Head bobbing

When I take M&J for a brisk walk up and down the street, I notice Mitzi's (7YO) head bobs as she walks, somewhat like a horse, but not so pronounced. Is this just her gait or is it a sign of pain or arthritis? When she walks in the house or in the yard, she doesn't bob the head. Also, in her yard off leash she runs like the wind - can outrun Judy. If she were injured or in pain, she wouldn't run like that. Am I concerned about nothing?