Even perfect body mechanics cannot eliminate all physical stress to your body when doing massage. Ideal body mechanics merely minimize the stress. For this reason, when doing massage, especially deep pressure massage, it is wise to alternate which treatment hand contact you use during a session.
When working the low back, a good strategy is to begin with a smaller treatment contact such as thumb or finger pads to assess and begin treatment of the lumbar region, and then switch to a larger contact such as the palm or elbow to deliver deeper pressure.
The flexor pollicis longus is a deep muscle of the anterior forearm and hand that flexes the thumb and is often overused with digital devices.