It's been proven that with the right focus and adrenaline we can increase our strength to unfathomable amounts. For instance, look at the mother who lifted a car to pull her child from under it. One man lifted a helicopter enough to pull his buddy who was trapped under it out.
The Norse would use Beserking to put themselves into trance rages.
We are more capable than we are aware of. But a few lines of a spell won't increase strength and stamina. You would have to be able to enter a trance at will or use chant to induce trance, and be capable of exciting ourselves at will to produce adrenaline.
Magick is first and foremost about being in control of our mind, body and spirit. So, yes, this would fit into magickal practice. Be aware though that performing feats of strength that are above our normal capabilities does not omit the ability to get severely hurt from the strain on our body. You could psych yourself up to lifting that box up the stairs, but that doesn't mean in a few hours you won't be sore as hell! And you can still injure yourself, especially if you already have any past injuries.
Its fun to play around with on pickle jars and carrying groceries home, but I wouldn't suggest pushing yourself with something extreme unless it's an emergency situation.