Exercise #4: Weight-Free Rows.
To create resistance with a weight-free row, all you need is your own fist. Though typically done with a dumbbell, making a tight fist activates the muscles of your arm enough to make for a substantial workout.

To do it, bend slightly forward by hinging at the hip. Bend your elbows at your sides, and pull your arms back without unbending at the elbow, creating a rowing motion.  This works biceps and triceps in addition to activating the muscles of your upper back. Repeat for a full minute.

Tip: Make sure to keep your arms close to your sides to maximize the effectiveness of this workout.