In the course of my experience working and training in gyms, Ive seen people doing some incredibly "interesting" exercises. Unfortunately, its usually because these people have not been properly instructed in exercise technique.
Here are some of the top winners. Remember, these are actual exercises that Ive seen people do. I made the names of the exercises up to match the lunacy of how they look.
DO NOT TRY THESE AT HOME! OR AT THE GYM! OR ANYWHERE!!!
1. Dumbbell Hair Combs - Start by holding a dumbbell in front of you. Do a front raise with it then whip the dumbbell back and over the top of your head like you're combing your hair with it. Make sure to just miss your skull.
2. Hog-tied Face-Rubs - Lie on your stomach on the floor. Grasp your ankles behind your back and rub your face back and forth on the ground repeatedly. Continue until you've had enough.
3. Abdominal Earthquakes - Lie on the floor on your back in the classic start position of a crunch. Now yank as hard as you can on the back of your head up and down and thrash your legs around in the air like you're fending off starving dogs. This evidently works your abs. You will resemble Jello that has just been dropped on the floor. Your face should be as red as a tomato by the time you're done.
4. The Arm Wrecker - Do one cheating, momentum-filled rep of an arm exercise with ridiculously heavy weight then swing your arms around as fast as you can in a circle to get blood to the muscle. This technique will either help your arm grow or will smack the person waiting to use the machine/weights next.
5. Pelvic Demolisher - Stand with your fingers interlocked behind your head. Do a pelvic thrust forward and drop your spinal column down and backwards about 6 inches. This exercise is best done in front of a large group of people.
6. Dumbbell Doggy Digs - Bend over at the waist so that your back is rounded completely over like an arch. Your legs should be completely straight and locked out. You should look like you are trying hard to touch your toes but not really succeeding. Hold two dumbbells down at arms-length. Now spin them round and round repeatedly just off the floor so that you resemble a dog digging a hole.
7. Pec Rockets - Set the pec deck machine with far too much weight for you to handle safely. Make sure you are very sweaty and slippery before attempting this one. First, use your entire bodyweight to get one arm pad up to the front. Then, throw yourself at the other one to get it to the front. Hold them there for a half-second then get shot four feet out across the floor as you squirt from the machine like a greased banana.
8. Rush-Hour Bench Press - This exercise is done on the vertical seated chest press machine that has a foot pedal to help raise the weight to the starting position. Use this pedal at the bottom of every single rep to bounce the weight back up. Your footwork will resemble that of someone in rush-hour traffic going from 0 to 60 to 0 every 3 seconds.
9. Close-Grip, Behind-The-Neck Shoulder Press - Sit in a shoulder press station, gripping the bar overhead with about 6 inches between your hands. Bring the bar down directly behind your head. Be sure to lean forward 45 degrees and round your back over so that your shoulder joints and lower back each get their fair share of trauma.
10. C.P.R. Bench Press - Start by loading your safe maximum bench press weight onto the bar. Now add 20 more pounds just to be safer. Have your spotter lift the bar off the rack for you. Lower it 2 inches on your own power then allow it to drop and cave in your rib cage. Be sure your spotter is a strong dead lifter before attempting this exercise as you will need them to pull the bar off you at the bottom of every rep. When your spotter has pulled the bar off you after the first rep and is trying to put it back on the racks, yell out "Ive got six more reps!"
Remember that this is just a small sample of things I have actually seen people doing. Please be sure when you do your exercises that you take the time to learn proper form and, if you do see someone performing an exercise that is potentially harmful, tactfully assist them.