Exercisers and exercise professionals alike continue to cling tightly to the long-held axiom that performing a lot of reps with a lighter weight will preferentially improve muscle endurance and create a toned appearance. While performing fewer reps with heavier weights will preferentially improve muscle strength and increase muscle size. In reality, the amount of weight you use and the number of reps we perform has no bearing (or an imperceivable bearing) on the results we produce from strength training. Our neuromuscular system experiences momentary muscle failure the same whether it occurs on the 5th rep, the 15th rep, or the 45th rep. Although this has been evident in the scientific literature for nearly two decades, it remains one of the most pervasive myths in fitness.
All rep ranges produce the same fitness results, but there are two compelling, research-based reasons you may want to avoid a light weight and extremely high rep ranges:
First, Discomfort. Authors of a study conducted at Solent University in the United Kingdom put students into one of two groups; one performed a sophisticated leg extension machine (a $50,000 machine) to muscle failure with a moderate weight; they completed 10 reps on average. The second group performed a set to muscle failure using a very light weight on the same machine; they completed 42 reps on average. The fitness results between the groups were the exact same (as we would expect). However, the group who performed 42 reps reported physical discomfort that was off the charts. They found the 42 reps to muscle failure to be almost unbearably uncomfortable.
Our take home message: Muscle failure is our real goal. Using a light weight and performing a lot of reps makes reaching muscle failure more unlikely because we are likely to terminate the set due to discomfort rather than an actual inability to perform another rep. Regardless, muscle failure is going to be uncomfortable, we should aim to make it as tolerable as possible and we do this by using a more moderate or heavier weight.
Second, Longer recovery. A more recent study also conducted at Solent University discovered that utilizing lighter weights and performing more repetitions necessitated longer recovery periods between workouts. Based on this research, if someone wanted to work out more often, they should opt for heavy weights and fewer reps. Lighter weights with more reps leave our muscles exhausted for longer. And even leave our unexercised muscles/limbs exhausted.