You can get superbly fit by practising the fitness 'triad' -- which is:
- Resistance, strength training
- Aerobic training - running, biking, rowing etc
- Anaerobic training - interval, short to medium high-intensity exercise
However, timing these activities for best effect can get complicated, depending on your available time and goals. When, and in what order to do resistance and cardio in a single session is a very popular question asked of personal trainers. Opinions vary. The same issue arises if you do high-intensity interval training as well. To simplify concepts, for this article let's stick to cardio and weight training.
Decide on Goals
Your fitness goals could affect how you combine weight training and cardio training, especially if you are inclined to do both in a single session -- at the gym for example. Thirty minutes of cardio on the treadmill and 10 weights exercises at 3 sets of 10 repetitions might be a common general program. If you alternate weights and cardio on different days, or even morning and afternoon with plenty of rest in between, the sequence is not so critical.
In mixed sessions, the general rule is to do your preferred target discipline last. That will favor your preference yet also provide some benefit from your earlier workout phase. See more below.
Fat Loss/Weight Loss
In this scenario, some experts suggest doing cardio first because you will use up muscle and liver glucose stores and then burn fat preferentially in the immediately following weights workout. When the body is short of glucose in muscle it turns to fat for fuel.
This is false reasoning and a misunderstanding of exercise physiology because you don't have to target fat burning to burn fat. All you need to do is expend energy and the deficit of energy intake and energy expenditure will ensure you burn fat. See more in this article.
Second, 30-40 minutes is not enough to use up all your glucose stores; you need 75-90 minutes of relatively high work load to get to the point where fat becomes a substantial preferred fuel, and, as with endurance athletes, this takes training to achieve.
Even so, if you've got limited time and have to combine cardio and weights in one session, doing cardio first or weights first will not harm your fat burning goals.
If your goals are more inclined to aerobic fitness for running or sports, you should do cardio last. Weights work can reduce arterial compliance (elasticity). You don't want to end a session with weights if you're training for endurance. In addition, you need to target the aerobic properties of the cells energy systems to favor aerobic fitness.
Muscle building and body shaping usually requires you to lose fat at the same time, so a good 30 minutes of cardio a few times each week can help keep body fat low by increasing energy expenditure. However, opinions differ as to the best sequence of cardio and weights.
One of the common justifications for not doing cardio first is that you might get fatigued for your weights workout and get injured, or that you won't be able to lift with sufficient energy to achieve good results. There is no evidence whatsoever that injuries increase if you do cardio first in a workout.
Nor is being fatigued from a moderate cardio session likely to diminish your muscle-building results because you're tired. In fact, lactate and fatigue in muscle is more likely to do the opposite because your goal is muscle damage, repair, and muscle enhancement. Many bodybuilding programs are designed inherently to do just that, with high-volume work.
In addition, to ensure this process of rebuilding and enhancing muscle takes place, you need to create the ideal muscle physiological environment post workout. Muscle needs to be able to absorb protein and carbohydrate and to rest so that it repairs and rebuilds. Doing 30-40 minutes of cardio after a bodybuilding workout is not conducive to such an environment. Rest and re-build!
For general fitness goals, see both of the above. However, if your goals are predominantly to build strength -- Olympic lifters and Powerlifters for example -- you do need to concentrate on the heavy lifts, that is, the 1-5RM programs in which you lift heavy with few repetitions. You need all your strength for such a program. For this, a light cardio warmup is appropriate but you will be better served by allocating aerobic workouts to another session or another day.
The same principles apply if you were to do cardio training after a strength workout. You need to let that neuromuscular system settle down and encapsulate your in-session gains. Cardio done immediately after a strength session is likely to be disruptive because you're creating interference in physiological environments.
You can see that for most goals, except serious strength training, and endurance training, doing cardio first is unlikely to be detrimental to your goals. Strength training men and women will be best served by doing cardio in another session.