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Gait transitions are intuitive changes between gaits as locomotor speed increases or decreases. Most frequently, it is referring to the transitions between walking and running gaits in bipeds, but can also describe changes between gaits such as walking, running, trotting, and galloping in quadrupeds.

Defining the Gait Transition Point
The presence of a double-support phase, sometimes characterized as an inverted pendulum is a major determinant of walking gait in humans, where as running, considered to be a bouncing gait, sometimes characterized as a spring-mass model, contains an aerial phase. Transitions in humans, both from walking to running and running to walking, are not abrupt events. Rather the transition occurs over the course of one step to one stride. The stride in which transition occurs has been characterized as beginning at midstance and ending at approximately the next ipsilateral heelstrike.

Energetic

 * Both humans and horses have been shown to prefer gaits at a particular speed in order to minimize energy expenditure or cost of transport. Cost of transport is the amount of energy needed in order to move a unit of weight a unit of distance.


 * Falls and Humphrey also supported the idea of an energetic trigger, by showing that walk to run transitions happen when the rate of energy expended during walking surpasses the amount needed to locomote at the same speed during running.

Mechanical

 * Critical musculo-skeletal forces proposed as the trigger for the trot to gallop transition in horses. Evidence was found in a study where weighted horses transitioned from trot to gallop at much lower speeds, but at the same critical mechanical stress points (peak vertical force), as when unloaded.

Kinematic

 * Maximum ankle angular velocity was identified as a determinant for the preferred gait transition in humans in order to prevent the overexertion of dorsiflexor muscles during fast walking.


 * Thigh angle as been shown to severely diminish after transitioning from walk to run, supporting the ideal that a critical thigh angle me be a determinant in the transition.

Muscular

 * Increased perceived effort due to exaggerated muscle activation has been cited as a potential trigger for gait transitions in humans. Specifically, increased tibialis anterior, biceps femoris longus, and rectus femoris activation during the walk-to-run transition and increased soleus, medial gastrocnemius, and vastus medialis during the run-to-walk transition.


 * Plantarflexion resistance has been shown to result in lower walk to run transition speeds. This supports the notion of dorsiflexor muscle activation as a potential walk to run transition trigger.


 * An impaired plantarflexor force production, resulting in a decreased ability to produce propulsive forces, as walking speed increased has been identified as a determinant of the walk to run transition in humans.

Humans
It is widely accepted that humans walk comfortably at approximately 1.2 m/s. While the gait transition speed, often referred to as the preferred transition speed is approximately 2.0 m/s. This value can vary slightly based on a few factors including leg length, training status, protocol, grade and direction of the transition.



A 2003 study tested untrained, sprint-trained, and endurance-trained men, concluding that the acquisition of running techniques though training can effect the walk to run transition speed. However, this study’s gait transition speeds were theoretical, based on each subject’s metabolic energy expenditure and internal work. A 2005 study later showed that the actual gait transition speeds are lower than theoretical calculations based on metabolic energy expenditure, and furthermore that both the theoretical and actual gait transitions speeds were not dependent on training status or aerobic capacity. There are a number of different protocols used to determine the gait transition speed, one being calculating a theoretical value based on when it becomes more economic to locomote at one gait rather than another, as used in the previous study. Incremental, protocols in which subjects are asked whether it is more comfortable to walk or run at a given speed are also often used. In these protocols, the walk to run transition speed has been commonly identified as the lowest speed in which a subject prefers to run rather than walk. Conversely, the run to walk transition speed is often identified as the highest speed in with the subject prefers to walk rather than run. Continuous protocols a treadmill constantly accelerating or decelerating treadmills is used to determine the walk to run and run to walk transition speeds respectively. This method has been shown to be more difficult to determine the exact point of transition, making it a more variable measure of the gait transition speed. Often the walk to run and run to walk transitions speeds are averaged to determine a single gait transition speed.

Birds
Gait transitions are often difficult to determine in birds due to a lack of ariel phase during slower running. The guinea fowl for example runs at the fastest speed with a duty factor of 0.51. Duty factor is simply the percent of the total stride cycle that a given foot is in contact with the ground. A duty factor of less than 0.50 is indicative of running in humans, due to an aerial phase being present, where as a duty factor of about 0.50 is indicative of walking since there is a double support phase. This means that even at it's fastest running speed the guinea fowl still has a very brief double support phase. Birds have similar patterns of height adjusted stride frequency and stride length. However, when humans transition from walking to running, step length, limb excursion angle, and duty factor all decrease dramatically while in birds these measures do not decrease drastically but rather have smoother transitions.

It has been shown that like humans and horses, emus and ostriches have gait-specific optimal speeds in order to be most metabolically efficient and minimize cost of transport . Furthermore, ostriches have been shown to run at slower speeds with a double support phase in order to minimize energy expenditure. They are also capable of running with an aerial phase, and do so at higher speeds, however this transition is very smooth which has led researchers to not classify these two strategies as distinctly different gaits.

Horses
Horses most commonly ambulate at a walk, trot, or gallop. As previously stated, horses have been shown to prefer gaits at a particular speed in order to minimize energy expenditure, as well as transition from trot to gallop at a same critical mechanical stress point (peak vertical force). However, a study in 2004 demonstrated that both the biomechanic and energetic factors associated with the walk to trot transitions were very closely linked. The horses all switched between a walk at a trot at equivalent  Froude numbers as well as within a small range of the most energetically optimal transition speed.

Froude Number
Gait transitions can be compared between animals of different sizes traveling at different speeds by using the Froude number:


 * $$\mathrm{Fr}=\frac{V^2}{gl}=\frac{(lf)^2}{gl}=\frac{lf^2}{g}.$$

where $$f$$ is the stride frequency, $$l$$ is the leg length, $$g$$ is the acceleration due to gravity and $$V$$ is the velocity.

For more information refer to Froude number use in gait.