My opinion from what I have read -
'Strength' is usually measured as the maximum force that can be applied in a 1 repetition maximum effort, 1RM, or maybe isometrically. I believe that these forces are mostly produced by the muscle cells, sarcomeres consisting of sliding filaments, Actin and Myosin. 'Strength' involves relatively slow muscle shortening velocities.
Tennis strokes involve the
stretch shortening cycle and stretch is produced by another component of the sarcomere,
Titin.
Titin can supply force while shortening at higher shortening velocities than Actin & Myosin.
Tendons probably have a lesser role is supplying elastic shortening forces that previously thought. Tendons need to be conditioned enough to prevent injury.
http://tt.tennis-warehouse.com/showthread.php?t=427364