The energy cost of barefoot running

Running in bare feet or in light so-called foot gloves is trendy. Its proponents claim the practice is more natural (because our prehistoric ancestors lacked shoes) and easier on a runner’s musculoskeletal system (because without a cushioning sole, a runner adopts a gentler running style).

You might think that barefoot running is also more efficient, given that runners are spared the energy penalty of lifting shoes every stride. But that turns out not to be the case. In a recent blog post , Gretchen Reynolds of the New York Times reported on research conducted by Rodger Kram’s group at the University of Colorado at Boulder.

Kram, Jason Franz, and Corbyn Wierzbinski recruited 12 men who were experienced barefoot runners. As the volunteers ran at a steady pace on motorized treadmills, the rates at which they consumed oxgen, dV _O2/ dt, and produced carbon dioxide were measured. By attaching small, unobtrusive lead weights to the runners’ socks, Kram and his team could isolate the two factors most likely to affect the energy efficiency of barefoot running: weight and gait.

The volunteers ran with and without shoes. Weights were attached in both cases. As you might expect, adding weight raised the amount of oxygen the runners consumed to maintain their pace. Regardless of whether the runners were barefoot or shod, dV _O2/ dt increased by 1% for every 100 g of added weight.

However, running barefoot without added weight offered no efficiency advantage over running while wearing lightweight shoes. In fact, once footwear mass was taken into account, the shod runners consumed oxygen at a rate that was 3â4% lower than the rate for barefoot runners.

When my friend Rich posted a link on Facebook to Reynold’s blog post, I speculated that barefoot runners’ gentler gait might be the cause of the disparity in power. To cushion the impact of each footfall, barefoot runners bend their knees a bit more than shod runners do. Rising from that extra dip entails doing work against gravity and could therefore be responsible for the energy penalty.

An additional 1 cm in vertical dip would cost an 80-kg runner 8 J or 0.002 nutritional calories in mgh work per strideâprovided the runner were 100% efficient at converting food into mechanical work. I’m not sure how metabolically efficient runners are. Concept2, a manufacturer of sophisticated rowing machines, assumes a value of 25% for a 175-lb rower. Using the same efficiency for the 80-kg runner yields a cost per stride of 32 J or 0.0076 nutritional calories.

According to this online calculator , an 80-kg runner expends 475 nutritional calories by running for 30 minutes at a pace of 8 minutes per mile (3.35 m/s or 7.5 mph, the pace of the volunteers in Kram’s experiment). If the 80-kg runner has a cadence of 160 strides per minute, then the cost per stride is 0.1 nutritional calories.

Springs and shock absorbers

The mgh work that extra bending entails seems to be about the right size to account for the loss of efficiency when running barefoot, but that equivalence is not enough to clinch the case. For one thing, as McNeill Alexander, Robert Ker, and Michael Bennett of the University of Leeds have argued , mammalian tendons serve as springs that store and release mechanical energy. Some of the potential energy that barefoot runners give up when they make that extra dip could be recovered when they spring back up.

In their paper , Kram and his colleagues speculate that the energy penalty of barefoot running arises from the work done by runners’ muscles in absorbing the impact of their footfall. Shod runners don’t pay the full penalty because the soles of their shoes do some of the work. Indeed, according to tests done by Sadayuki Ujihashi and his colleagues from Tokyo Institute of Technology, running shoes typically absorb about 55% of impact energy.

Kram himself runs in shoes. In the video you can see him running beside the foothills of the Rocky Mountains near Boulder. I also favor running shoesâbut not when I exercise on a rowing machine. Even though rowing is easier on the feet than running is, I don’t want any of my energy being wasted on compressing cushiony shoes!