In vivo strain in the deep and superficial regions of the human patellar tendon

This study investigated strain differences within the patellar tendon (PT) mid‐region using an ultrasound‐based digital image correlation (DIC) method. Six healthy young participants performed six knee extensions to 60% of maximal voluntary isometric contraction on 2 days. Sagittal ultrasound videos recorded during each contraction were analyzed using the DIC method to determine the strain–torque relationships of the superficial, deep, and whole PT mid‐regions. Significantly greater strain was observed in the deep vs superficial layer of the PT mid‐region for all contraction intensities, with peak strains of 5.8% (SD 1.7) and 4.5% (SD 1.5), respectively. DIC‐based measures of peak tendon strain were repeatable within [intraclass coefficients (ICC) >0.97] and between sessions (ICCs >0.83) and agreed well with the conventional point‐to‐point method. This study confirmed that significant differences exist between deep and superficial layers of PT mid‐region during ramped isometric extensions. These findings support the use of DIC to examine regional strain patterns within the PT mid‐region that may be important in the context of tendon injury and adaptation.