[tackem (OP)]

Could one person on a treadmill supply a worthwhile amount of electricity to the grid?

[Bored chemist]

Simple answer: No.

Complicated answer; Still no.

[vhfpmr]

The issue isn't whether you can produce a useful amount of power (useful for doing what? cycle lights run off a dynamo), it's whether you can do it at a useful efficiency. The human body is about 20% efficient, far worse than a combined cycle power station (55-60%), and growing the food to power it accounts for a quarter of global CO2 emissions.

[alancalverd]

Most adults can manage about 60 watts for an hour or so - pedal radio transmitters were used by the Australian Flying Doctor Service and the "Gibson Girl" air-sea rescue beacon(1943 - 1960) used a hand-cranked 5W transmitter. Champion cyclists can manage 300W for long enough to fly across the Channel. Weighlifters can deliver over a kilowatt for a few milliseconds.

[Petrochemicals]

A horse power is equivalent to 0.75kw, a horse being able to sustain that level for hours, so 4 horses on treadmills can power your kitchen kettle.

[Bored chemist]

Weighlifters can deliver over a kilowatt for a few milliseconds.

Dropping a rock on the floor can also do that.

[alancalverd]

Congratulations on recognising the essential symmetry of the conservation laws.

[myuncle]

And yet human muscles can power a helicopter.

[Bored chemist]

A horse power is equivalent to 0.75kw, a horse being able to sustain that level for hours

Actually, they can't.
A "typical" horse can sustain about 2/3 HP.
Their peak power can be over 15 HP.

Congratulations on recognising the essential symmetry of the conservation laws.

Someone had to.

[Bored chemist]

And yet human muscles can power a helicopter.

...briefly.

[alancalverd]

The problem with dropping a rock on the floor is that you have to pick it up in the first place, and entropy gets involved.