletic Injuries l Joseph J. Knapik, Bruee H. Jones, Connie L. Bauman and John MeA. Harris
Occupational Medicine Division, US Army Research Institute of Environmental Medicine, Natick, Massachusetts, and Wellesley College, Wellesley, Massachusetts, USA
Sports medicine professionals have long suspected that particular deficits in strength or flexibility are risk factors for athletic injuries. It has been assumed that stronger and/or more flexible athletes are less likely to get injured (Cureton 1941; Klafs & Arnheim 1973). It has also been suspected that imbalances in strength and flexibility contribute to the likelihood of athletic injuries (Ferguson & Bender 1964; Klafs & Arnheim 1973; Klein 1971). Our primary purpose in this article is to critically review the scientific evidence of an association between strength, flexibility and athletic injuries. Prior to this review we present the criteria we used to evaluate studies in this area. We also examine the potential confounding influence of previous injuries and the evidence that injuries can be reduced by correcting muscular imbalances through strength training. We have included previously unpublished data that relate to some of these topics (Knapik et al. 1991). I The views, opinions, and/or findings contained in this report are those of the authors and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other official documentation. Human subjects participated in this study after giving their free and informed voluntary consent. Investigators adhered to AR 7025 and USAMRDC Regulation 70-25 on Use of Volunteers in Research. Citations of commercial organisations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organisations. Approved for public release; distribution is unlimited.
1. General Considerations This article evaluates studies based on considerations of: study design; measures of strength and flexibility; type of sport or sports; definition of injury; injury risk and relative risk; statistical analysis; and sample size. Each of these is discussed below. 1.1 Study Design The primary focus of this article is on prospective studies. Prospective studies attempt to identify possible deficits in strength and flexibility before the actual occurrence of the injury. Once an injury has occurred, it is not possible to determine whether the deficit predisposed the athlete to the injury, or the injury caused the deficit. This issue is complicated by previous injuries an athlete may have had. The athlete 'may have fully recovered from t~~ injury and it does not affect their athletic performance; however, the previous injury may still result in a strength or flexibility deficit. 1.2 Measures of Strength and Flexibility (Risk Factors) To determine the influence of a risk factor on injury, that risk factor must be defined so it is clearly understood. Strength is the maximum force or torque exerted by a muscle group in a single voluntary contraction (Knapik et al. 1983). Strength
278
imbalances involve either a difference in strength between the muscle groups on the right and left side of the body (R/L imbalances) or a low strength ratio between an agonistic muscle group and its antagonist. Antagonist/agonist imbalances have been studied mainly in the knee flexor (hamstring) and knee extensor (quadriceps) muscle groups. Strength can be measured using a variety of methods that involve isometric, concentric or eccentric muscle contractions (Knuttgen & Kraemer 1987). Studies on strength and injuries have measured strength during isometric (e.g Burkett 1970; Ekstrand & Gillquist 1983) or concentric contractions. Concentric strength has been measured in 2 ways: isokinetically, through the use of a special dynamometer (Hislop & Perrine 1967) that controls the velocity of movement and allows the subject to exert as much torque as possible at the preset velocity (e.g. Grace et al. 1984; Knapik et al. 1991); and as the maximal weight that subjects can lift (e.g. Cowan et al. 1988: Laird 1981). Aexibility is the amount of movement of a joint through its normal plane of motion. Hypermobility is an aspect of flexibility where joints are moved in their normal plane of motion but beyond some predefined extreme point. A flexibility imbalance involves a difference in the range of motion between the right and left side of the body. Most studies on flexibility and injuries examined flexibility at specific joints using a goniometer" to measure the range of motion (e.g. Agre & Baxter 1987; Ekstrand & Gillquist 1983). Other studies employed a simple technique called the sit-andreach test which measures flexibility across several joints (e.g. Burkett 1970; Cowan et al. 1988). 1.3 Type of Sport Different sports and even different player positions within a sport may result in unique relationships between strength, flexibility and injury risk because of the type of movements required. For example, extremes in flexibility may be desirable and even necessary for a gymnast who performs repeated and well practised movements requiring wide ranges of motion. The same degree of
Sports Medicine 14 (5) 1992
flexibility may be detrimental for a soccer or football player. Also, in some sports like tennis or high jumping, athletes develop more strength in one limb than another. Such strength imbalances are due to the greater use of one limb and injuries may be more likely to occur in the stronger limb because of greater use rather than the imbalance. Sports that involve similar movement patterns might be expected to have similar relationships between strength, flexibility and injuries. For example, sports like soccer, lacrosse and field hockey all involve relatively continuous open field play, lateral movement and body contact. Participation in these sports may result in similar types and patterns of injuries. Studies on strength, flexibility and injuries have examined many different athletic populations. Some investigations have concentrated on specific sports like soccer (e.g. Agre & Baxter 1987; Ekstrand & Gillquist 1983), football (e.g. Godshall 1975; Grace et al. 1984) or track and field (Liemohn 1978). Other studies have examined more general athletic populations (Knapik et al. 1991; Lysens et al. 1984). 1.4 Injury Definition To determine the influence of a risk factor on injuries, the injuries of interest must be defined so they can be recognised. It is not important that all studies define injuries in the same manner; it is important that the definition be clearly explained and understandable (Powell et al. 1986). It is possible that either specific injuries or clusters of injuries may be related to different aspects of strength and/or flexibility. For example, muscle strains (a specific injury) may be more likely to occur in athletes who lack flexibility. Sprains, strain or dislocations (cluster of injuries) may be more likely among highly flexible athletes. Studies on strength, flexibility and injuries have used many different operational definitions of injury. These include specific types of injuries (e.g. Burkett 1970; Merrifield & Cowan 1973), injuries at particular joints (e.g. Ekstrand & Gillquist 1983; Grace et al. 1984) and injuries in specific body areas
Strength, Flexibility and Injuries
(Cowan et al. 1988; Knapik et al. 1991). Some studies provided no clear definition (e.g. Grana & Moretz 1978; Klein 1971). 1.5 Injury Risk Injury risk is the number of individuals injured (the numerator) divided by the population at risk f0r the injury (the denominator). The denominator is extremely important because without it risk of injury cannot be calculated and no assumptions regarding risk can be made (Powell et al. 1986). For example, assume that over 3 seasons a football team suffered injuries to 14 players with muscular imbalances and 36 players without imbalances (numerators). This could lead to the erroneous assumption that athletes with imbalances are injured less often. However, assume that there were only a total of 18 players with imbalances and I SO players with no imbalances (denominators). The risk of injury to players with the imbalance is 78% (14/ 18) and risk to those without the imbalance is 24% (36/1 SO). Relative risk (risk with versus risk without the imbalance) can be calculated as 78/24% = 3.2. Thus, in this example, football players with the imbalance are 3.2 times more likely to be injured than those without the imbalance. Few studies (Cowan et al. 1988; Knapik et al. 1991; Nicholas 1970) examining the association of strength and flexibility with injuries have calculated risk or relative risk. However, several studies (e.g. Burkett 1970; Ekstrand & Gillquist 1983) provide sufficient information for these calculations to be made. Because of the paucity of studies in this area, injury risk and relative risk were calculated wherever possible for the purposes of this article. 1.6 Statistical Analysis To examine the relationship between strength, flexibility and athletic injuries a commonly used statistical technique is the x-square. x-Square separates subjects into groups that either do or do not exhibit a risk factor (e.g. smokers and nonsmokers). If everyone in the population has the risk factor to some extent, subjects can be separated into
279
groups with different levels of the risk factor (e.g. strength or flexibility). The number of subjects injured and those not injured in each group are compared. For example, subjects may be separated into those that have R/L strength difference ~ 10% and those that have differences > 10%. The x-square statistic tests the assumption that there is no difference in injury incidence between the 2 groups (Hennekens & Buring 1987). A second but possibly misleading technique used by some investigators is to separate subjects into those injured and those not injured and look at the mean value of the risk factor in each group. The assumption that there is no difference between the groups is often examined using a t-test (Edwards 1972). While this technique may show differences between groups, it cannot be used to make assumptions or conclusions about injury risk. Subjects with and without the risk factor (or different levels of the risk factor) may be included in both groups. 1.7 Sample Size
The number of subjects required to demonstrate a statistically significant relationship between strength, flexibility and injuries can be determined by sample size estimation techniques (Cohen 1977). The required number of subjects is based on 3 factors: the proportion of the population with the risk factor; the size of the difference in injury incidence between those with and those without the risk factor; and the amount of error the investigator is willing to accept (Hennekens & Buring 1987). As an example, assume the researcher is interested in examining the relationship between R/L strength imbalances and the incidence of sprains and muscle strains. The literature suggests that about 20% of the population has the risk factor (Grace et al. 1984; Knapik et al. 1991). The incidence of strains and sprains is likely to be 45% in subjects at risk (Cowan et al. 1988; Knapik et al. 1991) and 20% in subjects not at risk (Cowan et al. 1988; Garrick 1977; Knapik et al. 1991; Splain & Rolnick 1984). On the basis of these figures about
Sports Medicine 14 (5) 1992
280
180 athletes will be required, at an a error level of 0.05 and {j error level of 0.20 (Cohen 1977). A lower prevalence of R/L imbalances in the population or a lower injury rate will obviously require a greater number of athletes. Published studies on strength, flexibility and injuries have reported sample sizes ranging from 23 (Laird 1981) to 628 (Meeuwisse & Fowler 1988).
2. Studies on Strength and Injuries Table I displays studies that have examined relationships between athletic injuries, strength and strength imbalances, showing the number of subjects, type of sport and injury definition. Discussed below are the strength measures, injury risk and the statistical analysis in these investigations. Studies that have compared mean values in injured and noninjured groups are treated only briefly because inferences from their data must be interpreted with caution (see section 1.6). 2.1 Strength and Injuries The relationship between strength and risk of injury is not clear. The only study that compared
Injury risk in groups of different strength was Cowan et al. (1988). They measured the maximal weight subjects could lift from the floor to head height. They found that risk of injury was similar in strong and weak subjects in terms of both absolute strength and strength per unit body mass. In our study of female collegiate athletes (see Knapik et al. 1991 for details) we separated subjects into quartiles of strength and looked at the number of subjects in each quartile who suffered lower extremity injuries (table II). We found no relationship between absolute strength and injuries, although there was a tendency for stronger subjects to be injured more often. Athletes with more strength per unit body mass at the faster isokinetic velocity were more likely to get injured. Although we are not able to fully explain this finding, it may be that various structures within the joints or even the muscles themselves can be damaged by high forces developed by the knee extensor and knee flexor muscle groups when those muscles are very strong relative to body mass. However, it may also be that stronger athletes play the sport more often than weak ones and the additional exposure results in the higher injury incident.
Table I. Studies examining relationships between strength. strength imbalances and injuries Study
No. of subjects and type of sport
Injury definition
Burkett (1970)
37 professional male football players 502 football players 54 high school and college hockey players 27 track and field athletes 23 soccer players 180 male soccer players (age 25 ± 5y) 172 male high school football players (age 13-18y) 25 college soccer players
Hamstring strains
Klein (1971) Merrifield & Cowan (1973) Liemohn (1978) Laird (1981) Ekstrand & Gillquist (1983) Grace et al. (1984)
Agre & Baxter (1987) Cowan et al. (1988) Knapik et al. (1991)
303 male army basic trainees 132 female college athletes in 8 sports (age 19 ± 1y)
Not clearly defined Hip adductor strains Hamstring strains Not clearly defined Time-loss contact and noncontact knee injuries Time-loss knee injuries
Noncontact low back strains and knee sprain Lower body and back musculoskeletal problems requiring medical attention First occurrence of a time-loss lower extremity injury
Strength, Flexibility and Injuries
281
Table II. Risk (%) of lower extremity injuries to female athletes by quartiles of strength and strength corrected for body mass Measure
01 (low)
02
27 25 24 34 19 21 27 27
43 27 27 28 38 33 21
34 40
46 39 46 41 44 41 41
36
34
33
03
04 (high)
p-Valuea (x-square)
Strength KE KE KF KF KE KE KF KF
300 /sec (R) 300 /sec (L) 300 /sec (R) 30° /sec (L) 1800 /sec (R) 180°/sec (L) 180°/sec (R) 180°/sec (L)
24 40 33 27 31
0.25 0.39 0.25 0.60 0.15 0.33 0.24 0.87
Strength/body mass b KE 300 /sec (R)
39 32
KE 300 /sec (L) KF 300 /sec (R)
29 36 27
22
26
42
30 30
32
36
27 21
KF KE KE KF KF
30·/sec (L) 1800 /sec (R) 180·/sec (L) 180·/sec (R) 180·/sec (L)
38 24 29 37 24
a
Overall probability of an association in 2 x 4 x-square analysis (n
29 21 35
sports). b Strength values were divided by body mass. Abbreviations: KE knee extensors; KF knee flexors; R
=
=
24 23 18 42
0.23 0.96
42
0.55
35 55
0.43 0.02
50 51 29
0.05 0.03 0.43
= 138 female collegiate athletes in 8 weight-bearing varsity
= right; L = left; 01,2,3,4 = first, second, third, fourth quartile.
Grace et al. (1984) found that injured subjects had greater knee flexion strength; however, they presented only average strength values for injured and noninjured groups and injury risk could not be calculated from their data. 2.2 Strength Imbalances and Injuries 2.2.1 Right/Left Side Strength Imbalances Some studies suggest that R/L imbalances are associated with greater risk of sprains and muscle strains. However, sample sizes in most of these studies were small and few used appropriate data analysis. Burkett (1970) measured the isometric strength of the knee extensor and knee flexor muscle groups using cable tensiometer methods. His data show that athletes with knee flexor R/L imbalance of > 10% were 20.0 times more likely to suffer a
hamstring strain in the weaker leg than subjects without this imbalance. Merrifield & Cowan (1973) measured the isokinetic peak torque and power of the hip extensors. Eight subjects with R/L imbalances of at least 25% subsequently suffered adductor strains and in all 8 the injured leg was the weaker. Knapik et al. (1991) measured isokinetic knee extension and knee flexion peak torque. They found that athletes with knee flexion R/L imbalances > 15% were 2.6 times more likely to suffer lower extremity injuries than subjects without this imbalance. Lower body sprains and strains were more common on the weaker side. On the other hand, Laird's (1981) results indicated no differences in injury incidence between athletes with R/ L strength imbalances < 10% and those with imbalances> 10%. He measured strength as the maximal weight that could be lifted with the knee flexor and knee extensor muscle groups.
282
Sports Medicine J4 (5) 1992
Table III. Studies examining relationships between flexibility, flexibility imbalances, hypermobility and injuries Study
No. of subjects and type of sport
Burkett (1970)
36 pro football players
Hamstring strains
Nicholas (1970)
139 pro football players 426 high school football players High school football players
Third degree knee sprains
Clark et al. (1971) Godshall (1975) Jackson et al. (1978)
Grana & Moretz (1978)
32 male high school basketball players 27 track and field athletes
Liemohm (1978) Moretz et al. (1982) Ekstrand & Gillquist (1983) Lysens et al. (1984) Agre & Baxter (1987) Meeuwisse & Fowler (1988) Cowan et al. (1988) Knapik et al. (1991)
Abbreviation: pro
850 West Point cadets 108 high school football players College football players 166 high school football players 84 female high school basketball players
155 football players 180 male soccer players
Injury definition
Ankle and knee injuries Knee injuries requiring surgery or casting Ankle, knee, shoulder and elbow injuries Not clearly defined
Hamstring strains Knee sprains Lower extremity muscle ruptures or tendinitis
138 physical education students (age 18 ± 1y) 25 college soccer players 628 male and female athletes in 15 collegiate sports 303 army basic trainees (age 17-35y) 132 female college athletes in 8 sports (age 19 ± 1y)
Not clearly defined Low back strains and knee sprains Hamstring strains, patellofemoral syndromes Lower body and back musculoskeletal problems requiring medical attention First occurrence of a time-loss lower extremity injury
= professional.
Five other studies reported only average R/L imbalance values in injured versus noninjured groups. Injury risk could not be calculated from their data. Klein (1971, 1974) and Ekstrand and Gillquist (1983) found that mean R/L imbalances were larger in injured subjects but Liemohn (1978), Agre and Baxter (1987) and Grace et al. (1984) found no differences between injured and non injured subjects. Right and left side strength imbalances of the knee extensor and knee flexor muscle groups are not uncommon. Imbalances of 10% or more occurred in 31 to 41 % of male and female athletes (Burkett 1970; Knapik et al. 1991; Grace et al. 1984). Imbalances of 15% or more were seen in 20 to 30% offemale athletes (Knapik et al. 1991). Imbalances of 20% or more were seen in 3 to 18% of male and female athletes (Grace et al. 1984; Knapik et al. 1991).
2.2.2 Antagonist/Agonist Strength Imbalances A large number of studies (Appen & Duncan 1986; Murray et al. 1980; Stafford & Grana 1984; Wyatt & Edwards 1981) reported on antagonist/ agonist ratios of the knee flexors/knee extensors because of an assumed relationship between this ratio and athletic injuries (Cooper & Fair 1978; Garrett 1983; Knight 1980). Reported normative values range from 0.43 to 0.90 (Nosse 1982). This wide range probably results from a lack of standardisation in terms of the type of strength measured (isometric versus isokinetic), joint angle, body position, body stabilisation and other factors (Nosse 1982). Interpretation of isokinetic studies is further complicated by the fact that many have not controlled for gravitational moments or changes in the angle of peak torque as the contractile velocity increases (Prietto & Caiozzo 1989). Compounding these problems is the fact that
Strength, Flexibility and Injuries
283
antagonist/agonist imbalances may actually reflect R/L imbalances. Burkett (1970) reported that athletes with lower isometrically measured knee flexion/knee extension ratios were 20 times more likely to be injured than subjects with higher ratios. Knapik et al. (1991) found that subjects with knee flexion/knee extension ratios
Strength, Flexibility and Athletic Injuries l Joseph J. Knapik, Bruee H. Jones, Connie L. Bauman and John MeA. Harris
Occupational Medicine Division, US Army Research Institute of Environmental Medicine, Natick, Massachusetts, and Wellesley College, Wellesley, Massachusetts, USA
Sports medicine professionals have long suspected that particular deficits in strength or flexibility are risk factors for athletic injuries. It has been assumed that stronger and/or more flexible athletes are less likely to get injured (Cureton 1941; Klafs & Arnheim 1973). It has also been suspected that imbalances in strength and flexibility contribute to the likelihood of athletic injuries (Ferguson & Bender 1964; Klafs & Arnheim 1973; Klein 1971). Our primary purpose in this article is to critically review the scientific evidence of an association between strength, flexibility and athletic injuries. Prior to this review we present the criteria we used to evaluate studies in this area. We also examine the potential confounding influence of previous injuries and the evidence that injuries can be reduced by correcting muscular imbalances through strength training. We have included previously unpublished data that relate to some of these topics (Knapik et al. 1991). I The views, opinions, and/or findings contained in this report are those of the authors and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other official documentation. Human subjects participated in this study after giving their free and informed voluntary consent. Investigators adhered to AR 7025 and USAMRDC Regulation 70-25 on Use of Volunteers in Research. Citations of commercial organisations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organisations. Approved for public release; distribution is unlimited.
1. General Considerations This article evaluates studies based on considerations of: study design; measures of strength and flexibility; type of sport or sports; definition of injury; injury risk and relative risk; statistical analysis; and sample size. Each of these is discussed below. 1.1 Study Design The primary focus of this article is on prospective studies. Prospective studies attempt to identify possible deficits in strength and flexibility before the actual occurrence of the injury. Once an injury has occurred, it is not possible to determine whether the deficit predisposed the athlete to the injury, or the injury caused the deficit. This issue is complicated by previous injuries an athlete may have had. The athlete 'may have fully recovered from t~~ injury and it does not affect their athletic performance; however, the previous injury may still result in a strength or flexibility deficit. 1.2 Measures of Strength and Flexibility (Risk Factors) To determine the influence of a risk factor on injury, that risk factor must be defined so it is clearly understood. Strength is the maximum force or torque exerted by a muscle group in a single voluntary contraction (Knapik et al. 1983). Strength
278
imbalances involve either a difference in strength between the muscle groups on the right and left side of the body (R/L imbalances) or a low strength ratio between an agonistic muscle group and its antagonist. Antagonist/agonist imbalances have been studied mainly in the knee flexor (hamstring) and knee extensor (quadriceps) muscle groups. Strength can be measured using a variety of methods that involve isometric, concentric or eccentric muscle contractions (Knuttgen & Kraemer 1987). Studies on strength and injuries have measured strength during isometric (e.g Burkett 1970; Ekstrand & Gillquist 1983) or concentric contractions. Concentric strength has been measured in 2 ways: isokinetically, through the use of a special dynamometer (Hislop & Perrine 1967) that controls the velocity of movement and allows the subject to exert as much torque as possible at the preset velocity (e.g. Grace et al. 1984; Knapik et al. 1991); and as the maximal weight that subjects can lift (e.g. Cowan et al. 1988: Laird 1981). Aexibility is the amount of movement of a joint through its normal plane of motion. Hypermobility is an aspect of flexibility where joints are moved in their normal plane of motion but beyond some predefined extreme point. A flexibility imbalance involves a difference in the range of motion between the right and left side of the body. Most studies on flexibility and injuries examined flexibility at specific joints using a goniometer" to measure the range of motion (e.g. Agre & Baxter 1987; Ekstrand & Gillquist 1983). Other studies employed a simple technique called the sit-andreach test which measures flexibility across several joints (e.g. Burkett 1970; Cowan et al. 1988). 1.3 Type of Sport Different sports and even different player positions within a sport may result in unique relationships between strength, flexibility and injury risk because of the type of movements required. For example, extremes in flexibility may be desirable and even necessary for a gymnast who performs repeated and well practised movements requiring wide ranges of motion. The same degree of
Sports Medicine 14 (5) 1992
flexibility may be detrimental for a soccer or football player. Also, in some sports like tennis or high jumping, athletes develop more strength in one limb than another. Such strength imbalances are due to the greater use of one limb and injuries may be more likely to occur in the stronger limb because of greater use rather than the imbalance. Sports that involve similar movement patterns might be expected to have similar relationships between strength, flexibility and injuries. For example, sports like soccer, lacrosse and field hockey all involve relatively continuous open field play, lateral movement and body contact. Participation in these sports may result in similar types and patterns of injuries. Studies on strength, flexibility and injuries have examined many different athletic populations. Some investigations have concentrated on specific sports like soccer (e.g. Agre & Baxter 1987; Ekstrand & Gillquist 1983), football (e.g. Godshall 1975; Grace et al. 1984) or track and field (Liemohn 1978). Other studies have examined more general athletic populations (Knapik et al. 1991; Lysens et al. 1984). 1.4 Injury Definition To determine the influence of a risk factor on injuries, the injuries of interest must be defined so they can be recognised. It is not important that all studies define injuries in the same manner; it is important that the definition be clearly explained and understandable (Powell et al. 1986). It is possible that either specific injuries or clusters of injuries may be related to different aspects of strength and/or flexibility. For example, muscle strains (a specific injury) may be more likely to occur in athletes who lack flexibility. Sprains, strain or dislocations (cluster of injuries) may be more likely among highly flexible athletes. Studies on strength, flexibility and injuries have used many different operational definitions of injury. These include specific types of injuries (e.g. Burkett 1970; Merrifield & Cowan 1973), injuries at particular joints (e.g. Ekstrand & Gillquist 1983; Grace et al. 1984) and injuries in specific body areas
Strength, Flexibility and Injuries
(Cowan et al. 1988; Knapik et al. 1991). Some studies provided no clear definition (e.g. Grana & Moretz 1978; Klein 1971). 1.5 Injury Risk Injury risk is the number of individuals injured (the numerator) divided by the population at risk f0r the injury (the denominator). The denominator is extremely important because without it risk of injury cannot be calculated and no assumptions regarding risk can be made (Powell et al. 1986). For example, assume that over 3 seasons a football team suffered injuries to 14 players with muscular imbalances and 36 players without imbalances (numerators). This could lead to the erroneous assumption that athletes with imbalances are injured less often. However, assume that there were only a total of 18 players with imbalances and I SO players with no imbalances (denominators). The risk of injury to players with the imbalance is 78% (14/ 18) and risk to those without the imbalance is 24% (36/1 SO). Relative risk (risk with versus risk without the imbalance) can be calculated as 78/24% = 3.2. Thus, in this example, football players with the imbalance are 3.2 times more likely to be injured than those without the imbalance. Few studies (Cowan et al. 1988; Knapik et al. 1991; Nicholas 1970) examining the association of strength and flexibility with injuries have calculated risk or relative risk. However, several studies (e.g. Burkett 1970; Ekstrand & Gillquist 1983) provide sufficient information for these calculations to be made. Because of the paucity of studies in this area, injury risk and relative risk were calculated wherever possible for the purposes of this article. 1.6 Statistical Analysis To examine the relationship between strength, flexibility and athletic injuries a commonly used statistical technique is the x-square. x-Square separates subjects into groups that either do or do not exhibit a risk factor (e.g. smokers and nonsmokers). If everyone in the population has the risk factor to some extent, subjects can be separated into
279
groups with different levels of the risk factor (e.g. strength or flexibility). The number of subjects injured and those not injured in each group are compared. For example, subjects may be separated into those that have R/L strength difference ~ 10% and those that have differences > 10%. The x-square statistic tests the assumption that there is no difference in injury incidence between the 2 groups (Hennekens & Buring 1987). A second but possibly misleading technique used by some investigators is to separate subjects into those injured and those not injured and look at the mean value of the risk factor in each group. The assumption that there is no difference between the groups is often examined using a t-test (Edwards 1972). While this technique may show differences between groups, it cannot be used to make assumptions or conclusions about injury risk. Subjects with and without the risk factor (or different levels of the risk factor) may be included in both groups. 1.7 Sample Size
The number of subjects required to demonstrate a statistically significant relationship between strength, flexibility and injuries can be determined by sample size estimation techniques (Cohen 1977). The required number of subjects is based on 3 factors: the proportion of the population with the risk factor; the size of the difference in injury incidence between those with and those without the risk factor; and the amount of error the investigator is willing to accept (Hennekens & Buring 1987). As an example, assume the researcher is interested in examining the relationship between R/L strength imbalances and the incidence of sprains and muscle strains. The literature suggests that about 20% of the population has the risk factor (Grace et al. 1984; Knapik et al. 1991). The incidence of strains and sprains is likely to be 45% in subjects at risk (Cowan et al. 1988; Knapik et al. 1991) and 20% in subjects not at risk (Cowan et al. 1988; Garrick 1977; Knapik et al. 1991; Splain & Rolnick 1984). On the basis of these figures about
Sports Medicine 14 (5) 1992
280
180 athletes will be required, at an a error level of 0.05 and {j error level of 0.20 (Cohen 1977). A lower prevalence of R/L imbalances in the population or a lower injury rate will obviously require a greater number of athletes. Published studies on strength, flexibility and injuries have reported sample sizes ranging from 23 (Laird 1981) to 628 (Meeuwisse & Fowler 1988).
2. Studies on Strength and Injuries Table I displays studies that have examined relationships between athletic injuries, strength and strength imbalances, showing the number of subjects, type of sport and injury definition. Discussed below are the strength measures, injury risk and the statistical analysis in these investigations. Studies that have compared mean values in injured and noninjured groups are treated only briefly because inferences from their data must be interpreted with caution (see section 1.6). 2.1 Strength and Injuries The relationship between strength and risk of injury is not clear. The only study that compared
Injury risk in groups of different strength was Cowan et al. (1988). They measured the maximal weight subjects could lift from the floor to head height. They found that risk of injury was similar in strong and weak subjects in terms of both absolute strength and strength per unit body mass. In our study of female collegiate athletes (see Knapik et al. 1991 for details) we separated subjects into quartiles of strength and looked at the number of subjects in each quartile who suffered lower extremity injuries (table II). We found no relationship between absolute strength and injuries, although there was a tendency for stronger subjects to be injured more often. Athletes with more strength per unit body mass at the faster isokinetic velocity were more likely to get injured. Although we are not able to fully explain this finding, it may be that various structures within the joints or even the muscles themselves can be damaged by high forces developed by the knee extensor and knee flexor muscle groups when those muscles are very strong relative to body mass. However, it may also be that stronger athletes play the sport more often than weak ones and the additional exposure results in the higher injury incident.
Table I. Studies examining relationships between strength. strength imbalances and injuries Study
No. of subjects and type of sport
Injury definition
Burkett (1970)
37 professional male football players 502 football players 54 high school and college hockey players 27 track and field athletes 23 soccer players 180 male soccer players (age 25 ± 5y) 172 male high school football players (age 13-18y) 25 college soccer players
Hamstring strains
Klein (1971) Merrifield & Cowan (1973) Liemohn (1978) Laird (1981) Ekstrand & Gillquist (1983) Grace et al. (1984)
Agre & Baxter (1987) Cowan et al. (1988) Knapik et al. (1991)
303 male army basic trainees 132 female college athletes in 8 sports (age 19 ± 1y)
Not clearly defined Hip adductor strains Hamstring strains Not clearly defined Time-loss contact and noncontact knee injuries Time-loss knee injuries
Noncontact low back strains and knee sprain Lower body and back musculoskeletal problems requiring medical attention First occurrence of a time-loss lower extremity injury
Strength, Flexibility and Injuries
281
Table II. Risk (%) of lower extremity injuries to female athletes by quartiles of strength and strength corrected for body mass Measure
01 (low)
02
27 25 24 34 19 21 27 27
43 27 27 28 38 33 21
34 40
46 39 46 41 44 41 41
36
34
33
03
04 (high)
p-Valuea (x-square)
Strength KE KE KF KF KE KE KF KF
300 /sec (R) 300 /sec (L) 300 /sec (R) 30° /sec (L) 1800 /sec (R) 180°/sec (L) 180°/sec (R) 180°/sec (L)
24 40 33 27 31
0.25 0.39 0.25 0.60 0.15 0.33 0.24 0.87
Strength/body mass b KE 300 /sec (R)
39 32
KE 300 /sec (L) KF 300 /sec (R)
29 36 27
22
26
42
30 30
32
36
27 21
KF KE KE KF KF
30·/sec (L) 1800 /sec (R) 180·/sec (L) 180·/sec (R) 180·/sec (L)
38 24 29 37 24
a
Overall probability of an association in 2 x 4 x-square analysis (n
29 21 35
sports). b Strength values were divided by body mass. Abbreviations: KE knee extensors; KF knee flexors; R
=
=
24 23 18 42
0.23 0.96
42
0.55
35 55
0.43 0.02
50 51 29
0.05 0.03 0.43
= 138 female collegiate athletes in 8 weight-bearing varsity
= right; L = left; 01,2,3,4 = first, second, third, fourth quartile.
Grace et al. (1984) found that injured subjects had greater knee flexion strength; however, they presented only average strength values for injured and noninjured groups and injury risk could not be calculated from their data. 2.2 Strength Imbalances and Injuries 2.2.1 Right/Left Side Strength Imbalances Some studies suggest that R/L imbalances are associated with greater risk of sprains and muscle strains. However, sample sizes in most of these studies were small and few used appropriate data analysis. Burkett (1970) measured the isometric strength of the knee extensor and knee flexor muscle groups using cable tensiometer methods. His data show that athletes with knee flexor R/L imbalance of > 10% were 20.0 times more likely to suffer a
hamstring strain in the weaker leg than subjects without this imbalance. Merrifield & Cowan (1973) measured the isokinetic peak torque and power of the hip extensors. Eight subjects with R/L imbalances of at least 25% subsequently suffered adductor strains and in all 8 the injured leg was the weaker. Knapik et al. (1991) measured isokinetic knee extension and knee flexion peak torque. They found that athletes with knee flexion R/L imbalances > 15% were 2.6 times more likely to suffer lower extremity injuries than subjects without this imbalance. Lower body sprains and strains were more common on the weaker side. On the other hand, Laird's (1981) results indicated no differences in injury incidence between athletes with R/ L strength imbalances < 10% and those with imbalances> 10%. He measured strength as the maximal weight that could be lifted with the knee flexor and knee extensor muscle groups.
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Table III. Studies examining relationships between flexibility, flexibility imbalances, hypermobility and injuries Study
No. of subjects and type of sport
Burkett (1970)
36 pro football players
Hamstring strains
Nicholas (1970)
139 pro football players 426 high school football players High school football players
Third degree knee sprains
Clark et al. (1971) Godshall (1975) Jackson et al. (1978)
Grana & Moretz (1978)
32 male high school basketball players 27 track and field athletes
Liemohm (1978) Moretz et al. (1982) Ekstrand & Gillquist (1983) Lysens et al. (1984) Agre & Baxter (1987) Meeuwisse & Fowler (1988) Cowan et al. (1988) Knapik et al. (1991)
Abbreviation: pro
850 West Point cadets 108 high school football players College football players 166 high school football players 84 female high school basketball players
155 football players 180 male soccer players
Injury definition
Ankle and knee injuries Knee injuries requiring surgery or casting Ankle, knee, shoulder and elbow injuries Not clearly defined
Hamstring strains Knee sprains Lower extremity muscle ruptures or tendinitis
138 physical education students (age 18 ± 1y) 25 college soccer players 628 male and female athletes in 15 collegiate sports 303 army basic trainees (age 17-35y) 132 female college athletes in 8 sports (age 19 ± 1y)
Not clearly defined Low back strains and knee sprains Hamstring strains, patellofemoral syndromes Lower body and back musculoskeletal problems requiring medical attention First occurrence of a time-loss lower extremity injury
= professional.
Five other studies reported only average R/L imbalance values in injured versus noninjured groups. Injury risk could not be calculated from their data. Klein (1971, 1974) and Ekstrand and Gillquist (1983) found that mean R/L imbalances were larger in injured subjects but Liemohn (1978), Agre and Baxter (1987) and Grace et al. (1984) found no differences between injured and non injured subjects. Right and left side strength imbalances of the knee extensor and knee flexor muscle groups are not uncommon. Imbalances of 10% or more occurred in 31 to 41 % of male and female athletes (Burkett 1970; Knapik et al. 1991; Grace et al. 1984). Imbalances of 15% or more were seen in 20 to 30% offemale athletes (Knapik et al. 1991). Imbalances of 20% or more were seen in 3 to 18% of male and female athletes (Grace et al. 1984; Knapik et al. 1991).
2.2.2 Antagonist/Agonist Strength Imbalances A large number of studies (Appen & Duncan 1986; Murray et al. 1980; Stafford & Grana 1984; Wyatt & Edwards 1981) reported on antagonist/ agonist ratios of the knee flexors/knee extensors because of an assumed relationship between this ratio and athletic injuries (Cooper & Fair 1978; Garrett 1983; Knight 1980). Reported normative values range from 0.43 to 0.90 (Nosse 1982). This wide range probably results from a lack of standardisation in terms of the type of strength measured (isometric versus isokinetic), joint angle, body position, body stabilisation and other factors (Nosse 1982). Interpretation of isokinetic studies is further complicated by the fact that many have not controlled for gravitational moments or changes in the angle of peak torque as the contractile velocity increases (Prietto & Caiozzo 1989). Compounding these problems is the fact that
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antagonist/agonist imbalances may actually reflect R/L imbalances. Burkett (1970) reported that athletes with lower isometrically measured knee flexion/knee extension ratios were 20 times more likely to be injured than subjects with higher ratios. Knapik et al. (1991) found that subjects with knee flexion/knee extension ratios
