Effects of resistance and stretching training programmes on the viscoelastic properties of human tendon structures in vivo

Abstract

The present study examined whether resistance and stretching training programmes altered the viscoelastic properties of human tendon structures in vivo. Eight subjects completed 8 weeks (4 days per week) of resistance training which consisted of unilateral plantar flexion at 70 % of one repetition maximum with 10 repetitions per set (5 sets per day). They performed resistance training (RT) on one side and resistance training and static stretching training (RST; 10 min per day, 7 days per week) on the other side. Before and after training, the elongation of the tendon structures in the medial gastrocnemius muscle was directly measured using ultrasonography, while the subjects performed ramp isometric plantar flexion up to the voluntary maximum, followed by a ramp relaxation. The relationship between estimated muscle force (F(m)) and tendon elongation (L) was fitted to a linear regression, the slope of which was defined as stiffness. The hysteresis was calculated as the ratio of the area within the F(m)-L loop to the area beneath the load portion of the curve. The stiffness increased significantly by 18.8 +/- 10.4 % for RT and 15.3 +/- 9.3 % for RST. There was no significant difference in the relative increase of stiffness between RT and RST. The hysteresis, on the other hand, decreased 17 +/- 20 % for RST, but was unchanged for RT. These results suggested that the resistance training increased the stiffness of tendon structures as well as muscle strength and size, and the stretching training affected the viscosity of tendon structures but not the elasticity.

Figure 1. Ultrasonic images of longitudinal sections of medial gastrocnemius muscle during isometric contraction

A marker (X) was placed between the skin and the ultrasonic probe as the landmark to confirm that the probe did not move during measurements. The cross-point (P) was determined from the echoes of the deep aponeurosis and fascicles. P moved proximally during isometric torque development from rest (P₁) to 50 % MVC (P₂). The distance traveled by P (L) was defined as the length change of tendon and aponeurosis during contraction.

Figure 2. The relative changes in the muscle volumes of m. medial gastrocnemius (MG), m. lateral gastrocnemius (LG) and m. soleus (SOL) before and after resistance training for 8 weeks

All the muscle volumes increased significantly. However, there seemed to be no differences in the degree of increase in the muscle volumes among the plantar flexor muscles.

Figure 3. The relationships between F_m and L in RT and RST

For both sides, there were no significant differences in L values at any force levels.

Figure 4. The relationship between % MVC and L before and after training

For the RST side, the % MVC-L loop tended to became smaller after training.

Figure 5. The relationship between passive torque and ankle joint angle during the passive stretch of the triceps surae muscle in RT and RST

For the RT side, the passive torque values at all ankle angles increased significantly after training. On the RST side, there were no significant differences in the passive torque values at any ankle angles. * Significantly greater than Before at P < 0.05.