Science behind safethrow measurements
Measurements taken at the time of Screening:
- Glenohumeral Internal Rotation at 90 degrees of abduction - with the arm out at the side (a starting position of 90 degrees of abduction) and the examinee in the lying-down face-up position (the "supine" position), the examiner rotates the arm inwards (hand toward examinee's feet) until mild resistance is met. This indicates the end of the normal "internal rotation" arc of movement. Because of the position of the examinee, the scapula or shoulder blade is stabilized and does not move in relation to the rib cage, and thus the only movement is from the ball-and-socket (glenohumeral) joint. The examiner measures that arc of movement in degrees, by measuring the angle formed by the forearm in the vertical (0 degree) starting position, and it's ending position.
- Glenohumeral External Rotation at 90 degrees of abduction - same procedure, only the forearm is taken backwards (externally rotated).
|Sage Rosenfels, internal rotation
||Dr. Metzger checks external rotation
Calculations Derived from those measurements:
- Both arms are measured as above, and compared. Throwing athletes offer us the unique opportunity to compare one arm that is used ALOT (the throwing arm) to the arm that is not used for that motion (the non-throwing arm). By comparing, we can see what throwing does to one's arm!
- Total arc of motion, TAM - internal rotation plus external rotation
- GIRD, Glenohumeral Internal Rotation Deficit - Internal rotation of the non-throwing arm minus the internal rotation of the throwing arm. It is normal for throwers to have less internal rotation in their throwing arms
- TMAD, Total Motion Arc Deficit - the TAM of the non-throwing minus the TAM of the throwing. It is normal for throwing arm to have less total motion than the non-throwing
- GIRD/TAM-NT, GIRD divided by the Non-Throwing arm's TAM - indexes the GIRD to the "normal" or non-throwing arm's total motion
- GIRD/IR-NT, GIRD divided by the Non-Throwing arm's Internal Rotation - another way to index the GIRD to the "normal" or non-throwing arm's total motion
- These parameters are used to determine if a player might be losing too much internal rotation in the throwing arm, which is known to lead to problems and likely increase the risk of serious injury which causes time out of baseball, and can result in need for surgery.