Obesity decreases patellar tendon stiffness in females but not males Introduction Patellar tendon (PT) injuries are frequent due to excessive mechanical loading during strenuous physical activity. PT injury incidence is higher in females and obese individuals. The reason behind higher tendon injury incidence in females and obese individuals might be structural changes in tendons such as stiffness or elasticity. Tendon stiffness can recently be quantified using shear wave elastography (SWE). We aimed to examine the stiffness of PT in healthy sedentary participants using this new technology.

This prospective study was carried out with 58 (34 female, 24 male) healthy sedentary participants between the ages of 18–44 years (27.5±7.7 years). Body mass and body fat percentage were measured with the Bioelectrical Impedance method using Tanita BC-418 MA Segmental Body Composition Analyser (Tanita Corporation, Tokyo, Japan). Participants were subsequently categorized into ‘normal-weight’ (BMI < 23 kg/m2) and ‘obese’ (BMI>27.5 kg/m2). SWE of the PT was measured with the ACUSON S3000 (Siemens Medical Solution, Mountain Wiew, CA, USA) ultrasound device using the Siemens 9L4 (4–9 MHz) linear-array probe with the Virtual Touch Imaging Quantification® method. The measurement was performed by placing the US probe longitudinally on patellar tendon with knee flexed at 30°. The region between about 1 cm distal of patellar bone-tendon junction and 1 cm proximal of bone-tendon junction of tibia was used for PT stiffness measurement (Figure 1). Average of three successive measurements at 10 sec intervals was recorded as PT stiffness. PT stiffness was quantified with MATLAB Version 2015 (Mathworks, Massachusetts, USA) by converting colour data into numbers.

PT stiffness, in males, in females, in normal males, in obese males, in normal females, and in obese females was 8.6±1.0 m/sec, 7.4±1.1 m/sec, 8.6±1.1 m/sec, 8.5±1.0 m/sec, 7.9±0.9 m/sec, and 6.2±0.9 m/sec, respectively. Average body fat percentage in males, in females, in normal males, in obese males, in normal females, and in obese females was 20.1±7.4 kg/m2, 30.1±8.1 kg/m2, 15.4±5.2 kg/m2, 24.7±4.6 kg/m2, 25.6±5.5 kg/m2, and 38.1±5.0 kg/m2, respectively. Males PT stiffness was higher when compared to that of females (p=0.000). PT stiffness was similar in obese and normal males (p=0.962) but obese females had lower PT stiffness compared to normal females (p=0.001).

PT stiffness of females was lower than males and obesity decreased PT stiffness in females but not in males. The possible explanation of lower PT stiffness in females might be due to their higher estrogen levels that lead to a decrease in estradiol level and collagen synthesis. Lower tendon stiffness in obese females might be metabolic effects due to the increased adipose tissue that contains proteins such as adipokinome, chemerin, lipocalin 2, serum amyloid A3 and adiponectin. These proteins lead to disturbance of tendon homeostasis and decreased collagen content. Altered tendon homeostasis and decreased collagen content may lead to a decrease in tendon stiffness. Decreased PT stiffness in especially in obese women might be associated with increased risk of PT injury.