Different muscles play different roles in posture and movement, and effectively assessing their function can only be achieved with the help of professionals such as vets and therapists. Therefore, work with a professional to get effective therapeutic and performance enhancing aims for your horse…BUT understanding more about these muscles and their roles, will help you work them correctly to prevent injury in the first place!
For example, in the hindlimbs, the hamstring group (biceps femoris and semitendinosus– figure 1) is most active during late stance-mid swing, generating energy and propelling them forwards (figure 6). Whereas the quadriceps and gluteus medius (quadriceps figure 2, gluteus medius under gluteal fascia/superficial gluteal in figure 1) assist with braking and stabilisation during late swing and early stance (Tabor and Williams, 2017) so that control of energy is maintained. Therefore, the gluteals can become chronically contracted in some hind end lameness’s as they try to stabilise and control movement to prevent lower joints flexing (Keegan, 2005), with greater activity of this muscle in the trot than in non-lame horses (Zaneb et al. 2009) causing hypertrophy (over-development of the muscle).
Additionally in the forelimb, the brachiocephalicus (from the poll to the forearm, Figure 1), is used to assist in propulsion of the forelimbs; however, if a horse does not use their hindlimbs effectively, and does not have the core stability to transfer the energy forwards, the horse will overuse this muscle to pull themselves forwards and end up with a thick underside of their neck which we call a ewe neck!
And finally, paraspinals (back muscles along the spine) often become very painful with back pathologies and become contracted and look atrophied (under-developed) as they become tense try to prevent excessive spinal movement.
For proper balance a horse needs to be able to use its core to assist with stabilisation and energy transfer from the hindlimbs, and needs to utilise their hamstrings for power, and their forelimbs for stabilisation/transference of power. Additionally, the thoracic sling needs to provide spring momentum back to the hindlimbs during stance to make movement energy efficient (Wagner, 2016).
Core stability is an important concept to ensure appropriate biomechanical function (figure 3). It assists with the dorsal ventral chain (top and bottom line connection); and by achieving core stability, contraction of the long back musculature is reduced, thus reducing the risk of back pain (de Oliveria et al. 2015). Core engagement involves the abdominal musculature (under the belly), and by engaging muscles such as the rectus abdominis, collection of the hindlimbs increases, resulting in an increase in protraction and range of motion due to its attachment to the hip joint (Wagner, 2016).
Forward motion is initiated through caudal (backward) displacement of the centre of gravity through forward motion of the hindlimb which tilts the pelvis, rising of the chest (like they are trotting uphill), and with abdominal engagement the spine can stabilise and propel motion from the hindlimbs forwards. Once the forelimb touches the ground, energy is then transferred up the limb to the neck muscles than adjoin to the trunk, such as the scalenus (figure 4), through the back to muscles which connect the back and hindlimb such as iliopsoas complex (figure 4); making movement incredibly efficient (Wagner, 2016; Higgins, 2019).
If any aspect of this chain is imbalanced it impacts motion as discussed above, limiting motion, a lack of energy transfer, and creating dysfunction such as a lack of hindlimb engagement (Tabor and Williams, 2017).
To get this correct motion the muscles need to work in different roles, in different parts of the stride. In figure 5 you can see how propulsion is generated when the limbs are in late stance to pull them forwards, and the forelimbs assist with braking motion in late swing to control the power. If a horse is leaning on the forehand, it disrupts the forelimb’s ability to control power, and you can feel like the horse is running in to the ground!
This is where physiotherapy and targeted exercises like pole work can assist with this, working on length of frame, lifting of the forelimbs, and “roundness” prior to collection. However, it is important to take it slowly, it takes weeks to build muscle strength, and to focus on changing one thing at a time, rather than trying to force their body in to the “desired” shape using training aids before it is ready as this can cause pain and injury. Therefore, if you horse is not working correctly, work with a physiotherapist to see if there are barriers to them working effectively, and an instructor to assist with correct training!
Any questions feel free to get in touch!
References:
de Oliveira, K., Soutello, R.V.G., da Fonseca, R., Costa, C., de L. Meirelles., P.R., Fachiolli, D.F., 2015. Gymnastic Training and Dynamic Mobilization Exercises Improve Stride Quality and Increase Epaxial Muscle Size in Therapy Horses. Journal of Equine Vet Science, 35: 888–93
Higgins, G., 2019. The Pelvis and Iliopsoas. Horses Inside Out [online]. Available: https://www.horsesinsideout.com/post/the-pelvis-and-iliopsoas
Keegan, K.G., 2005. Pelvic Movement Pattern in Horses with Hindlimb and Forelimb Lameness. AAEP Annual Convention – Seattle. Available: https://www.ivis.org/library/aaep/aaep-annual-convention-seattle-2005
Tabor, G., and Williams, J., 2017. Equine Rehabilitation: A Review of Trunk and Hind Limb Muscle Activity and Exercise Selection. Journal of Equine Vet Science, 60: 97-103
Wagner, K., 2016. Equine Biomechanics and Locomotion. Keith Wagner Equine Veterinarian [online]. Available: www.wagnerhorsedoc.com
Zaneb, H., Peham, C., Licka T., 2009. Quantitative differences in activities of back and pelvic limb muscles during walking and trotting between chronically lame and non-lame horses. American Journal of Veterinary Research. 70(9): 1129-1134