The Effect of Foot Structure and Range of Motion on Musculoskeletal Overuse Injuries

The Effect of Foot Structure and Range of Motion on Musculoskeletal Overuse Injuries

Kenton R. Kaufman

It is estimated that 30 million Americans run for exercise and that 10 million do so on a regular basis.[15] While estimates vary, approximately one-half to two-thirds of runners will sustain an injury that will result in cessation of training for at least 1 week.[24,33,36,46] Running involves repetitive loading of the lower extremity by repeated impact between the foot and the ground. During running at 3.8 m/sec, the acceleration of the shoe at footstrike can be in excess of 50g, although typical values are about 25g.[7,18] The resultant shock waves are thought to be associated with a number of injury problems including stress fractures,[37] shin splints,[12] cartilage breakdown,[52] osteoarthritis,[42,43] and low back pain.[55]

A simple method of classifying feet is by the medial longitudinal arch, an important structural feature associated with training injuries. It has been suggested that high-arch feet are inflexible, while fiat feet are hypermobile and susceptible to a large degree of pronation.[8,35,53] The literature is contradictory regarding the relationship between foot structure and exercise-related overuse injuries. Subotnik[54] suggested that a greater percentage of athletic injuries occur in people with fiat feet than in those with normal or high-arch feet. On the other hand, Cowan et al.[11] reported an increase of athletic activity-related injuries with increased arch height in US Army recruits. Greater subtalar range of motion has been observed in fiat feet than in high-arch feet,[10] yet several studies have not supported the hypothesis that a relationship exists between rearfoot motion and arch height.[19,28]

The purpose of this study was to prospectively determine whether there is an association between foot structure and the development of an overuse injury in a well-defined cohort undergoing high-intensity physical training.

MATERIALS AND METHODS

Study Group

This study was a 2-year prospective investigation of risk factors that contribute to overuse injuries. All subjects were Navy Sea, Air, and Land (SEAL) candidates who began training between May 1993 and July 1994 at the Basic Underwater Demolition/SEAL (BUD/S) School, Naval Special Warfare Center, Coronado, California. All candidates were 18- to 29-year-old male volunteers who passed a rigorous physical skills screening test. Before official training, all candidates engaged in a structured and mandatory fitness preparation for a period of 2 to 7 weeks, depending on reporting date and degree of fitness on arrival at the school. The 25-week formal training period could be extended for performance, medical, or administrative reasons. Training consisted of three instructional phases emphasizing different skills. Phase I involved 9 weeks of physical conditioning, including pool and ocean swims of up to 3.3 km, 6.7-kin timed runs, an obstacle course, and inflatable-raft seamanship. The 6th week of this phase (known as hell week) consisted of 5.5 virtually sleepless days of nonstop training. The 7 weeks of phase II emphasized basic-combat diving and swimming. Phase III of formal training involved 9 weeks of land warfare, including activities such as rappelling. During all three phases, physical training continuously intensified.

Data Collection

This study was approved by the Committee for the Protection of Human Subjects at the Naval Health Research Center, San Diego, California. Ail participants signed informed consent forms and were studied within 2 weeks of reporting to the Naval Special Warfare Training Center and before initiation of any training.

We measured the biomechanical characteristics of the subjects’ feet. Foot and ankle ranges of motion were measured by a registered physical therapist using a handheld goniometer. To evaluate the impact of gastrocnemius muscle tightness, ankle dorsiflexion was measured both with the knee fully extended and flexed to 90 [degrees]. One arm of the goniometer was aligned with the long axis of the lower leg and the other with the plantar surface of the foot. The transverse tarsal joint was locked by bringing the forefoot into a neutral position while measuring dorsiflexion. This was done to more accurately reflect ankle motion. Talocalcaneal (subtalar) inversion and eversion were also measured using a handheld goniometer. The zero starting position was defined as the position with the heel aligned with the midline of the tibia and the ankle joint in gentle dorsiflexion (that is, the Achilles tendon became taut).

Arch characteristics of both feet were studied both statically and dynamically while the subject was bearing weight. We used the ratio of navicular height to foot length as a static measurement of the bony arch index. The foot length was measured from the back of the calcaneus to the metatarsophalangeal joint. The bony arch index was chosen because it has been shown to be associated with risk of developing an overuse injury.[11] Static standing radiographs of the foot were not obtained because the Committee for Protection of Human Subjects considered collection of this type of data to be invasive and did not permit it.

A dynamic assessment of the arch characteristic was performed in the Motion Analysis Laboratory at Children’s Hospital-San Diego, using a foot-pressure measurement system (TEKSCAN, Boston, Massachusetts). This system uses a sensor with 960 sensing elements that covers the entire plantar surface of the foot. The system’s spatial resolution was 5 mm, and the sampling speed was 100 Hz. The sensing cells, formed by a conductive grid printed on polyester film, create a sensor that is 0.1-mm thick. A grid is formed where the intersection points are separated by a pressure-sensitive semiconducting layer. The sensing elements sense the location, timing, and pressure distribution of any contact that registered a pressure of 0.03 to 5.5 MPa. This system was designed to measure the pressure exerted by the foot on the ground. Data were collected while a subject walked under two different conditions: barefoot and with military footwear (jungle boots). The dynamic arch index was calculated as the ratio of the area of contact of the midfoot to the total contact area of the foot. The total foot contact area was defined as the region from the back of the calcaneus to the metatarsal heads. The midfoot was defined as the region from the point between the calcaneus and the cuboid to the point between the cuboid and the base of the metatarsals.[21]

Injury Tracking System

All subjects were observed prospectively during training for outcomes of musculoskeletal injury or noninjury until graduation or attrition.[5] Data were maintained in a data collection system based on an expanded orthopaedic International Classification of Disease, 9th Revision, Clinical Modification (ICD-9-CM) code.[23] Clinical personnel at the Navy Special Warfare Training Center (NSWTC) medical clinic all attended a 2-week course on sports injuries to optimize standardization of diagnoses based on these codes. Overuse injuries were defined as musculoskeletal problems of insidious onset (stress fractures, periostitis, iliotibial band syndrome, patellofemoral syndrome, and Achilles tendinitis) associated with repetitive physical activity.

Diagnosis of stress fractures required both clinical presentation of localized pain of insidious onset, without prior trauma, aggravated by repetitive weightbearing activity, and relieved with rest, and a positive radiograph, nuclear bone scan, or both at a site consistent with clinical presentation. A positive radiograph was defined as one that showed the presence of periosteal reaction, endosteal callus formation, a fracture line in otherwise normal bone, or any combination of these. A positive bone scan was defined as one that indicated the presence of 3+ to 4+ fusiform uptake of radioactive marker at the painful site.

Periostitis was defined as exertional pain of insidious onset that was localized to the tibia, without radiographic periosteal changes or positive bone scan findings. Iliotibial band syndrome was diagnosed as insidious-onset irritation of the iliotibial band at the level of the lateral femoral epicondyle with tenderness to palpation localized to the lateral femoral condyle, Gerdy’s tubercle, or both. Patellofemoral syndrome was defined as an ill-defined ache of insidious onset localized to the peripatellar area, including the medial or lateral patellofemoral joint and retinaculum. Palpation of the patella and peripatellar soft tissues

Statistical Analysis of Data

Means and standard deviations of continuous variables, such as body height, body weight, and joint ranges of motion, were calculated. The injury rate was calculated by dividing the total number of injuries by the total number of subjects studied. The injury incidence (percentage of enrollees injured) was calculated by dividing the number of subjects with one or more injuries by the total number of subjects studied.

For risk-factor analysis of continuous variables such as arch characteristics or joint ranges of motion, subjects were divided into three equal-sized groups with values for these variables ranging from low to high. The injury incidence (percentage) of musculoskeletal overuse injuries occurring in each group was calculated. The midgroup, with intermediate (normal) values, was chosen as a reference group. The other groups were then compared with this reference level, or baseline, risk group. Risk ratios were calculated to quantify the relative incidence of overuse injuries in the groups with low or high values for arch structure and joint range of motion compared with the reference group.[1] Confidence intervals of 90% and 95% were calculated for all risk ratios. [1] Risk ratios statistically significant at the 0.10 and 0.05 level were noted. The statistical power achieved to obtain a relative risk of two times the normal value was calculated using a one-sided test with a significance level of 0.05.[45] A Fisher’s exact test was performed to determine whether the association between stress fractures and arch height and foot range of motion differed for the different anatomic locations of the lower extremity.

RESULTS

Subjects

Of the 559 trainees arriving at the Naval Special Warfare Training Center between May 1993 and July 1994, 487 (87.1%) were briefed, and 449 (80.3%) elected to enroll in the study. All subjects in the study were young (mean age, 22.5 _+ 2.5 years) physically fit men (Table 1). The mean height was 177 [+ or- ] 6.8 cm, and the mean weight was 78.0 [+ or -] 8.8 kg. Ethnic background was 88.5% white, 5.7% hispanic, 2.0% black, 1.5% Native American, and the remainder were of other ethnic origins. Eighty-three percent of the study subjects reported having never used tobacco products of any type. About 88% of the subjects said they ran or jogged three or more times per week, and 73% reported having run or jogged on a regular basis for a period of 3 or more months before reporting to training. The overall reported incidence of a previous lower extremity injury was 62.9%, with 7.6% of these injuries occurring within the previous 2 months. About half (49.4%) of the subjects reported that the severity of their injury prevented full participation in normal physical activities for at least 1 week. Thirty subjects (6.7%) reported a lower extremity stress fracture in one or both limbs before reporting to the Naval Special Warfare Training Center.

TABLE 1

Physical and Performance Data on Navy SEAL Trainees

Factor N Mean SD Median

Age (years) 452 22.5 2.5 22

Height (cm) 427 177.0 6.76 176.5

Weight (kg) 427 78.0 8.8 77.5

2.5-km run (min) 359 9.93 0.69 10.08

Sit-ups (N) 360 73.94 11.66 72

Push-ups (N) 360 76.50 16.19 75

Pull-ups (N) 359 12.93 5.26 12

455-m (500-yd) swim (min) 361 12.93 5.26 10.08

Factor Minimum Maximum

Age (years) 18 29

Height (cm) 141.3 193.5

Weight (kg) 45.7 103.6

2.5-km run (min) 8.23 12.15

Sit-ups (N) 41 128

Push-ups (N) 29 149

Pull-ups (N) 1 80

455-m (500-yd) swim (min) 7.59 13.15

Anatomic and biomechanical measurements were collected on 423 of the total 449 (94.2%) enrollees in this study (Table 2). Biomechanical measurements of foot structure were performed on 334 of the 449 enrollees (74.4%) before the commencement of training. There were no significant differences in the age, race, height, and weight, or in the fitness scores in the subset of enrollees on whom biomechanical measurements were performed. All anatomic and biomechanical variables displayed a low variance. The static arch measurement displayed less variability than the dynamic arch measurement. The dynamic arch measurement of feet in shoes had a larger mean value than the dynamic arch measurement of bare feet, indicating increased ground contact in the midfoot region, most likely due to increased forces on the arch from the lacing of the boots.

TABLE 2

Anatomic and Biomechanical Characteristics of the Feet of

Navy SEAL Trainees

Variable N Mean SD

Arch(a)

Static 423 21.52 3.55

Dynamic, barefoot 334 6.59 4.34

Dynamic, with shoes 333 8.57 3.87

Ankle dorsiflexion, knee flexed (deg) 407 20.5 5.5

Ankle dorsiflexion, knee extended (deg) 409 13.3 4.5

Hindfoot inversion (deg) 423 27.9 9.6

Hindfoot eversion (deg) 423 11.4 5.3

Variable Median Minimum Maximum

Arch(a)

Static 21.49 7.06 33.04

Dynamic, barefoot 5.75 0.12 23.66

Dynamic, with shoes 8.27 0.58 21.05

Ankle dorsiflexion, knee flexed (deg) 20.5 -12.5 34.5

Ankle dorsiflexion, knee extended (deg) 13.0 -5.5 29.0

Hindfoot inversion (deg) 29.0 0.0 46.5

Hindfoot eversion (deg) 11.5 0.0 40.0

(a) Arch values are ratios: static arch index equals navicular height to foot length; dynamic arch index equals ratio of area of contact of the midfoot to total contact area of the foot.

Injury Incidence and Distribution

The study population had a high incidence of overuse injuries. Injuries were reported most often in phase I (first 9 weeks) of training. Of the entire cohort of 449 subjects, 149 (33.2%) suffered 348 lower extremity overuse injuries during training (Table 3). The most common overuse injuries were stress fractures, iliotibial band syndrome, patellofemoral syndrome, Achilles tendinitis, and periostitis (Table 3). The most common site of stress fractures was the lower leg (49%), followed by the foot (39%), and the femur (12%).

TABLE 3

Rate and Incidence of Musculoskeletal Overuse Injuries Among

the 449 Subjects in this Study

Injury Injury rate(a) Injury incidence(b)

N (%) N (%)

All overuse injuries 348 (77.5) 149 (33.2)

Stress fracture 60 (13.4) 39 (8.7)

Iliotibial band syndrome 49 (10.9) 42 (9.4)

Patellofemoral syndrome 42 (9.4) 35 (7.8)

Achilles tendinitis 30 (6.7) 23 (5.1)

Periostitis(c) 14 (3.1) 14 (3.1)

(a) Injury rate: injuries per 100 enrollees.

(b) Injury incidence: percentage of enrollees with one or more injuries.

(c) Excludes stress fractures.

Risk Factors for Overuse Injuries

The relationship between foot structure characteristics and risk of developing a stress fracture is shown in Table 4. We found an increased relative risk of developing a stress fracture for people with either pes planus or pes cavus. However, the association between arch type and risk of injury was statistically significant for the pes planus foot only when assessed dynamically in shoes. There was no association between ankle or hindfoot motion and relative risk of stress fracture (statistical power [is greater than or equal to]70%).

TABLE 4

Incidence and Relative Risk of Stress Fractures Associated with

Foot Structure and Range of Motion

Structure/range of motion by group N Incidence

(in percent)

Arch(a)

Static

Pes cavus (>22.8) 141 9.9

Normal (20.0-22.8) 138 5.8

Pes planus (<20.0) 139 10.8

Dynamic, barefoot

Pes cavus (<4.14) 106 8.5

Normal (4.14-8.10) 100 5.0

Pes planus (>8.10) 110 10.9

Dynamic, with shoes

Pes cavus (<4.14) 105 8.6

Normal (4.14-8.10) 106 4.7

Pes planus (>8.10) 104 11.5

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 127 8.7

Normal (11.5 [degrees]-15.0 [degrees]) 137 9.5

Flexible (> 15.0 [degrees]) 140 8.6

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 129 7.8

Normal (18.5 [degrees]-23.0 [degrees]) 135 8.9

Flexible (>23.0 [degrees]) 138 10.1

Hindfoot

Inversion

Tight (<26.0 [degrees]) 127 7.9

Normal (26.00-32.5 [degrees]) 145 8.3

Flexible (>32.5 [degrees]) 146 10.3

Eversion

Tight (<9.5 [degrees]) 132 9.1

Normal (9.5 [degrees]-13.5 [degrees]) 145 9.0

Flexible (>13.5 [degrees]) 141 8.5

Structure/range of motion by group Risk ratio

Arch(a)

Static

Pes cavus (>22.8) 1.71

Normal (20.0-22.8) 1.00

Pes planus (<20.0) 1.86

Dynamic, barefoot

Pes cavus (<4.14) 1.70

Normal (4.14-8.10) 1.00

Pes planus (>8.10) 2.18

Dynamic, with shoes

Pes cavus (<4.14) 1.82

Normal (4.14-8.10) 1.00

Pes planus (>8.10) 2.45

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 0.91

Normal (11.5 [degrees]-15.0 [degrees]) 1.00

Flexible (> 15.0 [degrees]) 0.90

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 0.87

Normal (18.5 [degrees]-23.0 [degrees]) 1.00

Flexible (>23.0 [degrees]) 1.14

Hindfoot

Inversion

Tight (<26.0 [degrees]) 0.95

Normal (26.00-32.5 [degrees]) 1.00

Flexible (>32.5 [degrees]) 1.24

Eversion

Tight (<9.5 [degrees]) 1.01

Normal (9.5 [degrees]-13.5 [degrees]) 1.00

Flexible (>13.5 [degrees]) 0.95

Structure/range of motion by group 95% confidence interval

Arch(a)

Static

Pes cavus (>22.8) 0.74, 3.95

Normal (20.0-22.8)

Pes planus (<20.0) 0.82, 4.25

Dynamic, barefoot

Pes cavus (<4.14) 0.59, 4.89

Normal (4.14-8.10)

Pes planus (>8.10) 0.80, 5.98

Dynamic, with shoes

Pes cavus (<4.14) 0.63, 5.24

Normal (4.14-8.10)

Pes planus (>8.10) 0.89, 6.70(b)

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 0.42, 1.96

Normal (11.5 [degrees]-15.0 [degrees])

Flexible (> 15.0 [degrees]) 0.43, 1.91

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 0.39, 1.95

Normal (18.5 [degrees]-23.0 [degrees])

Flexible (>23.0 [degrees]) 0.55, 2.38

Hindfoot

Inversion

Tight (<26.0 [degrees]) 0.43, 2.13

Normal (26.00-32.5 [degrees])

Flexible (>32.5 [degrees]) 0.60, 2.56

Eversion

Tight (<9.5 [degrees]) 0.48, 2.14

Normal (9.5 [degrees]-13.5 [degrees])

Flexible (>13.5 [degrees]) 0.45, 2.01

(a) Group is based on arch index. Static arch index equals navicular height to foot length; dynamic arch index equals midfoot contact area to total foot-contact area.

(b) p < 0.10.

Subsequently, a separate analysis was done to determine whether stress fracture rates for the femoral, tibial, or foot (tarsal/metatarsal) regions were affected differently by the various foot structural characteristics. There were no statistically significant differences in stress fracture rates with respect to static (P = 0.4) or dynamic (P 0.17) foot arch characteristics. Similarly, heel cord tightness did not have a differential effect on stress fracture rates in the femur, tibia, or feet (P [is greater than or equal to] 0.07). In contrast, the amount of foot inversion did have a differential effect on the location of the stress fracture (P = 0.03). Subjects with restricted hindfoot inversion had more femoral stress fractures, while subjects with increased hindfoot inversion had more tarsal/metatarsal stress fractures. There was no difference in rates of stress fractures by anatomic locations due to differences in hindfoot eversion (P = 0.12).

A statistically significant association was found between Achilles tendinitis and either a tight gastrocnemius or increased hindfoot inversion (Table 5). There was no statistically significant relationship between foot structure and iliotibial band syndrome or patellofemoral syndrome (Tables 6 and 7).

TABLE 5

Incidence and Relative Risk of Achilles Tendinitis Associated with

Foot Structure and Range of Motion

Structure/range of motion by group N Incidence

(in percent)

Arch(a)

Static

Pes cavus (>22.8) 140 5.7

Normal (20.0-22.8) 140 3.6

Pes planus (<20.0) 138 5.8

Dynamic, barefoot

Pes cavus (<4.14) 104 4.8

Normal (4.14-8.10) 104 3.8

Pes planus (>8.10) 108 7.4

Dynamic, with shoes

Pes cavus (<4.14) 103 5.8

Normal (4.14-8.10) 106 4.7

Pes planus (>8.10) 106 5.7

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 127 7.9

Normal (11.5 [degrees]-15.0 [degrees]) 136 2.2

Flexible (>15.0 [degrees]) 141 5.7

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 127 4.7

Normal (18.5 [degrees]-23.0 [degrees]) 137 4.4

Flexible (>23.0 [degrees]) 138 5.8

Hindfoot

Inversion

Tight (<26.0 [degrees]) 128 4.7

Normal (26.0 [degrees]-32.5 [degrees]) 146 2.7

Flexible (>32.5 [degrees]) 144 7.6

Eversion

Tight (<9.5 [degrees]) 134 3.7

Normal (9.5 [degrees]-13.5 [degrees]) 145 5.5

Flexible (>13.5 [degrees]) 139 5.8

Structure/range of motion by group Risk ratio

Arch(a)

Static

Pes cavus (>22.8) 1.60

Normal (20.0-22.8) 1.00

Pes planus (<20.0) 1.62

Dynamic, barefoot

Pes cavus (<4.14) 1.25

Normal (4.14-8.10) 1.00

Pes planus (>8.10) 1.93

Dynamic, with shoes

Pes cavus (<4.14) 1.23

Normal (4.14-8.10) 1.00

Pes planus (>8.10) 1.20

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 3.57

Normal (11.5 [degrees]-15.0 [degrees]) 1.00

Flexible (>15.0 [degrees]) 2.57

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 1.08

Normal (18.5 [degrees]-23.0 [degrees]) 1.00

Flexible (>23.0 [degrees]) 1.32

Hindfoot

Inversion

Tight (<26.0 [degrees]) 1.71

Normal (26.0 [degrees]-32.5 [degrees]) 1.00

Flexible (>32.5 [degrees]) 2.79

Eversion

Tight (<9.5 [degrees]) 0.68

Normal (9.5 [degrees]-13.5 [degrees]) 1.00

Flexible (>13.5 [degrees]) 1.04

Structure/range of motion by group 95% confidence interval

Arch(a)

Static

Pes cavus (>22.8) 0.54, 4.77

Normal (20.0-22.8)

Pes planus (<20.0) 0.54, 4.84

Dynamic, barefoot

Pes cavus (<4.14) 0.35, 4.52

Normal (4.14-8.10)

Pes planus (>8.10) 0.60, 6.20

Dynamic, with shoes

Pes cavus (<4.14) 0.39, 3.92

Normal (4.14-8.10)

Pes planus (>8.10) 0.38, 3.81

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 1.01, 12.68(b)

Normal (11.5 [degrees]-15.0 [degrees])

Flexible (>15.0 [degrees]) 0.70, 9.49

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 0.36, 3.26

Normal (18.5 [degrees]-23.0 [degrees])

Flexible (>23.0 [degrees]) 0.47, 3.71

Hindfoot

Inversion

Tight (<26.0 [degrees]) 0.49, 5.93

Normal (26.0 [degrees]-32.5 [degrees])

Flexible (>32.5 [degrees]) 0.91, 8.55(c)

Eversion

Tight (<9.5 [degrees]) 0.23, 2.02

Normal (9.5 [degrees]-13.5 [degrees])

Flexible (>13.5 [degrees]) 0.40, 2.70

(a) Group is based on arch index. Static arch index equals navicular height to foot length; dynamic arch index equals midfoot contact area to total foot-contact area.

(b) p < 0.05.

(c) p < 0.10.

TABLE 6

Incidence and Relative Risk of Iliotibial Band Syndrome Associated

with Foot Structure and Range of Motion

Structure/range of motion by group N Incidence

(in percent)

Arch(a)

Static

Pes cavus (>22.8) 141 9.9

Normal (20.0-22.8) 137 8.0

Pes planus (<20.0) 140 9.3

Dynamic, barefoot

Pes cavus (<4.14) 105 9.5

Normal (4.14-8.10) 102 7.8

Pes planus (>8.10) 109 10.1

Dynamic, with shoes

Pes cavus (<4.14) 105 9.5

Normal (4.14-8.10) 107 6.5

Pes planus (>8.10) 104 11.5

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 127 7.9

Normal (11.5 [degrees]-15.0 [degrees]) 134 11.2

Flexible (>15.0 [degrees]) 143 7.7

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 126 10.3

Normal (18.5 [degrees]-23.0 [degrees]) 138 9.4

Flexible (>23.0 [degrees]) 138 8.0

Hindfoot

Inversion

Tight (<26.0 [degrees]) 128 9.4

Normal (26.0 [degrees]-32.5 [degrees]) 147 9.5

Flexible (>32.5 [degrees]) 143 8.4

Eversion

Tight (<9.5 [degrees]) 134 9.0

Normal (9.5 [degrees]-13.5 [degrees]) 140 10.0

Flexible (>13.5 [degrees]) 144 8.3

Structure/range of motion by group Risk ratio

Arch(a)

Static

Pes cavus (>22.8) 1.24

Normal (20.0-22.8) 1.00

Pes planus (<20.0) 1.16

Dynamic, barefoot

Pes cavus (<4.14) 1.21

Normal (4.14-8.10) 1.00

Pes planus (>8.10) 1.29

Dynamic, with shoes

Pes cavus (<4.14) 1.46

Normal (4.14-8.10) 1.00

Pes planus (>8.10) 1.76

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 0.70

Normal (11.5 [degrees]-15.0 [degrees]) 1.00

Flexible (>15.0 [degrees]) 0.69

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 1.10

Normal (18.5 [degrees]-23.0 [degrees]) 1.00

Flexible (>23.0 [degrees]) 0.85

Hindfoot

Inversion

Tight (<26.0 [degrees]) 0.98

Normal (26.0 [degrees]-32.5 [degrees]) 1.00

Flexible (>32.5 [degrees]) 0.88

Eversion

Tight (<9.5 [degrees]) 0.90

Normal (9.5 [degrees]-13.5 [degrees]) 1.00

Flexible (>13.5 [degrees]) 0.83

Structure/range of motion by group 95% confidence interval

Arch(a)

Static

Pes cavus (>22.8) 0.58, 2.63

Normal (20.0-22.8)

Pes planus (<20.0) 0.54, 2.49

Dynamic, barefoot

Pes cavus (<4.14) 0.50, 2.95

Normal (4.14-8.10)

Pes planus (>8.10) 0.54, 3.07

Dynamic, with shoes

Pes cavus (<4.14) 0.58, 3.68

Normal (4.14-8.10)

Pes planus (>8.10) 0.72, 4.30

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 0.33, 1.51

Normal (11.5 [degrees]-15.0 [degrees])

Flexible (>15.0 [degrees]) 0.33, 1.44

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 0.53, 2.27

Normal (18.5 [degrees]-23.0 [degrees])

Flexible (>23.0 [degrees]) 0.39, 1.82

Hindfoot

Inversion

Tight (<26.0 [degrees]) 0.47, 2.05

Normal (26.0 [degrees]-32.5 [degrees])

Flexible (>32.5 [degrees]) 0.42, 1.84

Eversion

Tight (<9.5 [degrees]) 0.43, 1.87

Normal (9.5 [degrees]-13.5 [degrees])

Flexible (>13.5 [degrees]) 0.40, 1.74

(a) Group is based on arch index. Static arch index equals navicular height to foot length; dynamic arch index equals midfoot contact area to total foot-contact area.

TABLE 7

Incidence and Relative Risk of Patellofemoral Syndrome Associated

with Foot Structure and Range of Motion

Structure/range of motion by group N Incidence

(in percent)

Arch(a)

Static

Pes cavus (>22.8) 140 9.3

Normal (20.0-22.8) 139 7.2

Pes planus (<20.0) 139 5.8

Dynamic, barefoot

Pes cavus (<4.14) 104 6.7

Normal (4.14-8.10) 106 8.5

Pes planus (>8.10) 106 4.7

Dynamic, with shoes

Pes cavus (<4.14) 105 5.7

Normal (4.14-8.10) 107 5.6

Pes planus (>8.10) 103 7.8

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 128 7.0

Normal (11.5 [degrees]-15.0 [degrees]) 134 8.2

Flexible (>15.0 [degrees]) 143 6.3

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 127 10.2

Normal (18.5 [degrees]-23.0 [degrees]) 137 5.1

Flexible (>23.0 [degrees]) 138 7.2

Hindfoot

Inversion

Tight (<26.0 [degrees]) 127 7.1

Normal (26.0 [degrees]-32.5 [degrees]) 145 6.9

Flexible (>32.5 [degrees]) 146 8.2

Eversion

Tight (<9.5 [degrees]) 135 8.9

Normal (9.5 [degrees]-13.5 [degrees]) 141 7.1

Flexible (>13.5 [degrees]) 142 6.3

Structure/range of motion by group Risk ratio

Arch(a)

Static

Pes cavus (>22.8) 1.29

Normal (20.0-22.8) 1.00

Pes planus (<20.0) 0.80

Dynamic, barefoot

Pes cavus (<4.14) 0.79

Normal (4.14-8.10) 1.00

Pes planus (>8.10) 0.56

Dynamic, with shoes

Pes cavus (<4.14) 1.02

Normal (4.14-8.10) 1.00

Pes planus (>8.10) 1.39

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 0.86

Normal (11.5 [degrees]-15.0 [degrees]) 1.00

Flexible (>15.0 [degrees]) 0.77

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 2.00

Normal (18.5 [degrees]-23.0 [degrees]) 1.00

Flexible (>23.0 [degrees]) 1.42

Hindfoot

Inversion

Tight (<26.0 [degrees]) 1.03

Normal (26.0 [degrees]-32.5 [degrees]) 1.00

Flexible (>32.5 [degrees]) 1.19

Eversion

Tight (<9.5 [degrees]) 1.25

Normal (9.5 [degrees]-13.5 [degrees]) 1.00

Flexible (>13.5 [degrees]) 0.89

Structure/range of motion by group 95% confidence interval

Arch(a)

Static

Pes cavus (>22.8) 0.59, 2.84

Normal (20.0-22.8)

Pes planus (<20.0) 0.33, 1.97

Dynamic, barefoot

Pes cavus (<4.14) 0.31, 2.05

Normal (4.14-8.10)

Pes planus (>8.10) 0.19, 1.60

Dynamic, with shoes

Pes cavus (<4.14) 0.34, 3.06

Normal (4.14-8.10)

Pes planus (>8.10) 0.50, 3.85

Ankle

Dorsiflexion, knee extended

Tight (<11.5 [degrees]) 0.37, 2.00

Normal (11.5 [degrees]-15.0 [degrees])

Flexible (>15.0 [degrees]) 0.33, 1.79

Dorsiflexion, knee flexed

Tight (<18.5 [degrees]) 0.83, 4.86

Normal (18.5 [degrees]-23.0 [degrees])

Flexible (>23.0 [degrees]) 0.56, 3.62

Hindfoot

Inversion

Tight (<26.0 [degrees]) 0.43, 2.45

Normal (26.0 [degrees]-32.5 [degrees])

Flexible (>32.5 [degrees]) 0.53, 2.67

Eversion

Tight (<9.5 [degrees]) 0.56, 2.80

Normal (9.5 [degrees]-13.5 [degrees])

Flexible (>13.5 [degrees]) 0.37, 2.13

Group is based on arch index. Static arch index equals navicular height to foot length; dynamic arch index equals midfoot contact area to total foot-contact area.

The foot arch was characterized both statically and dynamically while bearing weight. Apes planus foot would have a low static arch index. In contrast, apes planus foot would have a high dynamic arch index. The data demonstrate that a dynamic assessment differs considerably from a static assessment of the foot arch. There was a low correlation (r = 0.22 to 0.24) between the static arch index and the dynamic arch index when measured bare foot (Fig. 1A) or in-shoe (Fig. 1B). There was a higher correlation between the dynamic arch index measured in the bare foot and the in-shoe foot (r = 0.55) (Fig. 1C). All correlations were statistically significant (P = 0.0001) Further, the weak association between the two dynamic arch measurements emphasizes that a person’s structural characteristics (intrinsic factor) interact with the footwear (extrinsic factor). This interaction alters the inherent structural characteristics of the foot.

[Figures 1A-1C ILLUSTRATION OMITTED]

DISCUSSION

Our results demonstrate an association between foot structure and the risk of overuse injury. Subjects with either pes planus or pes cavus, as measured both statically and dynamically, consistently had nearly twice the incidence of stress fractures compared with subjects with average arch height. The results of the present study concur with the reports on military trainees by Giladi et al.[16] and Simkin et al.,[51] showing that people with high arches are at an increased risk of incurring a stress fracture. These findings are in contrast, however, with two previous studies that reported a protective effect of pes planus on the incidence of overuse injury in military trainees.[11,16] Cowan et al.[11] reported a significant linear trend for increasing risk of injury with increasing arch height in Army infantry recruits. They did not, however, perform dynamic biomechanical measurements. Giladi et al. [16] also reported that Israeli Army recruits with low arches were less likely to experience stress fractures. In that study, however, data were collected in a nonweightbearing position, which underestimates the true frequency of pes planus. Further, classification of the arch type was subjective, so there were no distinct boundaries for low- , average-, or high-arch feet. Simkin et al.,[51] found femoral and tibial stress fractures were more prevalent in the presence of high-arch feet, whereas metatarsal fractures were more common in feet with low arches. The present study found that men with either pes planus or pes cavus were at increased risk for stress fractures in any of the lower extremity anatomic locations.

A parallel may be drawn between Navy SEAL trainees and civilian endurance athletes. The Navy trainees are expected to report to the Naval Special Warfare Training Center in optimum physical condition, and thus should be at lower risk for specific overuse injuries resulting from prior inactivity, yet there was an 8.7% incidence of stress fracture. In contrast, previous estimates are that 0.9% to 2.4% of military trainees suffer a stress fracture.[6,14,25,30,40,41,43,47] This difference is most likely because of the extreme physical demands incurred by trainees during Basic Underwater Demolition/SEAL (BUD/S) school. Further, other authors studying this population have suggested that injury rates among these trainees may be underreported because of high motivation to continue training despite injury.[2,38] Nevertheless, it is important to note that 14% to 48% of civilian runners will reduce mileage or seek medical attention for an injury attributable solely to running or overuse,[17, 24, 29, 34, 36,46, 56] which is in very close agreement with the proportion of the Navy trainees in this study (33.2%) who reported a lower extremity overuse injury.

The medial longitudinal arch of the normal foot is supported by both passive structures (bones and ligaments) and active structures (muscles).[20] During a standing-at-ease posture, little intrinsic or extrinsic muscle activity occurs,[3] and the arch is maintained primarily by static supporting elements. Huang and colleagues[22] found the highest relative contribution to overall stability is provided by the plantar fascia, followed by the plantar ligaments and the spring ligament. The plantar fascia plays a major role in maintaining the medial longitudinal arch and accounts for approximately 25% of the arch stiffness.

The passive structures of the foot have energy-storage capabilities that depend on both their geometry and elastic properties.[50] Much of the energy needed for running is stored by means of elastic structures in the leg and foot, particularly the longitudinal arch.[27] Kinetic and potential energy from the body in the first half of stance are stored briefly as elastic strain energy and then returned in the second half by elastic recoil. During walking, however, muscle activity may also play an important role in arch stabilization. Kayano[26] demonstrated that during walking, dynamic change of the medial arch occurred through a complex relationship of body weight, bone structure, and ligament and muscle force.

The role of the muscles of the foot in arch stabilization is not clear. The foot muscles vary in their activity level depending on the specific phase of the walking cycle. The present study obtained both static and dynamic measurements of the foot arch characteristics to include these possible differences in the mechanisms of arch support. In general, most of the support is provided by the passive factors. However, if the tarsal bones are less favorably placed during weightbearing, sufficient support must be obtained by increasing the contributions from muscular contraction. When the tarsal bones are poorly aligned, the load is carried more by the muscles which, in certain cases, leads to arch collapse. It was thought that for people who require muscular effort to maintain the foot structure, dynamic measurements obtained while walking may be more indicative of true structural characteristics of the foot. This is supported by our findings of a statistically significant relationship between the dynamic in-shoe assessment and stress fractures. The static measures also supported this relationship but were not statistically significant. The results of this study agree with those of Hamill and colleagues,[19] who found that static lower extremity measures have limited value in predicting dynamic lower extremity function. Thus, classification of the foot characteristics of an athlete based on dynamic factors during performance is important.

It has been suggested that a functional relationship between arch height and knee injury may exist. Nigg and colleagues[39] demonstrated that the transfer of foot inversion to internal leg rotation was found to increase significantly with increasing arch height. It was suggested that this relationship may explain the cause of knee pain. Lutter,[32] in his study of 213 runners, demonstrated that 164 (77%) knee injuries had a relationship to biomechanical dysfunction of the foot. The present study does not support the association between general foot type and overuse injuries at the knee. In the current study, no significant relationships were identified between foot structure and either iliotibial band syndrome or patellofemoral syndrome.

Faulty foot biomechanics is an important factor contributing to Achilles tendon overuse injuries. Segesser and colleagues[48] found that ankle joint instability and hyperpronation predispose people to Achilles tendon disorders. In contrast, Kvist[31] demonstrated that limited subtalar joint mobility and rigidity of the ankle joint were found more frequently in athletes with Achilles tendinitis than in other athletes. In general, different malalignments and biomechanical faults seem to play a causative role in 58% (239 of 411) of athletes with Achilles tendon overuse injuries.[31] Our results indicated that hindfoot inversion is increased in subjects with Achilles tendinitis. We also confirmed that decreased ankle dorsiflexion with knee extension is associated with Achilles tendinitis.

Our average measurements and standard deviations are in agreement with reports from other investigators for both ankle and hindfoot motion.[4,13,44,49] These studies did not report if the knee was flexed or extended while measuring ankle motion, except for that of Roaas and Anderson,[44] who flexed the knee to 45 [degrees].

Cavanagh and Rodgers[9] presented a method of measuring footprints for the purpose of classifying foot types. They defined the arch index as a ratio of the area of middle third of the footprint to the entire footprint area. They suggested criteria for classification of footprints with high arch (arch index [is less than] 0.21), normal arch (0.21 [is less than] arch index [is less than] 0.26), and flat arch (arch index [is greater than or equal to] 0.26). The distribution of arch types in the present study differs from this definition because of the difference in the way that we defined the midfoot region in the present study. Rather than dividing the footprint equally into three regions, the midfoot was defined as the distance from the point between the calcaneus and cuboid to the point between the cuboid and the base of the metatarsal.[21] Thus, the midfoot comprised approximately 20% of the total length of the foot in this study, as compared with 33% in the Cavanagh and Rogers study. We believe that this modification to the definition of the arch index more closely represented the true midfoot region, since the segmentation is based on the anatomic structures that compose the hindfoot, midfoot, and forefoot.

This study is the first to use both the previously studied anatomic measurements and the more precise biomechanical methods to quantify the structural characteristics of the foot. Application of these methods makes it possible to better define arch characteristics and the interaction with modifying factors such as footwear that contribute to injury. The results of this study may be used to identify high-endurance athletes who may be at increased risk of lower extremity overuse injuries. Further research needs to be done to develop strategies, particularly footwear modification, to prevent lower extremity overuse injuries.

ACKNOWLEDGMENTS

This work was supported by the Office of Naval Research, Department of the Navy, under work unit 6302. The authors acknowledge Janet Buttermore, Diane Ambrosini, Frederick Cei, and AnnaLisa Lauer of the Motion Analysis laboratory, Children’s Hospital, San Diego, California, for collecting data for this study.

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Kenton R. Kaufman,(*)([dagger]) PhD, Stephanie K. Brodine,([double dagger]) CAPT, MC, USN, Richard A. Shaffer,([double dagger]) CDR, MSC, USN, Chrisanna W. Johnson,([double dagger]) MPH, and Thomas R. Cullison,([sections]) CAPT, MC, USN

From the (*)Biomechanics Laboratory, Mayo Clinic/Foundation, Rochester, Minnesota, ([double dagger])Department of Epidemiology and Health Sciences, Naval Health Research Center, San Diego, California, and ([sections])Naval Hospital, Camp Lejeune, North Carolina

([dagger]) Address correspondence and reprint requests to Kenton R. Kaufman, PhD, PE, Biomechanics Laboratory, 128 Guggenheim, Mayo Clinic/Foundation, 200 First Street, SW, Rochester, MN 55905.

The views expressed in this article are those of the authors and do not reflect the official policy or position of the Department of Navy, Department of Defense, or the US Government. This report is approved for public release, distribution unlimited.

No author or related institution has received any financial benefit from research in this study. See “Acknowledgments” for funding information. caused discomfort. Achilles tendinitis was defined as localized tenderness, swelling, and pain over the involved tendon.3

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