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Although the two types of training produce similar metabolic stress, muscle activation is different [fapesp.br]:

Which kind of resistance training promotes more muscle growth: low load with many repetitions or high load with fewer repetitions? According to a study conducted at the State University of Campinas (UNICAMP) in São Paulo state, Brazil, it makes no difference.

The study lasted eight weeks and involved 18 volunteers in two different training protocols. One group performed high-load (HL) exercises with fewer repetitions, while the other did low-load (LL) exercises with more repetitions. Muscle mass was measured in the first and last exercise sessions. A comparison of the two groups did not show any difference in muscle growth or metabolic stress, measured in an analysis of substances released into the bloodstream by the exercises.

In the HL group, each individual lifted up to 80% of their own weight. In the LL group, the limit was 30%, but they repeated the exercises until their muscles could no longer lift the loads.

[...] "Resistance training is known to promote muscle growth, but it's still not completely clear whether the key to muscle hypertrophy is the load or the number of repetitions. Our study supports the theory that both types have the same effect. We also showed that muscle activation occurs in a different manner in each type, although metabolic stress is the same and the effect on hypertrophy is therefore also the same," said Renato Barroso, a professor at UNICAMP's School of Physical Education.

[...] Although the researchers found no differences in the overall metabolic response, they observed that some metabolites correlated with muscle hypertrophy in both groups. Some of these correlations, they concluded, may be associated with the characteristics of the muscle fibers activated by exercise (type 1 or type 2) as well as the metabolic demands of the training protocols used in the study.

"Some of the metabolites studied come from anaerobic energy systems and are produced by glycolysis [glucose breakdown] in the muscles or the breakdown of creatine and phosphocreatine, which supplies sufficient energy to maintain exercise intensity for a few seconds. Asparagine and acetoacetate are associated mainly with the Krebs cycle, which uses oxygen and nutrients such as fat, protein and carbohydrate to produce energy for the muscles and lasts a great deal longer," Barroso said.

[...] In HL training, activation of type 2 muscle fiber predominates because of the higher load. "These muscle fibers have low oxidative activity but high glycolytic activity and may be more responsive to hypertrophy than type 1 fibers. On the other hand, LL training, which has more repetitions, more preferentially activates type 1 fibers, which have low glycolytic capacity and high oxidative capacity, and are highly fatigue-resistant," he said.

Journal Reference:
Valério, Denis F., Alex Castro, Arthur Gáspari, and Renato Barroso. 2023. "Serum Metabolites Associated with Muscle Hypertrophy after 8 Weeks of High- and Low-Load Resistance Training" Metabolites 13, no. 3: 335. https://doi.org/10.3390/metabo13030335 [doi.org]

Original Submission