Strength can be defined as the ability of a muscle or muscle group to produce tension and a resulting force. Strength is required in sporting dogs that need speed (Fig. 1). These dogs have a higher muscle mass–to–body weight ratio than other dogs. Strength is closely linked to speed, for example, in racing Greyhounds, which are extremely strong and may run faster than 70 km/h (45 mph). Growth hormone increases muscle mass in dogs. Strengthening exercises follow a number of basic principles, including the principle of specificity and the overload principle. The principle of specificity refers to the need to train emphasizing the body systems that are used during the sport, and to train them in a manner consistent with how they are used during the sporting activity. In terms of strengthening exercises, the specific muscle groups whose strength is required to perform the desired activities need to be trained. These muscles also need to be strengthened in a way similar to how they are used during the activity (eg, aerobic versus anaerobic, duration of exercise). As an example, a dog involved in flyball needs to accelerate as quickly as possible in a straight line for just a few seconds and then jump four times, decelerate rapidly, turn, accelerate as quickly as possible, and jump over the hurdles again. In designing a strengthening program, there is a need to focus on strengthening pelvic limbs for acceleration and jumping and on strengthening the forelimbs for braking and turning. Specificity also should be present in the exercising environment (ie, surfaces, temperature, humidity, surroundings). The overload principle is arguably the most critical factor in training. It states that to increase strength (or endurance), a load that exceeds the metabolic capacity of the muscle system or cardiopulmonary system must be achieved during exercise. The systems must also be exercised to fatigue to promote improvement. The principles of specificity and overload apply to conditioning sled dogs, in which pulling is an important specific exercise to illustrate the overload principle. A conditioned sled dog does not improve its fitness level by running on a treadmill or outside at 3 mph for 1 hour five times per week. This level of exercise does not increase its cardiopulmonary endurance or strength for competition, because it does not stress these systems enough to cause adaptation. If improving fitness is the goal, these dogs should exercise while pulling a sled at reasonably high intensities (race speed) for reasonably long periods. If time available during training is an issue and the dog has only 3 hours per training session, the speed could be increased 10% over normal race speeds to stress these systems adequately. Swimming, although a good endurance exercise, does not work the musculoskeletal system of a sled dog in the same manner as the sport. The stresses and subsequent adaptations on the bones, ligaments, and cartilage that occur with weight-bearing exercises are not provided to the same degree with swimming or underwater treadmills. Strengthening exercises include trotting, trotting uphill, pulling weight or a cart, swimming, galloping, controlled ball playing, retrieving, dancing, and wheelbarrowing. Speed exercises include rapid acceleration and deceleration on level uphill and downhill terrain, ball playing, and playing and racing with other dogs.
Endurance is critically important to sporting dogs that perform prolonged efforts, for example, long-distance races (ie, sled dogs) and herding. Aerobic endurance exercises usually target large muscle groups for a prolonged period (more than 15 minutes). They are performed several times per week. Long-term changes occurring in muscle undergoing aerobic training include increased vascularization, which increases the amount of oxygen brought to muscle. In conditioned endurance athletes, several other important changes occur. These changes include decreased resting heart rate and increased stroke volume because of increased vagal tone, which allows greater time for ventricular filling; decreased resting blood pressure, thought to be attributable to a decrease in circulating catecholamines in the bloodstream; an increase in enzymes involved in the oxidative pathways so that ATP can be generated more rapidly; and increased capillary density within the muscle to allow for more efficient delivery of oxygen and greater oxygen uptake, resulting in improved performance. Training also positively affects the strength and stiffness of all musculoskeletal tissues. It makes cartilage and ligaments stiffer, and bones, muscle, and tendons are stronger. Exercise does not seem to increase the likelihood of osteoarthritis in dogs free of predisposing factors (ie, obesity, limb malalignment). Endurance exercises are performed for sustained periods of 15 minutes or more. They include trotting, swimming, land or water treadmill activity, and sled pulling. Monitoring of variables, such as heart rate, during exercise is commonly performed in people but rarely done in dogs. Recommendations of percentage of maximum heart rate that dogs should train at for optimal conditioning are unknown but may help to determine more efficient training regimens.
Balance and Proprioception
Exercise enhances balance and proprioception. Balance is the ability to adjust equilibrium at a stance or during locomotion to adjust to a change in direction or ground surfaces. Proprioception is the unconscious perception of movement and spatial orientation originating from the body. Sporting dogs need to have welldeveloped balance and proprioception to adjust to the specific challenges of their activities. Proprioception decreases with age in people. Proprioceptive training includes activities that may be performed at low or high speed and require an awareness of limb position in space. They include walking in circles or a figure-ofeight and walking across obstacles of various shape, height, and spacing. Balance exercises are exercises requiring rapid responses to changes in slopes, such as walking on a trampoline, balance or wobble board, and swimming. Other balance and agility exercises include cavaletti rails, exercise balls, rapid changes of direction while trotting and galloping, ball playing, tug-of-war, dancing, and wheelbarrowing. Adequate rest is important during conditioning to prevent muscle fatigue and lower the likelihood of overuse injuries. Conditioning, however, decreases rest requirements in dogs. Conditioning also minimizes the circulating lactic acid after intense muscular activity. It decreases the likelihood of rhabdomyolysis, a syndrome resulting from hydrogen ion accumulation in muscles during exercise; muscle swelling and ischemia; erythrocyte death in muscles; myoglobinuria; and potential renal failure. Rhabdomyolysis is primarily seen after intense exercise in poorly conditioned dogs. Sporting dogs often exercise all year long, with periodic higher intensity times corresponding to their competition season.