Cheat Sheet

Biomechanics For Dummies Cheat Sheet

From Biomechanics For Dummies

By Steve McCaw

Biomechanics has all kinds of practical applications — from the construction of running shoes to ankle braces to low-back pain to weightlifting. Knowing how the body moves because of the forces applied to the body is key to getting the most out of your athletic performance, and your daily life.

How Running Shoes Work

Humans have been running for millions of years. Large forces are produced at the foot–ground interface when running. The force from the ground stops the downward motion and slows the forward motion of the runner during the first half of ground contact, and then propels the runner upward and forward into the next running step during the second half of ground contact. Larger forces are produced to run faster and when running on harder surfaces, like concrete or asphalt (as opposed to softer surfaces, such as grass or dirt).

The foot is a structural marvel because of its anatomy. The 26 bones of the foot are aligned in two arches: one extending the length of the foot (the longitudinal arch) and the other traversing across the foot (the transverse arch). The arches are supported by muscles and ligaments. The foot’s anatomy allows it not only to serve as a flexible lever to help absorb energy during the first half of ground contact, but also to become a rigid lever to push the body into the next step during the second half of ground contact.

During the first half of ground contact, the foot pronates, a combination of inward rotation along the length of the foot (eversion), upward rotation of the foot toward the shin (dorsiflexion), and outward rotation of the foot relative to the tibia (external rotation). Muscles pulling on the foot act eccentrically (pull while getting longer) to control the rate and extent of pronation. The second half of ground contact is a reversal of the pronation. During this phase, the foot supinates, a combination of outward rotation along the length of the foot (inversion), downward rotation of the foot away from the shin (plantarflexion), and inward rotation of the foot relative to the shin (internal rotation). Muscles pulling on the foot act concentrically (pull while getting shorter) to cause the supination.

Pronation is a critical part of absorbing energy, and supination is a critical part of generating energy, and the two actions of the foot are coordinated with the flexion and extension occurring at the knee when running. The amount of pronation and supination differs among individuals, because of differences in skeletal structure, muscle strength and endurance, and running style.

Running shoes provide an interface between a runner’s feet and the ground. A main purpose of shoes is to protect a runner from the dangers on the ground surface like sharp rocks, jagged pavement, or broken glass. A tough material called the outsole on the bottom of the shoe provides this protection. The rest of the shoe is a manmade attempt to improve on the evolutionary design of the foot itself by increasing energy absorption (a feature called cushioning) and controlling the pronation and supination of the foot (a feature called stability).

There is a major trade-off in creating a shoe to provide both cushioning and stability: A shoe with more cushioning provides less stability, and a shoe with more stability provides less cushioning. This tradeoff results from the materials used to make the shoe and how they’re put together.

No one shoe is ideal for everyone. If you currently run in shoes that are comfortable and you’ve been injury free, buy another pair just like them when it’s time to replace your shoes. (Better yet, buy several pairs at the same time, because shoe manufacturers have the tendency to replace their current models with “newer and better” models every year or two.) When you first start using a new pair of shoes, don’t make big changes in the distance, speed, or terrain you run on for at least a few training sessions. You want to make sure you maintain a consistent running routine so that if you develop pain, you know for sure it’s the shoe causing the problem, not the fact that your routine has changed.

Why Ankle Supports Help Prevent Sprains

An ankle sprain is one of the most common injuries in sport and recreation. Typically, the ligaments on the outside of the ankle are sprained when someone “rolls” his or her ankle.

Ligaments are tough connective tissue running from bone to bone to help support a joint. Ligaments consist primarily of the fibers elastin and collagen, aligned to provide support and flexibility to the joint. A sprain occurs when a ligament is stretched so far that the arrangement of the elastin and collagen fibers gets disrupted. Sprains range from mild (a slight disruption of the fibers) to severe (a complete tear of the ligament). When a ligament is sprained, the joint swells, it’s painful to move or to touch, and it takes a while for the joint to become stable and usable for walking. For some people, the joint never feels the same again, and repeat sprains occur more easily than the first one.

Many participants try to prevent ankle sprains — either an initial sprain or a reoccurrence — by wearing high-top athletic shoes or braces, or by having the ankles taped before activity. Research has shown that the use of ankle support helps reduce the risk of ankle sprains. However, the mechanism of how the additional ankle support prevents a sprain is still under investigation.

The support may increase the proprioception, or sensory feedback, from around the joint by stimulating sensors in the skin over the ankle. For this reason, the hair is not shaved off the leg before the tape or brace is applied on the joint (and the brace is worn under, not over, a sock). The idea is that the stimulation to the skin increases activity in the muscles crossing the joint so the muscles respond more quickly to restrain the joint and prevent the ligaments from getting stretched to the point of injury.

Ankle support may provide additional mechanical support to the joint, beyond that provided by the ligaments and muscle. Various designs and materials have been used in the manufacture of braces, and research continues to work on improving the design to provide better support for the ligaments. An ideal brace would not limit joint motion until the ligaments are stretched to the point just before where the elastic and collagen fibers are disrupted.

Another proposed idea is that people who choose to wear support for injury prevention are not as reckless as those who don’t wear support. Choosing not to wear equipment known to prevent injury, such as ankle support, may indicate that a person chooses performance over protection. For the same reason, such a person may hold back from getting into situations from which an injury is more likely to result.

What Causes Low-Back Pain

Low-back pain affects many people. It’s often said that a person with low-back pain suffers from a “slipped disk,” but the better term is a bulging disk. Regardless of what it’s called, low-back pain is very debilitating, causing both pain and muscle weakness.

The spine is the backbone of the body. It consists of 24 individual bones called vertebrae. Each vertebra is adapted to provide support, protection, and sites for muscle attachment. Between each pair of vertebrae is an intervertebral disk, a structure made of layers of tough connective tissue called the annulus fibrosus. The annulus surrounds a gelatinous center called the nucleus pulposus. The structure of the intervertebral disks is uniquely adapted to help keep the vertebrae in alignment while allowing for limited motion between each pair of vertebrae. The motion of the spine reflects the combined motion between the pairs of adjacent vertebrae.

The spinal cord runs from the brain down the length of the spine within a protective channel formed by the vertebrae. A pair of nerves branch off the spinal cord and pass out of the spine between each pair of vertebrae, one to the left and one to the right. A nerve contains both motor neurons (which send signals away from the spinal cord) and sensory neurons (which bring signals to the spinal cord). Each nerve goes to a specific region of the body. The nerves in the low back, the lumbar portion of the spine, bring sensation from and control muscles in a region of the leg.

Low-back pain can develop when the tough outer layer, the annulus, breaks down and the gelatinous center, the nucleus, pushes it out, creating a bulge. The vertebral disk is sort of like a jelly-filled donut. If you step on one side of a jelly donut, the gooey center squishes out the opposite side of the donut. A bulge in the disk occurs similarly, although not quite as dramatically. When the spine bends forward, the vertebrae squeeze (apply a compressive load) on the front of the disk and pull (apply a tensile load) on the back of the disk. The compressive load on the front pushes the nucleus pulposus toward the back of the disk, where the annulus fibrosus has been stretched. If there is a weakness in the annulus fibrosus, from a congenital defect or from a breakdown of the connective tissue, the repetitive pushing of the nucleus pulposus can eventually cause the connective tissue to bulge out and push on the nerve (sometimes called a pinched nerve). The push disrupts the signal transmission along the nerve, leading to muscle weakness, pain, and numbness in the area of the body served by the nerve.

A bulging disk can occur from a single incident, such as a fall or violent collision that loads the back. However, the most common mechanism of a bulging disk is repetitive forward flexion of the spine. This form of overuse can lead to a gradual breakdown of the annulus, and then an identifiable event (leaning forward to pull an item out of the trunk of the car) triggers the rupture of the annulus and produces the bulge that presses on the nerve. Maintaining the inward curve of the low back while standing and sitting, and particularly while lifting with the arms, is a valuable preventive step to avoid low-back pain.

Positive and Negative Phases of Weightlifting

Talk in a weight room among experienced lifters may revolve around “doing negatives.” This doesn’t mean they’re going to stop exercising and go for a snack. “Doing negatives” refers to a particular way to perform an exercise. It’s weight room jargon, but it’s also talking biomechanics.

Positive work is performed when a force is applied to a body, and the body moves in the direction of the applied force. Negative work is performed when a force is applied to a body, but the body moves opposite to the direction of the applied force.

When lifting weights, each rep consists of a positive and a negative phase of work performed by the lifter on the bar. Consider the bench press, an exercise where the lifter lies on her back holding a bar in her hands and alternately lowers it and raises it above her chest (a complete rep consists of a lowering phase and a raising phase). While lowering the bar to her chest, the lifter pushes up on the bar and the bar moves down. The lifter does negative work on the bar. While raising the bar above her chest, the lifter pushes up on the bar and the bar moves up. The lifter does positive work on the bar.

Muscles are producing force eccentrically while the bar is lowering. The same muscles produce force concentrically while the bar is rising. Muscle can produce more force when it’s active eccentrically than it can while active concentrically. Practically, this means that you can lower more weight than you can lift.

“Doing negatives” in the weight room refers to completing sets of just the lowering phase of a lift, using a heavier bar than what can be used through a complete rep of down and up. A partner assists the lifter to raise the bar back up.