A recent study compared ten healthy young males walking at a set pace in running shoes, flip-flops, sandals (think Adidas slippers), and barefoot1. Shoes were found to produce less Ground Reaction Force (GRF), meaning less force going through the foot as the foot contacts and pushes off from the ground. Barefoot walking produced the greatest GRF, with sandals and flip-flops somewhere in the middle. Obviously cushioning for shock absorption factors in, but the increased heel height (think sneaker versus flip-flop) lends to a more pointed foot when contacting the ground. Less work is then required from your anterior tibialis (shin muscle) to slow your ankle so as not to slap your foot onto the ground. Meaning that flip-flops may not be the best choice for those with shin splints, stress fractures, or anterior compartment syndrome. However, this particular study maintained a constant walking speed, or cadence, regardless of footwear. Differences may have been more pronounced or skewed if the required cadence was faster than what is comfortable for your average -person.
Lack of arch support is often the biggest objection to flip-flops. However, the amount of arch support flip-flops do or do not provide is a bit mystifying. In the abovementioned study, the more “shoe” there was, the more mediolateral (side to side) motion noted. Shoes may provide more support than flip-flops, and certainly more than barefoot walking. But the amount of eversion, motion associated with what we think of as arch collapsing, was the same from shoe to barefoot. Thus, shoes help decrease side to side motion but, according to this study, do not appear to effect collapsing of the arch.
Another interesting point was the possible change in source of propulsion. Open toed shoes saw more hip flexion during gait. This may mean that a sandal clad or flip-flopping walker may propel him or herself forward via a hip flexion strategy, picking up the leg, rather than by pushing off of the toes. As we already tend towards tight hip flexors with how much we sit, this does not bode well. Those with hip impingement, hip flexor dominance or tendonitis may want to take note.
But no matter what the speed or gait pattern, mechanics will be different with flip-flops because of their structure. Recent observations from Zinkin and Sutera², noted toe involvement, namely via gripping, with flip-flop wear. The constant flexion at various toe joints, in attempt to keep the flip-flop on, may aggravate the band of tissue spanning the bottom of your foot (your plantar fascia) or create a constant position of flexion (such as hammertoes or bunions). A strap across the back of the sandal may reduce these effects.
This information was based on a study that, though most recent, only had a sample size of ten subjects of a specific gender and age range. Thus, it should not be considered generalizable across all demographics. More research is required for us to form valid opinions on whether or not wearing flip-flops can be damaging. Currently the most you can glean from the literature out there is that gait is different when walking in shoes versus flip-flops. Until then, stop walking at the typical NYC fast pace to reduce the fall out!
1) Zhang X, Paquette MR, Zhang S. (2013 Nov 6). “A comparison of gait biomechanics of flip-flops, sandals, barefoot and shoes.” J Foot Ankle Res;6(1):45. doi: 10.1186/1757-1146-6-45.