Shoulder and Arm Muscle Development
Using a speed bag on a regular basis builds up and tones the muscles in the wrists, arms, and shoulders. Regular speed bag practice develops muscle endurance, which is equally important. According to Balasz Boxing, muscular endurance is the ability of a muscle "to do repeated contractions against a less-than-maximum resistance for a given period of time." Boxers, in particular, are required to keep their arms up for long periods of time during a match. Speed bag practice trains a boxer's shoulder and arm muscles to work for extended periods.
If you see an exhausted-looking boxer who just can't seem to keep his hands up past round five, there's a good chance he's been avoiding the speed bag.
Using a speed bag continuously and rhythmically for as little as three minutes can have a positive cardiovascular effect, but to get more dramatic results, you'll want to build up to longer practice times. By using the speed bag with rhythmic intensity for 15 minutes or longer, one can start increasing his aerobic capacity and actually begin burning fat. To ensure the best results, you will need to eventually do speed bag drills for a minimum of 15 minutes, divided into five three-minute rounds.
Eye and Hand Coordination
Much like boxing with a human partner, a speed bag requires the user to respond quickly and accurately. Regularly sparring with the speed bag hones reaction time to a noticeable degree and sharpens reflexes. A sense of timing and rhythm are also improved through three-minute rounds of repetitive punching with a speed bag, because the only way to keep the speed bag rolling in a way conducive to exercise is to hit at the speed bag with an unbroken rhythm, and with consistent force. Any wasted, excessive, or irregular motions will cause the user to have to stop the bag and start over, breaking the rhythm of the exercise, and rendering the exercise ineffective.
Ease and Safety of Use
Speed bags, unlike free weights, do not require a spotter, and pose almost no risk of injury. A securely set-up speed bag is completely safe. The only sorts of injury possible are wrist strain or bruised knuckles, though one would have to be punching pretty hard to suffer either of these. To use the speed bag effectively is to use it with controlled force and rhythm, not to pummel it off of its platform. The odds of hand injury with a speed bag are very low.
Things to Remember when Starting
Common complications for the beginning speed-bag user are that the bag is too small or fast for a beginner, and the bag is either too high or too low. The bag should be set at face level. A bag that is too high will force you to move your arms more than they should just to reach the bag. Specifically, the fattest part of the bag should be set at mouth level. This setting lets you concern yourself with whether your fists are hitting the "meat" of the bag, rather than what your arms are doing. Your arm movements when using a speed bag should be minimal.
It is also important to remember to wrap the knuckles with a gauze bandage to prevent severe chafing and bruising. Nothing fancy is needed here--a simple, cheap gauze bandage will work nicely. Use enough gauze to wrap the knuckles a few times, with enough left to firmly tuck the end of the bandage into the wrapping.