Re: The Effect of Running Shoes on Lower Extremity Joint Torques

Letter to the Editor Re: The Effect of Running Shoes on Lower Extremity Joint Torques To the Editor, The methodically excellent study of Kerrigan and coworkers [1] reports the external moments acting at the joints of the lower limbs, which are caused by external forces and body masses. At a running speed of about 11 km/h, they measured external moments that were higher when running with shoes than when running barefoot. Notably, the adduction moment at the knee and the rotational torque at the hip joint were increased by 38% and 54%, respectively. The authors correctly state that the data on the external joint moments “provide only an estimate of the net difference between the forces on either side of the joint rather than an estimate of the actual joint contact forces.” We strongly support the authors who rightfully alluded to the need to carefully interpret the study findings. The following comments and some recent results from our own research might provide some additional insight into why this is indeed required. The external moments are balanced by the bone-on-bone contact forces together with the forces created by the muscles and the action of the ligaments [2]. In principle, an increased external adduction moment at the knee causes a shift of the axial tibiofemoral bone-on-bone contact force toward medial, and thus increases the loading of the medial condyle. An unknown fraction of this moment can be compensated by modulating the forces in the muscles and by ligaments, however. A direct conclusion regarding the magnitude of the bone-on-bone contact forces transferred in the knee joint is therefore not possible from the external moments alone. For normal walking, measurements with an instrumented knee implant in a single subject showed that only 7%–12% of the external adduction moment resulted in a shift of the axial tibial force to the medial side [3]. During stance phase, the correlation between the external adduction moment and the force on the medial condyle was not very strong and became even weaker with increasing speed. During high-flexion activities, this force even decreased at higher moments [4]. It is likely that changed muscle activation patterns and antagonistic muscle activities are major factors that contribute to this weak correlation. Whether or not muscle activation patterns were changed between the shoe and barefoot running conditions was not reported by Kerrigan and coworkers. Although findings from patients with instrumented knee implants during walking cannot be readily transferred to healthy subjects running fast, direct measurements of the internal forces indicate that caution should be taken when interpreting the reported torque data. Increased loading of the medial condyles when running with shoes is indeed PM&R

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likely, but the true extent of the load increase can only be quantified by direct measurements in the knee joint. In our institute, measurements with instrumented knee implants are being performed in 9 subjects. Preliminary unpublished results for normal walking with shoes have shown that the first of the 2 main peaks of the tibiofemoral force increases slightly on average while the second peak decreases. These changes vary substantially between subjects and shoes. For slow jogging, the (single) peak force was lowest when running barefoot. These discrepancies underline the necessity to not overinterpret the results of Kerrigan and her coworkers. Recent research on femoroacetabular impingement has provided new insight into possible biomechanical mechanisms that could contribute to the development of osteoarthritis at the hip [5]. Whether the increased rotational torque at the hip joint found in the work by Kerrigan and coworkers indeed results in an increased risk of hip osteoarthritis remains speculative. Even if the relative increase of the moment appears substantial, the rotational moment remains much smaller than the flexion-extension and abduction/adduction moments. Furthermore, an increased rotational torque is not per se associated with more rotational movement in the joint or impingement-related mechanisms that could foster the onset of osteoarthritis. The true implications of the observed increase in rotational torque during running can therefore only be determined by prospective, longitudinal studies in larger cohorts that specifically address the role of a change in the rotational torque at the hip for the onset and development of osteoarthritis at this joint. We congratulate Kerrigan et al for their stimulating work that rightfully raises the important issue of the role of shoes for modulating joint loads in the lower extremities, a question that is particularly important for maintaining joint health in long-distance runners. Not only did their study provide new insight into the joint mechanics during barefoot and shoe running, but it also clearly demonstrated the need for further research in this area. Georg Bergmann Charite Julius Wolff Institut Augustenburger Platz 1 Berlin 13353, Germany Markus O. Heller Charite Julius Wolff Institut Berlin, Germany G.B. Disclosure: nothing to disclose M.O.H. Disclosure: nothing to disclose DOI: 10.1016/j.pmrj.2010.02.009

© 2010 by the American Academy of Physical Medicine and Rehabilitation Vol. 2, 310-311, April 2010

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REFERENCES 1. Kerrigan DC, Franz JR, Keenan GS, Dicharry J, Croce UD, Wilder RP, Jr. The effect of running shoes in lower extremity joint torques. PM&R 2009;1:1058-1063. 2. Winter DA. Biomechanics and Motor Control of Human Movement. New York: John Wiley & Sons, Inc; 1990. 3. Zhao D, Banks SA, Mitchell KH, D’Lima DD, Colwell CW Jr., Fregly BJ. Correlation between the knee adduction torque and medial contact force for a variety of gait patterns. J Orthop Res 2007;25:789-797. 4. D’Lima DD, Patil S, Steklov N, Chien S, Colwell CW, Jr. In vivo knee moments and shear after total knee arthroplasty. J Biomech 2007; 40(Suppl 1):S11-S17. 5. Leunig M, Beaulé PE, Ganz R. The concept of femoroacetabular impingement: current status and future perspectives. Clin Orthop Related Res 2009;467:616-622.

REPLY To the Editor, We greatly appreciate these comments and hope that this study will become just the first of many to evaluate the effects of different types of footwear and individual design characteristics on joint biomechanics during running. Indeed, we recognize the limitations of standard 3-dimensional gait analysis and concur that studies with instrumented joint implants could provide a more direct estimate of joint forces. We should note, however, that the preliminary results reported here for normal walking using instrumented knee implants are very similar to what we have found (also unpublished) using 3-dimensional gait analysis. That is, we have seen only a slight increase in just the first of

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the 2 main peaks of the knee varus torque (or alternatively, the knee adduction moment) during walking with shoes, corresponding to a slight increase in tibiofemoral force. These findings suggest perhaps more similarities than differences between direct and indirect force assessments. Nonetheless, the potential differences, particularly with respect to increased speeds that might especially apply to running, may be important and clearly need to be explored further. There have been surprisingly few studies of the effect of different types of footwear on peak joint torques and forces, not just during running, but also during walking as well. Moreover, there is a dearth of information regarding the effectiveness or lack of effectiveness of individual footwear design characteristics on these peak values. For example, it would be useful to study the timing and extent of midsole cushioning in relationship to when these peak joint torques and forces occur. Given the potential profound and direct effect that footwear can have on musculoskeletal health, not just in running, we also believe footwear, especially in the athletic category, to be a most critical area in need of further scientific attention and development, ripe for thoughtful analysis and rigorous study. D. Casey Kerrigan, MD, and colleagues JKM Technologies Charlottesville, VA 22903 Disclosure: 1B, developed footwear design used by JKM Technologies, LLC, and the OESH brand; 7B, Brooks Sports, Inc. DOI: 10.1016/j.pmrj.2010.02.007