Gonna throw in my two cents as well
...this is a really fun question. And it gets technical pretty quickly. But to cover the basics (ish) ...
The important thing to keep in mind is that our bodies exist within spectrums; that is, there is no 'all or nothing', it's pretty much always 'mostly something'. Also, your body likes to be efficient.
To make things easier to explain, I'll talk about muscles in terms of 'fibres'. Just like when people talk about cutting 'against the grain' with a steak, or 'flaky' fish, our muscles are built in layers - or fibres. And the fibres are pretty much just big chains of protein.
There are two main types of fibres in your muscles. They are usually called Type I (slow twitch) and Type II (fast twitch - actually, there are types IIa and IIb, but we won't get into that today). Type I tends to do better with endurance work, or repetitive low-resistance contraction - like distance running, or high rep resistance work (as in, 18+ reps per set). And Type II tends to produce stronger contractions for shorter periods of time - like the heavy weight side of lifting (1-6 reps per set). Type I fibres are efficient in using oxygen to create energy (ATP) whereas Type II fibres create fuel without oxygen; this is where people get the terms aerobic and anaerobic activity
(again, it's actually more complicated than that: IIa fibres can actually metabolize energy both aerobically and anaerobically, while IIb are exclusively anaerobic - but we're trying keeping things simple).
But this isn't an on/off switch; for short bursts of movement, it's mostly
Type II fibres with a few Type I thrown in. And for more repetitive endurance work it's mostly
Type I with Type II pitching in a teeny tiny bit. The further along either side of the spectrum (eg. 1 rep vs 30, or sprint vs marathon), the bigger difference in which
fibres your muscles choose to recruit (use). Type II fibres tend to have a greater potential for growth, and are usually what people are talking about when discussing 'building muscle' in strength training.
To grossly generalize, there are several things that can happen in your muscle when you start to lift weights. A. Your body learns how to recruit more fibres at once (also sometimes called motor unit recruitment). B. Your body makes your fibres able to contract harder. C. Your body makes the fibres able to contract for longer. When you first start to lift weights, your muscles are forced to work harder (A), and very quickly they come to limit of what those fibres can accomplish. At which point, you undergo what's called hypertrophy - or an increase in muscle mass. This happens in two ways (B+C): your body adds more pieces that can contract to the fibres (B) so they can work harder, and your body adds more pieces that create energy in the fibres (C) so they can work longer. (C) tends to add more volume, and is often what people think they're talking about when using the term hypertrophy. Thinking logically, if you ask your muscles to work harder (ie. lift heavier weights) then your body will add more contracting bits (B) - but if you ask your muscles to work for a longer period of time (ie. lifting weights for more reps) then you body will add more energy bits (C).
So, actually, you are building your muscles whenever you increase the weight you move! The difference in how you SEE it in your muscles is affected by how your body adds to the fibres. This can be affected by 1.the type of exercise 2.the resistance and 3.how long your muscles work for. If you are using a full range of motion you actually get extra bits on the end
of the fibres, so your muscles literally get longer! If you use a shorter range of motion, your body might add more 'side by side' contracting pieces to the fibres. Again, this is a spectrum; your body does both regardless, it just swings from one side to the other depending on the variables.
You will sometimes hear people talking about 'strength' lifting routines or 'hypertrophy' lifting routines. You'll even get different rep ranges recommended for each.
It's often said that for:
Strength, lift with 2-6 reps per set
Hypertrophy, lift with 8-15 reps per set
This varies a bit depending on the muscle group, weight, individual, etc. And past the 15-18 rep mark, you're looking more at endurance work, or a greater focus on Type I fibres.
There are LOADS of different ways to structure your workouts; you can alternate rep ranges within your different weekly workouts, you can do 12 weeks of low reps and then switch to high reps, you can work in the middle range (5-10 reps). That is completely up to you, and the experimentation involved with what works best for YOU is half the fun!
If you want to continue to increase your strength (or muscle mass!
), you need to make sure you are always challenging your muscles. If you can lift a weight for 15 reps, but only do 6, you're not really challenging yourself. But, by the same token, it's not always a healthy thing to force yourself to do rep #7 if 6 is your absolute max with that weight (this is sometimes called 'lifting to failure', when you literally force your muscles to fail in their attempt to move the weight). Personally, I think it's healthier to lift until just before this (or when you can probably manage 1-2 more reps, but choose not to) so you don't burn out. How much you eat, and what you eat, also has a significant effect on building muscles and strength.
Being well hydrated, and/or supplementing with creatine, can also cause your muscles to 'swell'. This is just because there is more water in your cells and they literally get a bit bigger, but this is not considered to be hypertrophy.
Also, muscles have the potential to grow much faster than ligaments and tendons. While muscles produce the force to move your body and limbs, tendons are what hold the entire structure together, and they take on a great deal of stress to do so! That means that it's actually healthier to put on muscle at a gradual rate, because it allows your body the time to also develop your connective tissues; this keeps you stable, and helps prevent you from pulling or tearing stuff. Tendons (as far as I know) tend to respond better to the endurance side of the equation; that is, higher reps. The takeaway? Vary your workout routine to include both strength and endurance work, and don't be stupid about adding too much weight to your lifts at once. Connective tissue heals much slower than muscle, can be more painful to injure or irritate, and those sorts of injuries take WAY longer to heal than just straining a muscle - try not to do it
. Regular mobility work and foam rolling also help prevent strains and tears long-term.
Women can put on muscle just as quickly as men; it's just that (as I understand it) we tend to have lower 'genetic ceilings' for muscle growth. Which is to say our genes don't want us to grow as big as men, and our hormones make it harder to go past a certain point. A caveat on the speed of female muscle building: while women can usually match lower body muscle growth to men's (think squats, deadlifts, hip thrusts, etc.), men usually put on upper body muscle (push ups, pull ups/chinups, etc.) faster than women. Women also carry more essential fat ('cause of the whole baby making thing
), and will rarely show the same muscle definition as men because we have just a little bit more soft stuff.
alaskanlaughter, it sounds like you've made some pretty impressive gains, with the numbers you're talking about. After lifting with machines for a while, have you considered trying out some free weights? I like them because they allow for a more 'natural' range of movement, instead of restricting your body in the direction that the machine is built. Just a thought, anyway.
Erm, sorry for the essay. 'Hope this helps?
Disclaimer: I'm not a doctor, dietician or even a fitness professional – I just read too much. Take the time to research any new fitness/diet strategies, and make sure they are SAFE and EFFECTIVE for YOU!