The use of ankle weights during running is dangerous and unnecessary. Benefits from this added weight has been shown to be very small in recent studies. With every mile that is run, the feet must absorb 110 tons of energy. Ankle weights only add to that stress on your legs and feet. Use of these weights, even light ones, can result in not only acute injuries, but also chronic overuse injuries such as tendonitis. They can also disrupt your balance and proper running form.
There are smarter and safer ways to improve your running or increase the intensity of your runs. This can be achieved by adding hill repeats to your current running program. You can achieve the same intensity of ankle weight running without the same level of risk.
Find a hill that is a moderate-grade (5% to 8%) and is about one-quarter to one half-mile in length. It should take about two to five minutes to run up at your 5K to 10K race pace effort or slightly faster. If the hill is too long, the recovery coming back down will be too long. Do about three to five repeats, five to ten for more experienced competitors once a week.
*Journal of Sports Medicine and Physical Fitness showed that women ages 21-51 who used light ankle and hand weights (1/2-3 pounds) during step aerobics three times a week showed no muscle-strength increases at the end of the 12-week study