24/03/2023
It's never just “tight hamstrings”
Often when we think of a muscle, we imagine a specifically shaped part of the body, in a certain anatomical location, which functions to contribute towards the movement of the organism as a whole.
In the anatomy books, muscles are classified with relation to the relationship between their attachments to the body; their origin and insertion. For example, the hamstrings are classified as a retractor of the hindlimb; contraction pulls the insertion of the hamstrings on the distal femur and tibia, towards the origin in the pelvis, which in turn draws the limb backwards in the retraction phase of the stride.
However, the hamstrings are not the only muscle which act to retract the hindlimb; the middle gluteal also plays an integral role. When we consider the biomechanics of the body as a whole, we see that muscles often act as part of groups.
Moreover, muscles can belong to more than one group. The semitendinosus, which is part of the hamstring group (along with the semimembranosus and biceps femoris) and the hindlimb retractor muscle group, also acts as part of the hindlimb adductor group which functions to bring the leg towards the midline (inwards).
To further complicate things, the middle gluteal muscle, which is also part of the hindlimb retractor group, acts as a hindlimb abductor which brings the leg away from the midline (outwards). Both the semitendinosus and middle gluteal muscles are hindlimb retractors, yet when we consider their actions within the sagittal plane of the body they perform opposing actions.
Each action has a consequence, and as such each muscle group has an opposing muscle group, whose role is to perform the exact opposite action to balance the forces within the body and control movement. In the same way that we have agonist and antagonist pairs of muscles (ie the biceps brachii and triceps brachii muscles which act to bring your arm up and down), muscle groups have agonistic and antagonistic functions. The antagonists of the hindlimb retractors are the hindlimb protractors, which advance the hindlimb forwards in the swing phase of the stride.
So why is this important for us to know?
Because tension in a muscle will never only influence that muscle individually.
Tightness, spasm or atrophy (dysfunction) of an individual muscle will mean that its role within the muscle group as a whole will be impaired. As such, the other muscles within the group will have to take up a greater role in performing the function of the group, ie retracting the hindleg. This will increase their risk of fatigue.
Consequently, as the agonistic group begins to weaken, the synergy between the agonistic (hindlimb retractors) and antagonistic groups (hindlimb protractors) will become unbalanced. This will result in uneven forces being exerted on other anatomical structures of the body; the tendons, ligaments and joints. Furthermore, in the case of the hamstrings not only will protraction/retraction be affected, but also abduction/adduction.
These effects will not just be isolated locally to the hindlimb...
Each muscle is encased by the fascial network which expands across the entire body, connecting each muscle to their neighbours and globally linking each and every part of the body together into one unit. Often muscle origins and insertions are not as discrete as they are depicted in the anatomy books, thus the principle that muscles act alone within the body to perform a singular role has been challenged.
Through the work of both human and equine anatomists, it has been identified that muscles act within chains, known as myofascial chains/meridians/pathways, which span across the body linking certain muscles together within a pathway. Meaning that dysfunction within one muscle will not only have local effects to that region of the body, but will have further reaching global impacts across the body as a whole.
The hamstrings are part of the Superficial Back Line myofascial chain, which starts at the back of each hind hoof, then extends through the caudal muscles of the hindlimb, over the muscles of the croup, and through long back muscles, before each side then passes under the shoulder blade through the neck and over the poll to through the face to the muscles of the jaw (see the blue line).
Through this chain, we can see how dysfunction in the hindlimb can travel through the body to have a significant influence to the horse's back, neck, across the poll, and also extending to influencing the acceptance of the bit.
Conversely, myofascial chains work in both directions; tension in the hamstring group may not be due to dysfunction of the muscle itself, but may be as a result of issues elsewhere in the dorsal chain. For example, this may stem from hypertonicity and spasm through the epaxial muscle group (back) or of the dorsal cervical region (neck), which in turn may relate to a poorly fitting bridle or saddle.
And so we see that it is never just “tight hamstrings”. It is important to consider the posture of the horse as a whole, and not just fixate on tension in one muscle and how to fix it. Often by addressing the whole posture and mechanics of the horse, we will see the resolution of dysfunction in particular muscles as the body functions in improved balance.
Or...it might just be that your horse has been larking around in a wet slippery field the day before and has strained his hamstrings. It is always about how the horse presents on the day, and addressing these issues early on can intercept the cycle of compensation before it becomes a whole body issue. Listen to your horse and trust your therapist’s palpation and clinical reasoning to best address your horse 🐎
