August 2015

Running modifications to alter PFJ contact force

8PFP-iStock48337166-copyBy Collin D. Bowersock, BS, and John D. Willson, PT, PhD

Patellofemoral pain interventions have increasingly focused on running technique and training approaches—such as utilization of a forefoot strike pattern, a shortened step length, and manipulations to training pace—to reduce patellofemoral joint loading parameters.

Running is a very popular mode of exercise among people of all ages, with nearly 30 million Americans participating regularly. However, the high incidence of musculoskeletal injury associated with running represents a barrier to lifelong participation in this activity. Patellofemoral pain (PFP), which is among the most common of these running-related musculoskeletal injuries, typically requires treatment because patellofemoral joint (PFJ) symptoms do not naturally abate permanently.1 The importance of effective treatment for runners with PFJ symptoms is underscored by the reported finding that 24% to 30% of individuals with a musculoskeletal injury permanently stop participation in their exercise program.2

Although the etiology of PFJ pain among runners is believed to be multifactorial, a universally accepted causative factor is the rapid and repetitive microtrauma of elevated PFJ contact forces experienced while running. Indeed, the PFJ experiences a cumulative impulse of several hundred body weights per kilometer run.3 As such, interventions have increasingly focused on running technique and training approaches—such as utilization of a forefoot strike pattern, a shortened step length, and manipulations to training pace—that may reduce these PFJ loading parameters. Although limited evidence currently exists to support the effectiveness of such interventions in the prevention or treatment of PFJ pain, clinicians and runners alike tend to find these interventions appealing because they promote continued participation in running. Each of these conservative options has potential benefits and limitations from a biomechanical perspective, and the decision of which option, if any, to implement should be made on a case-by-case basis, with particular consideration of the runner’s short- and long-term goals.

Researchers have demonstrated effects of running modifications on PFJ kinetics, but have not yet conducted studies to determine whether there are effects on PFJ symptoms.

Step length

A reduction in step length during running appears to have favorable effects on PFJ kinetics. In laboratory environments, interventions to increase step rate while holding running speed constant are used to decrease step length. In the field, reductions in step length may be accomplished using mobile feedback devices to monitor both step rate and running speed.4 Increasing step rate without changing running speed (decreasing step length) by 10% has been reported to decrease peak PFJ contact force by 17% and peak PFJ stress by 10% to 15% per step compared with a runner’s preferred step length.3,5 Further, while running with a greater step rate, runners experience this force for less time with each step. Thus, runners may expect a 20% reduction in PFJ impulse (force multiplied by time) as a consequence of a 10% decrease in step length.3 Kinematic and kinetic changes that contribute to this reduction in PFJ impulse while running with a shortened step length include making initial contact closer to the runner’s center of mass (COM), decreased COM vertical excursion, decreased peak knee flexion during stance phase, and decreased braking ground reaction force.6

8PFP-iStock18190012-copyOver the course of a training run, these relatively large reductions in PFJ kinetics per step associated with reduced step length become less substantial due to the inverse relationship between step length and the number of steps required to run a given distance. For example, a 10% reduction in step length was found to decrease total PFJ impulse by 6% to 12% over the course of a kilometer.3,5 Although the increased number of steps partially mitigates the reduction in PFJ kinetics experienced with each step, the reduction in cumulative impulse may still have therapeutic effects. Running with a 10% reduction in step length is estimated to shield the PFJ from 21.5 body weights for each kilometer run.3 For a 150-lb runner, this equals a reduction of more than 16,000 lbs experienced by the PFJ for each 5-km run, compared with running at a preferred step length. Whether this reduction in cumulative joint contact force contributes to clinically relevant improvement in treatment outcomes for patellofemoral pain is a worthwhile area of future study.

Running with a shortened step length may also have other beneficial effects for individuals with PFJ symptoms. A 10% reduction from preferred step length has been found to result in a significant decrease in peak hip adduction moment and peak hip adduction angle,6 which are typically increased among individuals with PFP.7,8 Reducing step length may also increase preparatory gluteus medius activation prior to initial contact during running.9 This may help to address the delayed gluteus medius activation prior to initial contact during running experienced by runners with PFP, a delay that may be associated with increased hip adduction excursion during stance phase.10 The fact that reducing step length positively affects both PFJ kinetics and lower extremity alignment during running makes this gait modification an attractive rehabilitation option.

Reducing step length may be accompanied by unfavorable natural performance consequences, however. Runners tend to utilize a preferred stride length that minimizes metabolic cost, so reducing step length may decrease running efficiency, particularly among experienced runners.11,12 Self-reported perceived exertion also increases with a 10% reduction in step length, at least in the short term, which may reflect the additional metabolic demand as well as the novelty of the task.6 However, it is expected that the novelty and perceived difficulty of the task will decrease with practice. Previous gait modification interventions report significant improvement in both ratings of perceived effort and naturalness over as few as eight practice sessions.13 As such, runners with PFJ symptoms may be willing to accept a modestly increased metabolic demand, provided a reduction in symptoms accompanies their efforts—especially if the alternative is to decrease participation in running or discontinue it altogether.

Foot strike pattern

While running in traditional footwear with a cushioned heel, runners predominantly demonstrate a rearfoot strike pattern during training runs and endurance events.14 Two characteristics of running with a rearfoot strike pattern are that the foot is generally dorsiflexed relative to the running surface and that the ground reaction force vector acts at a point very close to the ankle joint. Together, these two characteristics put the ankle plantar flexors at a disadvantage for supporting the runner’s mass against gravity following initial contact with the ground. This, in turn, places greater demand on the quadriceps to stiffen the leg while running with a rearfoot strike pattern than if the runner’s foot made initial contact more anteriorly. Because the patella is a sesamoid bone within the quadriceps tendon, increased quadriceps force will increase PFJ contact force.

Several authors recently tested the hypothesis that using a mid- or forefoot strike pattern reduces PFJ contact force during running. Kulmala et al observed that female runners who habitually run using a forefoot strike pattern experienced 16% lower peak PFJ stress than those who run with rearfoot strike pattern.15 Vannatta and Kernozek16 and Willson et al3 also observed 27% and 11% reductions in peak PFJ stress while running with a forefoot strike pattern, respectively. Further, it was reported that cumulative stress per step (stress-time integral) was reduced by 11% to 12% during the forefoot strike condition.3,16 Although experimental evidence is lacking at this time, such decreases in peak stress and cumulative stress per step may assist with rehabilitation efforts for runners with PFP who wish to continue their preferred mode of exercise.

Some interaction appears to exist between step length and typical foot strike pattern during running. Some runners who utilize a rearfoot strike pattern while running in conventional footwear may adopt a forefoot strike pattern in response to a reduction in step length. A shorter step length has been associated with a more horizontal foot at initial contact and also decreased frequency of the vertical ground reaction force impact transients typically observed among runners with a rearfoot strike pattern.6 However, these changes were not ubiquitous among participants, and some runners continue to demonstrate a rearfoot strike pattern following instruction to run with a shortened step length.

8PFP-iStock6676593-copyAlthough interventions to reduce step length may affect foot strike pattern, changing foot strike pattern does not appear to affect step length. Interestingly, this appears to be true regardless of a runner’s experience with a forefoot strike technique. For example, Vannatta and Kernozek16 did not find an acute difference in step length among runners who preferred a rearfoot strike pattern when they were asked to run with a forefoot strike pattern. Almonroeder et al did not observe a difference in step length among runners who adopted a forefoot strike pattern following a two-week introduction to minimalist footwear.17 Finally, step length among experienced runners who habitually train with a forefoot strike pattern was similar to runners who habitually train with a rearfoot strike pattern.15,18 If adopting a forefoot strike pattern does not affect step length, the number of steps required to run a given distance is not expected to increase. Consequently, the magnitude of the potentially beneficial reductions in PFJ impulse observed during each step when switching from a rearfoot strike pattern to a forefoot strike pattern will be equivalent to the magnitude of the decrease in cumulative PFJ impulse for any given distance run. This is in contrast to running with a shortened step length, where the increase in the number of steps required to run a given distance reduces (but does not eliminate) these effects over the course of a mile.

Runners and clinicians considering a conversion to a forefoot strike pattern should be mindful that running with a mid- or forefoot strike pattern will introduce novel forces to the lower extremity, including an increased braking force impact peak and loading rate,19 increased metatarsal force,20 and increased Achilles tendon force.15,17 For example, the Achilles tendon is reported to experience an additional 48 body weights of force for each mile run with a forefoot strike pattern relative to a rearfoot strike pattern.17 Increased plantar flexor force contributes to increased distal tibia contact force while running with a forefoot strike pattern.21 And, because the gastrocnemius, which acts nearly parallel to the long axis of the tibia, crosses both the ankle and knee, increased tibiofemoral joint contact force may also be expected.

Despite the potentially adverse effects of adopting a mid- or forefoot strike pattern to reduce PFJ kinetics, to date, there is no experimental evidence of increased foot or lower leg injury incidence in response to this technique modification while running in conventional footwear. Regardless, rapid conversion to a forefoot strike pattern among people accustomed to running with a rearfoot strike pattern, without allowing sufficient time for tissue adaptation to these novel stresses, should be avoided. A systematic and progressive approach to this technique change that considers running frequency, duration, and intensity with adequate rest intervals for tissue adaptation is recommended.

Running speed

PFJ injury prevention and rehabilitation efforts may benefit from manipulations to training pace. Running speed is a product of step rate and step length. As such, increasing running speed may be accomplished by increasing step length or step rate. However, it has been observed that, during submaximal effort running, increases in step length tend to outpace increases in step rate as runners increase speed.18,22,23 Taking longer steps to increase speed requires a longer flight phase and shorter stance time, which is accomplished by generating larger lower extremity joint moments to increase vertical ground reaction force during the stance phase. The contribution of the ankle plantar flexors to the increased vertical ground reaction force at higher running speeds appears to be disproportionately large.23 Peak knee extension joint moment also increases per stride as running speed increases up to roughly 7 m/s, but to a lesser extent.24,25

Because quadriceps force is a primary contributor to knee extension moment, increased peak PFJ contact force during each step is a natural consequence of increased running speed. Further, the decreased stance time associated with increased running speed suggests these forces are applied at a higher rate. To the extent that repetitive exposure to greater peak PFJ contact force and loading rate exacerbates PFJ symptoms, it may be advisable for runners with PFP to avoid high-speed running.

Conversely, however, increased running speed may have beneficial effects for treatment or prevention of PFJ symptoms despite increased peak PFJ force and loading rate. This is possible due to two effects associated with running at higher velocity: decreased contact time with the ground and increased step length. Although, as described above, the magnitude of the peak PFJ contact force increases with increased running speed, the joint experiences this force for less time. Thus, the PFJ impulse per step changes very little as velocity increases. Increased step length while running at a higher velocity results in fewer steps per unit distance; therefore, the cumulative PFJ impulse per mile run may decrease with increased running velocity.

This premise is supported by the findings of Petersen et al,24 who reported no change in knee extension angular impulse per step and an 80% decrease in cumulative angular impulse per kilometer as running speed increased from 2.23 m/s to 4.38 m/s. That suggests that, to the extent that decreased cumulative PFJ impulse affects patellofemoral joint symptoms, increased running speed may be advisable in runners with PFP. However, as noted earlier, increasing running speed requires greater muscular effort per step and therefore may still exacerbate any existing PFJ injury.

Increasing running speed to reduce cumulative PFJ impulse per kilometer run will also have metabolic consequences that limit prolonged training runs at increased speed. Interval training techniques that incorporate rest periods between high speed bouts, however, can be easily integrated into a runner’s usual routine. Such routines provide energy expenditure that is similar to prolonged continuous bouts of running, with comparable short- and long-term health benefits.26,27

Despite the recent interest in these modifications to running technique and training routines as they relate to PFJ kinetics, it is worthwhile to note that none of these interventions have yet been demonstrated to reduce PFJ symptoms. This is not to say they are not effective. Rather, it is unknown if they are effective because they have not been tested using rigorous experimental research designs. Future studies to delineate the PFJ loading characteristics most significantly associated with PFJ injury and the effectiveness of interventions such as these are necessary and relevant for conservative prevention and treatment efforts.

Collin Bowersock, BS, is a graduate student in the Biomechanics Laboratory within the Department of Kinesiology at East Carolina University in Greenville, NC. John D. Willson, PT, PhD, is an associate professor working in the Human Movement Analysis Laboratory within the Department of Physical Therapy at East Carolina University.

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