A common belief in the fitness industry is that there is no need to lift heavy weights, because you can get just as effective of a workout lifting light weights (albeit usually for more repetitions). Is that true? The short answer is NO, it’s not true, so let’s take a look into why that is.

First off, it is important to clarify that “light weights” and “heavy weights” are both relative terms and the strength / fitness level of the person doing them of course needs to be factored in, as well as the exercise being performed. For example, a weight that is heavy for shoulder raises would feel laughably light for most people if they were asked to deadlift it. Similarly, it is common sense that on the same exercise a person new to the gym might find one weight to feel heavy, while a person who has spent years in the gym might not consider that weight to be challenging.

So, why is it that you can’t get the same workout with light weights compared to heavy weights … why is it important to include heavy weights in your strength training? There are a few reasons, and in the interest of brevity we’ll narrow it down to two of the most important ones.

First off, there is a quote I love that states “force is the language of cells,” (I first heard this quote at a Functional Range Conditioning certification I was at several years ago). As you probably know, your body is made up of a huge number of cells, including muscle cells, skin cells, those in your bones, your ligaments, and so on. Your cells adapt and turn over all the time – in fact many of the cells in your body today weren’t there just a few months ago. So, how do these new cells know where to go and how to grow / structure themselves? The answer is force; either forces imposed on them by your activities (like strength training), or a lack of forces due to lack of activity. The more force you can impose on your cells, the more input you give them on their development. So, this is good, this means we have control over how our tissues lay down.

So how do we do it? If you think back to high school physics, you remember that F = MA … Force = Mass x Acceleration. We know that our goal is a high level of force, which means we either need a high amount of mass (weight), acceleration, or both. So here it is plain to see that lifting heavy weights (high mass) will help us achieve our goal of force – this is good! If the mass is low, then the only way to make force high is then to have a high acceleration; you need to move the weights fast (kettlebell swings anyone?). The faster you move the weight, the more force you produce with a fixed mass, and thus we still accomplish our goal of high amounts of force. There is a catch though – light weights moved slowly DO NOT ACCOMPLISH OUR GOAL. Having worked in the fitness industry for a long time, I can’t tell you how many people (including personal trainers) advocate “light weights, slowly, for high reps” as their workout methodology. While these people are well meaning I’m sure, this advice doesn’t stand up to the test of basic anatomy and physics, and thus should be categorized in this instance as a myth / misrepresentation (in a future blog I’ll explain where this method can be helpful, but it should NOT be used as the exclusive style of training).

So, there is thing one … F = MA, and so we need either high mass or high acceleration in order to accomplish our goal of high force. If we don’t produce high amounts of force, our body doesn’t get the input it needs to lay down new tissues optimally, and leads to a myriad of problems. So what is thing two? Thing two is that not all muscles (or parts of your muscles) are created equally. Within your muscles you have actually have different types of fibers that are designed to help accomplish different tasks. A fairly simplified way to look at it is to break it down into “slow twitch” and “fast twitch” fibers (there are more subdivisions, but these two are sufficient for now). Slow twitch fibers are what we use all day every day – they are not particularly strong, but they can handle light to moderate loads for extended periods of time without getting too tired. When you start to move, be it just walking around or lifting light weights slowly, these are the fibers that do the job for you. The catch is, we REALLY want to recruit those fast twitch fibers. There is a TON of benefit to them, including getting a lot stronger and more powerful (surprise), but also they help you strengthen your bones, and challenging these fibers gives you fantastic hormonal changes that help you with things like burning fat and building muscle. These changes don’t just stop when you stop the workout, they’ll stay with you for an extended period afterwards, keeping your metabolism firing on all cylinders even when you’re not in the gym.

So, what’ the deal with these fast twitch fibers? The deal is they only respond to high enough stimuli – either heavy weights or fast movements. You need to create a lot of force in order to recruit these, so either weights that are quite heavy (think heavy deadlifts or bench press), do things that move very fast (box jumps, sprints), or something that is a mix of the two (kettlebell swings, medicine ball throws). Again, lifting light weights slowly will not do the trick, so you’re leaving SO MUCH benefit on the table by not recruiting these fast twitch fibers. There’s another catch though, and that is that these fibers inherently have poor endurance capabilities, so by trying to either go high repetition, or doing short / insufficient rest breaks, you start to recruit them LESS because their biology doesn’t jive with those styles of training. Don’t misinterpret, I’m not saying never do those other things, I’m just saying that we need to understand what we’re trying to accomplish, and if we want to recruit those fast twitch fibers (WHICH YOU REALLY WANT TO TO) then at least some of our training needs to be low repetition for heavy weights and/or explosive.

I understand this can be a lot to take in, so here are a couple simple solutions to help minimize confusion. In both examples below, give sufficient rest between sets so that fatigue does not compromise quality.

• Heavy, slower lifts (deadlift, bench press, etc.): Try for 5 reps / 3 sets using a weight that you could do 8-10 of if you tried your hardest
• Faster, explosive lifts (kettlebell swings, medicine ball throws): Try for 10 reps / 10 sets at a weight you can be very explosive with for all 10 reps, but if you tried to do 15-20 reps you’d lose explosiveness.

In future posts I’ll explain how some of the other styles of training can still be beneficial, but they’ll never be a replacement for developing high force and recruiting your fast twitch fibers.

-Tony Gracia