Both static and elastic side straps can be used on the Freemotion harness and Combined Harness. They are replaceable. The harnesses come with static side straps as standard, because they are more durable than elastic side straps.
Elastic side straps give the dog a few more centimeters for each stride. Because of this, athletes doing canicross and skijoring prefer elastic side straps. The elastic straps are not as durable as the static straps and are for race use.