Some turtles can move fairly quickly on land since they have to move away from predators, travel to find food, and move between bodies of water. These are usually semi-aquatic turtles that spend much time in the water but some time on land. They have short stubby legs ending in semi-square feet and toes. Their claws also help propel them along.
The Sea Turtles are entirely different creatures. While they can propel themselves on the beach as hatchlings and the females can do so during nesting, their bodies are not made to run around on land. They do not have legs and feet like an earth-dwelling tortoise or some fresh water turtles. They may move a little more quickly on sand than on other surfaces since their flippers have claws that enable them to move along a sandy surface.
These Sea Turtles have strong flippers that will enable them to swim with grace and dig large holes to lay eggs in. A Sea Turtle hatchling emerges from the shell ready to move to sea. Their appendages are curved somewhat from being inside the shell and they have a natural instinct to move as quickly as they can to evade predators and get out to sea.
Sea Turtles mature slowly and many of them swim many miles during their growing phase. Neither the males nor the females come completely out of the water for more than brief periods until they are breeding age. The males never come out of the sea again. Their bodies are streamlined and their shells are flat and slender unlike the domed earth-dwelling tortoises. Their muscles and their flippers are built to swim!
The skeleton of a Sea Turtle is composed of bone and cartilage. The skull is bone, the carapace (shell) is bonelike, and the vertebrae and ribs are bone. The plastron underneath is a composite of skeletal material, ribs and shoulder material. It remains somewhat flexible but strong. The rest of the Sea Turtle includes the front flippers, hind limbs and supporting structures.
The flippers of the Sea Turtle have specific muscles and ligaments that help move the flippers. These are called the flipper retractor muscles, flipper abductor muscles, and the flipper protractor muscles.
The forelimb has a humerus bone like the upper bone of the human arm and the lower limb has a radius and an ulna which are like the human wrist bones. Sea Turtles also have toes with carpals, metacarpals and phalanges, all similar bones to the human lower wrist and hand. In essence, the flipper isn’t construction to walk on, but is constructor to swim and in males these bones and muscles allow them to grasp the female firmly during mating. The bones of the turtle flipper that would compose the wrist and hand of a human are still quite different from humans since they are partially fused by fibrous connective tissue. In the same way that a person has fingernails, turtles have sharp claws for breeding, feeding, digging for food when necessary (rare) and digging nests to lay the eggs.
Due to the structure of the turtle flippers, the female turtle can move well enough on the beach to move through sand with great purpose, although it probably wouldn’t be considered fast by mammal standards. Most importantly she can use the flippers to powerfully dig the nests and then she can again navigate firmly back to sea, but she is not a speedy runner.
Essentially, Sea Turtles are probably fastest when they are hatchlings moving out to sea although even then, the speed is only relative to their small size and their natural desire to get into the water.
Female Sea Turtles are fast enough on land to take care of her nesting needs, but they are not intended to be land-dwellers and they do not move fast compared to shore animals or other reptiles with legs and feet for walking or running.