Equine Muscle Mass: Detecting and Monitoring Changes

A loss of muscle mass, which is also called muscle atrophy, can occur in horses for various reasons. For example, it can result from poor nutrition, changes in exercise routine, lameness, or several other medical conditions.

Older horses appear to be at a heightened risk of developing muscle atrophy. In part, this is because a continuous, slow reduction in muscle mass appears to be a component of the normal aging process in mammals, including in horses.

However, senior horses (15 years or older) are often also affected by medical conditions that might lead to more severe or rapidly developing muscle atrophy, such as pituitary pars intermedia dysfunction (PPID or Cushing’s disease) or osteoarthritis.

Detecting Muscle Mass Loss in Senior Horses

You can easily detect rapid and significant muscle mass loss by the eye. But slower or gradual changes are difficult to observe and monitor. Until recently, there was no simple and reliable tool available to practically monitor and detect muscle atrophy in horses.

However, the body condition scoring (BCS) system was sometimes incorrectly used to estimate muscle mass. The body condition scoring system was developed to monitor fat, not muscle mass.

There was an urgent need for a muscle scoring system that is as simple to use as the body condition scoring system. Therefore, our team developed the “muscle atrophy scoring system (MASS)” for horses.

How Does MASS Work?

With the muscle atrophy scoring system, a muscle score between 1 (no muscle atrophy) and 4 (severe muscle atrophy) is assigned to each of three body regions: the neck, back, and hind quarters.

When establishing the muscle atrophy scoring system, we also explored belly muscle scoring. However, it was found to be unreliable and is therefore not recommended today.

To score a horse, observe the shape of its muscle by eye. Then, feel the muscle tone with a flat hand and slight pressure.

Next, apply the score with the description that fits the observed muscle shape and tone best to the horse. The scoring system helps the person rating a horse distinguish between muscle mass and fat mass, which is often challenging. This distinction is very important to achieve an accurate muscle score. Some horses can have a high fat mass and a low muscle mass, and vice versa.

To help differentiate between muscle and fat mass, the MASS includes a set of questions at the start of the scoring system that guide the person rating a horse to one of two scoring tables for each body region, depending on the horse’s regional fat mass.

When Should I Use the MASS?

Owners and caretakers should routinely check horses for changes in muscle mass. This is important because a loss in muscle mass can alert a horse’s caretaker to potential management or health problems. In addition, if muscle mass is very low, it might have negative consequences for that horse’s ability to work or perform normal behaviors, such as getting up after laying down.

Changes in muscle mass can also indicate welfare issues, and routine checks are therefore important. Once you detect a change in muscle atrophy score, especially if it is rapid or progressively increasing, you should contact your veterinarian.

Where Can I Learn More About Muscle Mass?

You can read the summary of the scientific article Development and Evaluation of a Muscle Atrophy Scoring System (MASS) for Horses here https://doi.org/10.1016/j.jevs.2021.103771.

Spillers^TM has also provided an interactive online version of the scoring system, which can be accessed through this link.

To read more about risk factors for, and implications of, owner-reported low muscle mass in senior horses, use this link to the scientific article Retirement risk factors, exercise management and muscle mass in US senior horses.