Effect of Tendon Stiffness on Ankle Flexibility
Achilles' tendon injuries are fairly common. Getting ankle range-of-motion back can take quite some time. Finding a test that could be used to screen for injury-prone athletes could help prevent such injuries.

In this study, researchers from Japan look at the relationship between the Achilles' tendon and ankle joint flexibility. They show that stiffness of the Achilles' tendon account for decreased ankle joint motion.

Passive stretching of the muscle-tendon unit can help improve ankle joint flexibility. Stretching the gastrocnemius (calf) muscle belly also helps lengthen the Achilles' tendon.

Stretching should be done passively until the person feels a strong stretch. If there is a pain sensation, then muscle activity will increase reflexively to protect the joint. The end result is increased stiffness with a loss of flexibility.

Stretching should be done with enough load to alter the elastic properties of the tendon. The stretch must be held long enough to lengthen the soft tissue so that they cannot fully recover within one minute.

Despite the influence of tendon length on the ankle, joint flexibility has its own limits. Stiffness of the ligaments and joint capsule may contribute to ankle joint stiffness. This study did not look at the link between those soft tissue structures and ankle joint motion. It was a small study (six men, six women) so future research with a larger number of subjects is still needed.
References
Yasuo Kawakami, PhD, et al. The Relationship Between Passive Ankle Plantar Flexion Joint Torque and Gastrocnemius Muscle and Achilles Tendon Stiffness: Implications for Flexibility. In Journal of Orthopaedic & Sports Physical Therapy. May 2008. Vol. 38. No. 5. Pp. 269-276.