In vitro studies have shown that ageing decreases tendon stiffness (Nachemson & Evans, 1968; Vogel, 1991). However, physical activity has been found to alter the properties and/or the dimensions of tendons, yielding ultimately stiffer structures (Woo et al. 1982). The aim of the present study was to investigate the effect of strength training (ST) on the mechanical properties of an elderly tendon in vivo: the patella tendon (PT).
After receiving ethical approval and written informed consent, 18 elderly individuals (nine per sex) were randomly assigned to ST (means ± S.D.: age 74.3 ± 3.5 years, body mass 69.7 ± 14.8 kg and height 163.4 ± 9.1 cm) and control (age 67.1 ± 2 years, body mass 73.5 ± 14.9 kg and height 168.3 ± 11.5 cm) groups. Two sets of ~10 leg-extension and leg-press exercises at ~80 % of the five repetition maximum, were performed three times per week for 14 weeks. PT elongation was measured in vivo using B-mode ultrasonography (HDI 3000, ATL, USA) during a ramp isometric knee extension at 90 deg (cf. Maganaris & Paul, 1999). PT forces were calculated by dividing joint moments by MRI-measured PT moment arm length, after taking into account antagonist coactivation estimated from EMG activity. Stress and strain were calculated by normalizing forces and elongations to tendon dimensions, measured using ultrasound. Tendon stiffness (gradient of the force-elongation relationship) was multiplied by the ratio of tendon length to cross-sectional area to obtain Young’s modulus. All measurements were performed before and after the ST period. Results were analysed using a 2 X 2 ANOVA; level of significance was set at P < 0.05. Data are presented as means ± S.D.
Training induced a left shift of the stress-strain relationship indicating a decreased elongation and strain at all levels of force and stress (Fig. 1). Whereas at baseline, PT elongation and strain at maximal tendon load were 4.7 ± 1.1 mm and 9.9 ± 2.2 %, respectively (maximum force: 3346 ± 1168 N; maximum stress: 40 ± 11 MPa), after training these values decreased to 2.9 ± 1.2 mm and 5.9 ± 2.4 % (P < 0.01), respectively (maximum force: 3555 ± 1257 N; maximum stress: 42.1 ± 10.5 MPa). As a result PT stiffness increased by 65 % after ST (2187 ± 713 to 3609 ± 1220 N mm-1; P < 0.05) and Young’s modulus increased by 69 % (1.3 ± 0.3 to 2.2 ± 0.8 GPa; P < 0.01). In contrast, no significant changes in elongation, strain, stiffness or Young’s modulus occurred in the control group. There was no significant tendon hypertrophy following the training or the control periods. The rate of torque development increased by 27% following training (P < 0.01).
In conclusion, this study shows that ST alters the structural and material properties of human elderly tendons. The decreased tendon strain after training may help to reduce the risk of tendon injuries in old age. The increased tendon stiffness after training would increase the rate of force development and may enable the muscle to operate closer to resting length.
The authors would like to thank Technogym for supplying the resistance machines used in this study and the Italian Space Agency (ASI).