Why do we look at horses during trot?
During the dynamic part of the lameness examination, most information about the presence of lameness and the affected leg is gathered during trot. Trot is a symmetrical two-beat gait with limbs moving in diagonal pairs (Hildebrand, 1965) in near-synchrony. Assessment at trot takes advantage of the increased forces being exerted on the limbs (Merkens et al., 1986; Merkens et al., 1993) due to the increased speed and bouncing gait (Biewener, 2003). A painful focus in the limb can thus be exacerbated, leaving the horse little option to redistribute weight unless adopting an asymmetrical movement pattern. The resulting movement asymmetry, which is visually assessed by clinicians, is the main indicator of lameness.
The biomechanical foundation for our lameness training
Both kinematic (based on movement) and kinetic (based on forces) approaches have yielded sensitive measures for objective lameness quantification during trot on the straight line. Some of the most reliable objective features to detect lameness are based on upper body movement. Other very reliable indicators are differences in fetlock hyperextension or differences in peak vertical ground reaction force and vertical impulse compared between contralateral limbs (Buchner et al., 1996b; Ishihara et al., 2005; Weishaupt et al., 2006; Weishaupt, 2008).
Upper body movement lends itself to clinical lameness assessment, as changes are - within the bounds of perceptual limitations - readily visible to the eye. Studies into head and trunk movement have investigated the response of several upper-body landmarks to lameness. Investigations concerned head, withers, os sacrum and tubera coxae movement adaptations (Buchner et al., 1996a), selected locations such as only head or only the pelvis (Peham et al., 1996; Peham et al., 1999; Keegan et al., 2000; Kramer et al., 2000; Keegan et al., 2001; Peham et al., 2001; Keegan et al., 2003; Kramer et al., 2004; Pfau et al., 2007), locations distributed along the back (Audigie et al., 2002) and centre of mass movement adaptations (Buchner et al., 2001; Halling Thomsen et al., 2010). Alternatively, movement of the sternum has been measured (Barrey et al., 1994; Barrey and Desbrosse, 1996). These studies found that asymmetry in vertical acceleration, velocity and / or displacement of most upper body landmarks reflected lameness well (Buchner et al., 1996a; Peham et al., 1996; Uhlir et al., 1997; Church et al., 2009). Today, vertical head movement asymmetry is considered the most reliable kinematic lameness pointer for forelimb lameness (Peloso et al., 1993; Buchner et al., 1996a; Keegan et al., 1997; Peham et al., 1999; Keegan et al., 2000; Keegan et al., 2001; Keegan et al., 2003; Keegan et al., 2004) while vertical movement asymmetry of os sacrum and tubera coxae is the most reliable kinematic pointer for hind limb lameness (May and Wyn-Jones, 1987; Buchner et al., 1993; Buchner et al., 1996a; Kramer et al., 2000; Peham et al., 2001; Kramer et al., 2004; Pfau et al., 2007; Church et al., 2009).
Head movement can be used to detect forelimb lameness (Buchner et al., 1996a; Keegan et al., 2000; Keegan et al., 2001; Keegan et al., 2003). In a sound horse, vertical head displacement resembles a symmetrical, double-sinusoidal pattern (Buchner et al., 1993; Peham et al., 1999). With the onset of lameness, this pattern is disturbed (Peloso et al., 1993; Buchner et al., 1996a; Peham et al., 1996; Keegan et al., 1997; Peham et al., 1999; Peham et al., 2000), causing asymmetry between the two vertical excursions of the head during successive steps. A modelling study showed that head movement adaptations during forelimb lameness can actually account for the majority of limb unloading (Vorstenbosch et al., 1997). While this results in a straight forward indication of lameness, the pattern of head movement during lameness varies. For most forelimb lame horses, the head reaches its lowest position near mid-stance of the sound limb. Less commonly, horses for example show increased head elevation following stance of the lame limb while maintaining similar minimum positions, also characterised as ‘push-off lameness’ (Kramer and Keegan, 2004). In moderate to severe cases of forelimb lameness, head movement changes to a single sinusoidal excursion per stride (Clayton, 1987; Peloso et al., 1993; Christovão et al., 2007). This reduces the movement frequency by 50 % and likely contributes to the perceived ease of determining forelimb lameness.