The results of this study support the hypothesis that the carrying of dummies alters the ground reaction force distribution but not the paw contact area. When the dogs were carrying dummies, PFz and IFz increased in the forelimbs and decreased in the hindlimbs, with no change in contralateral limb symmetry. Notably, the SI of the hindlimbs revealed during W4 conditions was slightly higher than described in normal, non-lame dogs [6–8]. These results suggest that the weight of the dummy is transferred to forelimbs, which could lead to additional stress on corresponding joints, muscles and connective tissues.
It is known that weight bearing is accomplished via the humeral condyle [9] and that the load is then transferred to the antebrachium via the radial head and the trochlear notch of the ulna. It was shown that with applied load of 50, 100 and 150 N a significant difference exists in the force distribution between the proximal articular surfaces of the radius and ulna, for example if a 100 N load was applied, the measured radial force was 125. ± 26.3 N and 119.0 ± 29.15 N, but the ratio of the mean force remained close to a 50:50 distribution regardless of the applied load [10]. Supraphysiological cartilage pressures can damage the cartilage matrix and subchondral bone [10] and contribute to the pathogenesis of osteoarthritis [11]. Thus, only dogs with sound elbow joints should be used for retrieving, and they should be checked regularly by veterinarians for potential orthopaedic problems. Furthermore, excessive stress could lead to micro-damage or fracture of the subchondral trabecular bone, and thus play a role in the pathogenesis of fragmented medial coronoid process [12]. Although there are no currently available studies investigating the relationship between retrieving work and elbow dysplasia and/or osteoarthritis, veterinarians should advise owners and trainers to carefully train young hunting dogs to ensure that the retrieval weights do not overstress the musculoskeletal system. Even this study was performed with working retrievers, the results might be applied also to other sporting dogs (like obedience dogs) and any other dog carrying weights while playing.
The results of this study also demonstrate load redistribution, thus involving the muscles of the back and cervical spine. Further studies using electromyographic techniques should be performed to investigate this in detail.
One limitation of the present study is that the measurements were taken in a laboratory environment, and thus may not accurately reflect the motions performed by dogs in the field. Moreover, all measurements were taken while the animals were walking, whereas game retrieval typically occurs quickly, often at a gallop. The peak vertical forces increase with velocity, while vertical impulses decrease, due to the shorter stance phase [13, 14]. One study reported PFz values of 2.2× the bodyweight in the trailing forelimb and 1.6× the bodyweight in the leading hindlimb of galloping dogs [15], suggesting that the force redistribution described in the present study is likely to be more pronounced in dogs moving at a gallop. Although this study only included Labrador retrievers, the results are not influenced by breed differences, as has been reported previously [16, 17]. Furthermore, the walking velocity between animals and within trials was consistent, and it was shown that variance of PFz and IFz is low even in ranges between 1.5 and 2.2 m/s [18]. Finally, the pressure plates only register vertical forces, and thus the effects of carrying a weight on mediolateral and craniocaudal forces cannot be evaluated.
