Why Strength Training Can Be a Great Cardio Solution
There's one view that says you build strength with weight training, and get your cardio with running, jogging, cycling, etc., etc. This is false and limiting. Strength training is a good way to get cardio done, period. And it offers benefits beyond strict cardio.
We need some definition. A long-time definition says that aerobics is exercise fueled mainly by oxygen carried to muscles by the bloodstream, and obtained through steady-state, or continuous, unvaried and nonstop activity for an extended period of time (typically 20 minutes or more) (Anaerobic exercise is that which isn't steady-state, but done in bursts or very short periods and which uses other fuels such as ATP (adenosine triphosphate), in what's known as the phosphagen process. Example – a sprint; a max-weight squat or deadlift.
Strength training performed in circuits can build or maintain strength and give a great cardio pump.
Strength training performed in circuits (three sets each, say, of squats, deadlifts, dumbbell rows and bench presses, (say, 15 sets in total) performed with lighter-than-maximum weights, higher numbers of reps - 12, say - and short rest intervals between those sets, has long been known to be both strength-building (or at least strength-maintaining, if you're otherwise a heavy trainer) , and capable of delivering a great cardio pump. In fact, they're a mix of aerobic and anaerobic exercise, that is, using both oxygen carried to the muscles, (aerobic) and other fuels that are used for real muscle stress (anaerobic). In this way, much exercise is on a line between two extremes. At one end you’d have a couple of record-attempting lifts, and at the other, a half-hour's fast walk or jog.
But life can be much more interesting between those two extremes. A strength-training-based workout as described above, gives a great aerobic blast to your system, not only firing up the heart as a pump, but offering greater strength to the muscles pumping blood back. And where persistent runners certainly get their cardio workout, they risk developing the steady-state syndrome where their system adapts, and the unchanging pattern gradually reduces benefits, such as fat loss, and can lead to depletion of natural body muscle. Circuit-based strength training with can keep the muscle there, and with program tweaks, build it, even while doing the cardio/aerobic job. This is a major reason major professional sports that were originally associated with agility and cardio training alone, such European football, and basketball have taken up strength training big time. European professional football (soccer) gained very quickly from reduced muscle injuries and faster sprint times, when structured strength training was introduced.
Note: a 5 k run for the fresh air and cardio benefit is fine and good. It can be part of a great way of life, and is to be admired. (And if running's your thing, that's way to go. But don't forget strength. Its loss will gradually creep up. And many athletes who depend on cardio are adding strength training for more benefit. But the way many people spend hours a week on a treadmill or cross-trainer with the aim of losing weight is most often self-defeating. The body adjusts, stops dropping weight, saggy bits get saggier. There are better ways to exercise with variety, and better results.
Many people still think of of strength-training as being a roomful of grunting and shouting jocks pushing big lifts and squats. But more common styles of training now use less extreme effort, and higher quality execution. You don't have to go to an injury-risking limit in order to get strong (or even get more mass, if you're male). Going as far as completing your last repetition with proper form and well clear of injury does the job. (After all, by not quite taking it to the limit, you avoid all the wasted time of injury and save weeks of idle recovery) Underpin your exercise with base of strength training, add more variety, and build on circuit combinations. Even better, do your exercise more frequently and conveniently, in the way that's afforded by super versatile, space-efficient equipment, at home. Whatever your goals.
Conclusion: You can do most or all of your cardio training as part of your strength plan, if you want to. And strength training gives you additional benefits. (What are they? See here: Brief Benefits of Strength Training)