Patellofemoral Joint Stress during Running with Alterations in Foot Strike Pattern CHARLES NATHAN VANNATTA and THOMAS W. KERNOZEK Physical Therapy Program, Department o f Health Professions, La Crosse Institute fo r Movement Science, University o f Wisconsin - La Crosse, La Crosse, WI
ABSTRACT VANNATTA, C. N., and T. W. KERNOZEK. Patellofemoral Joint Stress during Running with Alterations in Foot Strike Pattern. Med. Sci. Sports Exerc., Vol. 47, No. 5, pp. 1001-1008, 2015. Purpose: This study aimed to quantify differences in patellofemoral joint stress that may occur when healthy runners alter their foot strike pattern from their habitual rearfoot strike to a forefoot strike to gain insight on the potential etiology and treatment methods of patellofemoral pain. Methods: Sixteen healthy female runners completed 20 running trials in a controlled laboratoiy setting under rearfoot strike and forefoot strike conditions. Kinetic and kinematic data were used to drive a static optimization technique to estimate individual muscle forces to input into a model of the patellofemoral joint to estimate joint stress during running. Results: Peak patellofemoral joint stress and the stress-time integral over stance phase decreased by 27% and 12%, respec tively, in the forefoot strike condition (P < 0.001). Peak vertical ground reaction force increased slightly in the forefoot strike condition (P < 0.001). Peak quadriceps force and average hamstring force decreased, whereas gastrocnemius and soleus muscle forces increased when running with a forefoot strike (P < 0.05). Knee flexion angle at initial contact increased (P < 0.001), total knee excursion decreased (P < 0.001), and no change occurred in peak knee flexion angle (P = 0.238). Step length did not change between conditions (P = 0.375), but the leading leg landed with the foot positioned with a horizontal distance closer to the hip at initial contact in the forefoot strike condition (P < 0.001). Conclusions: Altering one’s strike pattern to a forefoot strike results in consistent reductions in patellofemoral joint stress independent of changes in step length. Thus, implementation of forefoot strike training programs may be warranted in the treatment of runners with patellofemoral pain. However, it is suggested that the transition to a forefoot strike pattern should be completed in a graduated manner. Key Words: KNEE PAIN, FOREFOOT STRIKE, STRIDE LENGTH, RUNNING MECHANICS, REHABILITATION
Current interventions for PFP in runners attempt to reduce knee loading by altering step length or step rate to allow continued participation in running without exacerbating symptoms (12,16,22,38). More specifically, Willson et al. (38) showed that a reduction in step length of at least 10% resulted in an average decrease of 16.3% in PF joint stress. Even small increases in step rate have been shown to reduce energy absorption at the knee (17). A more recent investi gation found that a 10% increase from preferred step rate resulted in a 14% decrease in PF joint reaction force (22). PF loading rate and impulse were also reduced by 11% and 20%, respectively, with increased step rate (22). These re sults demonstrate that changes in running mechanics can lead to reductions in joint specific forces and stresses which may allow athletes to continue running without aggravating PFP symptoms. However, increased step rate seems to be associated with a decreased foot inclination angle at initial contact (16), suggesting a more anterior foot strike pattern (2). Increasing step rate under controlled running speed conditions neces sitates a shorter stride length (22), and decreasing step length results in increased shock attenuation at the ankle (12). This increased shock attenuation was attributed to a greater hor izontal distance of the ground reaction force vector from the ankle joint center. A more anterior location of the ground reaction force vector is indicative of a more anterior foot strike pattern based on the foot strike index proposed by
R
Address for correspondence: Thomas W. Kemozek, Ph.D., FACSM, Depart ment of Health Professions, Health Science Center, University of Wisconsin-La Crosse, 1300 Badger St., La Crosse, WI; E-mail: [email protected]. Submitted for publication May 2014. Accepted for publication August 2014. 0195-9131/15/4705-1001/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE® Copyright © 2014 by the American College of Sports Medicine DOl: 10.1249/MSS.0000000000000503
1001
APPLIED SCIENCES
unning is cited as a common form of recreational activity (30). Numerous forms of overuse injury re main largely unchanged (9,33) despite different in terventions (5,7,12,16,24,26,38). Patellofemoral pain (PFP) is one of the most commonly reported forms of pain among runners, especially in females (33). Yet, its specific pathoanatomical etiology and interventions used for treatment remain largely uncertain. A prevailing theory underlying the etiology of PFP is that an increase in patellofemoral (PF) joint stress occurs (4,15), leading to pain from the subchondral tissues of the patella or femur (13,17). The primary factors that determine PF joint stress are PF joint reaction force and PF contact area. These may be affected by quadriceps force (22,29), knee angle (8), hip internal or external rotation angle (31,32), knee rotation (31), or foot pronation (23).
APPLIED SCIENCES
Cavanagh and LaFortune (6). Thus, whether decreased stress at the knee is a result of shorter step length, increased step rate, or a more anterior foot strike pattern is difficult to as certain from these studies. Further, Roos et al. (29) reported that knee extensor mo ment predicted 93% of the variance in PF joint reaction forces during forward and backward running. They also stated that a center of pressure closer to the heel was a primary predictor of increased knee extensor moments. This suggests that an in tervention targeted toward training to run with a forefoot strike pattern may lead to even greater reductions in PF joint reaction force and PF joint stress. Forefoot strike running patterns have gained much interest with regards to running injury and treatment in recent years (1,7,9,21,24,36,37). More anterior foot strike patterns, or less foot inclination at initial contact, have been reported to reduce overall loading of the lower extremity during running as well as to decrease the stress on the knee joint (24,37). Cheung and Davis (7) reported in a case series that there was an improve ment in symptoms in three runners with PFP when using a landing pattern modification program. They attributed these improved symptoms to decreased ground reaction forces mea sured after training. Flowever, whether a corresponding de crease in PF joint stress occurred with the change in foot strike pattern could not be determined because they did not collect data specifically with regard to PF joint stress. A more recent study did investigate the differences in PF joint stress between forefoot strike and rearfoot strike patterns and found PF joint stress to be 15% less in those with a forefoot strike pattern (20). However, this difference occurred between groups rather than as a result of runners simply changing foot strike pattern. Although increased PF joint stress remains a prevailing theory with regard to the etiology of PFP (15,17), differ ences in PF joint stress between runners with and without PFP have only been suggested (39). However, these values of PF joint stress were calculated from the net knee mo ments using inverse dynamics to obtain the magnitude of quadriceps force, and thus, any coactivation of muscles surrounding the knee may lead to an underestimate of the actual quadriceps force produced (20,39). Therefore, the primary aim of this study was to determine whether a simple alteration of one’s foot strike pattern changes the PF joint stress calculated by a static optimiza tion technique estimating muscle forces of the lower extrem ity. Both peak PF joint stress and the PF joint stress-time integral will be investigated to gamer an understanding of the maximal stress and the accumulated stress to the PF joint during stance. Secondary objectives included investigating whether differences were present in other variables contrib uting to PF joint stress such as stride length, peak vertical ground reaction force, peak quadriceps force, knee flexion angle at initial contact, and peak knee flexion angle between rearfoot strike and forefoot strike running conditions. We hy pothesized that the forefoot strike condition would result in decreased PF joint stress, peak vertical ground reaction force, and peak quadriceps force; increased knee flexion angle at
1002
Official Journal o f the American College o f Sports Medicine
initial contact and peak knee flexion angle; and no change in step length. A post hoc analysis was completed to facilitate accurate interpretation of results and included investigation of such variables as the average hamstring force, average gastrocnemius force, average soleus force, and the lower ex tremity position at initial contact.
M ETHODS Subjects. Seventeen healthy females 18-35 yr were in cluded in this study (age = 22.8 ± 3.17 yr, height = 169 + 5.8 cm, mass = 63.7 ± 5.4 kg, weekly running distance = 33.2 ± 14.7 km). Inclusion criteria consisted of a selfreported running routine of >10 miles per week, selfreported rearfoot strike pattern (first contact with the ground made with the heel) while running, score of 5 or greater on the Tegner activity scale (a measure of regular participation with recreational sports which require running and jumping), and no reported knee symptoms similar to PFP, which limited regular participation in running in the past 12 months. Exclusion criteria consisted of pregnancy, reported cardiovascular pathology, surgery on either lower extremity in the last 12 months, and traumatic injury to ei ther knee in the past 6 months. All subjects gave their in formed consent to the testing protocol as approved by the institutional review board at the university. Protocol. After completion of a minimum of three practice running trials, participants ran down a 20-m runway under two conditions: using 1) their typical rearfoot strike pattern and 2) an adapted forefoot strike pattern after simple instruction to “contact the ground on the ball of the foot.” The order of these conditions was randomized. All partici pants were fitted with the same model of footwear (Model 629; New Balance, Boston, MA). Speed was restricted to a range of 3.52-3.89 nvs-1 using photoelectric timers. Run ning pattern was observed carefully, and any trials where targeting of the force plate occurred were repeated. Foot strike pattern was verified after each trial via the use of in sole pressure sensors (Novel GmbH, Munich, Germany). Rearfoot strike was defined as the subjects’ center of pres sure occurring in the rear most third of the overall foot length at initial contact as previously defined by Cavanagh and LaFortune (6). For the purpose of this study, we com bined the midfoot strike and forefoot strike categories of the foot strike index such that a forefoot strike was any pattern such that the center of pressure was located anterior to the 33% of foot length at initial contact. If a strike pattern was not the correct pattern, that trial was repeated. A total of 10 trials were completed under each condition. Foot strike pattern was then verified again during postprocessing by calculating the location of the center of pressure on the foot using force plate data as previously described (2). Instrum entation. Before all running trials participants were prepped for three-dimensional motion analysis using the Human Body Model (Motek Medical, Amsterdam, Netherlands). Reflective markers were placed on the right,
http://w w w .acsm -m sse.org
PATELLOFEMORAL JOINT STRESS AND FOOT STRIKE
The muscle forces from the Human Body Model were then used to quantify the total quadriceps force by summing the muscle forces o f the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius. To determine the PF joint reaction force, a k constant was calculated using the following equation derived by Brechter and Powers (4): k{x) = (4.62
ABSTRACT VANNATTA, C. N., and T. W. KERNOZEK. Patellofemoral Joint Stress during Running with Alterations in Foot Strike Pattern. Med. Sci. Sports Exerc., Vol. 47, No. 5, pp. 1001-1008, 2015. Purpose: This study aimed to quantify differences in patellofemoral joint stress that may occur when healthy runners alter their foot strike pattern from their habitual rearfoot strike to a forefoot strike to gain insight on the potential etiology and treatment methods of patellofemoral pain. Methods: Sixteen healthy female runners completed 20 running trials in a controlled laboratoiy setting under rearfoot strike and forefoot strike conditions. Kinetic and kinematic data were used to drive a static optimization technique to estimate individual muscle forces to input into a model of the patellofemoral joint to estimate joint stress during running. Results: Peak patellofemoral joint stress and the stress-time integral over stance phase decreased by 27% and 12%, respec tively, in the forefoot strike condition (P < 0.001). Peak vertical ground reaction force increased slightly in the forefoot strike condition (P < 0.001). Peak quadriceps force and average hamstring force decreased, whereas gastrocnemius and soleus muscle forces increased when running with a forefoot strike (P < 0.05). Knee flexion angle at initial contact increased (P < 0.001), total knee excursion decreased (P < 0.001), and no change occurred in peak knee flexion angle (P = 0.238). Step length did not change between conditions (P = 0.375), but the leading leg landed with the foot positioned with a horizontal distance closer to the hip at initial contact in the forefoot strike condition (P < 0.001). Conclusions: Altering one’s strike pattern to a forefoot strike results in consistent reductions in patellofemoral joint stress independent of changes in step length. Thus, implementation of forefoot strike training programs may be warranted in the treatment of runners with patellofemoral pain. However, it is suggested that the transition to a forefoot strike pattern should be completed in a graduated manner. Key Words: KNEE PAIN, FOREFOOT STRIKE, STRIDE LENGTH, RUNNING MECHANICS, REHABILITATION
Current interventions for PFP in runners attempt to reduce knee loading by altering step length or step rate to allow continued participation in running without exacerbating symptoms (12,16,22,38). More specifically, Willson et al. (38) showed that a reduction in step length of at least 10% resulted in an average decrease of 16.3% in PF joint stress. Even small increases in step rate have been shown to reduce energy absorption at the knee (17). A more recent investi gation found that a 10% increase from preferred step rate resulted in a 14% decrease in PF joint reaction force (22). PF loading rate and impulse were also reduced by 11% and 20%, respectively, with increased step rate (22). These re sults demonstrate that changes in running mechanics can lead to reductions in joint specific forces and stresses which may allow athletes to continue running without aggravating PFP symptoms. However, increased step rate seems to be associated with a decreased foot inclination angle at initial contact (16), suggesting a more anterior foot strike pattern (2). Increasing step rate under controlled running speed conditions neces sitates a shorter stride length (22), and decreasing step length results in increased shock attenuation at the ankle (12). This increased shock attenuation was attributed to a greater hor izontal distance of the ground reaction force vector from the ankle joint center. A more anterior location of the ground reaction force vector is indicative of a more anterior foot strike pattern based on the foot strike index proposed by
R
Address for correspondence: Thomas W. Kemozek, Ph.D., FACSM, Depart ment of Health Professions, Health Science Center, University of Wisconsin-La Crosse, 1300 Badger St., La Crosse, WI; E-mail: [email protected]. Submitted for publication May 2014. Accepted for publication August 2014. 0195-9131/15/4705-1001/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE® Copyright © 2014 by the American College of Sports Medicine DOl: 10.1249/MSS.0000000000000503
1001
APPLIED SCIENCES
unning is cited as a common form of recreational activity (30). Numerous forms of overuse injury re main largely unchanged (9,33) despite different in terventions (5,7,12,16,24,26,38). Patellofemoral pain (PFP) is one of the most commonly reported forms of pain among runners, especially in females (33). Yet, its specific pathoanatomical etiology and interventions used for treatment remain largely uncertain. A prevailing theory underlying the etiology of PFP is that an increase in patellofemoral (PF) joint stress occurs (4,15), leading to pain from the subchondral tissues of the patella or femur (13,17). The primary factors that determine PF joint stress are PF joint reaction force and PF contact area. These may be affected by quadriceps force (22,29), knee angle (8), hip internal or external rotation angle (31,32), knee rotation (31), or foot pronation (23).
APPLIED SCIENCES
Cavanagh and LaFortune (6). Thus, whether decreased stress at the knee is a result of shorter step length, increased step rate, or a more anterior foot strike pattern is difficult to as certain from these studies. Further, Roos et al. (29) reported that knee extensor mo ment predicted 93% of the variance in PF joint reaction forces during forward and backward running. They also stated that a center of pressure closer to the heel was a primary predictor of increased knee extensor moments. This suggests that an in tervention targeted toward training to run with a forefoot strike pattern may lead to even greater reductions in PF joint reaction force and PF joint stress. Forefoot strike running patterns have gained much interest with regards to running injury and treatment in recent years (1,7,9,21,24,36,37). More anterior foot strike patterns, or less foot inclination at initial contact, have been reported to reduce overall loading of the lower extremity during running as well as to decrease the stress on the knee joint (24,37). Cheung and Davis (7) reported in a case series that there was an improve ment in symptoms in three runners with PFP when using a landing pattern modification program. They attributed these improved symptoms to decreased ground reaction forces mea sured after training. Flowever, whether a corresponding de crease in PF joint stress occurred with the change in foot strike pattern could not be determined because they did not collect data specifically with regard to PF joint stress. A more recent study did investigate the differences in PF joint stress between forefoot strike and rearfoot strike patterns and found PF joint stress to be 15% less in those with a forefoot strike pattern (20). However, this difference occurred between groups rather than as a result of runners simply changing foot strike pattern. Although increased PF joint stress remains a prevailing theory with regard to the etiology of PFP (15,17), differ ences in PF joint stress between runners with and without PFP have only been suggested (39). However, these values of PF joint stress were calculated from the net knee mo ments using inverse dynamics to obtain the magnitude of quadriceps force, and thus, any coactivation of muscles surrounding the knee may lead to an underestimate of the actual quadriceps force produced (20,39). Therefore, the primary aim of this study was to determine whether a simple alteration of one’s foot strike pattern changes the PF joint stress calculated by a static optimiza tion technique estimating muscle forces of the lower extrem ity. Both peak PF joint stress and the PF joint stress-time integral will be investigated to gamer an understanding of the maximal stress and the accumulated stress to the PF joint during stance. Secondary objectives included investigating whether differences were present in other variables contrib uting to PF joint stress such as stride length, peak vertical ground reaction force, peak quadriceps force, knee flexion angle at initial contact, and peak knee flexion angle between rearfoot strike and forefoot strike running conditions. We hy pothesized that the forefoot strike condition would result in decreased PF joint stress, peak vertical ground reaction force, and peak quadriceps force; increased knee flexion angle at
1002
Official Journal o f the American College o f Sports Medicine
initial contact and peak knee flexion angle; and no change in step length. A post hoc analysis was completed to facilitate accurate interpretation of results and included investigation of such variables as the average hamstring force, average gastrocnemius force, average soleus force, and the lower ex tremity position at initial contact.
M ETHODS Subjects. Seventeen healthy females 18-35 yr were in cluded in this study (age = 22.8 ± 3.17 yr, height = 169 + 5.8 cm, mass = 63.7 ± 5.4 kg, weekly running distance = 33.2 ± 14.7 km). Inclusion criteria consisted of a selfreported running routine of >10 miles per week, selfreported rearfoot strike pattern (first contact with the ground made with the heel) while running, score of 5 or greater on the Tegner activity scale (a measure of regular participation with recreational sports which require running and jumping), and no reported knee symptoms similar to PFP, which limited regular participation in running in the past 12 months. Exclusion criteria consisted of pregnancy, reported cardiovascular pathology, surgery on either lower extremity in the last 12 months, and traumatic injury to ei ther knee in the past 6 months. All subjects gave their in formed consent to the testing protocol as approved by the institutional review board at the university. Protocol. After completion of a minimum of three practice running trials, participants ran down a 20-m runway under two conditions: using 1) their typical rearfoot strike pattern and 2) an adapted forefoot strike pattern after simple instruction to “contact the ground on the ball of the foot.” The order of these conditions was randomized. All partici pants were fitted with the same model of footwear (Model 629; New Balance, Boston, MA). Speed was restricted to a range of 3.52-3.89 nvs-1 using photoelectric timers. Run ning pattern was observed carefully, and any trials where targeting of the force plate occurred were repeated. Foot strike pattern was verified after each trial via the use of in sole pressure sensors (Novel GmbH, Munich, Germany). Rearfoot strike was defined as the subjects’ center of pres sure occurring in the rear most third of the overall foot length at initial contact as previously defined by Cavanagh and LaFortune (6). For the purpose of this study, we com bined the midfoot strike and forefoot strike categories of the foot strike index such that a forefoot strike was any pattern such that the center of pressure was located anterior to the 33% of foot length at initial contact. If a strike pattern was not the correct pattern, that trial was repeated. A total of 10 trials were completed under each condition. Foot strike pattern was then verified again during postprocessing by calculating the location of the center of pressure on the foot using force plate data as previously described (2). Instrum entation. Before all running trials participants were prepped for three-dimensional motion analysis using the Human Body Model (Motek Medical, Amsterdam, Netherlands). Reflective markers were placed on the right,
http://w w w .acsm -m sse.org
PATELLOFEMORAL JOINT STRESS AND FOOT STRIKE
The muscle forces from the Human Body Model were then used to quantify the total quadriceps force by summing the muscle forces o f the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius. To determine the PF joint reaction force, a k constant was calculated using the following equation derived by Brechter and Powers (4): k{x) = (4.62
