Strength is Specific: The key to optimal strength training for sports

by Chris Beardsley

However, muscle-tendon units might also increase their ability to produce force through other mechanisms, including: (1) an increase in tendon stiffness (stiffer tendons allow muscles to shorten at slower speeds, and therefore exert more force), (2) an increase in the density of myofibrillar parts of the muscle cells, also known as myofibrillar packing density (the sarcoplasm does not contribute to force production), and (3) increases in the number of lateral attachments between the muscle fiber and its surrounding collagen layer (this increases the amount of force that is transmitted laterally, but reduces the effective fiber length).