IN-DEPTH: HIND LEG FROM THE PELVIS DOWN


through more angulated joints”3 and the preemi- nence of the proximal pelvic limb musculature4 demonstrates its importance for the body’s for- ward motion. During loading of the limb all extensor muscles


get involved and the forward thrust is mainly pro- vided by energetic contraction of the hip, stifle, and tarsal extensors. These muscles, namely M. glu- taeus medius, M. quadriceps femoris, and M. gas- trocnemius, represent the main components of muscle levers primarily involved in generation of propulsion, described as trochanteric, patellar, and calcanean lever.1 They all represent first-class le- vers, with the joint as fulcrum being located between the muscular force application site and the moving extremity. The forward propulsion is supported by the “caudal femoral lever system,”1 another first-class lever. By activation of the hamstrings (M. semi- membranosus, M. semitendinosus, M. biceps fem- oris), the stifle is pulled caudally and the stifle and tarsal joint are simultaneously extended while the body is vigorously pushed forward over the hind leg throughout mid-stance. While the limb is ex- tended through muscle activity, the flexor tendons receive full tension and force the solar plane of the hoof onto the ground throughout the propulsion phase. The heel lifting off the ground marks the end of the propulsion phase and the swing phase is initiated as soon as the hoof breaks over the toe- pivot point. During the non-weight-bearing swing phase, an-


other type of lever system comes into action. The flexors of the hind limb predominantly represent third-class levers, with the muscular force applica- tion site being located between the moving extrem- ity and the joint as fulcrum. By activation of the hind-limb flexors, the lower extremity is lifted off the ground in a flexed manner as limb retraction and the mid-swing phase is initiated. Since the me- chanical advantage of a third-class lever is low, the extremity moves further than the force. This for- ward swing is supported by the tensile strength of the tensor fasciae latae, gluteus superficialis, sarto- rius, and iliopsoas muscles.5 The iliopsoas muscle (assisted by the deeper small


pelvic muscles) then actively pulls the extremity forward in an oblique craniolateral direction. Con- traction of the iliopsoas muscle is considered very important to the overall engagement of the hind limb at the end of swing phase and determines the overall amount of flexion of the hip joint as well as flexion at the lumbosacral junction. The forward swing of the hind limb is passively


limited by the elasticity of the medial thigh muscles (M. sartorius, M. pectineus, M. gracilis), which leads the extremity back toward midline, immediately be- fore ground contact of the hoof in the landing phase occurs and another stride begins.


3. Energy-Saving Mechanisms and Efficiency of Movement


Equine locomotion shows synergistic combinations between sprint performances and high endurance.5 However, as the horse is a flight animal, decrease of energy expenditure during movement is imperative. Equine evolvement has brought forward some unique biomechanical features and energy-saving mechanisms. By a series of muscles and ligaments, interacting as locking mechanisms to fix the joints in position, the horse is capable of constant prepared- ness for flight, as well as to stand upright with a minimum of muscular effort and relax or even sleep in a standing position. Since horses as herbivores need to carry consider-


able weight and be able to cover long distances in their natural habitat, economic locomotion is cru- cial. From a biomechanical point of view horses are equipped with extremely effective energy saving as well as shock-absorbing mechanisms within their locomotor apparatus.6 The extremity movements are well synchronized


with pelvic and spinal motion to minimize the ex- penditure of energy.7 The limbs can be described as pendulums “hanging from the body,” which get clenched between the body weight and ground reac- tion forces while the extremity is loaded. The pen- dular movement is considered an extremely efficient manner of motion, which can be compared to the natural oscillation of an isolated pendulum with equalized mass distribution (resonance vibration) in walking horses. With increasing speed of movement the swing


phase proportion of stride decreases, thus leaving an optimum range of energy efficiency. In trot this is compensated by spring mechanisms of the extremi- ties which enable higher resonance frequencies. By stretching and tensioning of tissues under limb loading, they accumulate potential energy,6 also de- scribed as a “pogo-stick-phenomenon.”8 By eccen- tric contraction during elongation, muscles are provided with power through stored potential en- ergy,1 which can be released almost free of losses when the tension decreases.6 Forward movement is characterized by eccentric (lengthening) contrac- tion of muscles in stance phase followed by concen- tric (shortening) contraction in swing phase. In gallop tissue elasticity is even more important


for the efficiency of movement, since temporarily three legs contact the ground simultaneously, thus shortening the resonance vibration phases com- pared to the other gaits. The high capacity of potential energy retention in


elastic tissue enables the horse to keep an optimi- zied “overall energetic efficiency” level.8 The ratio between “mechanical energy transport effort divided by metabolic energy transport effort” ranges on higher levels in horses compared to small animals (0.25 in horses vs. 0.03 in mice).8


AAEP PROCEEDINGS  Vol. 65  2019 325


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