I Trained One Side HEAVY vs LIGHT For 60 Days (Bad Idea)

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Should you lift heavy or light? You have to train with heavy weights for the best muscle growth; at least, that’s what many people say. But not everyone agrees. Light weights vs heavy weights for growth is one of the most debated questions in fitness. So, I’m putting lifting heavy vs lifting light to the test MYSELF. For 60 days straight, I’ll split my body in half. One side with heavy weights, the other with light weights.

Here’s my 60 day heavy vs light weights plan. Every major muscle will be trained unilaterally. Each side will receive the exact same number of total sets, and every set will be taken as close to failure as possible. But the heavy side must reach failure within 3-6 reps whereas the light side must reach failure within 15-30 reps. Plus, to make sure there’s actual changes in muscle growth and strength, I’ll also be eating in a small calorie surplus.

I have years of training experience; I’m curious if a beginner’s muscles would respond differently to this lift heavy or light experiment. So I roped in Dennis. His report shows that he’s also a right-hand frequent user, with a starting body fat of 18.4% with 113.6 lbs of lean mass.

Week 1: Given that the average beginner can gain around 8-10 lbs of muscle in their first year of training, I’m confident Dennis can gain at least a couple of pounds of muscle if we dial in his training during this lift light or heavy experiment.

Wondering about training volume in this heavy vs light weights experiment? By multiplying total sets × reps × weight from week 1, the light side came out to over 100 thousand lbs, whereas the heavy side was about half of that. Some people think total volume lifted and time under tension are keys to growth; if that’s the case, the light side should dominate. So, can the heavy side keep up despite lifting less total volume, taking half the time per set, feeling no burn, and having less soreness the next day?

Week 2: To help Dennis grow, we focused on his diet with a daily calorie and protein target. Both of us noticed quicker progress on our heavy weight sides. Why? When lifting heavy, going from 100 to 105 lbs is a 5% increase. With light loads, 50 to 55 is 10%, making it tougher to add weight when doing high reps. For the light side, the smart move is to beat last week’s reps rather than bumping weight too quickly. Both of us noticed the light side gets a WAY bigger pump. The pump happens when blood rushes into your muscles faster than it can leave; research is mixed on whether it helps long-term growth.

Weeks 3–6 of the lift heavy or light plan: Dennis’ heavy right side kept progressing with the weight, while his light side barely moved. My Bulgarian split squat went from 205 to 215 on the heavy side, whereas my light side stayed at the same weight. The extra fatigue from many high-rep sets was taking a toll on recovery and mentally burning me out. Then: sharp pain at the top of my knee during a heavy set.

Heavy weights put more stress on joints, which can make tendons stronger over time, but if you only ever use heavy weights—especially on exercises that aren’t a great fit—it can become too much. I switched my heavy leg to lighter weights for high reps and burned out WAY faster. That’s endurance adaptation. By consistently training with higher reps, your muscles create more mitochondria to reduce the “burn” and your brain increases its tolerance to pain.

The downside to training light? Your bigger, more powerful fibers grow your muscles the most, but they’re only called into action if needed. With heavy weights, they’re activated from the first rep. With light weights, they join only when smaller fibers fatigue, so if you stop because of the burn, you miss them.

Weeks 7–8 of the lifting heavy vs lifting light plan: Form creep and ego lifting can create the illusion of progress. I corrected Dennis when I caught it. Meanwhile, my elbow started acting up on isolation moves.

Day 60 Strength Test: Strength was movement-specific. On unpracticed tests, results were surprising; on practiced lifts like preacher curls, the trained pattern mattered. With the chest press machine and flat dumbbell press, stabilization and transfer told a nuanced story.

Time for results of the lifting heavy vs lifting light experiment. Overall, I gained about 2 pounds; just over half a pound (~0.66 lbs) was actual muscle. The lighter-weight side grew just slightly more in almost every muscle, but the differences were tiny and within measurement error.

Dennis gained ~3 lbs of lean mass; his body fat dropped from 18.4% to 17.8% because he added more muscle. Every muscle on Dennis trended toward the lighter side growing just slightly more, but again the difference was tiny and nowhere near statistical significance.