Allowing all of your limbs to work hard for a period of time will provide the best aerobic exercise possible. The best aerobic exercise involves raising your heart rate high. You must then keep it there for some length of time.
It can come about in countless ways. Any activity that uses large amounts of muscle with movement for enough time will work. Fancy equipment adds variety but forms no requirement.
Achieving this goal slowly or quickly matters little. Intervals carry unique benefits along with efficiency, so I prefer them over slow, long distance methods. Although most cardio incorporates the lower body, the upper body usually adds only a moderate effort to the movement.
Find a way to use your arms hard as well. Then you have discovered the best aerobic exercise. Keep in mind though that this will make the exercise feel very taxing.
VO2 max or maximal oxygen uptake appears as the best single measurement for the overall aerobic fitness. Although elite endurance runners have high VO2 maxes, cross-country skiers can achieve even higher values. This advantage likely comes from vigorous propulsion involving both the lower and upper bodies.
The lower body, containing the largest and most powerful muscles, alone can certainly raise the heart rate. Add in the arms too though, and you have a potent combination. Since the work spreads across more muscle groups, you will find an elevated heart rate with less by-products of fatigue. This means you gasp for air yet feel less of a burn in the muscles.
The VO2 max becomes the limiting factor.
Odd object lifting and carrying can work among the best aerobic exercise options. Consider sandbag and tractor tire drills. Both these items cost little. They involve a ton of muscle. Keep flipping, carrying, and lifting for enough time and you gain a powerful cardio workout. Any exercise involving your upper body even just to stabilize can work, such as a pushing or dragging sled. Without complex equipment, you can push a heavy box filled with miscellaneous stuff. You can get a tire, attach a rope, and place weight plates or other objects within if you build a platform within the tubing. Use your creativity.
Many choices tend to emphasize the upper or lower body. Boxing and swimming tend to focus on the upper body. Upper body work alone usually does not raise the heart rate as greatly. These may still carry other benefits. Anytime you use your lower body intensely for a length of time, you still have the potential for an excellent option. Running involves the upper body for arm swinging, but to a lesser degree. Any sort of sprinting tends to involve the upper body more aggressively. Adding resistance by holding weights in the hands and using a more aggressive arm swing can work for everyone of any fitness level. If you want to up the ante though, use all the limbs.
Use the best aerobic exercise methods, but practice your sport consistently if needed for specific adaptations. Keep in mind the muscles have to adapt to local fatigue. A runner will not develop specific local endurance through swimming. Using four limbs at a time focuses more so on the senders of oxygen: the lungs, heart, and other circulatory parts. The receivers, or muscles, tend to experience more of a challenge with harder work for fewer muscles.
You can always cross train to provide some variety. This can help avoid overworking any movement pattern. Although our bodies should only perform a few basic movements for proper strength training, conditioning has a greater selection. You can get creative, make it exciting, and change it up while staying safe. You may find a heart rate monitor useful for measuring progress. This would allow you to compare the efforts of various modes of exercise. Use this to determine what works best for you. Establish standards. Also realize that simply doing more work per unit of time for any exercise will mean you have grown more fit.
Perhaps you count yourself as an anaerobic athlete. You focus on explosive movements. You work hard toward gaining muscle strength and size. A high aerobic capacity will still help you. The aerobic system allows recovery from all exercise. A more efficient system speeds up recovery from resistance training as well. The cardiovascular system acts like internal plumbing. It improves the overall health and fitness of the body. Therefore, developing this creates synergy with other components. The whole is greater than the sum of its parts. Cardio alone will never develop a well-rounded and attractive body, but serves a valuable role. You can still specialize at speed and strength and get as aerobically fit as possible. You may just have a slower pace over longer times compared with someone carrying less mass.
I would still suggest against using machine methods as the best aerobic exercise option though. They carry similar disadvantages as using machines for resistance training. The user not controlling the motor pathway shows the greatest drawback. It may force you into an unnatural groove. This can cause joint injuries overtime. You have to determine if the motion feels right for you. Notice any sort of awkward pain? Move on to one of many alternatives. They can still have a place but should not form the only options.
The Best Aerobic Exercises Use all Four Limbs
Both intervals and resistance training can work together for complete fitness. Just as squats, dead-lifts, bench presses, rows, etc. form the best resistance training exercises because they work large muscle groups naturally and together, so do the best aerobic exercise choices. Include heart-rate raising activities involving both the lower and upper body at the same time. This truly forms the best aerobic exercise.