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The long digital extensor muscle is responsible for extending the digits of the foot, particularly in quadrupeds. Its primary function is to facilitate the extension of the second through fifth digits, making it crucial for various movements such as walking, running, and jumping.

The anatomy of the long digital extensor indicates that it extends from the tibia and fibula to the distal phalanges of the second to fifth digits. Therefore, the correct answer highlights that this muscle specifically targets the middle and distal phalanges of digits II to V. This focus on digits II to V is essential for understanding the muscle's role in facilitating extension and supporting locomotion in animals.

In contrast, the other choices suggest limitations to the involvement of other digits or solely the first digit, which does not accurately represent the muscle's anatomical and functional properties. The implications for movement are significant, as restricting the muscle action to fewer digits would compromise the full range of extension necessary for normal locomotion.