Both heavy and light weight can equally produce hypertrophy. Intensity of effort or high exertions is required for muscle growth to occur. It does not matter whether this is with a light or heavy weight; muscle hypertrophy is load-independent.
CAN YOU BUILD MUSCLE WITH LIGHT WEIGHTS? HIGH REP WORKOUTS CAN BUILD MUSCLE SUMMARY
- Both heavy and light weight can equally produce hypertrophy.
- Intensity of effort or high exertions is required for muscle growth to occur. It does not matter whether this is with a light or heavy weight; muscle hypertrophy is load-independent.
10/1 RESEARCH UPDATE: ANOTHER STUDY FINDS THAT MUSCLE GROWTH IS WEIGHT INDEPENDENT
For decades, bodybuilders assumed that lifting some heavy weight was the only way to get bigger. If you told a bodybuilder that he could gain just as much size doing 20-30 reps with a lighter weight as doing a moderately heavy weight for 8-10 reps, they would think that you are crazy!
The research has become overwhelmingly clear that muscle growth is weight independent as long as the higher rep group trains to failure. The newest study published in the Journal of Strength and Conditioning Research found that similar muscle growth can occur with heavier and lighter weights taken to failure.
Researchers had 23 untrained women divided into two groups: heavyweight- (80% of a 1RM) and lightweight (30% of 1RM) training groups for eight weeks. Both groups performed 2-3 sets of machine-based exercises (leg extensions, seated shoulder presses, leg curls, and lat pulldowns) to failure twice per week. They also measured perceptual responses to how hard they exercised after each session. They also measured the intention to exercise the next week and month after each training session. The researchers measured body composition, 1-RM strength, % body fat, and psychological factors (i.e., fatigue, desire to lift, etc.)
LIGHT WEIGHTS GROW MUSCLE JUST THE SAME AS HEAVIER WEIGHTS
At the end of the study, there were no differences in strength, body composition (i.e., lean mass and % body fat), or perceptual responses to the protocol.(9) The lighter weight group had a slightly greater increase in muscle mass (.5 pounds versus .05 pounds for the heavier weight group), but this was not statistically significant. Interestingly, the lighter weight group performed more total volume than the heavier group. The interesting finding was that the group’s intention to train was not different. This is unexpected, given the common gym myth that you can’t train heavy year-round because it’s too psychologically fatiguing.
Based on this study and other studies, you can train with either light or heavy weights and get equal increases in muscle growth. In terms of strength gains, it was unexpected that the light weight training group would gain equal strength gains as the heavier weight group. Training specificity usually dictates that lighter weight training will improve strength with lighter weights to failure and vice versa. If gaining muscle is your goal, you can train with heavy and light weights to make your training sessions more enjoyable.
However, if you prefer, you can stick to one way of lifting year-round. It makes sense to throw in a couple of lighter-weight sessions after a heavier-weight session to give the joints and tendons a break from the heavy lifting. Remember that this study used untrained female lifters, so that the applications may differ for trained lifters.
HIGH REPS OR LOW REPS FOR MUSCLE GROWTH?
Scientists once believed that a weight had to be greater than 60% of a 1RM to stimulate muscle growth; this was known as the hypertrophy zone. There is much controversy about this topic, you should check out the Reddit bodybuilding thread on the Best Rep Range for Natural Bodybuilders. In just about every journal for the past 10 years, it was stated that muscle growth occurs between 60-80% of a 1RM, with 8-12 repetitions. However, now we know that this is no longer true. Muscle growth can occur with both heavy (90% of a 1RM) and light weight (30% of a 1RM), provided the light weight reps are taken close to proximity to failure and the volume is sufficient.
In a meta-analysis of 23 studies with over 563 participants, it was found that muscle hypertrophy takes place with a wide spectrum of weights (i.e., 30 to 90% 1RM), and muscle failure seems to be an important component. The author also stated that heavy weight training to failure is demanding on the joints and tendons, possibly increasing the risks of overtraining. Thus, alternating periods with light, moderate, and heavy weight, is best for increasing muscle mass while minimizing excessive strain on the joints. 
LIGHTER WEIGHT TRAINING CAN CAUSE SIMILAR MUSCLE FIBER RECRUITMENT AS HEAVY WEIGHT TRAINING
Why isn’t weight a significant factor for muscle growth when performing repetitions until muscular failure? Henneman’s size principle explains this phenomenon, which states that muscles recruit fibers from smallest to largest 
Initially, the body utilizes small, slow-twitch fibers, but as the repetitions become more challenging, it activates more fast-twitch fibers. When you lift a light weight, the slow-twitch fibers tire out, leading to the recruitment of an increasing number of fast-twitch fibers.
By the end of a set, training to failure with a light weight fully engages all muscle fibers. However, using a lighter weight takes longer to activate the fast-twitch fibers. People once believed that muscle growth depended on recruiting fast-twitch fibers, which occurred when lifting weights above 60% of one’s one-repetition maximum (1RM). This notion is now subject to debate. Research shows that training with a light weight (e.g., 30% of 1RM) until failure results in complete recruitment of muscle fibers. On the other hand, lifting heavy weights (e.g., 70% of 1RM) recruits more muscle fibers.”
Training with heavy weight, light weight training at fast speeds, and light weight until failure all recruit fast-twitch fibers, but only training with heavy weights and light weights taken to failure results in muscle growth. This suggests there must be a certain amount of fatigue for muscle growth to occur. This may raise the question of whether plyometrics training performed with short bursts of explosive power can produce muscle growth?
Plyometrics for Muscle Growth?
Research documents that plyometrics increase jumping ability, sprinting speed, and strength, potentially due to improved neural adaptations.  Experts call the quick transition from the eccentric to the concentric phase of the movement the stretch-shortening cycle (SSC). Both plyometrics alone and resistance exercise promote muscle growth
A recent meta-analysis of 32 studies found that plyometrics training effectively increased the muscle thickness of the vastus lateralis, vastus medialis, rectus femoris, and triceps surae. Plyometrics also increased the fascicle length of the vastus lateralis and rectus femoris. Combining plyometrics and resistance exercise does not seem to produce additive effects on anabolic signaling or muscle growth compared to resistance training alone.
BEST REP RANGE FOR HYPERTROPHY
There must be a certain amount of fatigue for muscle growth to occur. Scientists now realize that it is less about how much weight is on the bar but more about effort level. In a paper published titled, “Training for strength and hypertrophy: an evidence-based approach,” which was published in Current Opinion in Physiology, the researchers placed the order of importance for muscle growth as Intensity of Effort > Volume > Training Frequency > Daily Protein Intake > Inter-Set Rest Periods.
You will notice that the top principles for stimulating muscle growth do not list weight or load; instead, they emphasize the intensity of effort. People assume that taking light weight sets (30% of a 1RM) to failure leads to similar muscle growth as using a heavier weight (80%). The authors claim that when individuals perform sets at a high exertion level and bring them too close to failure, the effort becomes more important than the weight. 
Not all PhDs subscribe to the philosophy of solely relying on effort level to build muscle. Some argue that you still need to train with heavier weights. Dr. Juneau expressed this view in an editorial responding to the previous review