Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
Unlike aerobic conditioning where duration and intensity is increased by 10%, increases in the weight you lift aren't prescribed with such precision, partly because muscle groups vary so much in size and strength, and partly because of the practical matter of the weights available at the gym. Typically you increase to whatever dumbbell is next on the rack (or plate on a weight machine), and so if you're lifting 12 pounds with biceps curls, then the next dumbbell available is usually 15 pounds. There is an option to increase in smaller increments with dumbbells by using an accessory called a donut, a magnetic 1¼ pound weight that attaches to the end of the dumbbell (they come in other weights besides 1¼ pounds as well). Weight machines have half weights for the same purpose. Ask your gym manager to purchase donuts if they don't have them.
Free weights vs. machines
Dumbbells and barbells are free weights. They are "free," or untethered, unlike a weight machine where the weight stack is connected by cables to cams and pulleys and only move in one direction. There are advantages to both styles of lifting.
Weight machines are easy to learn and use.
There are some exercises you can do with a machine that you can't do with a dumbbell. For instance, cable rows would be difficult to replicate with free weights. You could do bent over dumbbell rows, but they won't be quite the same. For my money, cable rows feel smoother than any exercise in the gym!
Free-weight training requires balance and coordination, and so if you are involved in a sport that requires balance, or you just need balance training, then free-weight training might be more effective.
Free-weight training may recruit more muscles than a machine because you have to stabilize your body when you lift a dumbbell, whereas the weight machine supports you. For example, a biceps curls is going to feel more natural and use more muscles in your torso (to support the weight) than if you did a seated biceps curl in a machine where the machine does some of the work and you can lean against it for leverage.
There are a variety of exercises that you can do with dumbbells that you can't do with machines. Lunges, step-ups, and many upper body exercises can be performed with free weights if you're creative.
There is no evidence to suggest that either method is superior to the other. My suggestion is to combine free weights and dumbbells to get the best of each. The ACSM weight training position stands states the following: "For novice to intermediate training, it is recommended that the resistance training program include free-weight and machine exercises. For advanced strength training, it is recommended that emphasis be placed on free-weight exercises, with machine exercises used to complement the program needs."