Patellofemoral joint stress during incline and decline running
Introduction
Patellofemoral pain (PFP) is a common overuse injury experienced by runners (DeHaven and Lintner, 1986, Taunton et al., 2002), and accounts for 18–33% of knee injuries (Callaghan and Selfe, 2007, DeHaven and Lintner, 1986, Devereaux and Lachmann, 1984, Kannus et al., 1987). In runners with PFP, increased patellofemoral joint (PFJ) stress has been theorized to cause retropatellar pain as a link between increased patella bone loading and elevated patella water content has been reported in this population (Ho et al., 2014a, Ho et al., 2014b). PFJ stress is defined as PFJ reaction force per unit area of contact of the patella against the femur (Heino Brechter & Powers, 2002). Thus, an increase in PFJ stress might be caused by reduced contact areas and increases in PFJ reaction forces, or from each component in isolation (Ho et al., 2012, Powers et al., 2017).
It is thought that the possible factors leading to elevated PFJ reaction force during running include increased knee flexion angle and elevated knee extensor moment (Powers et al., 2017). An increase in knee extensor moment might be the result of an upright trunk posture and/or a higher ground reaction force (Powers et al., 2017). Specifically, the external knee moment is generally estimated as the product of ground reaction force and knee moment arm (i.e., perpendicular distance from the knee joint axis to the ground reaction force) in the sagittal plane. Therefore, when one runs with a more upright trunk posture, the posteriorly shifted ground reaction force increases the knee lever arm, thereby elevating the knee extensor moment (Teng & Powers, 2014).
It has been observed that runners utilize different biomechanical features during incline and decline running. Specifically, a more extended trunk posture (Levine, Colston, Whittle, Pharo, & Marcellin-Little, 2007) and greater peak landing vertical ground reaction forces (Gottschall & Kram, 2005) have been found during decline running. On the contrary, a more flexed trunk posture (Levine et al., 2007) and smaller peak landing vertical ground reaction forces (Gottschall & Kram, 2005) have been observed in runners during incline running. The effects of altering trunk postures during running on PFJ stress have been studied by Teng and Powers (Teng & Powers, 2014). Their study has revealed that increased trunk flexion is associated with reduced PFJ stress, and decreased trunk flexion is related to increased PFJ stress.
Considering runners exhibit altered trunk flexion angles and vertical ground reaction forces during decline and incline running, the PFJ kinetic profile may be different when they run on a decline or incline slope. To date, however, the effects of incline and decline running on PFJ stress have not been quantified. The primary purpose of this study was to compare peak PFJ stress and PFJ stress-time integral (cumulative PFJ stress over the stance phase) between level, incline, and decline running. The secondary purpose was to determine the contributing factors that lead to altered peak PFJ stress during these three conditions. We hypothesized that peak PFJ stress and PFJ stress-time integral would be highest during decline running and smallest during incline running. Increased peak PFJ stress during decline running was hypothesized to be related to decreased trunk flexion and/or increased vertical ground reaction forces. In addition, reduced peak PFJ stress during incline running was hypothesized to be associated with increased trunk flexion and/or decreased vertical ground reaction forces.
Section snippets
Participants
Twenty recreational runners (10 males and 10 females) between the ages of 21 and 40 years were recruited in this study. The average age, height, weight, and running distance per week of this cohort were 24.9 ± 2.4 years, 1.70 ± 0.07 m, 67.0 ± 9.7 kg, and 13.8 ± 5.6 km, respectively. The data from an existing study were used to estimate the sample size for detecting changes in PFJ stress between running with different trunk postures (Teng & Powers, 2014). With 95% power, and an α level of 0.05,
Patellofemoral joint stress
The PFJ stress between the 3 running conditions was significantly different (P = 0.0001). The post-hoc test revealed that the peak PFJ stress during decline running was significantly higher than during level running (P = 0.0001) and incline running (P = 0.0001). There was no significant difference in PFJ stress between level and incline running (P = 1.000) (Fig. 1; Table 1).
Patellofemoral joint stress-time integral
The PFJ stress-time integral between the 3 running conditions was significantly different (P = 0.000). The post-hoc test
Discussion
This is the first study assessing the effect of decline and incline running on PFJ stress profile in recreational runners. This study aimed to compare peak PFJ stress and PFJ stress-time integral between level, incline, and decline running. In support of our hypothesis, runners experienced the greatest peak PFJ stress and PFJ stress-time integral during decline running. Compared to level running, peak PFJ stress increased by 36% and PFJ stress-time integral by 60% during decline running.
Conflicts of interest
None declared.
Ethical approval
All participants were informed of the nature of the study and signed a consent form approved by the Institutional Review Board of University of Nevada, Las Vegas.
Funding
This work was supported by University of Nevada, Las Vegas Physical Therapy Student Opportunity Research Grant.
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