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A comparison of plantar pressures in a standard flip-flop and a FitFlop using bespoke pressure insoles

Purpose: The study was undertaken to compare plantar pressures in a Havaiana flip-flop to a FitFlop, a flip-flop with a multi-density midsole designed to induce instability. It was hypothesised that in the Havaiana the toes are used to ‘grip’ the shoe in swing and the loose upper and thin sole provide limited protection to the foot, producing higher plantar pressures than the FitFlop. Methods: Twenty female subjects walked in the footwear conditions while a bespoke instrumented insole quantified plantar pressures. Data analysis grouped sensors into regions for the heel, 1st metatarsophalangeal joint and hallux to isolate pressures that have been linked to comfort and symptoms reportedly alleviated in the FitFlop. Additional analysis was undertaken to measure hallux ‘gripping’ during swing. Results: Significant reductions in plantar pressures in the FitFlop, particularly in peak pressure in the heel (3.6%) and pressure-time integral in the 1stmetatarsophalangeal joint (12.0%) were identified. These findings were attributed to the thicker midsole with different ethylene vinyl acetate (EVA) construction and a redistribution of load to the midfoot where contact area increased by 19.9% compared to the Havaiana. Also evident were reductions in anterior-posterior centre of pressure velocity in the FitFlop, attributed to its softer midfoot delaying progression. Hallux variables identified reductions in time spent ‘gripping’ as well as the magnitude of force applied by the hallux in swing in the FitFlop. Conclusions: Findings from the study identify that the FitFlop reduces pressure in key areas of the foot which are associated with walking comfort as well as clinical conditions. The ‘gripping’ mechanism postulated to hold flip-flops on is lessened in the FitFlop, potentially reducing the likelihood of overuse injuries.