Quantitative Motor Unit Action Potential Analysis in 2 Paraspinal Neck Muscles in Adult Royal Dutch Sport Horses

The use of electromyography interference pattern analysis to determine muscle force of the deep digital flexor muscle in healthy and laminitic horses

BACKGROUND

In equine laminitis, the deep digital flexor muscle (DDFM) appears to have increased muscle force, but evidence-based confirmation is lacking.

OBJECTIVES

The purpose of this study was to test if the DDFM of laminitic equines has an increased muscle force detectable by needle electromyography interference pattern analysis (IPA).

ANIMALS AND METHODS

The control group included six Royal Dutch Sport horses, three Shetland ponies and one Welsh pony [10 healthy, sound adults weighing 411 ± 217 kg (mean ± SD) and aged 10 ± 5 years]. The laminitic group included three Royal Dutch Sport horses, one Friesian, one Haflinger, one Icelandic horse, one Welsh pony, one miniature Appaloosa and six Shetland ponies (14 adults, weight 310 ± 178 kg, aged 13 ± 6 years) with acute/chronic laminitis. The electromyography IPA measurements included firing rate, turns/second (T), amplitude/turn (M) and M/T ratio. Statistical analysis used a general linear model with outcomes transformed to geometric means.

RESULTS

The firing rate of the total laminitic group was higher than the total control group. This difference was smaller for the ponies compared to the horses; in the horses, the geometric mean difference of the laminitic group was 1.73 [geometric 95% confidence interval (CI) 1.29-2.32], and in the ponies this value was 1.09 (geometric 95% CI 0.82-1.45).

CONCLUSION AND CLINICAL RELEVANCE

In human medicine, an increased firing rate is characteristic of increased muscle force. Thus, the increased firing rate of the DDFM in the context of laminitis suggests an elevated muscle force. However, this seems to be only a partial effect as in this study, the unchanged turns/second and amplitude/turn failed to prove the recruitment of larger motor units with larger amplitude motor unit potentials in laminitic equids.

The potential and limitations of quantitative electromyography in equine medicine

This review discusses the scope of using (quantitative) electromyography (EMG) in diagnosing myopathies and neuropathies in equine patients. In human medicine, many EMG methods are available for the diagnosis, pathophysiological description and evaluation, monitoring, or rehabilitation of patients, and some of these techniques have also been applied to horses. EMG results are usually combined with other neurophysiological data, ultrasound, histochemistry, biochemistry of muscle biopsies, and clinical signs in order to provide a complete picture of the condition and its clinical course. EMG technology is commonly used in human medicine and has been subject to constant development and refinement since its introduction in 1929, but the usefulness of the technique in equine medicine is not yet widely acknowledged. The possibilities and limitations of some EMG applications for equine use are discussed.

The equine neck and its function during movement and locomotion

During both locomotion and body movements at stance, the head and neck of the horse are a major craniocaudal and lateral balancing mechanism employing input from the visual, vestibular and proprioceptive systems. The function of the equine neck has recently become the focus of several research groups; this is probably also feeding on an increase of interest in the equine neck in equestrian sports, with a controversial discussion of specific neck positions such as maximum head and neck flexion. The aim of this review is to offer an overview of new findings on the structures and functions of the equine neck, illustrating their interplay. The movement of the neck is based on intervertebral motion, but it is also an integral part of locomotion; this is illustrated by the different neck conformations in the breeds of horses used for various types of work. The considerable effect of the neck movement and posture onto the whole trunk and even the limbs is transmitted via bony, ligamentous and muscular structures. Also, the fact that the neck position can easily be influenced by the rider and/or by the employment of training aids makes it an important avenue for training of new movements of the neck as well as the whole horse. Additionally, the neck position also affects the cervical spinal cord as well as the roots of the spinal nerves; besides the commonly encountered long-term neurological effects of cervical vertebral disorders, short-term changes of neural and muscular function have also been identified in the maximum flexion of the cranial neck and head position. During locomotion, the neck stores elastic energy within the passive tissues such as ligaments, joint capsules and fasciae. For adequate stabilisation, additional muscle activity is necessary; this is learned and requires constant muscle training as it is essential to prevent excessive wear and tear on the vertebral joints and also repetitive or single trauma to the spinal nerves and the spinal cord. The capability for this stabilisation decreases with age in the majority of horses due to changes in muscle tissue, muscle coordination and consequently muscle strength.

The effect of transient, moderate dietary phosphorus deprivation on phosphorus metabolism, muscle content of different phosphorus-containing compounds, and muscle function in dairy cows

Hypophosphatemia is a common finding in periparturient and anorectic cattle. Although the clinical relevance of hypophosphatemia in cattle is uncertain, it has been empirically associated with persistent recumbency, specifically in periparturient dairy cows. The objective of the present study was to determine if transient dietary phosphorus (P) deprivation over a course of 5 wk, by feeding an approximately 40% P-deficient ration to lactating dairy cows, would result in altered muscle function or muscle P metabolism severe enough to present a risk for animal health and well-being. In addition, we wanted to determine the association between the plasma phosphate concentration ([Pi]) and muscle tissue P content to assess to what extent intracellular P deprivation of muscle cells could be extrapolated from subnormal plasma [Pi]. Ten healthy multiparous, mid-lactating dairy cows received a ration with a P content of 0.18% over a period of 5 wk. Following the P-deprivation phase, the same ration supplemented with P to obtain a dietary P content of 0.43% was fed for 2 wk. Blood and urine samples were collected regularly and muscle biopsies were obtained repeatedly to determine the P content in muscle tissue. Function of skeletal and heart muscles was evaluated by electrocardiography and electromyography conducted repeatedly throughout the study. Feeding the P-deficient ration resulted in the rapid development of marked hypophosphatemia. The lowest plasma [Pi] were measured after 9 d of P depletion and were, on average, 60% below predepletion values. Plasma [Pi] increased thereafter, despite ongoing dietary P depletion. None of the animals developed clinical signs commonly associated with hypophosphatemia or any other health issues. Urine analysis revealed increasing renal calcium, pyridinoline, and hydroxypyridinoline excretion with ongoing P deprivation. Biochemical muscle tissue analysis showed that dietary P depletion and hypophosphatemia were not associated with a decline in muscle tissue P content. Electromyographic examination revealed increased occurrence of pathological spontaneous activity in striated muscles after 2 wk of dietary P depletion in several cows, which could be suggestive of neuromuscular membrane instability. No effect on heart muscle activity was identified electrocardiographically. These results suggest that counter-regulatory mechanisms were sufficient to maintain normal muscle tissue P content during transient and moderate P deprivation. Muscle function was not grossly affected, although the increased occurrence of pathological spontaneous activity suggests that subclinical neuropathy or myopathy, or both, may have occurred with ongoing P deprivation. The results presented here indicate that plasma [Pi] is unsuitable for assessing muscle tissue P content in cattle.

From maturity to old age: tasks of daily life require a different muscle use in horses

In vertebrates ageing is characterized by reduced viscoelasticity of the ligamentous and tendineous structures and fibre changes in muscle. Also, some vertebral joint degeneration develops with ageing. The aim of this study was to apply dynamic time warping to compare the temporal characteristics of the surface electromyography (sEMG) data and to illustrate the differences in the pattern of muscle use during tasks of daily life in old and mature horses. In vivo kinematics (24 skin markers) and sEMG measurements of neck extensors and flexors were taken in five mature horses (age 10 ± 2 years, half of mean life expectancy) and five old horses (age 25 ± 5 years, older than the mean life expectancy). All horses had the same level of activity in the 12 months prior to the measurement. Tasks measured were neck flexion and neck extension as well as neutral neck position. Muscle activation, minimum and maximum muscle activation were collected. Quartiles of muscle activity based on the maximum observed activity of each muscle were calculated to document the relative increase of activity level during the task. Kinematics as well as overall muscle activity patterns were similar across horses and age groups. However, in the neutral position old horses showed increased extensor activity compared to mature horses, indicating that old equine muscle requires more activity to counteract gravity. Dynamic time warping specified optimal temporal alignments of time series, and different temporal performances were identified. The age groups differed during the flexion task, while extension and neutral were more similar. The results of this study show that even in the second half of life and in the absence of muscle disuse the muscular strategy employed by horses continues to be adapted.