There is no consensus about the best time to start exercise after peripheral nerve injury. We evaluated the morphological and functional characteristics of the sciatic nerves of rats that began to swim immediately after crush nerve injury (CS1), those that began to swim 14 days after injury (CS14), injured rats not submitted to swimming (C), and uninjured rats submitted to swimming (S). After 30 days the number of axons in CS1 and CS14 was lower than in C ( ). The diameter of axons and nerve fibers was larger in CS1 ( ) and CS14 ( ) than in C, and myelin sheath thickness was lower in all crushed groups ( ). There was no functional difference between CS1 and CS14 ( ). Swimming exercise applied during the acute or late phase of nerve injury accelerated nerve regeneration and synaptic elimination after axonotmesis, suggesting that exercise may be initiated immediately after injury. 1. Introduction Peripheral nerve injury promotes motor, autonomic, and sensory alterations in the region of the affected nerve, among which loss of function and progressive muscular atrophy stand out [1, 2]. Regeneration speed and subsequent functional recovery depend on the extension, nature, and degree of injury [3, 4]. In many cases morphological and functional recovery are not fully achieved , causing limitations in daily life and work activities , and may lead to early retirement due to functional disability. Several studies have investigated the effects of physical treatments on peripheral nerve regeneration and functional recovery, including phasic electrical stimulation [6–8], chronic low-frequency electrical stimulation [9, 10], ultrasound [11, 12], and physical exercise [8, 13–15]. Studies with rabbits after sciatic nerve crush showed that swimming exercise aids in both the removal of degenerated myelin and in its synthesis during nerve regeneration . Physical exercise results in increased nerve impulse conduction speed and sensory-motor recovery  as well as in muscle property maintenance, assisting in trophism and minimizing muscle weakness after denervation . However, there is no consensus on the ideal time to start exercise after denervation. Considering that muscular reinnervation begins on the 14th day after injury, Herbinson et al. [18, 19] recommend that the stimulation of neuromuscular activity by exercise should begin approximately two weeks after nerve injury, leaving a rest period between the injury and the beginning of exercise. Gordon et al.  emphasized that, depending on the extent of the injury, exercise during the acute phase may
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