This causes early and excessive use of the ankle dorsiflexors (the muscles in front of the shin and point the foot up) and continually stretches the ankle plantarflexors (the muscles in the calf that point the foot down). If it didn't do this, the toes would hit the ground - causing you to stumble. This can create overuse injuries in the calf, in the achilles tendon, in the shin, in the plantar fascia, and also higher up the chain in the knee and hip.
There can be different causes for a swing gait. I typically see a two-fold reason for most: tight hip flexors and weak glute/hamstring activation in the "pawback" of the gait. The pawback occurs when the foot is forward and about to strike the ground. Many long distance runners have little to no pawback action as they attempt to conserve energy. However, without any energy created by the pawback, the runner is forced to toe off (pushing the foot off the ground) more forcefully to gain the same amount of ground (This usually causes the runner to "leap" upwards instead of forwards). And since there is not enough momentum produced at the end of the stride, the leg does not get picked up high enough and must swing around itself.
The hip flexors are also playing a role here. If the primary hip flexor (psoas muscle) is too tight, it will limit the runner's hip extension (when the leg goes backward) and won't produce enough power to cleanly bring the leg forward. What you see are other muscles acting in its place to produce a swing.
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