Symptomatic spondylolysis/spondylolisthesis is thought to be caused by repetitive lumbar extension. About 8.9% of baseball pitchers that experience back pain will be diagnosed with spondylolysis. Therefore, this study aims to identify and quantify lumbar extension experienced during baseball pitching. It was hypothesized that young pitchers would exhibit less lumbar extension than older pitchers. A total of 187 healthy pitchers were divided into 3 age groups: youth, adolescent, and college. Kinematic data were collected at 250 Hz using a 3-D motion capture system. Lumbar motion was calculated as the difference between upper thoracic motion and pelvic motion over the pitching cycle. Lumbar “hyperextension” was defined as ≥20° past neutral. College pitchers had significantly greater lumbar extension compared with youth and adolescent pitchers at the point of maximum external rotation of the glenohumeral joint during the pitch cycle (−25° [13°], P = .04). For all age groups, lumbar hyperextension was present during the first 66% of the pitch cycle. Most pitchers spent 45% of pitch cycle in ≥30° of lumbar extension. Understanding that lumbar extension and hyperextension are components of the complex, multiplanar motions of the spine associated with baseball pitching can potentially help in both the prevention and management of symptomatic spondylolysis/spondylolisthesis.