Bingo.

In fact, there are several instances where he’s shown to be
weakened.

His baseline strength is stronger than most of the other
nations regardless, but there’s an explicit economic reason for this. It’s not
just something inherent to him.

We first see his super strength emerge when he picks up the
wild bison and begins hurling it around. Notice how in the strip, the richness
of his culture and industrial strength are listed as reasons to account for
this.

Likewise, in “In just two minutes you can grasp the exterior
of the European Economy,” we learn that a nation’s health and well-being is
predicated on the stability of both their politics and economy.

“In this manga, a nation having a cold = something like a recession. By the way, when they’re physically unwell, the nation’s politics or economy is having problems.” [x]

It would follow, then, that if America gains strength from a
strong culture and economy, then he would also be weakened during political
turmoil and economic recessions. Now, to substantiate this claim…

1) The super strength is not inherent to America,
but is rather inherent to any personification who reaches enough political and
economic affluence.

Ex: Russia and America are referred to
as “super humans” by Finland, meanwhile the other nations aren’t that much
different from humans.

As I’ve stated before, I take this as a metaphor for how the
two nations emerged as dominant superpowers during the Cold War, both economically and politically (i.e. capitalist vs communist ideologies.)

Ex: Russia is
able to stop a tank with his body.

2)  America’s economy impacts his health

Ex: During the Great Depression, he’s notably low in spirits
and appears to be fatigued and/or out of sorts. He solemnly apologizes and
allows the other nations to berate him for causing the Stock Market Crash without
any protest– this is completely uncharacteristic of him. 

He even shivers in the face of their anger.

Each nation is given a negative number depending on how
badly their economies were affected as a result of the crash. Notice how
likewise to America’s presumably ill
health, their dependency on him and his [weakened] economy, strongly impacted
their own health and well-being.

Meanwhile, because socialism had an entirely different
production system, Russia is positively affected by the crash.

3)  America’s sovereignty impacted his strength and
overall health.

In this strip, it becomes clear that England’s taxation of
America after the French-Indian War caused America to experience considerable economic “strain.”

That said, look at the differences in age between the first
and last panel.

Independence from England, and therefore the freedom from
taxation and a full control over his economic and political affairs, allowed
America to reach adulthood.

I mean, he’s literally defined as “Young America” in the first
panel. Essentially, England’s rule stunted both his growth and strength.

Which brings me to my next point.

4) The Revolutionary War further weakened America’s
physical strength

Considering that England almost won the war, it’s no
surprise that America would grow weak from it.

If he’s so strong, how come he physically struggled to
defend himself when England came charging at him? Not to mention that England had the upper hand.

Yes, America’s “naive” and was evidently surprised by this, but there’s a clear assertion and confrontation of physical force here.

5)  Present politics are hinted at affecting
America’s strength

The dividing politics of America’s elections are enough for
England to concernedly inquire about America’s health.

 Of course, America
denies this and expresses confidence in his body’s strength.

If anything, this still confirms that America can be
weakened by his political situation, as is the same of any other
personification.

100% without a doubt.

Ex: When England
falls asleep during a meeting, America suggests that they leave him be. That opinion
is quick to change when England begins mumbling about America’s old bed-wetting
habit.

Ex: He asks
Lithuania to hide the toy soldier England gave him.