Alternating heavy days and light days - why?

My personal trainer set me up on a 3 day split where the first 3 days I lift lighter and more reps (12-15), and the second 3 days heavier weights and lower reps (6-8). What's the purpose behind this? What if I just lift heavy all the time?

Ask your trainer...duh.

There are many approaches to lifting. It frankly sounds like you know enough and are interested enough in all this to dispense with the trainer and experiment on your own, or at least discuss your ideas with your trainer.

There is a lot to be said for lifting every day or almost every day. Most of the benefit, as I understand it, is for strength, not size - it helps keep you 'in the groove' - your body remembers what to do with less layoff and progress comes more quickly. Some people, including me, use a technique Pavel calls Grease The Groove wherein we do a very easy session, typically half of what we'd normally do, but we do them all day long. I don't do this all the time but I use it as a plateau-busting strategy. I've heard of it giving very good results for things like pullups where a high rep number is the goal but it can work equally well where a high 1RM is the goal. I used this when I first was able to military press a 53 lb. kettlebell - I could only get one rep at a time, so I did my 1 rep 2 or 3 times throughout the day and soon enough I was able to get more reps at a time and switched back to a more traditional scheme. I've heard that Judd Biasiotto, a lifter I like to refer to a lot, put a bench in his kitchen while he was trying to overcome a plateau, and would do a few reps several times a day.

The idea of harder and easier days is to get the body to adapt and build muscle via the harder days while still keeping yourself 'in the groove' on the easier days.

Another precedent is the well-known Korte 3x3 program - there you do 4 weeks of easy but high rep lifting followed by 4 weeks of lower volume but higher intensity. All other things being equal, I would rather see you cycle between higher reps and higher weights on at least a 2 week basis to give your body time to adapt - I don't think switching more often will provide a whole lot of benefit - just my 2-cents worth.

Hmmm... So there's a name for the technique I've been using after all... ;-D

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Well, as far as trainers go, he's ok for a total beginner, but beyond that I think he's a little limited. And as trainers tend to be 100% biased as to the 'best' routine, I wanted to get someone else's opinion. It's hard to get good information for a female who wants to be a powerlifter. Not too many take me seriously. Thanks anyways...

WEll, I prefer the heavy days, and plan on changing up my routine, probably every couple months or so. I don't work out with the PT anymore, he kind of helped me get started, and I'm really greatful for his help, but feel he's a little limited and biased. I'm a 26 yr old female totally new to weights, and have a lofty ambition of becoming a powerlifter someday. When I tell people that, not too many take me seriously - I'm only 5'2' and don't come across as a serious weight lifter.

Yeah, I'd say so, but where are you in your development? Are your lifts still increasing smoothly, or are you running into plateaus? The heavy/light cycle you mention doesn't make a great deal of sense to me for either a beginner or intermediate lifter.

Being 5'2' is an advantage in powerlifting (unless you meant that you're too tall).

As the other poster said, ask your trainer. Could be an application periodization principles or exposing your muscles to different rep ranges... who knows unless you ask your trainer. What did you tell him/her about your training goals?

You'll make quick progress initially and then quickly burn out. Been there, done that...

Go heavy......or go home.

Then try Korte's routine.
http://www.deepsquatter.com/strength/archives/ korte.htm

It's how you come across in the gym that's important.

Stay that way!!