Steve McCaw

Cheat Sheet / Updated 12-05-2024

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.

Article / Updated 03-26-2016

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.

Article / Updated 03-26-2016

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.

Article / Updated 03-26-2016

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.

Article / Updated 03-26-2016

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.

Article / Updated 03-26-2016

The following ten principles of biomechanics provide a solid basis for looking at performance, whether it's coaching, teaching, rehabilitation, teaching a kid in the driveway, or watching a sporting event on TV. You can think of these principles as a list for quick reference. There may only be ten principles covered here, but seeing how they apply will keep you busy for the rest of your movement-analyzing days. The principle of force Force causes movement — that's the fundamental principle of biomechanics. All error detection should be based on this principle. The movement you see occurs because of the forces that were applied. Bad movement reflects bad force. When providing feedback, avoid simple descriptors of the body position you want to see (bend your leg, lead with the elbow, lean more) or bad performance (jump higher, throw farther, run faster) and focus on identifying and correcting the problem, with force production as the source of the problem. The principle of linked segments The simplest model of the human body is a series of linked sticks (individual segments), joined at frictionless hinges (joints). Muscle force pulls on a segment, causing it to rotate faster or slower. The combined action of the muscle force at each joint and the resulting speed of each segment affects the speed at the distal end of the linked segments, such as a foot at the end of a leg or a hand at the end of an arm. (Think of any implement held in the hand as simply an extension of the distal segment.) The speed of the distal segment determines how much force it can apply, like a foot on the ground or a hand on a ball. Considering the motion of the critical segments throughout the movement and not just the position of an individual segment at a specific instant within the movement gives greater insight into performance. The principle of impulse-causing momentum A body speeds up or slows down only while an external force is applied, and it speeds up or slows down only in the direction the force is applied. Impulse is the product of the force and its time of application. Impulse causes a change in the momentum of a body, or how fast it's going in a specific direction. This cause-effect relationship provides a useful approach to analyzing movement. If the body isn't traveling fast enough or isn't going in the desired direction, its momentum is wrong. The problem with the momentum comes from an error in the applied impulse. Errors in the applied impulse arise from force magnitude, force direction, and/or the length of time the force is applied — and these errors stem from the segment motions, not just the positions. The principle of the stretch-shorten cycle Because muscle force causes segment motions, it's important to optimize the force produced by the muscles. The key to producing optimal muscle force is the stretch-shorten cycle (SSC). More muscle force is produced when a muscle is stretched before it's shortened (hence, the name SSC). The muscle is stretched when a segment goes backward before it goes forward. The SSC begins with the windup at a joint, and it's probably the most critical component of skilled performance. Instead of focusing on an "ideal" start position, emphasize using a windup. Always. The windup should be used at all joints, and it should be performed fast — the quickness of the muscle stretching is more important than how much the muscle stretches. There should be no pause at the end of the stretching, just a quick reversal of the joint motion when the muscle shortens. The SSC increases the force produced during muscle shortening and increases the time of force application, both serving to increase the speed of the distal segment. The principle of summing joint forces Because the body consists of linked segments, the amount of force in the impulse applied by the distal segment is essentially the sum of the force from all the joints used. More joints contributing and more force from each joint increase the applied impulse. All joints that can contribute should contribute, and the force from each should be as much as is needed. If a joint is not used, or contributes less than its potential, the applied impulse is less. The visual key is the number of joints moving, with the important factor the rate at which they move. Faster joint action indicates more muscle force contribution and produces a greater applied impulse. The principle of continuity of joint forces When a movement is performed, look for a smooth continuity of the segment motions, starting from the larger, more proximal segments and flowing outward toward the smaller, more distal segments. This principle applies to both the windup and the shortening phase. The segments should not all move as one unit during either phase. The smooth, sequential timing of the motions from proximal to distal increases the applied impulse by the distal end of the segment. Any pause — evident as a jerkiness or hesitation in the motion — disrupts the smooth proximal-to-distal flow and causes a reduced impulse. The principle of impulse direction The change in momentum — the speeding up or slowing down — occurs in the direction of the applied impulse. If the body moves in the wrong direction, the problem comes from the direction of the applied force. In activities like walking, running, and jumping, the push on the ground must create an impulse directed opposite to the intended direction of travel. To go forward, push backward. To go upward, push downward. The principle of rotational motion A force must produce a torque to change the rotation of a body, which changes its angular momentum. Torque is produced when a force's line of action doesn't go through an axis of rotation, creating a moment arm. A jumper's center of gravity is the axis of rotation when rotating in the air. The torque that causes rotation in the air is produced before the jumper leaves the ground if the ground force has a moment arm to the center of gravity. A larger force and/or a larger moment arm create a larger torque and a greater change in angular momentum. The ground force is generated from the motions of the segments. It's not just a matter of "leaning" to create the moment arm. The moment arm occurs if the motion of the segments pushes the center of gravity ahead (for a forward rotation) or behind (for a backward rotation) of the jumper's feet while pushing the jumper upward into the air. The principle of manipulating the moment of inertia No angular momentum can be gained while in the air because no external force creates a torque on the body. However, angular momentum is the product of angular velocity and the moment of inertia, or how mass is distributed around the axis of rotation. A jumper in the air can control angular velocity by manipulating the moment of inertia. Bringing body segments closer to the axis of rotation decreases the moment of inertia and increases angular velocity, while moving segments farther from the axis of rotation decreases angular velocity. The angular momentum stays constant. Putting this principle into action is not as easy as the explanation. When a body rotates quickly, the segments tend to move away from the body because of inertia. The muscle force required to overcome inertia and pull body segments in close to the axis of rotation is considerable. Success in jumping rotations requires a lot of upper-body strength, as well as lower-body strength. The principle of stress causing strain Stress, the intensity of loading, is how an imposed load is distributed over a tissue. The loading causes a deformation, or strain, in the tissue. The strain from loading during regular physical activity typically causes changes increasing the strength of tissues like muscle, bone, ligament, and tendon if adequate time is provided for the tissue to adapt. If inadequate time is provided, an overuse injury can develop. To reduce overuse injuries, it's best to vary workouts and to provide rest between workouts to allow time for the adaptations to occur. Injury can also develop when a high level of stress causes more extensive damage to a tissue. Safety equipment like helmets and facemasks are designed to reduce the magnitude of, and redistribute an imposed force over, a larger area of a tissue that's better able to withstand stress. Improperly fitted or worn-out equipment won't provide safety by reducing stress.

Article / Updated 03-26-2016

Solving for the resultant force created when multiple forces act on a body involves several steps. The steps include using the tools of math and trigonometry to work with force vectors. Using a systematic approach makes it easier to arrive at the correct answer. With vector quantities like force, the direction of the vector is as important as the magnitude. A force of +50 Newtons (N) in the vertical direction is different from a force of –50 N in the vertical direction. Pay attention to the magnitude and the direction of every force given in a problem you’re trying to solve. Similarly, your answer must provide both the magnitude and the direction for the resultant force. When working with force vectors, be sure to first set a coordinate system to provide a reference for direction. Assign the positive and negative directions for both the horizontal and the vertical axis of your coordinate system. Sometimes this is set for you in the question, with words like “use upward as the + vertical direction.” Also identify the axis you’ll use when setting the direction of any vector with a direction given in degrees (for example, a force of 1,100 N at an angle of 38 degrees). Typically, the right horizontal axis represents 0 degrees, and the angle of a vector is measured as positive in the counterclockwise direction. On your coordinate system, sketch out each vector given in the question. Show the positive vectors pointing in the positive direction, the negative vectors pointing in the negative direction, and any vector given in degrees pointing in the general direction of the given angle. Beside each arrow, assign each a name and write in the magnitude and the direction of each force (for example F1 = 300 N at 20 degrees, F2 = –830 N vertical, F3 = 1,100 N at 38 degrees). This step is important because it gives you a visual image of each vector. Next, resolve each vector into its components. Components of a vector are at 90 degrees to each other. These are typically called the horizontal and vertical components. If the force is indicated as purely horizontal or purely vertical, this step is already done for you. For each vector with a direction that’s given as an angle, sketch out a right triangle to graphically show the two components. The given vector is the hypotenuse (H) of the right triangle. Assign the given angle as Ө, and use Ө to identify the side opposite (O) and the side adjacent (A). The next step is important: Using your reference system, make sure you identify which of the opposite and the adjacent sides is the horizontal and which is the vertical component of your vector. Name each of these components with the force name and the component name (for example, F1H, F1V, F2H, F2V, F3H, F3V). Be sure to correctly align the adjacent and the opposite sides to the reference system. If you don’t do this, even if you complete the next step correctly, your calculated resultant force in the final step will be wrong. Next, use one of the trigonometric functions — sine, cosine, or tangent — to calculate the magnitude of the individual sides of each right triangle using the given force (the hypotenuse) and the angle Ө. Use the anagram SOH CAH TOA to identify the correct trig function needed for each component of each vector. You can remember the three trig functions using the letters SOH CAH TOA, which is short for the first letter of the trig function and the first letter of the two sides defined by the function: When you calculate each component, make sure you identify both the magnitude and the direction (+ or –) of the force. The net force in each direction is the sum of all the forces acting in that direction, or Net ForceDirection = ΣForceDirection. For the horizontal direction, use ΣFH = F1H + F2H + F3H, and for the vertical direction use ΣFV = F1V + F2V + F3V. In each direction, use this format: ΣF = (Force) + (Force) + (Force). When entering the force vectors into the equation, enter both the magnitude and the direction (+ or –) within the parentheses. Now complete the summing to calculate the net force in each direction. The final steps involve calculating the magnitude and direction of the resultant force created by the combined effect of the net force acting in the vertical direction and the net force acting in the positive direction. A diagram will help here. Draw the vector arrow representing the net horizontal force in the correct direction, and draw the vertical force vector arrow pointing in the correct direction (+ or –) with the tail of the vertical vector starting at the tip (arrowhead) of the horizontal force vector. Correctly label each of these sides as horizontal and vertical, and write in your calculated magnitude and direction (+ or –) of each force. The resultant force you will calculate is the hypotenuse of the right triangle you’ve sketched. To calculate the magnitude of the resultant force, enter the net horizontal and vertical forces into the Pythagorean theorem (a2 = b2 + c2), or with your labeled sketch: To calculate the direction of the resultant force, enter the net horizontal and vertical force values into the trig function arctan: Present the answer in this format: The resultant force has a magnitude of (resultant magnitude) Newtons at an angle of Ө degrees.

Article / Updated 03-26-2016

Astronauts float around the interior of the space shuttle because they’re in a gravity-free environment. This creates a unique problem for the astronauts trying to get work done up there: how to turn around. Because they’re floating, when astronauts needs to turn around, they can’t do it as easily as you can on earth. On earth, if you’re standing on the ground facing one way and you want to turn around, you use the muscles of your legs to pull on the segments of your leg to create a force from the ground that pushes you in the direction you want to turn. To turn around in space, the floating astronauts can’t push off the ground because they aren’t always touching the ground! The astronauts could wait until the random floating motion brings them over to a wall, or the floor, or the ceiling, and then push off that surface, but this waiting is wasted time. One technique used to turn while floating in space is similar to the technique used by a cat. Cats, as the saying goes, always land feet first. As they do with all movement, Newton’s laws provide the explanation for turning cats (and astronauts). The angular versions of Newton’s laws are related to the turning effect of a force, called torque. Newton’s first law says that an unbalanced torque causes a change in the angular motion of a body, an angular acceleration. Newton’s second law says that the size of the acceleration depends directly on the size of the torque applied to the body — a larger torque causes a larger acceleration, and a smaller torque causes a smaller acceleration. But Newton’s second law also says that the size of the acceleration is inversely related to the resistance of the body to change motion — greater resistance means less acceleration, and less resistance means more acceleration. The resistance to changing angular motion is called the moment of inertia. The moment of inertia depends not just on the mass of the body, but on how the mass is distributed around the axis of rotation. Humans, and cats, can manipulate the moment of inertia by moving body segments closer to, or farther from, an axis of rotation. Moving segments farther from the axis increases the moment of inertia and increases the resistance to changing angular motion. Bringing segments closer to the axis reduces the moment of inertia and decreases the resistance to changing angular motion. The axis of rotation when an upright person turns around to face the other direction is called the vertical axis of the body. It’s an imaginary axis running the length of the body from head to foot (or foot to head). When a person is standing upright with her arms in close to the body and her feet together, the moment of inertia around the vertical axis is at its lowest value. Mechanically, a person consists of two separate bodies — the upper body (head, arms, and trunk, including the vertebral column, or backbone) and the lower body (pelvis and legs). The upper body and lower body can rotate independently around the vertical axis (like when you stand upright and twist from side to side — your upper body twists, but your feet stay planted on the ground), and each has its own moment of inertia. Consider an astronaut in an upright position facing to the right while floating in space. To turn the entire floating body to the left, the technique used by the astronaut involves the following movements: Raising the arms above the head while at the same time raising the legs in front to create an L position of the body: These movements reduce the moment of inertia of the upper body and increase the moment of inertia of the lower body, around the vertical axis. Twisting the upper body toward the left: This twisting motion is caused by muscles in the abdomen and lower back. One end of the muscles attaches to the lower body on the pelvis, and the other end attaches to the upper body on the vertebral column and ribs. The pull of the muscles is equal at both ends. When the muscles pull the upper body toward the left, they pull the lower body toward the right. The rotation of the upper body toward the left is more than the rotation of the lower body toward the right because the moment of inertia of the upper body is less than the moment of inertia of the lower body. Lowering the arms so they’re straight out in front of the body while at the same time lowering the legs. These movements create an upside-down L position to the body, increasing the moment of inertia of the upper body and decreasing the moment of inertia of the lower body around the vertical axis. Twisting the upper body toward the right: The pull of the muscles cause the upper body to rotate to the right and causes the lower body to rotate to the left. The lower body rotates more because it has the smaller moment of inertia. The body is now aligned in the original starting position. Repeating the sequence of movements until the astronaut faces the intended direction. Astronauts must learn the technique of manipulating the moment of inertia to turn while floating in space, although cats seem to be born with their version of the technique wired into their neuromuscular system (even kittens almost always land feet first). A similar version of twisting can be performed on the trampoline, negating the need to go to outer space to see how manipulating the moment of inertia can allow rotation while airborne.

Article / Updated 03-26-2016

Each muscle in the body includes many motor units. A motor unit consists of a group of individual muscle fibers that are activated by a single motor neuron. When stimulated by an action potential transmitted along the motor neuron, all the fibers in a motor unit develop muscle tension (a pulling force) at the same time. Although tension development in response to an action potential is common to all motor units, the response characteristics of the individual motor units are not the same. Motor units differ based on how quickly the motor unit develops muscle tension when stimulated (known as twitch time) and the resistance of the motor unit to fatigue. Based on response characteristics, two main categories of motor units are commonly used. One of the categories includes two subcategories: Type I: Type I motor units develop a low peak force in a relatively long period of time (about 60 to 120 milliseconds, or ms). Type I motor units are very resistant to fatigue because they’re nourished with an extensive blood supply to maintain aerobic metabolism. (Aerobic means using oxygen.) Another name for Type I motor units is slow-twitch oxidative, based on the slow tension development time and the use of oxygen. Because they’re fatigue resistant, Type I motor units are the first motor units recruited by the central nervous system when a muscle is activated, and they continue to be recruited as long as the muscle remains active. Type I motor units are well adapted for low-intensity work like maintaining posture. They’re sometimes called tonic motor units because they provide “muscle tone.” Type II: Type II motor units develop a high peak force in a relatively short period of time (10 to 50 ms). Type II motor units are called fast-twitch motor units because of this quicker response time. Another common name is the phasic motor unit, because Type II motor units are recruited after Type I motor units to provide short bursts, or phases, of higher muscle tension as required. Type II motor units have two subcategories. Both reach high peak force in a relatively short period of time, but they differ in resistance to fatigue: Type IIb: Type IIb motor units are very prone to fatigue, but they produce the most force when stimulated. These are the last motor units recruited when a muscle gets activated, and they’re the first to stop being recruited when the force from a muscle is no longer needed. Type IIb are sometimes called Type IIx in human muscle. Type IIa: Type IIa motor units are the intermediate motor units. Their peak force and their resistance to fatigue fall between the classifications of Type I and Type IIb. These motor units are also recruited after Type I but before Type IIb, and they stop being activated after Type IIb but before Type I. An individual muscle contains all three types of motor units. This provides each muscle with the ability to produce an increased force output from a low level to a high level, and it also provides each muscle with a certain degree of muscular endurance. The relative percentage of each motor unit type present affects the muscle’s overall endurance and power (rate of force production) capability. A greater percentage of Type I fibers means the muscle has better endurance capability, while a greater percentage of Type IIb fibers means the muscle has better power capability. There is still controversy on whether strength training and endurance training can cause a motor unit to change its response characteristics. What is fact is that strength training is needed to provide a training stimulus to maintain the Type IIb motor units. Another fact is that a decrease in physical activity causes the muscle fibers in Type IIb motor units to atrophy (lose size and force-producing capability) more quickly than the fibers in Type I and Type IIa motor units. This is very important in preventing falls in sedentary individuals, especially the elderly. Without the availability of rapidly developed force from Type IIb motor units, the quick response needed to regain balance will not be there, and an innocent stumble can result in a devastating fall.

Article / Updated 03-26-2016

In biomechanics, a common word problem to be solved involves calculating the magnitude of the muscle force required to hold a weight in the hand. A typical problem is worded something like this: A person holds a 500 Newton (N) dumbbell in his right hand. His forearm and hand are held stationery in the horizontal position with no rotation at the elbow joint. The forearm and hand segment weighs 17 N, and the center of gravity of the forearm/hand segment is 0.23 meters (m) from the axis of the elbow joint. The center of gravity of the dumbbell is 0.34 m from the elbow joint. If the muscle holding the arm in this position inserts 0.05 m from the elbow joint, how much muscle force is required to keep the forearm/hand from rotating at the elbow joint? Many students are perplexed by how to solve this type of a problem. A step-by-step solution involves first figuring out the biomechanics concept to apply and then selecting and solving the appropriate equation. "To keep the forearm/hand from rotating at the elbow joint" means to prevent angular acceleration. No angular acceleration is part of a situation called equilibrium. The basic point of equilibrium is that there are no unbalanced forces causing linear acceleration, and there are no unbalanced torques causing angular acceleration of the body. (The body, in this case, is the forearm/hand holding the dumbbell, which is free to rotate at the elbow joint.) So, the biomechanics concept to apply is equilibrium. Equilibrium, in equation format, is stated as ΣF = 0 (where F is forces) ΣT = 0 (where T is torques) The question describes preventing rotation of the forearm/hand at the elbow joint, which means to maintain ΣT = 0 at the elbow joint. The biomechanics concept to apply is torque. Torque is the turning effect of a force, calculated as the product of a force (F) and its moment arm (MA), written mathematically as T = F × MA. Before you can sum up the torques, you need to identify the forces that have a moment arm and can create a torque. To do this, go through the problem, identify each force, and give it a label: The weight of the dumbbell can be labeled WD (where D stands for dumbbell). The weight of the forearm/hand segment can be labeled WS (where S stands for segment). The muscle force can be labeled FM (where M stands for muscle). Weight is a force that always acts downward. Use a plus sign (+) for the upward direction, and a minus sign (–) for the downward direction. Weights are applied at the center of gravity of a body, and the location of the center of gravity for both the segment and the dumbbell weight are given. Create a table listing what you'll use to calculate the torques, and fill in the known information from the word problem, sort of like this: Force Magnitude and Direction Moment Arm (MA) Torque (T = F × MA) Torque Name WD –500 N 0.34 m –170.0 Nm TD WS –17 N 0.23 m –3.9 Nm TS FM Unknown, to be solved 0.05 m Unknown TM It's important that weights be listed as negative forces. The moment arm for each force is on the same side of the elbow joint axis, so set them all as positive. The moment arms for the segment weight and the dumbbell weight are the distance of each center of gravity from the elbow axis because the forearm/hand is in the horizontal position. The torque created by each force is calculated as the product of the force and moment arm. The weights (segment and dumbbell) create negative torques, and it's important to list the direction, as well as the magnitude of the torque in the table. Next, use the equation ΣT = 0 to solve for the torque created by the muscle (TM). To do this, expand the equation to list all the torques, like this: TM + TD + TS = 0 Now, isolate for the unknown muscle torque: TM = –TD – TS Fill in the known values from the table you created and solve: TM = –(–170 Nm) – (–3.9 Nm) = 173.9 Nm The muscle must create a torque of 173.9 Nm, opposite in direction to the torques created by segment and dumbbell weights, to prevent angular acceleration. The last step is to calculate the muscle force (FM), using the following equation: TM = FM × MAM Isolate for FM, making the equation: Finally, write out your answer: The muscle torque required to prevent rotation is 3,478 N. Don't be alarmed when you calculate a large force value from the muscle — the muscle force is always much larger than the force held in the hand, because of the short moment arm for the muscle at the joint.