Brain capacity may be a limiting factor in human muscle function per a study 'Maximal Voluntary Fingertip Force Production Is Not Limited by Movement Speed in Combined Motion and Force Tasks' by Valero-Cuevas et.al. which appears in July 8, 2009, Journal of Neuroscience.
"Valero-Cuevas and his collaborators, his former students Kevin G. Keenan of the University of Wisconsin/Milwaukee, Veronica J. Santos of Arizona State University, and Madhusudhan Venkadesan of Harvard University interpret the results to mean the brain is sufficiently occupied by the physical demands of combining motions and forces, so the muscle properties are not the limiting factors for how much force the fingers can create."
The results of the study were,
We find that maximal voluntary fingertip force is insensitive to finger movement velocity. This challenges the common hypothesis that the force–velocity properties of muscle are a primary limiting factor of force output during anisometric tasks. Therefore, it is necessary to consider other limiting factors, such as how the musculoskeletal structure of the fingers, the constraints of the task, and the nature of the neural controller conspire to reduce motor output even for ordinary manipulation tasks that combine motion and force production. Our musculature is not redundant in the context of combined motion and force tasks, and may begin to explain the vulnerability of dexterous function to development, aging, and even mild neuromuscular pathology.
Human muscular performance might be markedly inferior to that of chimpanzees and macaque monkeys per a study of metabolome evolution conducted in three brain regions and two non-neural tissues from humans, chimpanzees, macaque monkeys, and mice based on over 10,000 hydrophilic compounds by Katarzyna Bozek et.al. in 2014. The results suggest that "while humans are characterized by superior cognition, their muscular performance might be markedly inferior to that of chimpanzees and macaque monkeys."
We found that the evolution of the metabolome largely reflects genetic divergence between species and is not greatly affected by environmental factors. In the human lineage, however, we observed an exceptional acceleration of metabolome evolution in the prefrontal cortical region of the brain and in skeletal muscle. Based on additional behavioral tests, we further show that metabolic changes in human muscle seem to be paralleled by a drastic reduction in muscle strength. The observed rapid metabolic changes in brain and muscle, together with the unique human cognitive skills and low muscle performance, might reflect parallel mechanisms in human evolution.
Per Alan Walker in April 2009 issue of Current Anthropology, humans may lack the strength of chimps because human nervous system exerts more control over human muscles. Human fine motor control allows to perform delicate and uniquely human tasks while preventing great feats of strength when compared to chimps.
Walker's hypothesis is derived partly from a finding by primatologist Ann MacLarnon. MacLarnon showed that relative to body mass, chimps have much less grey matter in their spinal cords than humans. Spinal grey matter contains large numbers of motor neurons nerves cells that connect to muscle fibers and regulate muscle movement. Walker proposes that more grey matter in humans means more motor neurons and having more motor neurons means more muscle control.
According to Walker, human surplus motor neurons allows to engage smaller portions of muscles at any given time and the finely-tuned motor system makes a wide variety of human tasks possible such as manipulating small objects, making complex tools or throwing accurately. Due to conservation of energy by using muscle gradually, humans seem to have more physical endurance. "Great apes, with their all-or-nothing muscle usage, are explosive sprinters, climbers and fighters, but not nearly as good at complex motor tasks."