Jogging is an efficient form of exercise in that it targets several areas at the same time -- weight loss, cardio, stress relief and leg strengthening. It's easy, however, to go overboard, and any process that promises physical and mental rewards is laced with the potential for setbacks. Joggers often report having heavy legs for reasons they cannot identify, but it's usually easy to determine the cause or causes, with a little investigation.
Changes in Routine
Sometimes, you can make changes to your running routine that add to your workload, and hence your likely fatigue level, without trying to do so. One example is changing from mostly flat routes to hilly ones, either because you're traveling, have moved or are simply exploring new jogging locales. Another is when the weather suddenly becomes a lot warmer. Non-running-related lifestyle factors, such as increased job or family demands or other stressful events, can translate into feeling heavy-legged. During these periods, focus on getting your daily jog in without pushing yourself hard, and take rest days as needed.
Overtraining Syndrome
Being highly motivated carries the potential drawback of doing more than your body is currently capable of handling. Notice that what might be an easy routine for a very fit, experienced jogger -- for example, 45 minutes a day -- might be far too much for you, at least for the time being. Exercise physiologist Pete Pfitzinger describes overtraining as accumulating stress more quickly than your body can make positive adaptations to, leading to fatigue and a decline in jogging performance. If you are cranky on top of being heavy-legged and have lost motivation, chances are you need a break of several days or longer. You can always switch to an activity such as easy swimming, as you recuperate from bouts of overtraining, so that you can still get an exercise fix.
Glycogen Depletion
Glycogen is your body's storage form of glucose, the chief source of fuel for your muscles and the fuel most easily converted to usable energy. It is stored in liver as well as muscle. When you exercise at moderate to high intensity for longer than about two hours without taking in any carbohydrate, your body becomes glycogen-depleted and performance rapidly declines -- "hitting the wall," in marathon parlance. Some joggers are chronically glycogen-depleted as a result of trying to lose weight and thus underfeeding themselves, leading to dead legs. According to Peak Performance Online, glycogen depletion can cause not only exhaustion and heavy legs, but injury as well.
Systemic Illness
Being sick, as with a touch of flu, the common cold or some other typical ailment, can cause you to feel especially tired when you try to exercise even lightly. Dehydration accompanying infectious illnesses can cause your limbs to feel heavy, and the medications you may be prescribed for sicknesses, ranging from urinary tract infections to hepatitis, can compound leg fatigue. If you feel at all woozy or unsteady, take the day off and get medical clearance before heading out for your next jog.
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