Science behind exercise: muscle size vs muscle definition

In weightlifting I have often heard it said that using heavier weights with fewer repetitions promotes muscle size, where as lighter weights with higher repetition promotes muscle definition. But what exactly does this mean?

It seems like any type of exercise should increase muscle mass, while definition would be a result of having low body fat. Does this old adage just mean that heavy weights build more muscle while higher repetitions burn more fat? Or is there something more going on here? I know that there are two types of muscle fibers – slow twitch and fast twitch – but does exercising one type instead of the other actually change the size or shape of the relevant muscle? And if so, how?

just a wag but with the big weights you are causing big damage to a large part of the muscle group (the damage is why they hurt and come back bigger after the repair, thats how it works)
with a lighter weight you get less damage and its probably more spread out over the muscle group instead of confined to the largest portion…I dont know if that makes sense in text but it sounds right in my head. I guess its like the muscles on the edges get to work as well with the light weights with the heavy ones you just use the ones needed to get the weight up and down.

It doesn’t mean anything.

Nothing can change the shape of a muscle. You can make it bigger, and if you do, the muscle will be more apparent thru the skin, even if you don’t lose subcutaneous fat, but the shape of a muscle is genetically determined and can’t be changed with exercise.

The idea of “higher reps produce definition” was based on the misconception that you can spot reduce fat in one body part by working it selectively. You can’t. You can lose fat with exercise, and circuit training might be a good way of using up calories, but you will lose fat first from where ever you are genetically predetermined to gain it last, and lose it last from where ever you are genetically predetermined to gain it first.

Sustained, low-intensity exercise is thought to burn off fat, and you could do that with lots of sets, low weights, and no rest periods (which is what I meant by circuit training) but it might work better to do some higher-intensity lifting to build muscle size and then walk for an hour.

Regards,
Shodan

Increased reps can increase vascularity and muscle striations.

95% of all the muscle’s ‘‘look’’ comes from size and reduced body fat, but to achieve peak vascularity, doing more reps can increase the need for blood, so the supply lines are going to be more noticable.

The other ‘tweak’ you can get is to use extra reps very deliberately to stretch the muscles so that striations are visible.

Deliberately training for both vascularity and muscle striations is really left to the elite body builder to worry about. Everyone else who is working out is going to see their muscles change in size – marking the visual progress – and the appearance will be enhanced by low body fat percentages.

What about the differences in muscle fibers? Do muscles that contain relatively more or relatively fewer of a given muscle fiber have a different shape or appearance?

What we see as a muscle can actually be several muscles (a muscle group), so the type of work they get will dictate the appearance, but most of the appearance is just sheer size vs. the fat that camoflauges them.

You are in a tough spot here in GQ, because I’ll give you advice, but it won’t be based on double blind studies, and others will pounce.

Work out the major muscle groups and stick to 12 reps or less to build size. What does this mean? Say you have 135 lbs on a barbell for benching. If you do 12 reps and you could have done 20 reps, you aren’t building size just because you did 12 reps.

You should have enough weight that by the 12th rep, you are toast, or nearly toast. Then on your next set, you add some more weight and fail around 8-10 reps. On the last set, you have a little more weigfht and fail say around 6-8, maybe getting a little help to get the last 1-2 reps out. You do 3, 4 or 5 sets of this benching exercise.

For big muscle groups, like the chest, you can do this type of pyramiding of sets/reps…say 3-4 sets per exercise, and doing 3-5 exercises for that group.

At the end of working the chest group, you might have done 100-125 reps total, from 3-4 different types of exercises. You can call it a day…and maybe call it your chest workout for the week. Maybe you did flat bench, incline bech with dumbells, pec deck, incline flys, etc.

Some guys will alternate light and heavy days. On a ‘light day’, you shoot for 12-15 reps before failure.

…It seems like any type of exercise should increase muscle mass…

Nope. Marathon runners exercise like crazy and they don’t build muscle mass. Hypertrophy requires progressive loading resistance training.

Lower weight/higher rep training and higher weight/lower rep training both have advantages and disadvatages. As has been said, higher rep training promotes improved vascularization. This happens at the macroscopic and microscopic level so higher rep sets improve muscle blood dynamics and you gotta feed 'em if you want 'em bigger and so higher rep sets have a place in a complete hypertrophy regimen.

Higher weight/lower rep training improves fiber bundle recruitment, among other things and so it too has a place in a complete hypertrophy regimen.

Exercise physiology is a rapidly expanding science and you can get very deep into it and still have only scratched the surface.

The site at Hypertrophy Specific Training has a lot of good information. You can also get a lot of training information at T-Nation.

Slow twitch fibers are darker than fast twitch ones due to their higher mitochondria density. Other than that, no.

Ignoring the fact that the slow twitch and fast twitch distinctions are pretty meaningless when applied to humans, fast twitch muslces on average have a slightly higher mitochondrial density.

Any colour difference is due to vascularisation and myoglobin, nothing to do with mitochondria.

When I need to push a heavy couch up the stairs, I need to call a buddy to help. Together, we can lift the heavy load. If I just need to carry a letter up the steps, but a bunch of times, it’s best if I just had some endurance and leave my friend alone.

Likewise, if you give your muscles heavy things that they need help with (high weight, low rep), they’ll need to “call in the buddies” and will build more muscle fibers (myofibrils), which are basically cords that pull the two ends of the muscle cells closer together (contraction). If you only need to lift light things a bunch of times (low weight, high rep), your muscles aren’t going to bother building expensive (from a nutrition standpoint, anyway) fibers. They’ll just get more of what they need to endure, e.g. mitochondria (power factories) and vessels (supply lines).

So you’ll need to do both types of exercise sets because they’re two different types of growth. Don’t put your reps higher than 12 though…you wont be able to outpace your oxygen intake. That is, you’ll be able to breathe fast enough and your heart will be able to keep up, at least enough that you’re wasting time in the gym. It’s good for your heart and lungs to do 20 reps a set, but you’re better off running if that’s your goal.

As for ‘definition’, heavy sets are carried mostly by the big muscle, and light loads allow your posture muscles more of the action. With your light loads, go slower and focus on controlling your weight, to help develop those balance muscles. That’ll give you that ‘cut’ look as much as putting away the fatty cakes at lunch.