At the end of last year, there was a buzz in the running community about a study showing that highly cushioned shoes result in higher ground impact forces than lower cushioned shoes. Because studies often circulate in the general public as brief statements (e.g., Twitter) that over-simplify the facts, let’s clarify what was found. The paper was published in Scientific Reports* and compared ground reaction forces in runners wearing the Hoka Conquest (MAX) or the Brooks Ghost 6 (CON) at two speeds: 10 km/h (~6.2 miles/hr) and 14.5 km/h (~9 miles/hr). Heel-forefoot thickness were 43.9-38.5 mm for the Conquest and 34.6-24.1 mm for the Ghost.That’s still a well cushioned shoe; this was not a study of high vs low cushion but rather very high vs high. Note that the lower cushion shoe (CON) also had a much higher heel-forefoot difference (aka, ‘drop’) at 10. 5 mm (vs 5.4 mm for the MAX shoe).
*regularly and incorrectly referenced as a paper in the journal Nature. Scientific Reports is one of the many journals that is part of the Nature Research journal family, which includes the prestigious journal Nature.
What did they find?
There was no difference in step length, contact time, or cadence for runners between the two shoes. Greater leg stiffness was required by runners in the MAX shoes because there was less leg compression on impact and it’s this compression that assists in stabilizing the leg in that part of the gait cycle. With less compression, the soft tissues of the leg must make up the stabilizing difference meaning more strain on muscle, joints, and connective tissue. The take-home message was summarized by the authors: “Our results demonstrate that running in highly cushioned MAX shoes amplifies rather than attenuates impact loading“ by delivering a higher impact peak and loading rate. A big caveat here is that the differences were found only at the higher running speed. At the slower running speed there was little difference in impact loading. During an ultra-marathon, there will be few sections for which these pace-dependent differences might matter...how often do you run a 6.7- minute mile pace over flat ground during an ultra-marathon? And, of course there’s still the lingering question of how much the difference in shoe ‘drop’ contributed to the findings.