You raise a good question and one that has been debated by other interested parties as well. Throwing the ball while on flat ground is often used to warm-up for mound pitching. But there’s some concern that flat-toss throwing requires different shoulder and elbow biomechanics that may actually harm the pitcher.
A recent study at the American Sports Medicine Institute in Birmingham, Alabama might offer some helpful information to answer your question. Seventeen (17) healthy college athletes participated. No one in the study had any previous shoulder or elbow pitching injuries or problems. Information on pitching was collected using a three-dimensional (3-D) motion analysis system.
Measurements of shoulder rotation, trunk position, and elbow motion and velocity were compared for two different pitches: from the pitcher’s mound and on the flat surface. Comparison of joint forces and torques for pitches from these two locations were made. Measurements were taken with the arm in a cocked position (ready to throw) and at the point of ball release.
Understanding the forces on a pitcher’s shoulder and elbow with different types of baseball throws may be an important key in not only preventing injuries but also improving performance. This study was meant to see if there is a difference in force, load, and stress from one style of pitching to the other.
The results of this study support the continued use of flat-ground (long toss) throwing as a safe and effective rehabilitation exercise. But caution is advised when throwing as far as possible during the early phases of rehab and recovery after injury.
Throwing for distance requires the pitcher to lean forward much farther. This position increases the amount of force placed on the arm and may not be tolerated by injured tissue that is still on the mend. Further study is needed to determine how soon and how often greater distance throws can be used in training exercises for pitchers recovering from an arm injury.