Running Mechanics

The walking gait cycle consists of two phases, stance and swing. The stance phase has various components. It begins with initial contact, the moment when the foot contacts the ground. During initial contact, the loading response commences as forces are controlled eccentrically. Midstance starts as the contralateral limb toes off and enters swing phase. Once the center of gravity is directly over the stance foot, terminal stance begins. As the contralateral foot contacts the ground, preswing begins. Stance phase can also be viewed in terms of functional components—the absorption of forces on loading, followed by the propulsion of the body forward. During the swing phase of gait, initial swing begins at toe off and continues until the knee reaches a maximal knee flexion of approximately 60 degrees. Midswing follows and continues until the lower leg/shank is perpendicular to the ground. Terminal swing then proceeds until initial contact is made.

The running gait cycle (Fig. 7-1) is also divided into a stance phase and a swing phase. The stance phase may involve an initial foot contact which takes place as a heel strike, midfoot strike, or forefoot strike. Initial foot contact exists on a continuum with increasing gait speed, progressing from heel strike in walking to forefoot strike in sprinting. The percentage of the gait cycle spent in the stance phase varies depending on gait speed—60% with walking, 40% with running, and just 22% with world class sprinters. The walking gait cycle is distinct in that it involves a period of double limb support in which both of the feet are on the ground. The running gait cycle is distinct in that it involves a period of double float in which both of the feet are off the ground. The progression from walking gait to running gait is a continuum—from double limb support in walking to double float period in running.

Figure 7-1 Normal running gait cycle.

(Redrawn from Mann RA, Coughlin MJ. Surgery of the Foot and Ankle, 6th ed. St. Louis: Mosby, 1993.)

At a certain walking speed, there is a transition from walking to running gait which occurs in order to maintain biomechanical, metabolic, and aerobic efficiency (Fig. 7-2). The speed at which this transition occurs varies between individuals, although it tends to be at or near a velocity of 12:00 per mile (5.0 mph) for most. This becomes an important issue when 70% of the running population runs at a pace of 10:00 per mile or slower. Though fast walking and slow jogging have a similar cardiovascular response, slow jogging creates ground reaction forces and loading rates as much as 65% greater than fast walking (Table 7-2). The progression from walking to running involves certain requirements from the body including the ability to tolerate increased mechanical loads (i.e., ground reaction forces) and the strength not only to progress the body forward concentrically, but also to eccentrically control the stance leg. Running and sprinting require more power and range of motion at the hip, knee, and ankle as speed is increased.

Figure 7-2 Transition from walking to running.

(Redrawn from Besselink A. RunSmart: A Comprehensive Approach to Injury-Free Running, Morrisville, 2008, Lulu Press.)

Table 7-2 Ground Reaction Forces Associated with Walking and Running at Various Speeds

During the running gait cycle, the initial functional task of the stance leg is absorption—to eccentrically decelerate and stabilize the limb—before concentrically activating the lower limb for propulsion. The initial phase of stance involves absorbing the ground reaction forces. For walking and slow running up to 3.0m/s⁻¹ (6.7mph, or 8:57/mile), there are two notable peaks in ground reaction forces: the impact peak and the thrust maximum. This two-peaked configuration of the ground reaction curve is consistent in the literature for heel-strike runners. The impact peak occurs during the first 15% to 25% of stance phase. For faster running speeds involving a midfoot or forefoot strike, there is no initial impact peak but usually a single peak, the thrust maximum, and this occurs during the first 40% to 50% of the stance phase.

Ground reaction forces appear to increase linearly up to a gait speed of 60% of maximum speed (average of 4.0m/s⁻¹), but at higher speeds, ground reaction forces appear to stay at approximately 2.5–2.8 times body weight (Table 7-2). It is also noteworthy that during running, athletes that heel strike upon initial contact have a higher initial peak in vertical ground reaction force than midfoot strikers. There is a strong relationship between impact peak and loading rate. The loading rate associated with running has been found to be positively correlated with running velocity, finding an average rate of 77 BW/s⁻¹ (body weight) at slower speeds of 3.0m/s⁻¹, increasing to 113 BW/s⁻¹ at faster speeds of 5.0 m/s⁻¹.

For a runner who has a heel strike, these forces transmit directly through the heel and, therefore, are attenuated by the heel fat pad, pronation of the foot, and primarily passive, more than active, mechanisms in the lower extremity. However, for a runner with a midfoot or forefoot strike, these forces are primarily attenuated by the eccentric activation of the gastrocemius/soleous complex, the quadriceps, and to a lesser degree, the pronation of the foot. Doris Miller, in the book, Biomechanics of Distance Running noted that “initial contact with the heel does not appear to incorporate soft tissue and linked body segment shock absorption mechanisms to as great an extent as landing with initial contact in the midfoot or forefoot region.”

The anterior and posterior calf muscles, quadriceps, hip extensors, and hamstrings all work eccentrically during the stance phase. Of note is the function of the quadriceps, which is the primary shock absorber, absorbing 3.5 times as much energy as it produces. After the initial ground reaction forces are attenuated, the foot then supinates during the propulsion phase to provide a more rigid lever for push off. Winter (1983) noted that the gastrocneminus generates the primary propulsive force during the propulsion phase of running and produces forces between 800–1500 W, compared to 150 W for slow walking and 500 W for fast walking.

The primary purpose of the swing phase is to return the leg back to the stance phase as efficiently as possible. Flexion of the knee shortens the swing limb, effectively reducing the length of the “swinging pendulum”. The hip flexors (including rectus femoris), hamstrings, and ankle dorsiflexors are active both concentrically and eccentrically during the swing phase. There is a small vertical and horizontal translation of the whole body with running. The center of gravity will lower with an increasing velocity of gait. Arm swing is important for balance and for reciprocal running movement, as posterior arm swing corresponds with and assists the propulsive phase of the contralateral limb. The posterior deltoid muscle is very active during posterior arm swing.

Causes of Running Injuries

With the high incidence of running injuries, the suspected factors contributing to injury have been researched for decades. There are virtually as many perceived causes of injury as there are injured runners. A review of the scientific literature would reveal a plethora of perceived causes of and contributing factors to running injury including, but certainly not limited to gender, age, asymmetries and malalignment, leg-length discrepancy, flat feet, high arches, mileage per week, speed work, shoe wear, flexibility (too much or too little), running surfaces (too hard or too soft), gait deviations, history of prior injuries, “muscle imbalances,” training programs, running experience, orthotics, etc.

Review of the current scientific research does in fact yield a definitive answer. One primary factor has been directly associated with the onset of running injury—training or errors in training. James et al. (1978) noted that the primary etiology in two thirds of all causes of injury can be directly related to “training error.” Lysholm and Wiklander (1987) reported that training errors alone, or in combination with other factors, were implicated in injuries in 72% of runners. Simply stated, training error is most often an issue of “too much, too soon,” the importance of which is explained later.

Contrary to the commonly held beliefs of the medical and running communities, there is not any specific correlation between anatomic malalignment or variations in the lower extremity and any specific pathologic entities or predisposition to any “overuse” syndromes. In fact, Reid (1992) noted that “normal variations in the human body abound, and only a few percent of the population are actually good examples of ‘normal.’… Furthermore, all of these variations are found in world class athletes and seem to produce little adverse effect on their ability to perform their sports.… [T]he corollary of this enormous variation of body build among enthusiastic amateur and the professional athletes is that there is a poor correlation of specific malalignments with specific conditions.” Table 7-3 summarizes the sport sciences literature regarding the factors that have been noted to have a direct association with running injury and those that either have no direct association or do not presently have scientific evidence to support an association with running injuries.

Table 7-3 Evidence-Based Factors Associated and Not Associated with Running Injuries

Training error is the only factor that consistently displays a cause–effect relationship with running injuries. Reid (1992) has gone so far as to state that “every running injury should be viewed as a failure of training technique, even if other contributing factors are subsequently identified.” In addition, running distance of more than 25 to 40 miles per week, previous competition in running events, and a history of prior injury have been found to be strongly associated with running injuries.

There are two types of injuries: traumatic and overuse. A traumatic injury occurs when a single force applied to the tissues exceeds the critical limit of the tissues, such as a collision in football that results in a fractured leg or an ankle sprain while trail running. Overuse injuries occur when repetitive forces are applied to the tissues without allowing the tissues to recover.

Under-Recovery Not Overuse

For years, the health care community has pointed to the “overuse” running injury, but if “overuse” were the problem, then there would be a preset threshold at which point all runners would get injured—and this simply is not the case. Physiologic causes of running injuries can be explained by Wolfe’s law. The body aims to attain homeostasis at the cellular level. As a stimulus is applied to tissues (including bone, tendon, muscle, ligament, and collagen-based tissues), a cellular response is triggered and, over time and with sufficient recovery, an adaptation occurs. This adaptation could be greater tissue integrity, strength, or similar mechanical response. Tissues adapt to mechanical loading if given an environment in which to do so and sufficient metabolic capacity to allow this to occur (Fig. 7-3). This has been shown repeatedly with studies on astronauts and deep sea divers, two populations that face altered repeated and/or sustained mechanical loads. There is a precise balance between stimulus and response—or, for the athlete, the application of a training stimulus and the recovery and adaptation to this stimulus. With this in mind, “overuse” injuries should be more accurately described as “under–recovery” injuries because, given appropriate time for recovery, adaptation to the stimulus will take place successfully.

Figure 7-3 Training stimulus and response. This depicts the body’s ability to recover from and adapt to a single training stimulus.

(Graph originally published in UltraRunning magazine, April 2010.)

Figure 7-3 illustrates the body’s ability to recover from and adapt to a single training stimulus. Figures 7-4 and 7-5 display the effect of several training stimuli: Figure 7-4 with appropriate and sufficient recovery and Figure 7-5 with insufficient recovery and poor training adaptation. Injuries occur when the rate of application of training stimulus exceeds the rate of recovery and adaptation.

Figure 7-4 Repeated training stimuli and responses, given appropriate and sufficient recovery. This depicts the body’s ability to recover from and adapt to repeated training stimuli successfully.

(Graph originally published in UltraRunning magazine, April 2010.)

Figure 7-5 Repeated training stimuli and responses, given insufficient recovery and poor training adaptation. This depicts the body’s inability to recover from and adapt to repeated training stimuli when insufficient recovery time and poor training adaptations occur.

(Graph originally published in UltraRunning magazine, April 2010.)

The rate of recurrence of running injuries is as high as 70%. There is little scientific evidence to relate any specific biomechanical factors to the onset of these injuries, yet upward of 70% of running injuries have been found to be related to training errors alone. It becomes imperative for the clinician to understand the relationship between training stimulus and training recovery and adaptation, keeping in mind that the human body is well-adapted to respond to the demands required for running. Assessment and treatment should focus on the training error that disrupted the normal adaptation process. Using this information, the clinician can create an environment that promotes healing and builds the capacity to tolerate the demands of running.

Mechanical Assessment

Education

Education of the patient is a critical element in the effective treatment of the injured runner.

MDT™ uniquely emphasizes education and active patient involvement in the management of their treatment, which minimizes the number of visits to the clinic. Ultimately, most patients can successfully treat themselves when provided with the necessary knowledge and tools. Active approaches to care enhance patient self-responsibility, and education and empowerment of the individual become integral to effectively dealing with injury and the further goal of injury prevention. By learning how to self-treat the current problem, patients gain hands-on knowledge on how to minimize the risk of recurrence and to rapidly deal with recurrences.

The goal of the assessment process is to establish movements, positions, and exercises that will allow the patient to self-treat, if an injury responds successfully to a certain direction of movement. Self-care strategies can be used so that the athlete can be applying mechanical loads to the affected tissues on a regular and consistent basis to promote reduction of the mechanical problem (directional preference) or to stimulate tissue repair and remodeling. The athlete needs to be aware of how to apply safe and appropriate mechanical loads and how (and when) to progress them. By doing so, the athlete can be applying the right forces at the right frequency, far more effectively than a two- or three-times-per week clinical treatment approach. In this way, the practitioner becomes the “guide” and the patient takes an active role in implementing the prescribed treatment with increasing independence. This refines the role of the clinician in the health care spectrum—to one of problem solver, educator, and mentor.

As the patient recovers from injury and returns to running, the physical therapist thoroughly reviews the progression back to running to prevent reinjury (see Table 7-4). Runners, like most athletes, are eager to return to athletic training and competition. Because running injuries are generally training related, it is imperative that athletes understand how to modify their training to foster injury recovery and tissue repair, how to prepare their body to accept the increasing mechanical loads with running, and how to optimize their performance. Most runners are under the mindset that “more is better.” Because research clearly dictates otherwise for runners, it is imperative to educate the patient.

Progression of the program is based on appropriate symptomatic, functional, and mechanical responses to loading. Based on this loading response, the athlete is given the green light to progress the functional loading within his or her training program. Having knowledge of this allows the athlete to progress steadily within the timeline and limits of normal tissue repair, and under his or her own control.

Building Capacity

Related posts:

The Arthritic Lower Extremity Knee Injuries Foot and Ankle Injuries Hand and Wrist Injuries Spinal Disorders Shoulder Injuries

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