In the last few years there have been some pretty interesting studies on muscle and tendon behavior during running. Technology has improved to be able to study the whole muscle tendon unit (comprised of the muscle fibers/fascicles and the “series elastic component” or tendon fibers). We used to think the muscles in the legs would go through pretty large concentric (shortening) and eccentric (lengthening) contractions during running, but some recent studies are showing that this is actually pretty unlikely:
The newest paper (Monte et al. 2020) shows the roles of one of the calf muscles and one of the quad muscles with their respective tendons (Achilles and patellar). The subjects (15 male recreational runners) ran at 10, 13, and 16 km/hr. Interestingly, as speed increased, both muscles didn’t really change length that much (the calf got slightly shorter and quad slightly longer), indicating that they perform mostly isometric contractions and a majority of the range of motion change we see is coming mostly from the Achilles tendon. The contribution of energy towards running also increased much more in the Achilles tendon than the patellar tendon, quad, and calf.
While this is a lab study and there’s a long way to go before its importance is translated to the clinic, this has some pretty significant implications. Training muscles and training tendons aren’t always the same thing, whether it’s in the weight room or structuring our running drills. It also may not be entirely necessary to strengthen a muscle through its full range of motion if it only operates in a very reduced range during running anyway. I’ve been considering this during my own training and with programming strength training for other runners too. Finally, studies like this demonstrate why it’s pretty reasonable that we see Achilles tendinopathy in a large percentage of runners, especially as they age.
(Also feel free to move this thread @adamengst , I didn’t really know where else to put it)