Top end heel raises. The top end might matter.

Thought experiment . . .
If top end posterior compartment (loosely, the calf complex) strength is lacking, then heel rise may not be optimized to transfer body mass forward sufficiently and effectively.
This lack of forward progression, fails to move the body mass sufficiently forward enough to reduce the external moment arms and optimize the internal moment arms to take maximal advantage of the calf complex (I am talking about moment arms between the grounded 1st MTP joint and ankle mortise & ankle mortise and achilles tendon).
These are rough thoughts today gang, letting you inside our heads and how we juggle multiple parameters when we are struggling to solve a client's problems.

In the lower heel rise photo, The body mass does not progress forward enough over the grounded first MTP joint at the big toe (during gait, the heel doesn’t just rise up, the axis of the ankle joint moves both up and forward).
In this case, the foot may not be fully rigid in a supinated position to benefit from joint closed-packed positions. Thus, the foot may be more pliable and one might suppose that if not adequately supinated, they are inadequately still too much relatively pronated. This might put more load into the tibialis posterior and other soft tissue mechanical loading scenarios that are less optimally suited to do this job. Over time, might this lead to pathology? Likely.
Thus, when running on a weaker posterior mechanism (often found unilaterally) the higher up posterior chains might be overburdened, the tendon loads and loading response of the achilles, tibialis posterior, and long flexors will be most likely altered, likely negatively, the naturally occurring foot locking mechanisms might be less optimal than desired, subtalar and forefoot loading might be premature (ie. sesamoid malpositioning for one, as a simple example), etc etc. Loading a foot(the mid and forefoot) into heel rise that is still somewhat pronated creates a different moment arm around the subtalar joint axis (that moves through the 1st metatarsal), than a foot that is more supinated.

Now, put these ideas into the 2 photos from yesterday where one might be loading the forefoot laterally or more medially, and now make the top end strength more in one of those scenarios. Is it any wonder why so many struggle with posteiror mechanism tendonopathies ? There are so many parameters to consider and examine. And, if not examined in great detail, the key lacking parameter can be missed.
Hence, just forcing calf strength loading is too simple a solution, there is a needle in that haystack that upset the client's apple cart, it is the job of the clinician to find it and remedy it.

Today, looking into the research and finding some interesting things that are spurring some thoughts.

Shawn Allen, one of the gait guys

Effects of high loading by eccentric triceps surae training on Achilles tendon properties in humans.

Maximum isometric force had increased by 49% and tendon CSA by 17% !
Tendons can change their cross sectional area, if you load them.

Here I show lateral forefoot loading in a heel raise, and a medial forefoot loading in heel raise. This has to be part of the discovery process outlined below.

Isometrics are useful, they have their place. In a recent podcast to load up here in the future, we discuss the place and time to use isometrics, isotonics, eccentrics and concentrics.
One of the goals in a tendinopathy is to restore the tendon stiffness. Isometrics are a safe way to load the muscle tendon complex without engaging a movement that might have to go through a painful arc of movement. With isometrics here is neurologic over-spill into the painful arc without having to actually go there.
The key seems to be load. More load seems to get most people further along. Remember, the tendon is often problematic because it is inflamed and cannot provide a stiffness across its expanse. Heavy isometric loading seems to be a huge key for most cases. But, we have to say it here, not everyone fits this mold. Some tendons, in some people, will respond better to eccentrics, and strangely enough, some cases like stretching (perhaps because this is a subset of an eccentric it seems or because there is a range of motion issue in the joint that is a subset of the problem). Now the literature suggests that stretching is foolish, but each case is unique all in its own way, and finding what works for a client is their medicine, regardless of what the literature and research says.
Finding the right load for a given tendon and a right frequency of loading and duration of loading is also case by case specific. Part of finding the right loading position is a discovery process as well, as noted in the photos above. Finding the fascicles you want to load, and the ones you do not want to load (painful) can be a challenging discovery process for you and your client. Finding the right slice of the pie to load, and the ones not to load takes experimentation. When it is the achilles complex, finding the safe However, if one is looking for a rough template to build from, brief, often, heavy pain free loads is a good template recipe to start with.

Here, in this Geremia et al article, "ultrasound was used to determine Achilles tendon cross-sectional area (CSA), length and elongation as a function of plantar flexion torque during voluntary plantar flexion."
They discovered that, "At the end of the training program, maximum isometric force had increased by 49% and tendon CSA by 17%, but tendon length, maximal tendon elongation and maximal strain were unchanged. Hence, tendon stiffness had increased by 82%, and so had Young’s modulus, by 86%.

Effects of high loading by eccentric triceps surae training on Achilles tendon properties in humans. Jeam Marcel Geremia, Bruno Manfredini Baroni, Maarten Frank Bobbert, Rodrigo Rico Bini, Fabio Juner Lanferdini, Marco Aurélio Vaz
European Journal of Applied Physiology
August 2018, Volume 118, Issue 8, pp 1725–1736