Effects of footwear and heel elevation on tensile load in the Achilles tendon during treadmill walking

This study used transmission-mode ultrasound (US) to evaluate the effect of footwear, with differing heel elevation, on Achilles tendon loading and basic gait parameters. Axial transmission velocity of US was measured in the right Achilles tendon of 20 healthy adults (10 male and 10 female; age: 31 +/- 9 years; height: 1.72 +/- 0.04 m; weight: 67.8 +/- 14.2 kg) while walking barefoot (BF) and in four prototype athletic shoes (S1-S4) on an instrumented treadmill. Shoes were constructed from identical materials and differed only in heel elevation (S1: 0.4 mm; S2: 5 mm, S3: 9.9 mm, S4: 14.8 mm), which was achieved by varying the graded thickness of the midsole beneath the rearfoot. Temporospatial gait parameters, sagittal ankle movement, and vertical ground reaction force were simultaneously recorded at rate of 120 Hz. Shod walking conditions were characterized by a significantly lower cadence, longer stance duration, greater ankle dorsiflexion, larger peak vertical ground reaction force, and higher US transmission velocity in the Achilles tendon than BF walking (P < .05). Incremental heel elevation in footwear resulted in a progressive increase in peak vertical force, a reduction in peak external loading rate, and a small but progressive reduction in US transmission velocity in the Achilles tendon (P < .05). Simple regression modelling predicted that a shoe with 58 mm of heel elevation would effectively replicate values observed during BF walking. Peak US transmission velocity, and hence tensile load, in the Achilles tendon was higher during shod than BF walking and was partially countered by increasing the heel elevation of the shoe. These findings have important clinical implications for the use of footwear in performance applications and in the prevention and rehabilitation of Achilles tendon disorders.