Terrestrial locomotion by means of a running gait can be accomplished on level surfaces. However, in most outdoor environments an individual will experience terrain undulations requiring uphill running. Similar conditions can be mimicked in a controlled environment on a treadmill also. Additionally, running on inclines is used by runners, both distance and sprinter, to improve cardiovascular conditioning and lower limb strength.
Gait
One complete gait cycle is defined as beginning when one foot comes into contact with the ground and continuing until that same foot contacts the ground again. The gait cycle can be further broken down into a number of component parts. Running, by definition, involves a maximum of one foot in contact with the ground at any given time and often no contact with the ground. When the foot is in contact with the ground it is referred to as the “stance phase”. The “aerial phase” is the period between contralateral foot contacts when the body is airborne. For one specific leg, the time of toe-off until subsequent heel contact is known as the “swing-phase” for that leg. One complete gait cycle involves a stance and swing phase for each leg.
Running is characterized as a “bouncing gait” rather than the inverted pendulum mechanism of walking. The stance phase of running can be sub-divided into two parts; during the first half energy is used to perform the negative work of both slowing and lowering the center of mass. In the second half of the stance phase energy is used to perform positive work to lift and accelerate the body. Because of the synchrony of the fluctuations of kinetic energy and gravitational potential energy experienced by the center of mass, mechanical work during running is performed by optimizing the combination of conserved elastic energy in tendons from lengthening and muscle contraction.
Kinematics
Kinematics of running is concerned with describing the motion of the body and in particular the joint angles realized at the hip, knee, and ankle. In level running the hip reaches maximal flexion prior to the end of the swing phase, followed by extension as the leg moves to meet the ground. Throughout stance phase the hip is extending until toe-off to provide propulsion. Knee joint angle displays a biphasic plot. At initial ground contact the knee flexes to lower the body followed by extending to propel the body. Maximal extension is reached at toe-off, after which the knee again flexes to provide clearance. At the ankle maximal plantarflexion is reached at toe-off and is followed by dorsiflexion until mid-swing when the ankle of dorsiflexion remains nearly constant until initial ground contact is made and further dorsiflexion occurs to lower and support the body. In the second half of the stance phase the ankle begin rapid plantar flexion until toe-off.
A number of studies have been performed examining the kinematics of incline running. Swanson and Caldwell (2000) found greater joint flexion at all three joints during initial foot-strike. Also noted was an increase in extensor range of motion and angular velocity at all three joints during push off. Conversely, Klein et al. had found no changes in joint angles when running on an incline versus level surfaces. However, his subjects ran at a speed equal to anaerobic threshold (average of 3.5 meters/second) that was significantly slower than the subjects in Swanson and Caldwell’s study (4.5 meters/second). Also, grade was 5% in contrast to 30%.
Stride Frequency and Length
When running at a constant speed, it has been found that stride frequency increases during incline vs. level running with a concomitant decrease in stride length. At a speed of 30 meters/second Gottschall and Kram noted an increase in stride frequency from 1.45±0.06 Hz to 1.51±0.07 Hz at an incline of 9 degrees (15.8%). Telhan et al. validated this finding at a constant speed of 3.13 meters/second and a gradient of 4 degrees (6.98%) when they observed an increase in frequency from 168.5±8.1 steps/minute to 170.5±7.9 steps/minute. Both studies also saw significant decreases in stride length when running on an incline as compared to level running. The previous studies occurred at moderate speeds. When running velocity is increased 4.5 meters/second and grade increased to 30%, the same trends of increasing stride frequency and decreasing stride length are seen.