Training the equine respiratory muscles: Ultrasonographic measurement of muscle size

Abstract Background Limited information exists regarding changes in the size of respiratory and locomotor muscles in response to exercise training in the Thoroughbred racehorse. Objectives To describe and compare the responses of the respiratory and locomotor muscles to conventional exercise training and inspiratory muscle training (IMT). Study design Prospective randomised controlled trial. Methods Thoroughbred racehorses, in training for competition in National Hunt races, were recruited from two training establishments. Ultrasonographic images were obtained for selected muscles of the upper airway, diaphragm, accessory respiratory, and locomotor systems and their sizes measured. Examinations were performed at three timepoints: (A) when unfit, (B) following 12 weeks of conventional exercise training and (C) following 10–12 weeks continued training at race fitness. In addition, horses at yard 1 performed IMT, between timepoint B and C, and were randomly assigned into high‐load (treatment) or low‐load (control) group. Repeated measures models were constructed to compare the change in muscle measurements over time, and to investigate the effects of yard, previous airway surgery and IMT on the change in ultrasonographic size measurements obtained. Results Upper airway muscle size increased in response to conventional race training between timepoints A–C, and B–C. Diaphragm size increased in response to conventional exercise training between timepoints A and B. The diaphragm size of horses that undertook high‐load IMT was either maintained or increased, whereas diaphragm size decreased in horses that undertook low‐load IMT or no IMT between timepoints B and C. A significant interaction between gluteal muscle size and airway surgery status was observed, with greater gluteal muscle thicknesses measured in horses that had not previously undergone airway surgery (left gluteal 3.9%, p < 0.001; right 4.5%, p = 0.04). Main limitations Low number of horses underwent IMT. Conclusions Respiratory and locomotor muscles increase in size in response to conventional exercise training, with a further change in diaphragm size in response to inspiratory muscle training.

Photograph to demonstrate ultrasound transducer placement, for examination of the diaphragm, within the 10th intercostal space, 1cm below a marker placed on a line between the cranioventral aspect of the tuber coxae and the proximal margin of the olecranon, on the left side of the horse.

Description
Horse standing with head straight and slightly extended. Position the transducer horizontally at the dorsal-mid aspect of the larynx, with the rostral aspect of the transducer in contact with/underneath the caudal aspect of the mandible so you can see the thyroid, cricoid cartilage articulation with a gap in between. The caudal border of the thyroid cartilage should be in the middle of the screen. Slide the transducer ventrally if the cricoid and thyroid cartilages are touching or the thyrohyoideus muscle cannot be observer clearly.

Image Measurement
Measure the depth of the TH muscle from the most superficial portion of the thyroid cartilage to the adipose tissue border.

Ultrasound Machine Settings
Transducer

Description
Horse standing with head straight and slightly extended. Position transducer horizontally at the caudolateral window, with the rostral aspect of the transducer close/in contact with the caudal aspect of the mandible so you can see the thyroid, cricoid and arytenoid cartilages. Slide the transducer ventrally until there is a gap between the thyroid cartilage and cricoid cartilages.

Image Measurement
Trace the circumference of the muscle lateral to cricoid cartilage to obtain the cross sectional area.

Ultrasound Machine Settings
Transducer: Hockey

Description
Position transducer transversely across the ventral aspect of the larynx and record cine-loop while scanning from the caudal border of the basihyoid bone to the rostral tip of the lingual process.

Image Measurement
Go through cine-loop and select images at the junction between the base of the lingual process and the basihyoid bone where 'U' shaped dips are observed either side of the lingual process. Measure from the adipose/connective tissue junction to the BH bone, at the intersection with the fascial plane.

Ultrasound Machine Settings
Transducer

Description
Horse relaxed and not chewing. Position transducer longitudinally between the left and right mandible with the middle of the transducer in line with the junction between the curved and straight part of the mandible. Place the transducer to the left or right of the sagittal plane to measure the left and right muscles respectively. Obtain clear image of both muscles, with blood vessel running through genioglossus at ~6cm deep.

Image Measurement
Geniohyoideus: Measure from the skin surface to the junction with the genioglossus. Genioglossus: Measure from the junction with the geniohyoideus and the soft palate.

Ultrasound Machine Settings
Transducer:

Description
Horse's head in a neutral position with chin approximately at level of shoulder (relaxed as possible) and the front legs square. Position transducer at the base of the neck where it joins the body. Scan the sternothyrohyoideus muscle in transverse section between the left and right carotid arteries adjacent to the trachea.

Image Measurement
Trace the circumference of the sternothyrohyoideus muscle to measure the cross sectional area.

Ultrasound Machine Settings
Transducer

Description
With the horse standing square, position the transducer on the ECR at the point of maximal thickness with the transducer in transverse alignment with minimal pressure application. Position image such that the internal tendon crosses the middle of the screen, and the radius is horizontal.

Image Measurement
Measure from the middle of the screen from adipose/muscle tissue interface to radius, or to the edge of the blood vessel superficial to the bone surface if present.

Ultrasound Machine Settings
Transducer

Description
Horse standing square, position a marker above the line 1/3 distance from the dorsal aspect of the tuber coxae to the tail head. Position the transducer vertically, just ventral to the marker with minimal pressure application.

Image Measurement
In the middle of the screen, measure from the adipose/muscle interface to the bone surface Ultrasound Machine

Description
Horse standing square behind. Place a marker over the femur in line with the junction between the flank and thigh. Position the transducer transverse to the femur, with minimal pressure.

Image Measurement
Measure from the skin to the surface of the femur Ultrasound Machine Photograph to demonstrate ultrasound transducer placement for the vastus lateralis muscle. A marker is placed over the femur, on a line between the junction of the thigh and flank, to guide ultrasound transducer placement.