Load-displacement properties of the human triceps surae aponeurosis in vivo

Abstract

1. The present investigation measured the load-displacement and stress-strain characteristics of the proximal and distal human triceps surae aponeurosis and tendon in vivo during graded voluntary 10 s isometric plantarflexion efforts in five subjects. 2. During the contractions synchronous real-time ultrasonography of aponeurosis displacement, electromyography of the gastrocnemius, soleus and dorsiflexor muscles, and joint angular rotation were obtained. Tendon cross-sectional area and moment arm were obtained from magnetic resonance (MR) images. Force and electromyography data from dorsiflexion efforts were used to examine the effect of coactivation. 3. Tendon force was calculated from the joint moments and tendon moment arm, and stress was obtained by dividing force by cross-sectional area. Aponeurosis and tendon strain were obtained from the displacements normalised to tendon length. 4. Tendon force was 3171 +/- 201 N, which corresponded to 2.6 % less than the estimated force when coactivation was accounted for (3255 +/- 206 N). Aponeurosis displacement (13.9- 12.9 mm) decreased 30 % (9.6-10.7 mm) when accounting for joint angular rotation (3.6 deg). Coactivation and angular rotation-corrected stiffness yielded a quadratic relationship, R 2 = 0.98 +/- 0.01, which was similar for the proximal (467 N mm(-1)) and distal (494 N mm(-1)) aponeurosis and tendon. Maximal strain and stress were 4.4-5.6 % and 41.6 +/- 3.9 MPa, respectively, which resulted in a Young's modulus of 1048-1474 MPa. 5. The mechanical properties of the human triceps surae aponeurosis and tendon in vivo were for the first time examined. The stiffness and Young's modulus exceeded those previously reported for the tibialis anterior tendon in vivo, but were similar to those obtained for various isolated mammalian and human tendons.

Figures

Figure 1. Measurement set-up

A, the subjects were seated in a rigid steel frame with the knee extended and the hip flexed to 90 deg. The foot rested against an adjustable steel foot plate with a mechanical axis of rotation that corresponded to the lateral malleolus. (1) EMG activity of the medial gastrocnemius (not shown), soleus and dorsiflexor muscles were registered. (2) Joint angular change was monitored with an electrical goniometer. (3) To register plantarflexion force (N) a strain gauge load cell was attached between the foot plate and the steel frame. One computer was used to sample 1-3, while a separate computer was used to sample the ultrasound data. The two computers were interconnected electronically to ensure that all signal sampling was synchronous. Synchronisation between the computers was achieved with a custom-built device that provided a visual marker on the ultrasound image and simultaneously initiated data sampling of force, EMG and joint angular change via the A/D converter. B, the connective tissue length (Lo) of the triceps surae complex (free tendon and aponeurosis) was obtained by measuring the distance from the sole of the foot to the proximal (a) and distal point (b) of the fascicle aponeurosis cross-point on the ultrasound image at rest.

Figure 2. Sonography of the proximal aponeurosis

GA, gastrocnemius muscle; SO, soleus muscle. The measurement was performed along the length of aponeurosis from the white vertical bar to the end of the ultrasound (US) field. Note the shift in the displacement of the aponeurosis to the left during the graded isometric contraction effort from rest to 2000 N of tendon force. The US data presented correspond to subject a in Fig. 8. The aponeurosis between the gastrocnemius and the soleus muscle was seen on the ultrasound as two distinct entities with a small separating space. This was noted in all subjects.

Figure 8. The Load–displacement of the proximal aponeurosis (A) and stress-strain (B) data for 2 subjects (a and b)

Note that 2 two subjects have approximately similar tendon forces, but due to an appreciable difference in aponeurosis displacement subject a has a greater stiffness (667 N mm−1) than subject b (371 N mm−1). On the other hand, the stress was almost 50 % lower in subject a due to the larger tendon cross-sectional area. The Young’s modulus was 1806 MPa for subject a and 1556 MPa for subject b.

Figure 3. The synchronised data recorded during a 10 s plantarflexion effort for one subject

A, the dorsiflexor EMG activity normalised to its maximum amplitude during maximum isometric dorsiflexion. B and C, the gastrocnemius (B) and soleus (C) EMG activity normalised to its maximum amplitude during maximum isometric dorsiflexion. D, the proximal aponeurosis displacement recorded with the ultrasonography, and corrected for joint angular motion. E, joint angular motion (deg). F, the tendon force (N) has been calculated using tendon moment arm data from MR images, and corrected for antagonist coactivation.

Figure 4. The Load–displacement curve for one subject

○, without any correction for ankle joint rotation; •, corrected for tendon movement attributed to ankle joint rotation. ▵, corrected for ankle joint rotation and antagonist coactivation. The mechanical stiffness (N mm−1) was calculated from the Load–displacement relationship in the final 10 % of the force range, i.e. from 90 to 100 % based on the quadratic fit.

Figure 5. The mechanical stiffness for the proximal and distal aponeurosis

The bars show group means ±s.e.m. The mechanical stiffness (N mm−1) for the proximal (A) and distal (B) aponeurosis (1) without any correction for ankle joint rotation, (2) corrected for tendon movement attributed to ankle joint rotation, and (3) corrected for ankle joint rotation and antagonist coactivation. There was a sigificant increase in the stiffness when ankle joint angular rotation was accounted for (**P < 0.01) and when antagonist coactivation was accounted for (*P < 0.05).

Figure 6. Absolute Load–displacement data for the proximal and distal aponeurosis after correction for ankle joint rotatation and antagonist coactivation

Data are group means ±s.e.m. The mechanical stiffness (N mm−1) was calculated from the Load–displacement relationship after a quadratic fit was applied for each person. There was no significant difference between the proximal and distal aponeurosis.

Figure 7. Estimated stress-strain for the proximal and distal aponeurosis after correction for ankle joint rotatation and antagonist coactivation based on the quadratic fit

Data are group means ±s.e.m. The quadratic fit yielded somewhat lower ‘maximal’ values than the maximal values reported in Tables 1 and 2. The maximal strain and the Young’s modulus were significantly different for the distal (5.6 ± 0.4 %, 1048 ± 93 MPa) and proximal aponeurosis (4.4 ± 0.5 %, 1474 ± 100 MPa); P < 0.05.