Objective:​ To determine if a difference exists in the plantar soft tissue of patients with diabetes mellitus (DM) and peripheral neuropathy (PN) compared with age-matched controls.

Design:​ Case-control study with a parallel 3-element 1-dimensional viscoelastic model developed to characterize indentation data.

Setting:​ Data collection performed in an academic physical therapy laboratory.

Participants:​ Forty subjects were recruited into 2 groups (20 subjects with DM, PN, and history of plantar ulcers;​ 20 control subjects), matched for age (DM:​ 55.22±9.39y;​ control:​ 55.91±10.97y), gender (DM:​ 14 men, 6 women;​ control:​ 14 men, 6 women), and body mass index (DM:​ 32.96±8.39kg/m²;​ control:​ 32.58±7.69kg/m²).

Interventions:​ The plantar soft tissue stiffness was measured over the first, third, and fifth metatarsals, and heel of each subject using an indentor system that accurately measures force/displacement (F/D) data. A parallel 3-element viscoelastic mechanical model was then used to transform the F/D data into values that were used to make stiffness assessments.

Main Outcome Measures:​ The element coefficients of our model indicated the stiffness of the plantar tissue.

Results:​ The plantar tissue of the subjects with DM over the metatarsal heads was stiffer than the control population as indicated by one of the spring constants in the parallel 3-element model (first:​ 1.13±0.55N/mm vs .72±.32N/mm;​ third:​ .96±.32 N/mm vs .79±.17N/mm;​ fifth:​ .90±.31N/mm vs .69±.28N/mm;​ P.05).

Conclusions:​ The plantar tissue of subjects with DM, PN, and a history of ulcers was stiffer than control subjects. However, additional research is needed to determine the relationship among increased soft tissue stiffness, plantar pressures, and skin breakdown.