How Muscles Work

By Gillian Higgins and Horses Inside Out

With a background in Sports and Remedial therapies focussing on muscular skeletal problems and imbalances in both horses and riders, with a passion for anatomy and biomechanics, Gillian strongly believes that if we understand more about how our horse works, we can improve both our performance and our relationship with our equine partners.

As Gerd Heuschmann observes, ‘Many training mistakes could be prevented if riders learned to respect a horse’s physical and physiological make up.’

The Action of Muscles

Superficial muscles of the horse painted by Gillian Higgins digital extensor, digital flexor, achilles tendon, lateral crural fascia, lateral femoral fascia, semitendinosus, biceps femoris, tensor fascia lata, superficial gluteal gluteal fascia, thoracolumbar fascia, dorsal serrated, latissimus dorsi, trapezius, splemius, brachiocephalic tendon, jugular vein, sternomandibular, cervical ventral serrated, subclavian, omohyoid, triceps, descending pectoral, brachialis extensor carpi radialis, common digital extensor, flexor carpi ulnaris, thraosis ventral serrated, ascending pectoral, external intercostals, external abdominal oblique

Understanding how muscles work is one of the most fundamental concepts that can influence how we ride and train our horses. Movement is created by the skeletal muscles pulling on the bones to operate the joints. Every bone is moved by a muscle. The muscles cross either one joint or several. The longissimus dorsi for example, crosses all the joints between the thoracic and lumbar vertebrae. Electronic signals are sent from the brain along neural pathways telling the muscle when to contract and relax.

The longissimus dorsi, is the main muscle in the horse's back and underneath the saddle. However, it is not just local to this area. The longissimus dorsi starts at the 4th neck vertebra and and attached into the sacrum in the hind quarters. Branches of the longissimus dorsi also connect to the head and tail.

As a general rule, deep muscles and those close to joints are postural muscles responsible for supporting and stabilising joints. The muscles responsible for creating power and large gymnastic movement are those further away from the joints. The massive superficial muscles of the hindquarters for example provide the propulsive forces necessary for locomotion.

Muscle Pairs → Groups → Chains!

In its most simplistic form, muscles work in pairs. As one muscle contracts, the other lengthens. This allows the movement to happen. For example, in our arm, the biceps contracts and the triceps relax to flex the elbow and visa versa to extend it.

No one joint however is operated by one muscle alone. Movement therefore is dependant on groups of muscles working together to create fine precision and smoothness; when one group contracts, the opposite group relaxes. The two groups of muscles are known as agonists which move the body part by shortening or contracting the muscle or antagonists which relax or stretch to allow the movement to happen.

Not only do muscles work in pairs and groups, they also work in chains. This facilitates precise control and continuous flowing movement. The presence of muscle chains helps to explain compensation mechanisms and why, when there is restriction in a muscle in one part of the chain, movement is compromised in another.

There are two main muscle chains which affect the spine and hind leg, and are implicated in propulsion. They are:

1. The Extensor Chain

These muscles, which make up the top line, are situated above the spine and behind the hip. They are sometimes called the extensor chain as they extend the hip and spine, hollow the back and raise the head. This chain is largely responsible for forward propulsion. The better conditioned these muscles are, the more powerful the movement. For example the horse can only perform a really powerful extended trot or execute a perfect piaffe when these muscles are strong enough to do so.

Fact File – The Key Muscles of the Dorsal Chain

The Splenius Muscle originates from the spinous processes of the withers and Nuchal Ligament and inserts onto the poll and the first four cervical vertebrae. This muscles works to extend and elevate the neck.

The Longissimus Dorsi, part of the erector spinae group of muscles, is a long, strong muscle that runs along the top of the thoracic and lumbar spine to the pelvis attaching to each vertebrae. It supports and extends (hollows) the spine and contributes to lateral flexion.

The Gluteal Muscle Group, consists of the superficial, medial and deep gluteal muscles. The Medial Gluteal or Glutimus Maximus is the largest and most powerful gymnastic muscle of the hind quarter primarily involved in creating extension of the hip joint during propulsion.

The Hamstring Muscle Group consists of the Biceps Femoris, Semi-tendinosus and Semi-membranosus muscles. These powerful gymnastic muscles are involved in creating extension of the hip and stifle during propulsion.

2. The Flexor Chain

These muscles make up the bottom line and lie underneath the spine, in front of the hip and include the abdominal muscles. They are also known as the flexor chain as they help to flex the hip and vertebral joints, raising the back and withers and lowering the head. As part of the ‘core’ muscles they have an important role to play in supporting and maintaining the correct posture of the back. They are also important in all movements requiring collection.

Sam Rahmatalla and WM Macho

Fact File – The Key Muscles of the Ventral Chain

The Brachiocephalic Muscle,originates at the poll and inserts into the humerus just below the point of the shoulder. Its main functions are to flex the cervical vertebrae bringing the neck downwards and head backwards and to create lateral flexion in the neck moving the head from side to side. It is also the main muscle pulling the foreleg forwards during the swing phase.

The Sternocephalic Muscle, running from the sternum to the Mandible, flexes the neck and pulls head down as well as helping to open the mouth and support the jaw.

Abdominal Muscles, consist of the rectus abdominae, the transverse abdominae and the internal and external abdominal oblique muscles. As well as supporting the abdominal viscera, aiding breathing and defecation these muscles also work together to help to create flexion (lift) and latero-flexion of the thoraco-lumbar region of the spine.

The Iliopsoas Muscle Group consisting of Psoas Major, Psoas Minor and Iliacus lies and attaches onto the ventral side of the lumbar spine and pelvis as well as the minor trochantor of the femur. This muscle group is one of the main contributors to hip flexion and movement of the pelvis through flexion of the lumbo-sacral junction.

The Tensor Fascia Lata Muscle runs from the tuber coxae via fascia to the stifle. Along with the rectus femoris (part of the quadriceps muscle group) this muscle is the main hip flexor, really recruited when advancing the limb forwards.

Co-ordination of Muscle Chains

The flexor and extensor muscle chains work together and when balanced create a state of equilibrium. Because there often tends to be a greater focus on the top line muscles of the horse, the tone of the abdominal muscles is sometimes forgotten and even neglected.

Tension in any individual muscle in the chain may have a ‘knock on’ effect at any point throughout the chain. For example, if the long muscle of the back muscle is in spasm, it will affect the mechanics of the entire extensor chain and in turn inhibit the use of the flexor chain.

The importance of the abdominal muscles within the flexor chain cannot be overemphasised.

Understanding how muscles work synchronously in chains, enables us to appreciate that no one part of the horse’s body can be affected in isolation. Muscularly influencing one part of the chain will cause an effect within both that and the agonistic chain. For example, increased flexion in the hip joint has to affect the positioning of the pelvis, back and consequently the neck through the muscle chains.

Types of Contraction

Understanding muscle contraction is another concept that enables the rider to train more sympathetically. Muscles are signalled to contract via nerve impulses. Relaxation occurs when the nerve impulses cease. Very simply as muscles work they perform one of two actions. These are:

1. Isotonic,where the muscle contraction results in movement. This type of contraction can be subdivided into two categories although all movement uses a mixture of both. They are:

• Concentric,where a muscle shortens to create movement and

• Eccentric where the muscle gradually lengthens to control movement, support and stabilise joints. This type of contraction also acts as a shock absorber during an abrupt movement such as coming to a sudden stop.

2. Isometric,where the muscle is working hard but there is no change in the muscle length as it contracts to hold a position.

Isometric and Eccentric muscle contractions tend to create greater fatigue, tension and associated discomfort than concentric muscle contraction, particularly if the muscle is not appropriately conditioned. When relating these contractions to training it is particularly important to understand the significance of isometric contraction.

Horses commonly use isometric muscular contraction during dressage particularly when working in more advanced outlines. When top line muscles of the neck including the splenius muscle, contract concentrically (the muscle shortens in length) they extend the neck and raise the head. When the muscles work isometrically they contribute towards maintaining a flexed outline supporting the heavy weight of the head. When the horse is working in a more novice outline, or with the neck stretched long and low, the nuchal ligament plays a greater role in supporting the weight of the head and neck.

The horse uses isometric muscle contraction particularly in the abdominal muscles, deep back muscles and iliopsaos muscles to support the back when carrying a weight.

When the horse performs movements requiring high levels of collection and engagement the hamstring muscle group which are part of the extensor chain work isometrically both to support the joints in a more flexed position and carry a greater percentage of the weight.

Horses also use isometric muscle contraction to brace and support themselves whilst travelling. This is why 1 hour in the horse box can be attributed to 20 minutes trot work!

To perform isometric contraction consistently and over a longer period of time requires strength and appropriate muscle conditioning. Imagine you were asked to hold a weight up high at arms length for several minutes or to stand with all your leg joints in a flexed position. You would soon begin to feel a burn or an ache in the muscle as a result of performing these isometric muscle contractions. To relieve this you would need to move the body part.

As isometric muscle contraction requires strength and appropriate conditioning, problems may start to arise if young or relatively unconditioned horses are asked to work in too advanced an outline for too long. The horse may begin to compensate by trying to maintain the outline through concentric muscle contraction rather than isometric. The rest of the dorsal chain muscles may also start to shorten (concentrically) as part of the evasion mechanism. This helps to explain why it is so important to allow horses freedom to stretch and move his neck at regular intervals during training.

Sam Rahmatalla on WM Macho, As the horse moves he uses all types of muscular contraction.

There are many aspects to appreciating how muscles contribute to movement. Understanding the concept of muscle chains and how isotonic and isometric contraction affects training are only two very small contributory parts to a wide and complex subject.

Want to learn more about the Action of Muscles: Horse Anatomy for Performance and Posture and Performance will be of interest to you