The tendon junction (MTJ) is part of the tendon unit. The tendon unit, usually composed of bony attachments, tendon-tendon junctions and muscles, is responsible for generating skeletal motion .
The MTJ has a unique form in which the muscle membrane has many folds to which collagen fibers from the tendon attach (see Figure 1). This unique construction increases the force transfer area between muscles and tendons for better force distribution and less focus pressure.
The MTJ transmits enormous amounts of force from the muscle to the tendon during strenuous exercise and is therefore a common site for muscle strains. Most of these can be prevented with vigorous eccentric exercise .
Anatomy of the gastrocnemius-soleus MTJ
The weakest area of the tendon unit is the MTJ, so it is the most commonly injured part.
- Large pennate muscles that are multi-articular and generate high tensile stress are most likely to suffer MTJ injuries, such as the biceps femoris quadratus femoris biceps brachii .
- With age, the interdigitations of the MTJ shorten, reducing the contact area for force transmission and increasing the risk of injury. 
US and MRI
For proper diagnosis, there is a grading system for MTJ damage based on MRI or US scan findings.
- Mild strain: feathery interstitial edema and fluid/hemorrhage around MTJ
- Moderate strain: Intramuscular hematoma and perifascial effusion/hemorrhage
- Severe strain: MTJ tear, tendon and muscle flaccidity/discontinuity, sometimes contraction
Old blood products of scar tissue and atrophy/steatosis of muscle suggest an old strain .
- Radiopedia Myotendinous unit Available: https://radiopaedia.org/articles/myotendinous-unit?lang=us(accessed 12.6.2022)
- Radiopedia Myotendinous junction Available:https://radiopaedia.org/articles/myotendinous-junction?lang=us (accessed 12.6.2022)
- Jakobsen JR, Krogsgaard MR. The Myotendinous Junction—A Vulnerable Companion in Sports. A Narrative Review. Frontiers in physiology. 2021;12. Available;https://www.frontiersin.org/articles/10.3389/fphys.2021.635561/full (accessed 12.6.2022)
- Wikimsk MTJ Available:https://wikimsk.org/wiki/Myotendinous_Junction (accessed 12.6.2022)