You can sprint 100 yards or you can jog 10 miles. Try sprinting 10 miles and see how far you get.
– Arthur Jones
Strength, endurance (cardio), and mobility form the three pillars for fitness. Athletes need to go beyond this for their sport, but still should focus on these to develop their general ability which they then apply specifically.
If you could choose only one option for cardio, sprint. Beyond any extreme ultimatum used in writing to demonstrate a point, you could still drop everything else and focus on sprints alone. While this lacks variety, variety is overrated anyway. You could argue that sprinting embodies the cardio we adapted to perform.
Consider our evolution. Humans are versatile beyond just what we eat. Surely we elevated our heart rates by dragging animals, carrying equipment, slaying wimps, and making love (if long enough). We also walked a lot and possibly needed to jog as well. Sprinting gave us that rare but valued burst of speed to avoid a predator or close in to spear the kill.
Did our genes expect us to sit and perform bike rides and rowing? These rank as decent options since they combine fairly safe joint motions, but others fare less well. Machines can place your joints at unusual angles and your muscles at weak lengths. They provide less work for stabilization. Many ellipticals fit this category. Other functional training crap work far less muscle and compromise the joints, such as complexes and overhead carries. We can feel assured that sprinting works our muscles as intended.
The best cardio involves lots of muscle for movement. This occurs since the senders of oxygen circulate blood for all your limbs. Some options, such as riding a bike, can still work since they involve the large muscles of the lower body, but not as well as if you could also bring in the arms. Lactic acid will build up in the legs, causing muscular endurance to blunt the cardio effect. Sprinting on the other hand brings into play massive mounds of muscle.
Sprinting draws upon the upper body for swinging, involving the shoulders, chest, back, and arms. The entire thighs and legs drive the movement. Since sprints involve unilateral work, you resist rotation at the spine which works the core. The outer hips, inner thighs, and other big stabilizers get hit. The hip flexors receive only a small amount of work during lifting, which makes sense since raising your leg should occur unloaded. Nonetheless, sprints address them further. Nearly every major joint in the body either moves or works to stabilize. This polishes the odds and ends that get worked less intensely on the big three. This gets your heart rate up high, perhaps rivaled only by bringing a knife to a bazooka fight.
Sprinting requires no equipment. Sprinting on a flat surface carries serious risks though. Regular sprinting involves spikes in force that can harm you, leading to hamstrings tears and other issues. Getting strong in the weight room and then sprinting after a long lapse will spell disaster faster than you can in a spelling bee.
You must warm-up and consider some stretching. Break into it by jogging or doing less than 100% sprints initially. No formal system exists to prepare. Assess the situation honestly. If you feel any sort of cramp or slight tightness at all during your sprints, hold off on going all-out. Warm-up for longer and finish with a less than 100% workout.
You can slow your sprint down for safety. Use a hill, dragging sled, push sled, or band to smooth out the spikes that otherwise make the perfect conditioner dangerous. This also forces you to lean further forward, bringing the quadriceps into the fray. It can also make progress easier as you can increase the resistance and not just the speed.
While an athlete may need longer rest periods to build up their speed, sprinting for conditioning works well Tabata style. Use bursts of exercise followed by short rest periods. Work hard for 20 seconds, followed by 10 seconds of rest. This pattern continues for 6-10 cycles (3-5 minutes). Choose whatever number of cycles you wish within that range.
As you get in better shape, try to achieve more distance in each 20 second period or add weight to your sled for the same length. Avoid increasing either if you fail to achieve your goal on every interval of your 3-5 minute workout. This gives you a system for progress.
Keep in mind that some speed of movement seems important to raise your heart rate. Trying pushing against a wall as hard as you can for 20 seconds. It will get you going a bit, but not like a sprint. Make sure you avoid loading so heavily as to prevent speed. If your muscles seem as the limiting factor instead of feeling out-of-breath and fighting some burn in your muscles then reduce the resistance.
The disadvantages of sprinting are practical. Since you need a lot of space, going outdoors and to a local school or park is the only option for most trainees. Since weather can vary, it feel tough to standardize, as a damp ground could change how heavy for sled feels.
Sprint as a cardio option and perhaps as your only choice. Consider slowing yourself down for safety and apply intervals. Smile knowing you perform the original and best cardio, at least beforehand and probably at least good hour or so after the sprint workout.