Identifying factors related to Achilles tendon stress, strain, and stiffness before and after 6 months of growth in youth 10-14 years of age

Abstract

The purposes of this study were (1) determine if youth peak Achilles tendon (AT) strain, peak AT stress, and AT stiffness, measured during an isometric plantar flexion, differed after six months (mos) of growth, and (2) determine if sex, physical activity level (Physical Activity Questionnaire (PAQ-C)), and/or growth rate (GR) were related to these properties. AT stress, strain, and stiffness were quantified in 20 boys (13.47±0.81 years) and 22 girls (11.18±0.82 years) at 2 times (0 and 6 mos). GR (change in height in 6 mos) was not significantly different between boys and girls (3.5±1.4 and 3.4±1.1cm/6 mos respectively). Peak AT strain and stiffness (mean 3.8±0.4% and 128.9±153.6N/mm, respectively) did not differ between testing sessions or sex. Peak AT stress (22.1±2.4 and 24.0±2.1MPa at 0 and 6 mos, respectively) did not differ between sex and increased significantly at 6 mos due to a significant decrease in AT cross-sectional area (40.6±1.3 and 38.1±1.6mm(2) at 0 and 6 mos, respectively) with no significant difference in peak AT force (882.3±93.9 and 900.3± 65.5N at 0 and 6 mos, respectively). Peak AT stress was significantly greater in subjects with greater PAQ-C scores (9.1% increase with 1 unit increase in PAQ-C score) and smaller in subjects with faster GRs (13.8% decrease with 1cm/6 mos increase in GR). These results indicate that of the AT mechanical properties quantified, none differed between sex, and only peak AT stress significantly differed after 6 months and was related to GR and physical activity.

Figure 1

The basic setup used to quantify muscle-tendon architecture and tendon strain during isometric muscle efforts. A force transducer, located at the ball of the foot, is used to quantify ankle plantar flexion torque developed by the triceps surae muscles. The ultrasound system was used to quantify tendon CSA and AT length during muscle contraction.

Figure 2

Force-deformation curve averaged across all subjects for 0 mos and 6 mos. Standard deviation bars are shown for both deformation and peak F_AT. The passive force in the AT (e.g. at 0% strain) for a 90 degree ankle angle was 18.8 N (18.4 N standard deviation) and 23.1 N (24.7 N standard deviation) at 0 and 6 mos, respectively.

Figure 3

Stress-strain curve averaged across all subjects for 0 mos and 6 mos. Standard deviation bars are shown for both strain and stress.\