Chris Beardsley graduated from Durham University with a Masters Degree in 2001. He since contributed to the fields of sports science and sports medicine by working alongside researchers from Team GB boxing, the School of Sport and Recreation at Auckland University of Technology, the Faculty of Sport at the University of Ljubljana, the Department of Sport at Staffordshire University, and the College of Health Solutions at Arizona State University. He is also a Director at Strength and Conditioning Research Limited
Muscles are made up of individual muscle fibers, which are arranged into bundles of muscle fibers, called fascicles.
Each muscle fiber is surrounded by a layer of connective tissue made up of collagen proteins, called the endomysium. Muscle fascicles are surrounded by a layer called the perimysium, and the muscle itself is surrounded by a layer known as the epimysium.
Together, these collagen layers are known as the extracellular matrix (ECM) of the muscle. They provide support for the muscle fibers and muscle fascicles, and are also involved in the transmission of forces.
The endomysium, which surrounds single muscle fibers, is particularly important for force transmission. When muscle fibers contract, they transmit force laterally through structures that connect their basement membranes to the endomysium. The endomysium then transmits force laterally into the rest of the muscle and longitudinally to the tendon.
Since the ECM functions to provide support for the muscle, the collagen content of it should increase as muscle fibers increase in size after strength training. This suggests that we should observe an increase in collagen protein synthesis alongside the increase in muscle protein synthesis after a strength training workout.
And indeed, that is what happens.
As you can see from the above infographic, when matched for work done, eccentric and concentric training increase collagen protein synthesis similarly after strength training.
Since concentric training does not tend to cause very much muscle damage, this supports the idea that the post-workout increase in collagen content is mainly used for adapting the muscle structure in order to accommodate changes in muscle fiber size.
Even so, eccentric training leads to greater collagen protein synthesis than concentric training when using the same number of sets and reps (volume) and not the same work done, as you can see from the following infographic.
Together, these findings suggest that there is a dose-response effect for the increase in collagen content after strength training, such that greater work done leads to greater increases.
And interestingly, this further supports the idea that the collagen is being used to restructure the muscle to accommodate the greater muscle fiber size, because muscles also increase to a greater extent with higher training volumes.
It must be admitted that until recently, there was little evidence for increases in collagen content within a muscle after long-term strength training.
In the last few months however, it was discovered that a standard strength training program could produce increases in type IV collagen within the endomysium of human muscle fibers, and also displayed trends for increases in types I, VI, and XII.
As well as providing a larger structure for a larger muscle fiber, the increased collagen content of the endomysium could also increase muscle stiffness (which is a common adaptation to strength training).
In addition, it has been suggested that the increased collagen content in the endomysium could make the muscle more able to store elastic energy, potentially protecting it from future strain injury. However, this is still speculative, and we are still a long way away from understanding the interactions between increased collagen content, increased collagen cross-links, and changes in the lateral connections between the muscle fiber and the endomysium after various types of strength training.