Modelling activity dependent diametral strain in Achilles tendon

The Achilles tendon is a viscoelastic tissue that typically experiences 3500 ± 1700 cyclic loads per day from intermittent periods of ambulatory activity. Typically, peak tensile loads exceed three times body weight and average about 1500 N during stance, which lasts for approximately 0.6 s, followed by a 0.2 s unloaded swing phase. Viscoelastic materials respond to external load (stress) in a time-dependent manner commonly referred to as creep deformation (strain) and recover slowly when unloaded. This can be observed in vivo by monitoring changes in tendon diameter using quantitative ultrasonography and is referred to here as diametral strain. Diametral strains between −25% and +10% have been recorded over a 24 h period and are hypothesised to be associated with fluid movement within the tendon that corresponds with the creep and recovery histories. Changes in tendon diameter were taken at five times throughout a 24 h period in 11 subjects. Ambulatory activity was monitored as time stamped cadence periods by a Polar RS800sd module enabling time of day and activity duration to be used as indicators of creep and recovery histories. These archival records of diametral strain versus activity were then used to develop an adaptive non-linear viscoelastic model for interpolating tendon cross-sectional dimensions between observation points and from which, site specific volumetric flow rates can be estimated. This model has an error bound of less than 5% and has the potential for application to future studies linking fluid flow within the tissue to inherent biomechanical properties, injury status or pathological defects