Here's a neat little trick that all the aikidoka out there will doubtless know already. This is one of the first things the student of aikido will learn, as it's part of many if not all of the techniques.
How do I do this?
Stand relaxed and upright. Extend either arm out parallel with the ground, fingers pointing upwards like a cop directing traffic. The arm should not be rigidly locked, but should be very slightly bent (a locked joint is easily broken). Imagine that you're reaching out to touch the opposite wall of the room, or imagine a line of ki flowing from your center out past your fingertips. Now feel very foolish. You're standing there like a moron holding up imaginary traffic, after all.
It's easy enough to look like a dork, thankyouverymuch. Why should I do this?
To prove your physical superiority. Challenge the largest and strongest person present to bend your hand in to your chest, using one hand at your wrist and the other at your elbow. Laugh at his puny struggles, and notice that he absolutely cannot budge your arm more than a few inches.
Kewl beans! How does that work?
Ditching the fuzzy metaphysical babble about ki and internal power, it's all a matter of simple body mechanics. With your arm in this position (slightly bent, remember?) both the biceps and triceps muscles can act on the elbow joint. Unless you're unusually weak, this combination is enough to resist all but the strongest opponent.
Great trick. What, if any, are the real life applications?
As I said, this is one of the more important parts of aikido training. This technique allows maximum transmission of force through the arm, magnifying the energy of even a tiny combatant. Imagine a sack of cement equal in weight to your opponent slamming into your chest, and you'll get an idea of how effective this can be. During aikido practice, I've had a 4'11" 90 lb. attacker knock me flat by combining this technique with one well-placed shove. I myself am 5'10 and weigh 140 lbs., so you can imagine how powerful this technique can be.