Relationships between muscle atrophy scores, body mass, and fat-free mass in mature horses

Abstract

Various diseases, malnutrition, detraining, and aging can cause muscle atrophy in horses. Although even small reductions in muscle mass can adversely affect performance, larger losses can significantly impair a horse's welfare due to decreased strength. Thus, routine muscle mass monitoring is important to detect changes in muscle mass early when they are most treatable. The equine muscle atrophy scoring system (MASS) is a standardized muscle scoring system with good interrater reliability. However, the system is currently not fully validated, and relationships to other body composition measurements are not fully understood. The objectives of this study were therefore to assess the relationships between muscle atrophy scores (MAS), body mass (BM), and fat-free mass (FFM; a muscle mass indicator) at individual measurement time points (objective 1) and over time (objective 2). We hypothesized that MAS, BM, and FFM were significantly associated across measurement time points, and that changes in these parameters over time were also related. Linear mixed-effects models were used for data analysis. Nine Standardbreds (n = 4 geldings and n = 5 mares) aged between 13 and 21 years were used. The horses had Henneke body condition scores between 4.5 and 5.75, weighted 430 kg to 547 kg, were not in work, and received a diet consisting of ad libitum hay and pasture. The horses’ BMs and ultrasound-based FFMs were obtained, and the horses were scored for muscle atrophy (higher scores indicating more atrophy) by the same experienced rater. BM and MAS were recorded a total of 8 times over 10 wk, with a minimum of 7 d between data recordings. FFM was recorded 3 times, at the start, in the middle, at the end of the study, and on the same days on which the other parameters were recorded. There was no significant association between the horses’ neck and hind region MAS and their BM or FFM (P > 0.05), but the horses’ MAS of the back region were positively related to BM (P < 0.001) and FFM (P < 0.01). BM and FFM were significantly correlated (P < 0.001). There was no significant association between the changes in MAS in any body region and the changes in the horses’ BMs or FFMs over time (P > 0.05). The maximal changes in BM and FFM between 2 time points were 3.6% and 4.1%, respectively. This study suggests that the MASS is not sensitive enough to detect very small changes in BM or FFM over time. The findings of this study also suggest that ultrasound-based FFM may be biased by BM, and that BM, measured at a single time point, may not be a good marker for normal muscle mass, even within horses of similar body size, as higher MAS were not related to lower BM.