Biomechanics of Holding Phone

Every day, I check my cellphone for notifications. I first reach for my phone on a surface like my bedside table, or kitchen table. I start with my shoulder flexed, elbow extended, forearm pronated, and wrist and fingers extended. After I bring it towards my face, so I can view it, my shoulder is extended, elbow is flexed, forearm is supinated, wrist is extended, and my digits two through four are extended, while my thumb is flexed. Flexion and extension of the shoulder, elbow, wrist, and digits two through four at the proximal and distal interphalangeal joints all occur in the sagittal plane about the frontal axis. The osteokinematics of the shoulder (glenoid fossa of the scapula and humerus head) is flexion to extension in an open kinematic chain. When considering the arthrokinematics, the moving humerus rolls posteriorly and glides anteriorly on the stable, concave glenoid fossa of the scapula. The primary movers, or agonist, for shoulder extension in this situation are the anterior deltoid and coracobrachialis; these muscles would be eccentrically activating because they are performing negative work on the load and are helping decelerate the arm and phone during this movement. 

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