• Useful in the diagnosis of conditions affecting horse performance Poor performance: overview.
• Can be used to assess respiratory function, monitor heart rate and electrical activity (electrocardiography) and identify muscle damage (clinical or subclinical myopathy).
• Performance testing can assess fitness, physiological response to exercise tests and therefore training regimes.
• Exercise testing is currently most frequently used in racehorses.

Uses

Alone

• Exercise intolerance.
• Exercise-induced abnormal respiratory noise.
• Suspected cardiorespiratory disorder.
• Suspected clinical or subclinical myopathy - exertional rhabdomyolysis, polysaccharide storage myopathy, ‘tying up’, ‘Monday morning syndrome’ Muscle: myopathy - exertional rhabdomyolysis Muscle: myopathy - overview.
• Poor performance Poor performance: overview: multifactorial – should always be combined with a full physical examination (musculoskeletal) and gait assessment.

Clinical exercise tests

Muscle enzymes - serological sampling

Indications

• Suspected subclinical/chronic exertional rhabdomyolysis, eg stiffness during/after exercise.
• Where serum aspartate aminotransferase and creatine kinase are normal at rest.

Muscle enzymes

• Aspartate aminotransferase (AST) Blood: biochemistry - aspartate amino transferase (AST):
  • Not muscle-specific.
  • Highest activity in skeletal and cardiac muscle. Also in liver, red blood cells.
  • Peaks after 12-24 h and may persist for 2-3 weeks.
• Creatine kinase (CK) Blood: biochemistry - creatine kinase:
  • Muscle-specific.
  • Raised levels can be physiological, eg after strenuous exercise) or pathological (myonecrosis in muscle damage.
• Post-exercise levels of muscle enzymes are affected by:
  • Fitness of individual.
  • Intensity, duration and nature of exercise.
  • Sex- young mares may have higher resting CK than males.
  • Muscle damage (myonecrosis).
• Healthy horses sampled after strenuous exercise can have markedly increased levels of CK, eg endurance horses, racehorses, cross country phase of a three day event (1000-5000 U/L). Creatine kinase (CK) should decrease to baseline values in 1-2 days.
• Low to moderate intensity exercise tests (taking the fitness of the individual into consideration) is more appropriate than strenuous exercise, eg 15 min of trotting, unless clinical signs develop before this time, in which case exercise should be stopped.
• Pre-exercise (baseline) and post-exercise (4-6 h) serum CK and AST should be sampled.
• A post-exercise three- to four-fold increase (or greater) in CK from the baseline value is considered consistent with subclinical exertional rhabdomyolysis. 

Respiratory function

• Abnormal (usually inspiratory) respiratory noise or poor performance → exercise endoscopy of the upper respiratory tract à dynamic upper airway obstruction:
  • High speed treadmill exercise, or in the field (overground endoscopy).
  • To replicate the presenting complaint many horses need to be exercised under the same circumstances under which the abnormality is noted by the rider, ie head and neck position (some must be ridden to replicate complaint), or exercise intensity (replicate specific training/competition conditions).
  • Dorsal displacement of the soft palate +/- preceding palatal instability. Most common condition seen during exercise endoscopy.
  • Axial collapse of the arytenoid cartilage:
    • Can be partial or complete, unilateral or bilateral.
    • ‘Roaring’ or ‘whistling’.
    • Most commonly the left arytenoid cartilage (unilaterally) as a result of recurrent left laryngeal neuropathy:
      • +/- vocal cord collapse (can occasionally be seen in isolation).
      • +/- vocal fold collapse.
      • +/- axial collapse of the aryepliglottic folds (can also be seen in conjunction with palatal dysfunction).
  • Epiglottic entrapment:
    • Under the subepiglottic mucosa and aryepiglottal folds.
  • Pharyngeal wall collapse.
  • Collapse of the rostral soft palate.
  • Ventrorostral collapse of the dorsal laryngeal mucosa.

Cardiac

Indications

• To determine whether an arrhythmia noted at rest disappears with exercise.
• In horses with valvular regurgitation, to determine the risk of exercise associated collapse or sudden cardiac failure – ridden safety.
• In horses with persistent atrial fibrillation, determine cardiac response to exercise - ridden safety.

Heart rate/rhythm

• Heart rate and rhythm during and after exercise: ECG or heart rate monitor.
• Arrhythmias:
  • Atrial fibrillation (sustained or paroxysmal): reduced ventricular filling → reduced stroke volume → negative impact upon performance (especially at high speed/intensity).
  • Ventricular premature complexes (VPCs) and supraventricular premature complexes (SVPCs): effect of occasional/isolated premature complexes on performance is unclear. Interpret in light of intensity of exercise and the presence of other cardiac abnormalities, eg aortic regurgitation.
  • Abnormally increased heart rate at submaximal exercise intensity- may reflect decreased stroke volume, eg valvular insufficiency, cardiac disease.
  • Abnormally high heart rate at maximal exercise intensity: atrial fibrillation (>250 beats/min).
  • Prolonged recovery of heart rate during recovery: unfit horse, dehydration, seen in horses with atrial fibrillation.
  • Interpret with caution: many confounding factors can affect heart rate at exercise, for example external stimuli, pain, fitness, hydration status.
  • Physiological cardiac hypertrophy from exercise → predisposes to valvular insufficiency.

Performance exercise tests

• Can be used to assess or monitor the fitness of an individual or the response to training programs (comparison over time or response to an exercise test).
• Can include:
  • Time to cover a set distance/incline: measure of fitness.
  • Heart rate: measure of fitness, measure of intensity of exercise test.
  • Paired heart rate and velocity test.
  • Lactate: marker of aerobic and anaerobic capability.
  • Maximal oxygen consumption: measurement of aerobic capacity (usually involves treadmill exercise).
  • Recovery time (heart rate).