In my analysis of the possibility of growing taller with swimming, I pointed out how physiologically it is peculiar to have long arms and not long legs. The bones of the arms and the legs are long bones, why would they not grow at the same rate? It makes sense to have a long torso but short appendages as they can grow with different mechanisms but not to have long arms and short legs.
One hypothesis is that evolutionary it is advantageous to have longer arms than legs as that would increase your reach while adding less to your weight. But evolutionary advantage aside, their is still some sort of mechanism for the body to "order" the bones of the arms to be longer than the legs. What is that mechanism?
Is the mechanism local or systemic? Does the body say send more growth hormone to the bones of the arms and the legs? Or, are the chondrocytes of the arm bones more prone to proliferation than the leg bones?
One-year treatment with recombinant human growth hormone of children with meningomyelocele and growth hormone deficiency: a comparison of supine length and arm span.
Supine length refers to laying down height.
"Growth retardation and precocious puberty are frequently found in children with meningomyelocele (MMC). Lower limb contractions, spasticity and kyphoscoliosis may lead to disproportionate short stature. Most of these patients have structural brain defects or hydrocephalus which can cause growth hormone deficiency. In this study, 19 children aged between 3.5 and 12.8 years with MMC and growth hormone (GH) deficiency were treated with recombinant human GH for a period of 12 months. Supine length, arm span and growth velocity were compared before, and after 6 and 12 months of treatment with rhGH (daily dose 2.0 IU/m2 BSA s.c.). Mean supine length standard deviation score (SDS) increased by +0.8 SDS after 6 months and +1.2 SDS after 12 months of therapy. Mean arm span standard deviation score increased by +0.9 SDS and +1.3 SDS. Growth velocity increased in supine length from 3.3 cm/yr (-2.1 SDS) to 8.4 cm/yr (+2.4 SDS) and in arm span from 4.8 cm/yr (-1.3 SDS) to 8.6 cm/yr (+3.1 SDS) in the first 6 months and was 8.1 cm/yr (+2.4 SDS) and 8.3 cm/yr (+2.6 SDS) after 12 months of therapy. Linear correlation between SDS growth velocity supine length and SDS growth velocity arm span during one year of treatment was excellent (r = 0.65, p < 0.0025). We surmise that body proportions do not deteriorate when growth velocity is stimulated in MMC patients. Both supine length and arm span measurements are necessary to document growth in children with spinal dysraphism."
Now supine length incorporates both torso and leg length. We can see though that the growth velocity between the arm span and torso/leg height was about the same. So, that is one strike against a systemic factor making the arms longer than the legs as if the growth rate of the arms and the legs is the same than the only way for the arms to be longer is for the arms to grow for a longer period of time.
Here's what one paper suggests that the local mechanism is:
Developmental basis of limb length in rodents: evidence for multiple divisions of labor in mechanisms of endochondral bone growth.
"Mammals are remarkably diverse in limb lengths and proportions, but the number and kind of developmental mechanisms that contribute to length differences between limb bones remain largely unknown. Intra- and interspecific differences in bone length could result from variations in the cellular processes of endochondral bone growth, creating differences in rates of chondrocyte proliferation or hypertrophy, variation in the shape and size of chondrocytes, differences in the number of chondrocytes in precursor populations and throughout growth, or a combination of these mechanisms. To address these questions, this study compared cellular mechanisms of endochondral bone growth in cross-sectional ontogenetic series of the appendicular skeleton of two rodent species: the mouse (Mus musculus) and Mongolian gerbil (Meriones unguiculatus). Results indicate that multiple cellular processes of endochondral bone growth contribute to phenotypic differences in limb bone length. The data also suggest that separate developmental processes contribute to intraspecific length differences in proximal versus distal limb bones[proximal bones are closer to the body, distal bones are farther away], and that these proximo-distal mechanisms are distinct from mechanisms that contribute to interspecific differences in limb bone length related to body size. These developmental "divisions of labor" are hypothesized to be important features of vertebrate limb development that allow (1) morphology in the autopods to evolve independently of the proximal limb skeleton, and (2) adaptive changes in limb proportions related to locomotion to evolve independently of evolutionary changes in body size."
Interspecific refers to differences in bone length within the individual(Michael Phelps' long arms versus long legs). But, basically the scientists are just as perplexed by the Michael Phelps phenomena as we are. If we knew more about these variations in the cellular processes of endochondral bone growth we could do something to encourage growth by initiating these cellular processes in our children whom we wish to be tall. But the key word is phenotypic and phenotype means the potential to manipulate(see bodybuilders). The study is saying that how we use our limbs can affect our growth rates!
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