Non‐uniform displacement and strain between the soleus and gastrocnemius subtendons of rat Achilles tendon

Abstract

Achilles tendon (AT) comprises of 3 subtendons arising from the soleus (SOL) and the lateral (LG) and medial (MG) heads of the gastrocnemius muscle. While recent human studies show differential displacement within AT, these displacements have not been attributed to specific subtendons. We tested the hypothesis that the SOL and LG subtendons show differential displacement and strain during various combinations of SOL, LG, and MG excitations. Movement of knots, sutured onto SOL and LG subtendons of 12 Wistar rats, was videotaped, while the muscles were stimulated intramuscularly and ankle torque was assessed. When SOL only was stimulated, the plantar flexion torque was the smallest among the different conditions (P < .001). In this condition, from passive to active state, the displacement (0.57 vs 0.47 mm, P = .002) and strain (8.4% vs 2.4%, P < .001) in the SOL subtendon were greater than in LG subtendon. When LG only was stimulated, a higher ankle torque was measured as compared to SOL stimulation (P < .001); the displacement was similar in both subtendons (~0.6 mm), while the strain was greater in LG than in SOL (4.7% vs 1.7%, P < .001). When all 3 muscles were stimulated simultaneously, ankle torque was highest and the displacement (0.79 vs 0.74 mm, P = .002) and strain (7.7% vs 4.4%, P = .003) were greater in SOL than in LG. These data show that the different subtendons of AT can experience relative displacement and differential strains. Together with anatomical dissections, the results revealed that such uniformities may be due to a lower stiffness of SOL subtendon compared to LG.

Biplanar ultrasound investigation of in vivo Achilles tendon displacement non-uniformity

“…This could be explained by lateral force transmission within the Achilles tendon or via intermuscular myofascial structures that would serve to limit non‐uniform distribution of forces within the Achilles tendon and differential displacements of the tissue regions—a hypothesis recently presented by Maas & Finni . The hypothesis that alterations in triceps surae muscle forces induce only limited changes in Achilles tendon non‐uniform motion due to lateral force transmission is supported by a recent experiment in rat in which isolated contractions of different triceps surae muscles via electrical stimulations resulted in only small variations in displacements of the subtendons . Although rat Achilles tendon may not be functionally representative of human Achilles tendon, it nevertheless serves as a model for a tendon in which the distal tendon is a union of tendon fascicles arising from muscles with different anatomical and functional properties similar to human Achilles tendon.…”

Neuromechanical coupling within the human triceps surae and its consequence on individual force sharing strategies

“…As such an imbalance of muscle force would not directly impose an imbalance of strain distribution between the subtendons. In this way, recent studies performed in rats (Finni et al, 2017) and humans (Pekala et al, 2017) have highlighted different mechanical properties between the three subtendons, with large differences between individuals (Pekala et al, 2017). Finally, the twisted structure of the Achilles tendon might also participate in reducing differences in strains between the subtendons (Edama et al, 2015).…”

Mechanical Coupling Between Muscle-Tendon Units Reduces Peak Stresses

“…Within the AT in particular, these differential displacements may be possible because of the presence of distinct subtendons for SOL, LG, and MG muscles (26), but with the applied imaging methods, it is not possible to identify the specific anatomic locations of the displacements in humans. New evidence from in vivo measurements in the rat indicates differential lengthening of the distal tendons of SOL and LG muscles during controlled muscle stimulations (30) and during locomotion (31). This suggests relative displacements between Achilles subtendons, which could, given the material properties of the intersubtendon matrix, facilitate intersubtendon force transmission.…”