Six million cells that never stop twitching, and the control group that could still run

Start with the control group. Every animal in the study, grafted or sham-injected, was free to move around its cage for the duration ^[4]. So the gains in muscle mass and immune function were measured on top of ordinary rodent activity, not in place of it. The population the work is aimed at, people whose conditions stop them exercising ^[12], is the one state the experiment did not model. Nothing in the data therefore fixes an effect size for a body that is not moving at all.

The mechanism on offer is not mechanical. Muscle stem cells were taken from mice, cultured until they matured into myocytes, and injected under the skin of the same animals ^[2]; the resulting patch built its own network of small blood vessels and pulsed on regardless of whether the mouse was awake ^[3]. Tang Hong-Wen of Duke-NUS Medical School, who was not involved in the work, reads the distant effects as evidence that contracting muscle is acting as an endocrine organ, changing circulating factors that reach other tissues ^[11].

That reading is where the study argues with its own commentary. In 18-month-old mice and in mice fattened by diet, the patch improved bone density along with strength and endurance ^[5], yet Tang also cautions that a small subcutaneous graft cannot fully reproduce the cardiovascular, neuromuscular, mechanical and bone-loading effects of exercise ^[8]. If denser bone appeared in animals whose skeletons were never loaded any differently, the route was humoral, and that is the single result most worth attacking with a replication.

Durability is thinner than the framing suggests. Imaging confirmed surviving patches to 81 days, when measurements stopped, and Ng Shyh-Chang says unpublished follow-up shows them stable at six months with little volume loss and no sign of tumour growth ^[9]. The published window is under half the period being claimed ^[10], which is a slim base for his remark that there is no apparent limit to the graft's lifespan ^[9].

Then there is the patient's side of it. Matthew Stroud at King's College London calls exercise mimicry for people who cannot exercise an appealing idea and asks how comfortable a continuously contracting graft would be ^[13]. For someone with limited mobility who cannot easily shift position, a lump under the skin of the back that pulses through the night is the part that will decide adherence, not the cytokine panel. Talks with several hospitals are under way and trials could start within a year ^[12].