Degenerative changes in one or other of the three biceps tendons may be the cause for proximal or distal tear (usually SLAP lesion in men after 50 y.) when flexing elbow (even without extreme resistance).
In young people (i.e.athletes) injuries to the biceps tendon at the elbow usually occur when the elbow is forced straight against resistance and the forearm is in supination ! giving additional tension to the distal biceps insertion at the radial tuberosity. In pronated forearm the tension is bigger at the glenoidal labrum (especially when the shoulder joint is internally rotated) and SLAP lesion is more probable.