To narrow this down somewhat, by cardio we'll assume that this is going to be something reasonable and either one of the two most gainz friendly forms:
Low intensity (65% or below max heart rate):
- fast walking, stepmill, exercise bike, cross trainer...
- continuous intensity
- the type of exercise you could still breathe only through your nose and be ok
- anywhere from 15-60 minutes is generally reasonable
High Intensity Interval Training
- sprinting, bodyweight circuit, battleropes etc...
- high intensity followed by period of low intensity
- the type of exercise you are breathing very heavy and can feel borderline sick after
- anywhere from 4-30 minutes is generally reasonable
1. Which goal should be the priority?
With weight training and cardio, two primary goals we desire are muscle gain and fat loss. Yes even if you just want to “tone”, that is the process of gaining muscle and losing fat.
It is far harder, slower and requires more skill to build quality muscle than it does to lose fat. In the vast majority of situations muscle building or retention should be the number 1 priority. Anything that compromises your ability to weight train as effectively as possible should be avoided where possible.
Even if your goal is to lose fat, you still want to maintain as much muscle as possible, or you’ll end up with the dreaded skinny fat look.
2. Which is technically more difficult?
Again weight training will usually require more technically ability and performing this when your mind and body is fresh would be beneficial so that you can maintain good form and avoid potential injuries.
Cardio before training
This is not something that I would recommend for either low or high intensity cardio. The only exception would be if the duration was very brief. Doing 10 minutes of low intensity as part of your 'warmup' or 1-2 sprints beforehand, isn't going to negatively affect your weight training.
Low intensity before
The main issue with low intensity cardio before training is that you will often find that your blood sugar levels can be low by the end. Being Hypoglycemic (having low blood sugar) is a terrible feeling entering a weight training session, and potentially even dangerous.
High intensity before
The main issue with high intensity cardio before training is that your muscle glycogen stores can be greatly reduced. Glycogen is a crucial energy source for your weight training sets, and as mentioned before anything that will compromise your weight training (main priority) should be avoided.
Cardio after training
If you have to do cardio in the same session as weight training, this would definitely be my preference. Firstly you do not compromise your weight training and have got the hardest part of your session out of the way.
Low intensity after
Now mentally this can be difficult after a hard weight session, but most low intensity cardio, doesn’t require much skill and you can use this time to keep the blood flowing to help remove waste products and promote recovery.
High intensity after
Here you will need to be conscious of the type of cardio performed and also your nutrition. Attempting to do high intensity sprints after leg day may not be the best idea from an injury prevention standpoint.
"But I read..."
I recently wrote about conflicting information and often you have to take scientific studies with a pinch of salt, as while they are useful and can point you in the right direction, many are very limited in their application and conflict with other studies on the exact same subject. One I want to highlight as people may point it out was a study in 2012 looking at the hormonal responses to performing cardio 1st or 2nd. What they found was in their particular study was that testosterone remained elevated for longer when cardio was performed 1st. While it does seem as if the form of exercise performed last could influence your bodies hormonal adaptations. Whereby ending on anabolic exercise (weights, hiit) could be beneficial.
The issue to be had with this study is that the cardio performed was at a heart rate of 75% for 30 minutes, which is the exact type of medium intensity cardio I would not recommend for making gainz. I have not seen the same negative effects using low or high intensity. This doesn't mean the study should be ignored or is not of use, but can be seen as part of the indication of why cardio at this mid range of intensity is rarely performed by those looking to maximise their physiques.
If your goal is to run a 5k personal best or similar, then some training at this mid-range will be required, and ideally scheduled separately away from weight training.
If your goal is to be the next Mo' Farah and compete in longer duration races then your priorities will differ, but for the rest of us I would personally recommend doing cardio after weight training. While neither situation would be deemed as ideal, (the ideal being separate sessions) I believe that this one will provide you with the best results.
Nutrition can play a key role here and whether you choose to do your cardio pre or post weight training, see how you feel between sessions and consider having a small shake during training with easy to digest carbs and protein, such as dextrose and BCAAs. This can help negate some of the negative impacts on energy and blood sugar levels.
What I personally do when needing to perform both in one session is:
1. Make sure I have a good pre-workout meal around 60 minutes before
2. Perform the Weight training session. This is my priority
3. Drink a post workout shake of around 50g carbs, 25g protein either towards the end of my weight session or straight after. This is for two main reasons to help counter muscle protein breakdown, maintain blood sugar levels and lower cortisol.
4. Perform cardio session. Either HIIT (5-10 minutes) or LISS (20-30 minutes)