Hi Jes

Reflexes drive very simple motor behaviour. Reflexes are really the simplest neural networks, and play a role in the building blocks of more complex motor behaviour. So although some reflexes are clearly protective (eg flexor withdrawl reflex) reflexes have other functions and protection is only one of many possible roles that reflexes may take. what makes reflexes different from other neural networks are their simple stimulus-response nature and that they can be modulated from other neural networks.

The Achilles tendon reflex is one of the group called deep tendon reflexes, which are considered to be driven by the monosynaptic stretch reflex. the theory goes that when you strike the tendon you mechanically stretch the musculotendinous unit the muscle spindles are momentarily stretched, sending a barrage of afferent information to the alpha motor neurons (AMNs) supplying the same muscle, in the ventral horn of the spinal cord. the afferent information excites the alpha motor units and so a momentary contraction is elicited. As the muscle contracts the muscle spindles are put on slack and so there is a great reduction in the afferent information to the AMNs immediately following the contraction.

At least that is the theory but reality could be somewhat more complicated. Historically the monsynaptic stretch reflex was significant because it represented an example of the most simple reflex the nervous system could provide and it's discoverer, Charles Sherrington went on to postulate the reflex theory of motor behaviour. The stretch reflex was seen as a building block for modulating muscle stiffness via the gamma motor system during voluntary movement. Descending motor pathways can act directly on the alpha motor units or indirectly on the gamma motor neurons to influence the alpha motor units via the stretch reflex.

Does the stretch reflex have a protective role? Not directly. However it may play a role in assisting a muscle to provide appropriate torque when required. For example if you are doing a biceps curl and the weight is quite heavy for you the stretch reflex jumps in to hyperexcite the biceps and brachialis muscles so it no doubt assists your voluntary intention to contract the biceps.

Best of luck with your research

Here is a the original article:

E G. T. Liddell and Sir Charles Scott Sherrington. Reflexes in response to stretch (myotatic reflexes). Proceedings of the Royal Society of London. Series B, Biological Sciences, 1924, 86: 212-242