Hey Jacob,
Two questions in here, one relating to tendon physiology and the other about handling client concerns. Admittedly, the first is easier to answer than the second.
For tendons, this is from a soon-to-be-published article:
How do tendons adapt to training?
Tendons attach muscle to bone and transmit contractile force into movement at the joint. Ligaments stabilize joints. Both adapt to mechanical loading by remodeling toward the demands they consistently experience, a principle articulated as Davis’s Law in the nineteenth century and now built out by the Berlin and Copenhagen tendon labs.
Two findings from this body of work matter for training. First, mechanical properties change before structure. A tendon gets stiffer and stronger before it gets thicker, because the changes happen at the molecular level inside the existing tissue before the tissue itself enlarges. Second, the loading has to be heavy. A pooled meta-analysis of 61 studies and 763 participants found that high-strain protocols, contractions at roughly 85 to 90% of maximum effort held for about three seconds, produced significantly larger gains in tendon stiffness than light-load training.¹⁶ The recommended dose for measurable adaptation is at least twelve weeks at this intensity.
Why does tendon adaptation lag muscle adaptation?
Achilles tendon stiffness determines how efficiently the force a muscle generates gets transmitted to the bone it moves. Stiffer tendons transmit force more efficiently without reducing flexibility or range of motion. In untrained adults, this stiffness takes roughly six to eight weeks of heavy loading to measurably change.¹⁷ In a longitudinal isometric calf training study, muscle strength rose significantly by two months while Achilles tendon stiffness did not reach significance until three months, with a 50% increase from baseline.¹⁸ Even under conditions designed to drive tendon adaptation, there is a six to eight week lag behind the muscle and neural changes.
The same pattern from strength and hypertrophy holds for tendons too: the closer the tissue is to its ceiling, the smaller the short-term response to a new intervention. The Lazarczuk meta-analysis noted that the three included studies recruiting well-trained athletes showed no change in tendon stiffness even with high-intensity training.¹⁶ The same machinery is running, but the room left for it to operate is smaller.
There is no data I’m aware of showing that high reps and low weight do much of anything beneficial for tendons. Underloading aging adults is a problem, too.
For the client, I would ask what concerns he has about his programming as they relate to him the individual, his goals, etc. Unpacking that may help shore up the disconnect between your prescription and his understanding. Ideally, a client wouldn’t be seeking tons of changes to their program because you guys are aligned.
My 0.02.
-Jordan