The extensibility or otherwise of tendon ....

Most studies of muscle/tendon extensibility in live subjects consider the muscle and tendon as a unit - for obvious reasons.  Histological studies of cadaver tissue have to replicate the complex biomechanics of the living animal - which is always difficult. 

Plus there's the fact that, just because something can stretch in certain situations, doesn't mean it should. 

When considered as a unit - the horse's digital flexors are highly extensible and it is agued that the digital flexor tendons themselves stretch by up to 10% of their resting length. 

Most skeletal muscle has a voluntary action - ligament doesn't and nor does tendon in isolation from its muscle; their action is automatic.  Joints have a certain range and direction of motion - if they go too far away from that, damage occurs.  To avoid excess strain (to the joint and other bones and muscle/tendon units), the voluntary control of the muscles which act across a given joint needs to be precise. 

This is why - as I always understood it - tendon is practically (in the sense of essentially) inextensible. Any significant degree of involuntary stretch in the tendon would reduce precise control.

Muscle both effects and controls movement by contracting (pulling) and playing out (lengthening).  It also can have a passive, stabilising (semi-automatic) action - and that's nowhere more obvious and important than in the role of the digital flexors in the horse. 

The front limb digital flexor muscles are part of the fetlock suspensory apparatus and, although they are normal contractile tissue and their action is voluntary,  the DDF muscle in particular has an important passive action and, because of the check ligaments, both muscle systems can act automatically - to a degree.

The digital flexor muscles have check ligaments that run from tendon to bone (cannon and radius) which allows the muscles to back up the suspensory ligaments by cutting off the muscle belly from excess strain -ie diverting it to bone. 

The suspensory ligaments can stretch massively because they are not true ligament - ie they contain muscle fibres - but their action is completely automatic i.e. the horse doesn't voluntarily lengthen or shorten them - which is why they are backed up by the flexor muscles. Without strong and balanced muscle systems, the suspensory ligaments will receive too much strain - acute or chronic - and break down. If the suspensory ligaments are too slack or are injured, the digital flexors receive too much strain.

If the digital flexor tendons were capable of a significant degree of automatic stretch, in addition to the extensibility of the muscle itself, why would the check ligaments be needed? And wouldn't an automatic stretch of the flexor tendons place the check ligaments at greater risk of strain? Ditto the suspensory ligaments?

In addition to aiding the automatic function of the suspensory ligaments in the fetlock suspensory apparatus, and the dynamic function of flexing the coffin joint, the deep digital flexor has a passive role in keeping the elbow joint in optimal extension in rest stance.  A certain tension of the DDF tendon and the weight of the DDF muscle (plus correct operation of the SDF system) hold the elbow joint in an optimal extension in rest stance - without which the horse cannot maintain optimal shoulder joint and scapula angles - and as a result cannot fully relax its skeletal muscle whilst upright. The consequences should be self evident but sadly are not.

The extensibility (the ability to lengthen and shorten) of the flexor unit  is (or should be) in the muscle - and a 10% automatic stretch of the tendon itself would reduce the flexor muscle / tendon unit's efficiency and precision in all its critically important functions.

This does not mean that the tendons are just passive cords - they store and release energy but not in any simplistic sense. Also - is the energy store and release of a compressed and released 'spring' different from that of a stretched and released spring?

We always need to remember that the horse has evolutionary imperatives which do not always fit with our ideas of what it should be able to do.  Left to its own devices it will seek to conserve energy and avoid injury and it will not normally choose to place the excessive stress on the suspensory apparatus of the fetlock joints which we impose on it by asking it to 'bounce' up and down with a rider on its back, gallop long distances or leap huge obstacles.

Have we bred greater tendon elasticity into horses as we have bred for longer legs, greater size? 

The greater the elasticity in a tendon - the less precise the control over the joint it attaches to. For the horse, the added force that might be gained from elastic tendons may be paid for in the greater potential for injury of a reduced control of position.