Furrowed brow?

[Individual]

Is it normal or natural to furrow one's brow when concentrating? When meditating or concentrating in general, I hotice I have a tendency to do this. It seems to help me because I enter into "concentration mode," sort of the same way that psychologists have shown that merely smiling or frowning releases chemicals in the brain that cause a mild sense of joy or sorrow.

[Ben]

Its association. Your associating the vedanas of furrowed brow with mental focus.
But if you look at the commentary on anapana-sati in Vism, you'll see that Buddhaghosa talk about the quality of the concentrated mind. Its without physical or mental tension. Buddhaghosa says that one should place the mind on the object. Its relaxed, a gentle firmness of application. If you apply yourself in that manner you may find a better quality of concentration.
Kind regards

[Individual]

But if you look at the commentary on anapana-sati in Vism, you'll see that Buddhaghosa talk about the quality of the concentrated mind. Its without physical or mental tension. Buddhaghosa says that one should place the mind on the object. Its relaxed, a gentle firmness of application. If you apply yourself in that manner you may find a better quality of concentration.
Kind regards

Tension, in what sense? Of course, emotional extremes like anger, fear, anxiety, sadness, desire, etc., disturb concentration, but the act of developing concentration can feel quite tense.

If this tension is entirely fabricated, is there no limitation on the immediate potential of concentration?

i.e., when trying to fully realize anatta, sunatta, or simply memorize something for school, there is tension when there is the inability to achieve a certain level of focus. Self-view cannot simply be focused away, rationality cannot be continuously maintained, suffering cannot be immediately seen through. Seeing through anger and ignorance is difficult.

As you mentioned: It's a "gentle firmness" of application. The "firmness" suggests at least some degree of tension, like using a muscle to lift something instead of snapping the muscle (in a punch or kick), a muscle spasm, or not moving it at all... the former two being metaphors for anxiety, the last one a metaphor for torpor...

Up to a certain point, it feels as though concentration can be "used up," that the ability to direct attention is like a currency with a fixed limit and you can't simply direct mindfulness outwardly in all directions infinitely.

[Jechbi]

when trying to fully realize anatta ... or simply memorize something for school, there is tension

The following is a personal observation, so discard it if it's not useful for you, and challenge it if that would aid your own better undertanding:
These are 2 very different pursuits. From a certain perspective, concentration only derives from not-self conditions. It comes together as a blend of viriya, nekhamma and khanti built on a foundation of sila. Sure, you might furrow your brow. But that's probably not going to be a relevant factor. fwiw

[genkaku]

There are plenty of representations of Bodhidharma in which he is not scowling, but there are also plenty in which he looks as if he might bite your head off for a nickel.

This ferocious aspect, I have always thought, relates to the wily forces of greed, anger and ignorance and the fact that it takes some pretty gritty fire to straighten things out.

FWIW

[Individual]

There are plenty of representations of Bodhidharma in which he is not scowling, but there are also plenty in which he looks as if he might bite your head off for a nickel.

After I remarked on my appreciation of Vulcan philosophy for its similarities to Buddhism, I remember Ven. Dhammanando once remarked that Spock was a good example of the type of mentality and behavior an Arahant might exhibit.



When we say there is no "tension," in concentration, I would agree about emotional tension, but there is mental tension, in the same manner as there is muscle tension when flexing an arm.

[Jechbi]

Spock was constantly in denial. Actually he did feel. That's why his character was interesting from the cinematic standpoint. Half human, whatever that means.

The phrase "as a strong man might might flex his extended arm" is a metaphor for how the Buddha can accomplish seemingly impossible mental feats with effortless ease.