Strength rose 79.5% in ten weeks — one muscle grew

Start with the numbers. After ten weeks the trained group could climb carrying 79.5% more relative load — close to a doubling of strength capacity. When the muscles were weighed, exactly one had gained significant relative mass: the flexor hallucis longus (FHL). The soleus was unchanged.

Hypertrophy was not the point of the paper. The point was a new rodent resistance-training model. The standard approach straps weight to the animal's tail, and the tail carries blood flow, so loading it interferes with circulation. The authors built a pulley system driving a load-bearing vest, delivering progressive overload without touching the tail. The ten-week protocol above was run to check whether the device actually works.

How was the study set up?

The animals were spontaneously hypertensive rats (SHR), split into a control group of four and a resistance training group of six. Training was ten weeks of ladder climbing with load in the vest, progressively increased. Functional and morphometric measures were taken afterwards, in two muscles: the soleus and the flexor hallucis longus (FHL).

Strength and mass moved separately

• Relative maximum load — up 79.5% (p = 0.0007). The protocol demonstrably built strength, which was the model-validation result.
• FHL relative mass — significantly increased (p = 0.002). The one muscle that grew.
• Soleus — no significant change. Same animals, same ten weeks.
• Gene expression — no significant difference between trained and control animals (p > 0.05).

Why did only one muscle grow?

The two muscles do different jobs. The soleus is a slow-twitch-dominant postural muscle working at low intensity all day. The FHL flexes the toes to push off and carries a higher proportion of fast-twitch fibres. Ladder climbing is heavy load for short bouts, and that is the stimulus fast-twitch-dominant muscle responds to first. A muscle already saturated with low-grade postural work has less to gain from the same protocol.

And 79.5% is not explained by size alone. A large share of any early strength gain is skill and neural adaptation. Humans show the same ordering — the number moves before the body does. How that plays out in the squat specifically is in squat strength rises before technique changes.

What does a null gene-expression result mean?

Not that training leaves genes alone. It is a timing problem. Expression responses to a training stimulus swing hard in the hours to days after a session and then settle. Measuring once at the end of a ten-week programme samples a new steady state, not the response. On top of that the groups were four and six animals — at that size, only a very large difference clears statistical significance.

What carries over to anyone logging lifts

This is a rat study, but one thing transfers: log strength and size as separate things. Here strength climbed 79.5% while one of two muscles grew. It is easy to conclude a programme has failed when the bar goes up and the mirror does not, and repeated models show the two indices simply move at different rates. How closely muscle size and strength actually track each other is covered in muscle size and strength correlation.

In practice, keep three records apart — load (1RM or estimated 1RM), bodyweight, and measurements or photos. The muscle index calculator runs off the first two, so during a phase where strength leads, the index moves faster than your appearance does. That gap is normal. The separate problem of tendons trailing behind strength is in tendons lag behind strength.