All muscle shortening forces are developed in the sarcomere.
1) Titin
This animation represents Titin in the muscle sarcomere, blue. Some forces are produced by elastic Titin, a giant molecule. The Titin forces are called 'Passive Forces'
2) Actin & Myosin (Titin not shown)
Better known and understood shortening forces are produced by other structures in the sarcomere called Actin and Myosin. The Actin & Myosin forces are called 'Active Forces'. Nerve signals release calcium ions that cause the Myosin to reach over to the Actin, form 'cross bridges', and cause muscle shortening.
Sarcomeres are the microscopic engines of muscle motion. Both the characteristics and limitations of the Titin and of the Actin & Myosin are observed in muscle motion. In 2019 not all features of muscle movement are well understood. Especially the features of Titin are curently being researched and discovered.
The answer to the OP is in the use and capabilities of Titin. The use involves stretching muscles and their Titin and then using stretched muscles to strike the ball at high speed. The mix of Titin vs Actin & Myosin vs time during tennis strokes is not well known.
1) Look at the mechanisms of Titin - do they resemble the rubber in a sling shot?
2) Look at the mechanisms of Actin & Myosin - do they resemble a caterpillar walking?
These two very different mechanisms will have different speeds for shortening the sarcomere.
Which mechanism would you think has the fastest maximum speed of sarcomere and muscle shortening?
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Complete Sarcomeres with Actin, Myosin and Titin as they contract and are stretched by external forces.