An almost identical running speed was achieved between BR and FR. PFJCF
was significantly lower in BR than in FR and this difference was not due to running speed, confirming our first hypothesis. The reduction in PFJCF
was lower than that found by Flynn and Soutaslittle (1995)
(27% versus 46% decrease). The higher decrease in PFJCF
observed by Flynn and Soutaslittle (1995)
was most likely due to the difference in running speed between their FR and BR trials.
Kinematics at the peak knee moment (Mk(max)
) were similar between BR and FR which seems consistent with an earlier study (van Deursen et al., 1998
). However, kinetics differed with higher Mk(max)
in FR and higher Mh(max)
in BR. This predominantly agreed with Flynn and Soutaslittle (1995)
, as knee flexion angles were similar (FR: 44° and 38°, BR: 41° and 38°) and peak knee moments were similar for BR (124 and 123 Nm) but lower in FR (157 and 246 Nm).
TIP (telescopic inverted pendulum) analysis demonstrated that in both BR and FR Mk(max) occurred during the loading response, with the body upright and leaning slightly forward; Mk(max) thereby positively contributed to the support moment. The forward lean indicated that Mk(max) contributed to push-off in FR only, as in BR a backward lean would be required for Mk(max) to contribute to push-off. We propose that BR was predominantly generated by pendular movement. This was demonstrated by the reduced Mk(max) and increased Mh(max) in BR, which would be expected in pendular movement. Also, in FR the stance leg extended more during the push-off phase (and flexed slightly more during the deceleration phase) than in BR, consistent with our interpretation that FR involves a more telescopic movement.
Regression analysis showed that Mk(max) could predict the majority of the differences in PFJCF, confirming the first part of the second hypothesis that reduced PFJCF in BR was caused by a reduced Mk(max).
Although the magnitude (|GRF|) and orientation (θGRF) of the GRF at Mk(max) did not differ between BR and FR, the location of the GRF was further back on the foot in FR (larger COPloc). This would result in a larger moment arm between the GRF vector and the knee joint, as the knee flexion angle (θK) at Mk(max) was similar between BR and FR. Subsequently, this would result in a larger Mk(max).
Stepwise regression analysis showed that the variance in Mk(max) was best predicted by θK, COPloc and |GRF|. θK and COPloc both influence the magnitude of the moment arm of the GRF vector relative to the knee joint. Mk(max) therefore relied most on the position and magnitude of the GRF, partly confirming the second part of our second hypothesis. |GRF| was however not smaller in BR than in FR, as we hypothesized. The main factor influencing the peak knee moment was therefore COPloc, indicating that foot strike has a large impact on PFJCF. Although angular accelerations of the lower limb segments and joint angles were different between BR and FR trials, these were not significant predictors of Mk(max), and therefore are considered to have minimal influence on PFJCF.
The differences in PFJCF
observed between BR and FR were not consistent in all subjects (). Investigation of the COP location at foot strike (COPlocFS
) showed that this was closer to the heel in FR. There was a weak correlation between COPlocFS
when FR and BR data were pooled. PFJCF
was reduced if at foot strike the COP was closer to the forefoot. The relatively low PFJCF
observed in some of the subjects during FR may therefore be due to running style (such as heel versus forefoot strike). We would expect lower knee moments resulting in lower PFJCF
in forefoot strike runners, as they have lower loading rates of the foot (Oakley and Pratt, 1988
) and a lower GRF (Lieberman et al., 2010
). This agrees with our findings that PFJCF
was reduced if the COP was closer to the forefoot. However, this conclusion did not apply to all subjects during FR (see , right lower corner). Clearly, further research is required to investigate whether it is the BR style that resulted in a reduced PFJCF
or whether an adapted FR style could also be advised to PFP patients.
This study had several limitations; as PFJCF
cannot be measured in vivo it was estimated with simplified models. This study focused on compressive forces only and did not include the direction and location of the forces acting on the patellofemoral joint. The use of more complex models of the knee and the additional calculation of patellofemoral joint stresses (ratio of PFJCF
to the contact area (McGinty et al., 2000
)) would have provided insight into the distribution and direction of the forces acting on the joint surface. There is however a strong relationship between the patellofemoral contact area and knee flexion angles (Salsich et al., 2003; Besier et al., 2005; Escamilla et al., 2008
). As Mk(max)
occurred at similar knee flexion angles in BR and FR, it can be assumed that the patellofemoral contact area would be comparable, and patellofemoral joint stresses would be directly related to PFJCF
. Estimation of joint stresses requires complex and computationally intense methods (Farrokhi et al., 2011
); as similar trends could be expected in patellofemoral joint stresses and compression forces between BR and FR, this study included compression forces only. Future research may involve more detailed analysis of the forces acting on the patellofemoral joint during backward and forward running.
The patellar tendon moment arm was important in the calculations of the PFJCF
, as it defined the magnitude of the PFJCF
relative to the knee moment. There is controversy in literature on how this moment arm should be estimated (Tsaopoulos et al., 2006
). We assumed the patellar tendon moment arm depended on knee angle, as the majority of studies demonstrated that the patellar tendon moment arm changes significantly during the first 45° of knee flexion (Smidt, 1973; Herzog and Read, 1993; Baltzopoulos, 1995; Kellis and Baltzopoulos, 1999; Tsaopoulos et al., 2006, 2007
), and only limited studies found the moment arm to change little with knee angle (Gill and O'Connor, 1996
This study demonstrated that PFJCF
was reduced in BR compared to FR, and that this was not due to a difference in running speed. It can be concluded that BR can be used as part of rehabilitation of PFP patients, to continue to exercise without increased PFJCF
. Although BR can be suggested for rehabilitation, only a limited number of studies investigated BR as part of rehabilitation of knee injured patients. A case study showed that BR allowed exercising with decreased PFP; however if implemented incorrectly it can lead to overuse injury (Satterfield et al., 1993
). Care therefore needs to be taken when implementing BR. Obviously, rehabilitation programs need to include other components, such as muscle strengthening (Dixit et al., 2007; Crossley et al., 2008
), specific exercise therapy (Heintjes et al., 2003; Dixit et al., 2007
) and/or taping (Dixit et al., 2007
The reduced PFJCF in BR compared to FR may also prevent overloading and thereby the development of chronic conditions such as osteoarthritis. However PFJCF was not decreased in BR compared to FR in all subjects, and PFJCF was lower when the COP was closer to the forefoot. The COP location, that was closer to the heel at peak knee moment in FR than in BR, was the main predictor of the increased knee extensor moments. Certain FR styles may therefore also be able to reduce PFJCF, and could be useful in injury prevention or rehabilitation.