Landing Impact Forces Revisited

While I was at the Gait and Clinical Movement Analysis Society Conference in Cincinnati, OH a few weeks back, I saw an interesting presentation on landing impact loading titled: "The effect of plantarflexion angle on landing mechanics using a within-subjects real-time feedback protocol."  This was not the usual bit on clinical gait analysis or the study of walking and its clinical implications.  Rather it was more oriented towards sports performance and using form to mitigate injuries from landing.

It's long been known that pointing your toe, or increasing your plantarflexion results in a lower impact upon landing [1][2].  The theory is that the ankle can flex faster and thereby distribute the load to your muscles (which can take it) rather than to the skeletal system (which can't).  Dr. Jim Richards, my Ph.D. advisor, noticed that ice skaters are particularly at risk from high impacts due to the skating boot itself. Ice skates generally do not flex *at all* at the ankle and thus require that landing impacts be absorbed primarily at the knee and hip and in the bones and ligaments.  This causes all manner of mayhem in the anatomy of skaters: Tara Lipinski, an Olympic class skater, has had a number of problems in her hips and back [3].  Richards addressed the limitations in skates by developing ankle articulated ice skates in the early 1990s so that the ankle could take more of the load. Recent work has shown that these can significantly reduce loading rate on landing from skating jumps [4].

My own masters thesis work reflects the value of plantarflexion on landing.

"Jump toe tap" kick from contemporary wushu

We looked at the landings from three kicks commonly done in Chinese Wushu: the jump toe tap, the jump inside crescent (or tornado kick), and the butterfly kick.  Eight subjects who were ranked at the regional or national level were recruited to perform the jumps.  Subjects were marked up with retro-reflective markers and then filmed doing their jumps onto force plates at the University of Delaware Sports Science Lab (now the Human Performance Lab).  You can find the full results in the University of Delaware Library or selected portions in the Journal of Asian Martial Arts [5].  But the key points were that these eight subjects landed in two distinct ways: some were heel landers, and some were toe landers.  Those who landed on their heels had nearly twice the force at the ankles and knees (up to 3 x body weight) compared to those who landed on their toes (who peaked at about 1.5 body weights).

What's interesting about the recent work by Mike Rowley and Jim Richards is that it quantifies the degree of plantarflexion that is most effective in reducing joint forces [6].

From: Rowley & Richards: The effect of plantarflexion angle on landing mechanics using a within-subjects real-time feedback protocol

Rowley & Richards recruited 26 subjects, marked them up with retroreflective markers for video capture, and then had them drop from a hang onto force plates.  A feedback system informed the subjects of their ankle flexion angles which then were held as they dropped onto the force plates.  Figure 1 shows the ground reaction force (GRF) in body weights (BW), and the peak loading rate in BW*100 per seconds.  Both GRF and peak loading rates decrease as ankle flexion increases.  In these graphs 90 degrees is flat footed, so larger angles imply more plantarflexion.  Although the returns diminish as plantarflexion increases, it still seems that more plantarflexion is better.  Except that in Figure 2, we see a graph of peak support moments or joint torque as a function of angle.  Remember that with the ice skaters, the chronic injuries tended to be in the hips and lower back.  Therefore, it is desirable to decrease the hip moment the most.  The point at which this occurs is at 120 degrees.  After this, the hip flexion moment increases again. 

What this study seems to imply is that under these conditions, roughly 30 degrees of toe point (or 120 degrees of plantarflexion) will go a long way towards minimizes forces, loading rate and hip torque in landing.  Mike Rowley mentioned to me that he would be recruiting more subjects to look at this in a more sport specific way.  I don't know if he will be recruiting from the wushu crowd, but if you're interested, I can always ask.

References:
[1] Cavenaugh & LaFortune (1980)  J. Biomech 13, 397-406.
[2] Kovacs et. al. (1999) Foot placement modifies kinematics and kinetics during drop jumping. Med Sci Sports Exerc 31 (5): 708-16.
[3] Wilner (2000) Lipinski retires over hip injury. http://www.cbc.ca/sports/story/2000/12/19/lipinski001219.html
[4] Richards & Bruening (2006) Analysis of an articulated figure skating boot.  American Society of Biomechanics. http://www.asbweb.org/conferences/2006/pdfs/344.pdf
[5] Niiler (1998) Landing impact loading and injury risk to the lower legs in Chinese Wushu.  http://www.journalofasianmartialarts.com/product/china/other-practices/landing-impact,-loading,-and-injury-risk-to-the-lower-legs-in-chinese-wushu-detail-161
[6] Rowley & Richards (2013), The effect of plantarflexion angle on landing mechanics using a within-subjects real-time feedback protocol.  Proceedings of GCMAS.