Category Archives: muscle

On Leanness

Although many acknowledge that some people are inherently (perhaps genetically) leaner than others, it remains unclear what the biological basis for the body’s “set-point” might be. A study appearing in the open-access journal PLoS One suggests one possible factor1.

from reference 1

Both rat and human data were collected in this work, which concludes that “the lean phenotype is characterized by high endurance capacity and high activity and may stem from altered skeletal muscle energetics.” These researchers gathered data from a population of people who they categorized as non-exercisers (less than 1 hour per week of activity exceeding 4 METS). They subjected these individuals to a treadmill test to determine their endurance (as assessed by oxygen consumption during exercise) and kept track of their average daily activity over a period of 10 days; finding that there was a significant relationship between endurance and leanness, as well as average daily activity and leanness. Furthermore, they found that there was no significant difference in the amount of food consumed by lean versus non-lean rats, and fascinatingly, that the skeletal muscle tissue of lean rats has significantly higher levels of the enzyme PEPCK-C.

Of course, it is not surprising that those individuals with higher daily activity are leaner in general, rather, this study is suggesting that there may be a fundamental, biological reason why certain individuals are more active: they simply have a greater capacity for activity. Indeed, if one fatigues more easily, it wouldn’t be surprising if they were less active; it is also conceivable that reduced activity could feed-back on behavior in the sense that an individual with lower endurance might progressively reduce the amount of physical activity they engage in so as to reserve energy for other tasks. This is especially true in contemporary society, where mental activity is often the basis for work and play.

It is unclear as yet, however, whether the differential muscle-properties found in rats extend to humans; further work will be required to clarify what the molecular-biological basis for increased human endurance might be.

1. Novak CM, Escande C, Gerber SM, Chini EN, Zhang M, et al. (2009) Endurance Capacity, Not Body Size, Determines Physical Activity Levels: Role of Skeletal Muscle PEPCK. PLoS ONE 4(6): e5869. doi:10.1371/journal.pone.0005869

On sticking out your tongue.

Muscles can only pull, not push, so how is it that you can stick out your tongue? In other words, since we have no muscle outside our mouths to pull our tongues out, the fact that muscles are incapable of pushing seems to imply that there is some sort of mechanism at work in producing this movement which doesn’t fit in with our general conception of all other movements (arms, legs, eyes, peristalsis, breathing, et cetera).

i am currently (4/29/08) attending the 18th Annual Neural Control of Movement (NCM) meeting in Naples, Florida, and this tongue question was brought up as a way to remind the attendants that we should be careful not to let dogma affect our thinking too much as such adherence to well-established ideas can prevent us from reaching new ways of understanding and new modes of analysis.

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The simple answer is that the tongue has constant volume, so if you sufficiently contract the muscles in your tongue laterally (from molar to molar), the tongue must increase its volume longitudinally (from throat to lips). Notice also that to really stick your tongue out, you must protrude your mandible significantly.

In any case, it was a perhaps frivolous but highly stimulating and poignant aside in a meeting otherwise devoted to the serious analysis of experimental data, thus kicking things off in a congenial tone.