Dumbbells: These are more expensive, though there are plenty of affordable options. You'll eventually want to get a variety of weights, but you can easily start with three sets of dumbbells: A light set (3 to 5 pounds for women, 5 to 8 pounds for men), a medium set (5 to 10 pounds for women, 10 to 15 pounds for men), and a heavy set (10 to 20 pounds for women, 15 to 30 pounds for men).

K. Aleisha Fetters, M.S., C.S.C.S., is a Chicago-based personal and online trainer. She has a graduate degree in health and science reporting from the Medill School of Journalism at Northwestern University and regularly contributes to Men's Health, Women's Health, USNews.com, TIME, and SHAPE. When she's not lifting something heavy, she's usually guzzling coffee and writing about the health benefits of doing so.
When Katula started his research on whether weight training would improve quality of life for seniors, he realized that many had never even picked up a dumbbell. “They first had to learn how to use these big intimidating weights and machines,” he says. He recalls the story of one woman who protested that she couldn’t do the leg press machine. Finally, Katula persuaded her to sit in the machine and set the weight at 50 pounds. “I couldn’t believe how fast she whipped out 10 reps,” he says, “When she got out of that machine, she was two inches taller just from increased pride.”
It is known that intracellular energy depletion (assessed by a depletion of ATP) stimulates AMPK activity in order to normalize the AMP:ATP ratio,[333][334] and when activated AMPK (active in states of low cellular energy[335] and colocalizes with creatine kinase in muscle tissue[336]) appears to inhibit creatine kinase via phosphorylation (preserving phosphocreatine stores but attenuating the rate that creatine buffers ATP). While phosphocreatine technically inhibits AMPK, this does not occur in the presence of creatine at a 2:1 ratio.[334] It seems that if the ratio of phosphocreatine:creatine increases (indicative of excess cellular energy status) that AMPK activity is then attenuated, since when a cell is in a high energy status, there is less AMP to directly activate AMPK.[334][336][337]