Equine Muscle Mass: Detecting and Monitoring Changes

A loss of muscle mass, which is also called muscle atrophy, can occur 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. Conditions that might lead to this include pituitary pars intermedia dysfunction (PPID or Cushing’s disease) or osteoarthritis.

Detecting Muscle Mass Loss in Senior Horses

Rapid and significant loss of muscle mass can be easily detected 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. Unfortunately, the body condition scoring system was developed to monitor fat and 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 one of three body regions. Those are the neck, back, and hind quarters.

When the the muscle atrophy scoring system was established, belly muscle scoring was explored as well. However, it was found to be unreliable and is therefore not recommended today.

When scoring a horse, the shape of the horse’s muscle is observed by eye. The muscle tone is felt with the flat hand and slight pressure.

The score with the description that fits the observed muscle shape and tone best is given to the horse. The scoring system is also set up in a way that helps the person rating a horse to distinguish between muscle mass and fat mass. That is often a challenge. 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?

Horses should be checked for changes in muscle mass routinely. 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 found to be very low, it might have negative consequences for that horse’s ability to work or to perform normal behaviors. That might be as simple as getting up after laying down.

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

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.