Physical development considerations should be central to every decision about a yearling's groundwork program, because a yearling's musculoskeletal system is still in active development and the demands placed on it during this period directly affect whether the horse develops soundly or accumulates the early joint and soft tissue damage that shows up as lameness and shortened careers later.
The growth plates — areas of active bone growth at the ends of long bones and at the vertebral apophyses — are particularly vulnerable in the yearling year. The growth plates of the distal cannon bone and pastern close earliest, typically within the first year of life. Others, including the proximal radius and the vertebral growth plates of the spine, remain open significantly longer — the vertebral column in horses is not fully mature until five or six years of age in some regions. This means that repetitive concussive loading and extreme flexion or extension demands on the spine during the yearling year carry real developmental risk.
Practical guidelines for protecting yearling physical development include keeping lunging circles large enough to reduce the lateral loading stress on developing leg joints — a minimum of fifteen meters and ideally larger. Limiting the amount of trot work on hard or deep footing reduces concussive loading. Avoiding demanding collection that requires extreme hindquarter engagement before the horse has the muscular development to support it protects the developing lumbar and sacral regions.
Conversely, appropriate light work — walking on varied terrain, short sessions of active trot, and the lateral bending exercises of yielding — actually promotes healthy joint and tendon development by stimulating appropriate tissue remodeling. The goal is not to eliminate all physical demand but to keep the demand within the range that promotes development rather than causing damage.