Biomechanical Basis for Movement

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Biomechanical Basis for Movement

Mitchell A. Collins and Michael Hales

LEARNING OBJECTIVES

1. Define and apply biomechanical concepts in the description of rudimentary movement patterns.

2. Discuss the difference between the kinematics and kinetics of movement.

3. Identify and discuss the kinematic principles as related to movement in a rehabilitation setting.

4. Discuss both the linear and angular kinematics and kinetics of movement, and explain how angular motion translates to linear movements.

5. Describe the differences among the different levers, and explain the concept of mechanical advantage as related to levers.

6. Describe Newton’s laws of motion.

7. Identify and discuss the different forces that act on objects and how the forces affect movement.

8. Discuss the concepts of mechanical loading, and describe how loading is associated with different types of injuries.

9. Discuss the principles of mechanical energy.

10. Describe the concept of equilibrium and identify the factors that contribute to stability.

Biomechanics is the study of the internal and external forces acting on the body and the effects these forces produce. The data obtained from biomechanical research provide a great deal of insight into human movement interactions in fields such as physical and occupational therapy, sports medicine, human factors, prosthetics, orthotics, and ergonomics.12,31 Clinical biomechanics, a subdiscipline, provides direct measures of human motion which influence our knowledge of injury mechanics, rehabilitation, treatment, and prevention. This information can directly affect how a physical therapist assistant (PTA) rehabilitates a patient, an orthopedist repairs a broken limb or ruptured ligament, an athletic trainer implements modalities for treatment, or a clinician evaluates an individual’s gait.^19,21,24

The field of biomechanics combines the study of applied human anatomy with that of mechanical physics. These combined sciences allow for detailed descriptions of how and why the human body moves the way it does and why a person may or may not have sustained an injury. Understanding the neuromuscular and mechanical factors associated with human movement provides the PTA with the knowledge and skills necessary for administering rehabilitative techniques correctly and performing patient assistive lifting tasks safely. These biomechanical descriptions influence health professionals to refine their knowledge and, therefore, their approach to injury rehabilitation as well as to consider new and innovative techniques that may lead to improved rehabilitation processes. This information also provides insight into the mechanical causes of injuries, potentially leading to safer participation as individuals interact with the environment.

Biomechanics can provide the health professional with a better understanding and a broadened knowledge of the causes and degrees of severity associated with an injury. In order to improve upon a current rehabilitative technique, repair procedure or treatment process, the mechanics associated with the causation of the injury must be fully understood. To help the health professional accomplish this task, several factors need consideration when determining the cause and severity of an injury ³⁶:

How much force was applied? (magnitude)

Where on the body was the force applied? (location)

Where was the force directed? (direction)

How long was the force applied? (duration)

How often was the force applied? (frequency)

Does the force application vary in magnitude? (variability)

How quickly is the force applied? (rate)

The major role of a PTA revolves around both the causes and effects of human motion and therefore it is imperative to have a fundamental understanding of the mechanical basis for whole body or segmental movement. Since biomechanics is concerned with the effects of forces on the human body, biomechanical principles are involved when a force is present. The following chapter will offer an overview of the science of biomechanics with practical applications for the PTA.

REFERENCE TERMINOLOGY

To facilitate the process of describing human movement, standardized terminology has been adopted to identify body positions and directions of motion (Box 16-1). Movements are defined based on a reference or starting position often referred to as the anatomic position (Fig. 16-1, A), which is a standing position with arms at one’s side and palms facing forward. From the anatomic position, three imaginary cardinal planes bisect the body along three dimensions (Fig. 16-1, B). The transverse or horizontal plane segments the body into upper and lower parts; the frontal or coronal plane separates the body into front and back parts; and the sagittal or anteroposterior plane divides the body into right and left parts. It is important to note that the planes do not necessarily divide the body into equal parts. However, when the segments are equal, the mid-intersection point of the transverse, frontal, and sagittal planes is referred to as the center of mass (COM) or center of gravity (COG). Although human movement is not restricted to a single plane, most named movements (e.g., flexion and abduction) are described based on the three cardinal planes.

Fig. 16-1 A, Anatomical reference position for human motion. B, Sagittal, frontal, and transverse planes. (From Cameron MH, Monroe LG: *Physical rehabilitation: evidence-based examination, evaluation, and intervention*, St Louis, 2006, Saunders.)

Movements in the transverse plane occur around the longitudinal axis, which runs superiorly–inferiorly while perpendicularly intersecting the transverse plane. These movements include medial and lateral rotation of the leg, thigh, and shoulder; supination and pronation of the forearm; and horizontal abduction and adduction of the shoulder. Movements in the frontal plane occur around the sagittal axis, which runs anteriorly–posteriorly while perpendicularly intersecting the frontal plane. These movements include abduction and adduction of the shoulder and hip, lateral flexion of the neck and trunk, elevation and depression of the shoulder girdle, and inversion and eversion of the foot. Movements in the sagittal plane occur around the frontal axis, which runs from left to right while perpendicularly intersecting the sagittal plane. These movements include flexion and extension of the knee, hip, trunk, elbow, shoulder, and neck; and dorsiflexion and plantar flexion of the foot as well as hyperextension movements. A key role of the PTA is to facilitate patient rehabilitation through the incorporation of various exercises using basic movement patterns. Therefore it is important to be familiar with the appropriate terminology for these movements (Fig. 16-2).

Fig. 16-2 Major movements of body segments. (With permission from *NSCA Journal* 14 (1) by Harman, Johnson, and Frykman, pp 47-54.)

BASIC CONCEPTS

To facilitate the discussion of biomechanical principles, a working understanding of various rudimentary concepts is essential. The following are definitions of some common terms along with their appropriate unit of measure. Most of these terms will be discussed in more detail as various biomechanical concepts are introduced.

Mass (m) is the amount of matter an object possesses within its physical boundaries; generally, the denser the material that comprises the object, the greater the mass. For example, muscle tissue is denser than fat tissue; therefore, two persons of equal size or volume may differ in mass if one is more muscular than the other.

Inertia is the resistance an object offers to a change in its state of motion (velocity) or direction of motion and is directly related to its mass. The greater the mass of the object, the more resistance it offers to any attempt at changing its velocity or direction of motion.

Force (F) is a push or pull acting on an object. A force will have both direction and magnitude, and it is commonly expressed in newtons (N). Forces applied to objects, if sufficient to overcome their inertia, will cause them to accelerate in direct proportion to the magnitude of the force.

Friction is created when two objects are in direct contact with one another and a force acts to impede motion of the objects. Frictional force can be increased or decreased by adding substances between the two surfaces, such as the installation of tennis balls on the rear support for walkers. Joint damage (osteoarthritis) caused by chronic exposure to high frictional forces can lead to arthroplasty.

COM or COG is the point within which the weight and mass of an object is equally distributed or balanced in all directions. The COM is important because when a force is applied to an object, the movement pattern will vary based on the relation of the point of force application to the COM.

Kinetic energy (KE) is energy by virtue of an object’s motion. The units for KE are typically expressed as a joule (J), however, one may also see units of newton-meters (N-m), which is an equivalent unit (i.e., 1 N-m = 1 J). An injury mechanism is predominantly due to the transfer of KE to the body arising from different sources under a variety of conditions: from blunt trauma (impact of object’s colliding with the body), penetrating trauma, acceleration/deceleration motion (rapidly moving forward and backward), and crushing weight (high compression forces).

Potential energy is energy generated by virtue of the position or shape of an object. Potential energy may be affected by how much elastic energy is generated by either stretching or compressing the object (e.g., cartilage, tendon, connective tissue) such that, if the distorting force is removed, the object will recoil to its resting length. The units for potential energy are typically expressed as a J; however, one may also use units of N-m.

Torque (T) is the product of the force and the perpendicular distance from the line of action to the axis point, also called lever arm length. Torque is considered a rotary force, but more specifically a measure of the ability of a force to cause rotation. Consequently, torque can be increased or decreased easily by altering the length of the moment arm of the force. A force couple is formed in situations where there are two torques that are equal in magnitude but opposite in direction. The resultant action of a force couple is rotation without any translation. Torque is typically expressed in units of N-m.

Work (W) is the product of force and the distance the object moves. If no displacement of the object occurs, even though force may have been applied to the object, no work was done. The unit of measure is a N-m or a J.

Power (P) is the rate of performing work, and can be expressed algebraically as the product of force and displacement over time. If work is accomplished very quickly (i.e., in a very short amount of time) then a higher magnitude of power is generated as compared to the same amount of work being done over a longer interval of time. Given this explanation, power is work divided by time. Power is expressed in watts (w), and 1 w is equal to 1 J of work per second.

Pressure (p) is a measure of the distribution of a force over a given area (force/area), and it is expressed in newtons per meter ² (N/m²). An example of the concept of pressure in a rehabilitation setting is the development of decubitus ulcers that commonly occur among diabetics.^3,32,38 Innovations in shoe design help dissipate the forces applied to the foot over a larger area (e.g., reduced pressure) during locomotion, thus minimizing soft-tissue damage and the incidence of ulceration.^6,10,35

Momentum is the product of mass and velocity used to determine the outcome of collisions between two objects of mass as well as to determine the ease with which one can stop or change the direction of travel when velocity is present. A motionless object has no momentum. The unit of measure is kilogram × meters/second (kg × m/sec).

Impulse is the product of the force magnitude and the force application time interval expressed in newton × seconds. The direct relationship between an applied force and the change in momentum it creates is known as the impulse-momentum principle. Consider a high force applied to the musculoskeletal system over a very short duration, as is often the case in force related injuries.

BIOMECHANICAL PRINCIPLES

which mathematically illustrates that the acceleration of an object is directly proportional to the force applied and inversely related to mass of the object. Newton’s second law is relatively simple, but it has many applications to physical therapy. Because acceleration (motion) is inversely related to mass, it is clear that basic weight-bearing movements tend to be more challenging for larger patients. In addition, a greater force application is needed for these patients to accomplish said movement. The difficulty in locomotion is compounded if larger mass of a patient results from too much fat, because fat does not contribute to force production.

Newton’s third law of motion is commonly referred to as the law of reaction, which states that to every action there is always opposed an equal reaction; or, the mutual action of two bodies upon each other are always equal and directed to contrary parts.⁵

The concept of reaction can be more difficult to visualize than inertia and acceleration, but when a force is applied to an object such as a wall, there is a force opposite in direction and equal in magnitude to the force applied. As the applied force increases, the reaction force likewise increases. From a clinical perspective, reaction forces are of interest during gait analysis. When a patient walks or runs, clinicians may analyze the differences in gait patterns among individuals with varying levels of disability and the subsequent ground reaction forces that occur when the foot strikes the floor. These forces can achieve three times one’s body weight during running, and patterns vary based on running style. Physicians and therapists use devices such as orthotics to alter ground reaction force patterns to help minimize foot injuries.³³

ADVANCED BIOMECHANICAL CONCEPTS

The following advanced concepts highlight several of the major areas that must be considered when evaluating movement skills. These biomechanical principles have direct implications on rehabilitation based on injury causation and level of severity: reaction forces, levers and resistive torque, kinetic link principle, and balance and stability.

Reaction Forces

Reaction forces are forces applied on a person by a surface with which the person’s body is in contact. These forces are applied as an equal and opposite reaction force to the force applied to the surface by the person. The more force generated by the person onto the contact surface, as a result of their body weight and musculoskeletal activity, the more force the surface returns to the performer. When analyzing gait abnormalities, ground reaction forces (GRFs) should be measured in relation to a fixed three-dimensional coordinate system oriented at the surface of the platform.³⁰ The GRF planes of motion are termed anteroposterior, which describes forces imposed horizontally directed forward or backward; mediolateral, which describes forces generated horizontally side to side; and vertical, which describes forces directed upward or downward. In other words, if the person pushes downward on the surface the surface reacts by pushing upward with equal force. If the person pushes backward on the surface the surface pushes forward with an equal force. If the person pushes rightward on the surface the surface pushes leftward with equal force. Other sources for force application are wheelchair pushrims, which are referred to as pushrim reaction forces. These multidirectional forces are applied to the hand when in contact with the wheelchair pushrim in an equal and opposite manner. The forces specific to a pushrim are termed tangential, which describes forces applied tangent to the pushrim (causes motion); radial, which describes the forces directed toward the axis of rotation (frictional force); and mediolateral, which describes forces directed parallel to the axis of rotation. The forces imposed at the pushrim are transferred to the hand and used to determine subsequent upper extremity joint forces and moments during wheelchair ambulation. Chronic and acute shoulder pain of manual wheelchair users is one of the foremost issues facing PTAs.^2,4,8

Levers and Resistive Torque

The human body consists of a very poor leverage system when one considers its need to generate large, forceful movements against heavy objects. Humans are designed, however, with leverage, which allows us to generate high speeds and produce large ROM. In more meaningful terms, because of the musculoskeletal system’s structure with muscle tendon insertion attachment sites occurring very close to the proximal joint of the segment to which they apply force, the human body has very short force lever arms for the muscle to produce torque around the joint. The resistive forces that the muscle must compete against, the weight of a body segment and/or external object, often have much longer lever arm lengths with which to generate resistive torques. The muscles, therefore, must be very strong to overcome their poor mechanical leverage when dealing with objects which have great weight or momentum. This is the case when the external object contacts a body segment distal to the body’s major torque producing joints (hip, knee, shoulder, and elbow).

With regard to the ability of the human body to generate speed and ROM, the close attachment of the inserting tendon to the proximal joint of a segment means that a small amount of muscle shortening through a concentric contraction will generate a relatively large rotation of the segment. Were the tendon to attach farther away from the joint, the same amount of muscle shortening would result in a lesser rotation around the joint. When the force production of the muscle is rapid, the large amount of angular displacement generated and the small time in which this displacement occurs means a fast angular velocity for the segment. Therefore the linear velocity generated at the end of the lever (i.e., the feet or hands) may be quite large in magnitude.

A lever system includes a fulcrum, which is the point or axis of rotation; an applied force; and a resistance (resistive force). The perpendicular distance from the line of action of the force to the fulcrum is termed the moment arm of the force (MF). Likewise, the perpendicular distance from the line of action of the resistance is termed the moment arm of the resistance (MR). This is important because a lever may be evaluated based on the computation of its mechanical advantage (MA).

MA=MFMR

In a first-class lever, when the fulcrum is located halfway between the point of force and point of resistance on the lever, the MA is equal to 1 (e.g., MF = MR). If the fulcrum is closer to the point of the resistance than the point of force (e.g., MF > MR), the MA becomes greater than 1; thus a smaller force is necessary to overcome a constant resistance. In the opposite scenario where the MF is less than the MR, the MA is less than 1 and a larger force is necessary to move a given resistance. The head tilting backward, initiated by a concentric contraction of the extensor neck muscles (splenius capitis, semispinalis capitis, suboccipitals, trapezius) to overcome the resistive force imposed by the weight of the cranium where the fulcrum is the atlanto-occipital joint, is an example of a first-class lever system (Fig. 16-3, A).

Fig. 16-3 Anatomic examples are shown of first-class **(A),** second-class **(B),** and third-class **(C)** levers. The muscle activation (depicted in red) is isometric in each case, with no movement occurring at the joint. (From Neumann DA: *Kinesiology of the musculoskeletal system: foundations for rehabilitation*, ed 2, St Louis, 2010, Mosby.)

In a second-class lever, the point of resistance is located between the fulcrum and the point of force. The MF is always greater than the MR, yielding a MA greater than 1. Hence this arrangement favors the effort of force, because less force is necessary to cause movement of a given resistance. Performing a toe raise exercise demonstrates a second-class lever. The plantar and dorsiflexion movement can be employed by the PTA to regain ankle ROM due to injury or surgery. Resistance can be added to create an exercise designed to strengthen the calf muscles (soleus and gastrocnemius), which represents a second-class lever system (Fig. 16-3, B).

In a third-class lever, the point of force is located between the fulcrum and the resistance. The MF always is less than the MR, yielding a MA less than 1. Therefore this arrangement favors the effort of resistance, because more force is necessary to cause movement of a given resistance. This is the more common type of lever found within the human body (Fig. 16-3, C). Therefore muscles within the body tend to work under a mechanical disadvantage, resulting in larger internal muscular forces than the mass of the external object (resistance) being moved. It is also important to realize that within some joints of the skeletal system, the actual fulcrum point changes through the ROM. Consequently, mechanical advantage and muscle force vary as the length of the moment arm changes. A person performing elbow flexion during a bicep curl exercise depicts a third-class lever system. The olecranon represents the axis of rotation where the biceps muscle group (biceps brachii, brachioradialis, brachialis) exerts a force to overcome the mass of the forearm or any additional resistance held in the hand.

The resistive force, or more specifically the resistive torque, encountered during movements varies based on the length of the resistance arm (moment arm of the resistance). During the performance of a bicep curl exercise, length of the resistance arm is maximized when the elbow is at a 90-degree angle (Fig. 16-4). As the angle increases or decreases, the length of the resistance arm shortens, thus reducing the magnitude of the resistive torque.²⁶ Therefore it is important to realize that less force is necessary to lift a given resistance when the resistance is closer to the fulcrum (e.g., the resistance arm is reduced). This basic biomechanical principle has numerous applications, including the mechanical function of exercise equipment. Various manufacturers use a pulley system consisting of cam-based pulleys. The unique advantage of a cam compared with a traditional round pulley is the variation in the length of the resistance and force arms during rotation of the pulley. By simply increasing the length of the resistance arm or reducing the length of the force arm, the resistive torque can be increased to provide a variable resistance through the ROM. This relatively simple alteration in design has enhanced the effectiveness of exercise equipment to provide proper loading on muscles throughout the full ROM.

Fig. 16-4 Illustration of how changes in the angle at the elbow alter the length of the resistance arm, thus modifying the resistive torque. *D, Distance; W, weight.* (Redrawn from Baechle TR: *Essentials of strength training and conditioning*, ed 3, Champaign, Ill., 2008, Human Kinetics.)

Kinetic Link Principle

The coordination of body movements in sporting activities, occupational skills, or ADLs is critical for success. Many terms are used to refer to these coordinated movement patterns, such as perfect timing, fluid motion, natural, and graceful. Each of these terms means simply that the body’s nervous system is finely tuned for stimulating the body’s musculature to contract with appropriate intensity or to relax at just the right time to produce the necessary joint rotations required for a successful performance. Without this timing between the nervous and muscular systems, the skeletal system motions that result would be less than effective or efficient.

Body segments are connected in more ways than most individuals realize. When there is a perpetual orthopedic problem in one area of the body, chances are the problem is related to another area of the body in some form or fashion. For example, if patients complain their ankles or knees hurt when they walk, it could be linked to an orthopedic issue with their feet. The foot pain could be directly associated with a problem in the calf region. A taut gastrocnemius–soleus muscle group can often lead to problems in the knees, hips, and lower back area. If this is ignored, the problem can migrate to the upper back, shoulders, and neck. In a sense, everything in the body is connected, in other words, it is a kinetic chain. When a part of this chain is weak or damaged, it will affect other parts of the kinetic chain (Box 16-2).

BOX 16-2 Sequential Kinetic Chain

Common reasons for these issues are:

Compensation due to another injury. An example would be putting more weight on one leg to take pressure off of the injured leg.

Muscular imbalances around a joint. This occurs frequently in the shoulder. In many people the internal rotators are overdeveloped or overactivated, whereas the external rotators are left weak or underactivated. Many people have muscular imbalances all over the body that can cause many issues.

Scar tissue build up or adhesions from previous injury. Most active people have some scar tissue somewhere in the body due to a previous injury. The scar tissue can cause an obvious limit in ROM, thus causing improper joint mechanics.

Improper movement patterns. If a person swings a tennis racket incorrectly for long enough, not only will his or her tennis game suffer, but bad motor patterns or mechanics will quickly develop. This in time will lead to imbalances of strength and flexibility, pain, injury, and overcompensation patterns. Bad movement patterns can occur in anything a person does, whether it is as simple as walking or as complex as throwing a baseball.

The kinetic link principle is typically subdivided into two categories. First, the sequential kinetic link principle basically means segmental motions or joint rotations occur in a specific sequence such that time elapses occur between the peak rotational velocities of each involved segment. This coordinated effort typically leads to high velocity or momentum of the last segment involved in the performance. This principle is often observed in sports skills where the sequential kinetic link is employed for success, the energy or momentum flows from the core of the body (typically the trunk) distally to the appendages of the body (the leg segments to the foot or the arm segments to the hand). This flow is from the body’s more massive segments to its least massive segments. The building of momentum in the bigger, slower segments (trunk and upper legs) of the body leads to effective transference of momentum to the smaller, faster moving segments. In other words, failure to use the trunk appropriately adversely affects the velocity with which a ball is thrown; a club, bat, or racket is swung; or a ball is kicked. Running and wheelchair propulsion demonstrate sequential movement patterns. It is extremely important for individuals with spinal cord injuries to develop core muscle strength because they do not have lower body muscle force contributing to the kinetic chain. For example, wheelchair racing athletes rely on a strong core region in which to transfer momentum generated by the abdominals to the upper extremities.

Second, the simultaneous kinetic link principle states that primary segmental and/or joint movements occur within the same time period where no visible difference in time exists between the contributions of the involved segments and/or joints during the activity. This type of movement is generally employed when an individual is required to move objects, or even their own body, which offer great resistance, such as wheelchair propulsion.

Squat lifting is an example of movement requiring force generation by several muscle groups simultaneously. These types of movements generally need high force magnitudes to overcome relatively high inertial conditions.²⁵

Balance and Stability

Realize that biomechanically, stability and mobility are inversely related. A health professional can apply the principle of balance to provide a patient with an appropriate mix of stability and mobility for a particular activity or skill. Three biomechanical factors directly affect an individual’s balance for improving stability: body mass, COG reference height, and base of support area.1,17 The stability of the human body with respect to the base of support depends on the overall size of the base of support and the height of the body’s COG above the base of support. For example, a person recovering from a lower extremity injury may use crutches or a walking cane in order to relieve the load on the injured limb. The use of crutches also increases the area of the base of support and makes it easier for the user to maintain stability (Fig. 16-5).^29,34 Postural sway is normally under the control of automatic neuromuscular mechanisms that dictate the adequate base of support area. However, those mechanisms are diminished in patients with a muscle atrophy disease or neurological disorder. The use of crutches would greatly increase the stability of these individuals.^29,34

Fig. 16-5 A person ambulating with crutches, depicting stable **(A)** and unstable **(B)** positions.

BASIC BIOMECHANICAL ANALYSES OF INJURY MECHANISMS

Mechanical Loading

Load is defined as an outside force or group of forces that act on an object. For example, when a patient performs an exercise with sandbags on the foot, a load is being applied to the muscles that cause knee extension. Although loads apply to the point of contact (e.g., use of compression bandage), other loads are applied away from the point of contact, as described. Depending on the magnitude of the load (stress), there is a deformation in the object, termed a mechanical strain.

There are three types of stresses: compression, tension, and shear (Fig. 16-6). Compression occurs when two forces or loads are applied toward each other. A common example is the compression stress on the vertebral column during standing while supporting an object (e.g., dumbbell). Excessive compression stress can lead to contusions, fractures, or herniations.^23,25 Tension occurs when two forces or loads are applied in opposite directions. This type of loading is commonly applied to muscles and tendons during stretching activities to improve flexibility.

Fig. 16-6 Illustration of different types of loading. (From Hamill J, Knutzen KM: *Biomechanical basis of human movement*, Baltimore, 1995, Williams & Wilkins.)

When the loading exceeds the ability of the object to resist the stress, injuries such as strains, sprains, and avulsion fractures commonly occur.²⁰ Figure 16-7 illustrates the tension stresses acting on the Achilles tendon during movements of the foot. Injuries associated with tension are more common during activities that utilize the stretch-shortening cycle most often observed during actions that require fast, quick, change in direction movements. Shear occurs when there are two parallel forces or loads in opposite directions, causing adjacent points on the surface to slide past each other. Injuries such as vertebral disk problems, femoral condyle fractures, and epiphyseal fractures of the distal femur in children occur as a result of shear stress acting on the body.

Fig. 16-7 Tension stresses acting on the Achilles tendon during foot movements. (From Neumann DA: *Kinesiology of the musculoskeletal system: foundations for rehabilitation,* ed 2, St Louis, 2010, Mosby.)

The resulting action of compression, tension, and shear forces is dependent on how these forces are distributed, a concept called mechanical stress. Force applied over a smaller surface area results in a greater mechanical stress than force applied over a larger surface area. Given that the lumbar vertebrae typically are more load bearing than the vertebrae in the upper back, one notices that the load-bearing surface area in the lumbar vertebrae is greater than that found in the upper vertebrae.¹⁸ Accordingly, the greater surface area for the thoracic vertebrae translates into a lower mechanical stress for a given load.

Another type of loading on an object, called bending, occurs when nonaxial forces are applied, resulting in compressive stress on one side and a tension stress on the other. When an object is forced to twist along the longitudinal axis, a load called torsion is produced. Injuries commonly occur because of torsion in activities such as skiing, when a foot is planted and the body begins to twist.11,16 Another example is an anterior cruciate ligament (ACL) injury, which commonly occurs during a running activity when one suddenly stops and then turns, thereby causing a deceleration of the lower limb, a forced hyperextension of the knee, or a forced tibial rotation (Fig. 16-8). Other injury mechanisms include an internal rotary force applied to a femur on a fixed weight-bearing tibia, an external rotation force with a valgus (outward) force, or a straight anterior force applied to the back of the leg, forcing the tibia forward relative to the femur.

Fig. 16-8 An image of a young healthy woman immediately after landing from a jump. The inset on the left shows the increased tension in the anterior cruciate ligament (ACL) and the line of force of the quadriceps muscle. Note the relative lateral displacement of the patella relative to the intercondylar groove of the femur. (From Neumann DA: *Kinesiology of the musculoskeletal system: foundations for rehabilitation*, ed 2, St Louis, 2010, Mosby.)

Deformation or a change in shape can occur when an object is loaded. The load–deformation curve describes the relationship between the loading and corresponding degree of deformation (Fig. 16-9). Within the elastic region, the object deforms in direct relation to the force, and returns to the beginning shape once the force is removed. However, at the elastic limit point, the response becomes plastic, resulting in some degree of permanent deformation, even when the force is removed. Also within the plastic region, excessive loading can result in a point of failure. For a bone, this is the point where a fracture occurs. Although excessive acute loading as described can result in damage to the object, the effect of chronic or repetitive loading that commonly occurs in occupational settings is equally problematic.³¹ Repetitive loading can result in microtrauma to the point of stress. If this persists long enough, a chronic wound, termed a stress-related injury, may occur.²⁶

Fig. 16-9 Relationship between load and deformation of an object. (From Hall SJ: *Basic biomechanics*, ed 3, New York, 1999, McGraw-Hill.)

Another important consideration in predicting injury potential is the rate of loading. This variable reflects how rapidly an external force’s load is transmitted to an internal musculoskeletal tissue. There is a period of time during which muscles are incapable of producing tension to resist forces applied to the body. This period of time, referred to as passive loading, is approximately 50 ms (0.05 second). If a large force is applied to the body such that it reaches a very high magnitude within this 50 ms or passive period, the muscles are not able to resist the force and other musculoskeletal tissues must assume this role. These other tissues, such as bone, ligaments, cartilage, and tendons, are less capable of enduring these forces. Should the forces be very high in magnitude and occur very rapidly an acute injury may occur. On the other hand, if the force peaks quickly, though not necessarily extremely high in magnitude, overuse injuries to the bone or connective tissues may occur if these forces are applied repetitively, such as in wheelchair propulsion activities. Therefore a goal in reducing injury would be to reduce either or both the magnitude of the forces applied to the body and the rate at which they are applied.

Common Musculoskeletal Injuries

Soft Tissue Strains and Sprains

Strain and sprain injuries are usually caused by trauma (slip, fall, collision). Strain refers to an injury to a muscle, occurring when a muscle–tendon unit is stretched or overloaded. Sports that incorporate a running component have a relatively high incidence of soft tissue strains, which are quite common injuries treated by PTAs. The injury mechanism begins just before ground impact of the foot; the hamstring muscles contract forcefully to halt the knee extension, which occurs at the end of the recovery phase.²⁰ The action of the hamstrings causes knee flexion to occur and this continues through the early portions of the ground phase. In combination with the hip extension, this knee flexion serves to reduce the braking force (decelerating force), which occurs when the foot strikes the ground. The braking force is a result of a forward moving foot at initial ground impact, analogous to kicking the ground. Hamstring (semitendinosus, semimembranosus, biceps femoris) muscle group injuries are quite common in running activities.²⁰ Hamstring injuries, such as strains or ruptures to soft tissues, result when the muscle concentrically contracts while the muscle continues to undergo stretching during heel contact. A non-sport related soft tissue strain often occurs during slips on surfaces with low coefficient fiction.

Sprain is an injury to a ligament when it is overstretched. The most commonly injured ligaments are located at the ankle, knee, wrist, and low back region. The purpose of ligaments is to hold the adjacent bones together in a normal alignment and prevent abnormal movements by the bones. However, when too much force is applied to a ligament, such as in a fall, the ligaments can be stretched or torn, leading to an injury. Back pain or injury is a leading cause of disability among warehouse and construction workers. The diagnosis of back sprain (whether lumbar or cervical) implies that the ligamentous and capsular structures connecting the facet joints and vertebrae have been damaged.18,31

Rotator Cuff Injury/Shoulder Separation

The primary biomechanical goal of the baseball overhand fastball pitch is to throw a ball at near maximum horizontal velocity with accuracy. Though there are numerous styles of pitching windups, the delivery phase of the overhand pitch (when the ball is accelerated to the release point) tends to have great similarity.¹⁴ This complex series of body movements is characterized by the sequential nature in which the body segments contribute to the velocity of the ball. Basically, the lower body begins its contribution with the ground contact of the lead foot at the completion of the stride toward home plate. The forces created by the ground contact, when combined with the forward moving body, cause a rapid rotation of the lower trunk and pelvic area. This rotation is closely followed by the sequential movements of the upper trunk, the upper arm, forearm, and hand. When the hand reaches its maximum velocity the ball is released. The velocity of each successive segment exceeds that of its preceding segment. In other words, these successive segment rotations accept the momentum passed on by the preceding segments and add their momentum before passing it on to the next segment. As momentum builds with each successive segmental contribution, the kinetic energy of the ball increases and reaches a peak at the release point.¹³

One must consider the biomechanical issues associated with shoulder injuries in overhand baseball pitching. As a result of the very high shoulder rotational velocities attained during the acceleration of the ball toward the catcher, the pitcher must create tremendously large deceleration forces to stop this fast shoulder internal rotation. Relatively speaking, the pitcher spends much time building up shoulder internal rotation to throw the ball with high velocity to the batter, but must stop this motion in a very short time period in order to be ready to field his position. This very short time period for deceleration means the torques around the shoulder joint will be quite high.13,14

The rotator cuff muscles (supraspinatus, infraspinatus, teres major, and subscapularis) are a group of relatively small muscles that are challenged to maintain shoulder integrity and, particularly, to stop shoulder internal rotation and anterolateral distraction. Because the momentum for the shoulder with regard to both the internal rotation and the anterolateral distraction are high, this muscle group is challenged to the extreme with every ball thrown. With the onset of fatigue and the wear and tear of the forces associated with throwing a large number of pitches, injury to this muscle group occurs fairly frequently.¹⁴ If the injury is severe, it may very well end a pitcher’s career prematurely, much like an athlete suffering an ACL injury of the knee. Another common shoulder injury involves a disruption of the acromioclavicular (AC) joint. This joint is composed of the collar bone, or clavicle, and the highest portion of the shoulder blade, the acromion of the scapula. These two bones meet on top of the shoulder and form the AC joint, as mentioned earlier.^13,17 The most common cause of shoulder separation, or AC joint disruption, is a direct fall onto the shoulder. This fall injures the ligaments that provide stability to the joint. The laxity that results allows a degree of separation between the acromion and the clavicle. The degree of separation can range from mild to severe with a noticeable deformity depending on the momentum–impulse relationship. Treatment of this condition can vary from conservative management with a period of immobility followed by gentle shoulder strengthening, to surgery.