Basic Concepts in Biomechanics and Musculoskeletal Rehabilitation

Maureen Young Shin Noh

Benjamin C. Soydan

Anand B. Joshi

Clinical pain training has historically focused on the following categories: type, location (usually tied to a specific offending joint), and psychological components of pain. Musculoskeletal pain generators do not neatly fit into these categories. These pain generators are often regional, a consequence of the body’s biomechanical function against Earth’s gravitational pull, and subject to the will and desire of the individual person. For example, the pain generator in medial compartment knee osteoarthritis can be viewed as the knee joint complex in which targeted treatments such as medications, injections, or surgery can be considered. Alternately (or perhaps ideally, simultaneously), the pain generator could be viewed as all the biomechanical considerations of strength, flexibility, and gait which lead the patient to place increased mechanical stress on the medial knee compartment, therefore worsening pain and joint pathology. In that light, a combination of exercise, weight loss, and specific gait training can decrease medial knee joint loads and therefore treat the painful area.¹

By emphasizing biomechanical principles, the contents of this chapter represent a fundamental change from the customary emphasis on the use of passive physical therapy and physical modalities in treating musculoskeletal pain. Patients are often treated for musculoskeletal disorders with passive modalities such as hot packs, cold packs, massage, electrical stimulation, and deep heat. Unfortunately, these passive therapies are overused. Although they certainly can play an important role in providing symptomatic relief of musculoskeletal pain, passive modalities should be used only as methods to facilitate active rehabilitation. The patient most needs to become actively involved in a therapeutic exercise program specifically designed to improve musculoskeletal functioning.

Musculoskeletal rehabilitation is a process whereby poor posture, muscle imbalances, and other biomechanical deficits are corrected using specific exercises to gain better static and dynamic control of the musculoskeletal system. The physical restoration process may involve passive therapeutic modalities to facilitate the exercise program. Clinicians treating musculoskeletal pain and dysfunction must identify and work to correct deficits in the patient’s biomechanics.

The goal of this chapter is to provide an overview of a biomechanical approach to assessing and managing patients with musculoskeletal pain. This chapter reviews basic concepts in biomechanical assessment, followed by examples of common painful syndromes and their biomechanical considerations. Clinical applications of physical modalities are outside the intention of this chapter and are discussed elsewhere in this book.

Basic Considerations

Key considerations in musculoskeletal rehabilitation include but are not limited to the following concepts: kinetic chain theory, adverse neural tension, and neuromuscular control. In addition, biomechanical considerations in the setting of common physical examination techniques can aid the clinician in evaluating ineffective movement patterns which can precipitate or reinforce pain. As the patient is an active participant in the examination, he or she too is made aware of his or her alternate movement patterns which may positively reinforce the process for correction.

KINETIC CHAIN THEORY

The underpinning of modern kinetic chain theory was pioneered by Franz Reuleaux, a German mechanical engineer as well as author. Reuleaux’s works in the late 1800s, inclusive of The Kinematics of Machinery, emphasized the relationship among mechanical links or “kinematic pairings.” He theorized that any mechanistic movement could be broken down into these fundamental pairings and that the sequences of movement within and between these pairs produced a resultant “kinematic chain” directly related to constraints placed on them.² Therefore, movements at one location directly affected movements at another location in the mechanical link.³ Although kinematics relate more strictly to description of movement, it can be assumed that kinetics (forces that cause motion) can be used to explain kinematic relationships.

Arthur Steindler, an orthopedic surgeon and professor at the University of Iowa, adapted these theories into the analysis of human movement.⁴ Steindler was very involved in analysis of movement and pain associated with a variety of orthopedic diagnoses, including scoliosis, foot deformities, and back pain as well as upper and lower extremity reconstruction.⁵ Steindler proposed that the segments in the human body be thought of as rigid, overlapping units in series, where in successively arranged joints create an overlying kinetic chain and ultimately a collection of multisegmental movement patterns. He divided these movements into two categories: open kinetic (where the terminal segment moves freely) and closed kinetic (the terminal segment is restrained from free motion). It is acknowledged, however, that, unlike machines, no motion of the human body is “true” closed chain, as there always exist some component or segment that is unrestricted during movement (whether it be upper or lower body).⁴

Regardless of whether or not a kinetic chain is open or closed, in order to achieve a desired human movement pattern
two kinetic chain variables of interest are considered: adequate range of motion (ROM) (kinematic element) and adequate force production (kinetic element). In the assessment of musculoskeletal pain, it should not and cannot be assumed that each joint has adequate freedom and/or strength to achieve the desired movement pattern (neuromuscular control plays a significant role but is to be discussed in a later section in more detail). One may present with inadequate strength and/or inadequate ROM at a particular joint, thereby lending to a compensatory faulty movement pattern (and likely undue tissue stress) at any given joint involved in the movement.

A prime example of a faulty closed chain movement pattern is that of a flexible foot deformity leading to excessive subtalar joint (STJ) pronation and/or delayed return to supination during gait propulsion. In a weight-bearing position, STJ pronation is correlated with a position of knee flexion, valgus, and medial tibial rotation.⁶ If one is unable to adequately control STJ pronation during the initial loading phase or propulsion phase of gait, the knee will be placed in a position of excessive medial tibial rotation, valgus, and flexion. This may lend to excessive stress on supporting structures such as the medial collateral ligament and patellofemoral joint. However, if tissue structures at the knee fail to control motion adequately, the femur may then incur excessive internal rotational forces, and in such cases, the hip or even the sacroiliac (SI) joint may be affected.⁶

In this example, the variable of concern is not a restricted ROM but rather inadequate strength or control to adequately maintain proper positions within a closed kinetic chain movement. This is one isolated example but lends insight into concerns of any multijoint movement (running, walking, bending, throwing, etc.). Although a patient may present with a localized tissue or joint insult, it is paramount that any clinician receiving a patient reporting localized pain acknowledge the role of faulty movement mechanics in the assessment of pain etiology and appropriate management.

ADVERSE NEURAL TENSION

When observing an athletic contest or a performance art, one can appreciate movement of the human body. Although the primary function of the nervous system is to conduct impulses, the nervous system must also be able to move along with the rest of the body. However, in addition to movement alone, nervous system function is also dependent on normal physiology. Neurodynamics refers to the interactions between nervous system mechanics and physiology.⁷ Adverse neural tension is a subset of neurodynamic theory that specifically deals with abnormal mechanical responses of the nervous system as its tissues are taken through a ROM.⁸ Due to commonality of use, we use the term adverse neural tension (ANT) in this chapter.

Literature on “nerve stretching” can be found as far back as the 1880s.⁹^,¹⁰ Although a full review of the underpinnings of ANT theory is beyond the scope of this book, a bedrock principle for the pain clinician is that abnormalities in nervous system movement or physiology can provoke symptoms.⁷ Failure to engage in the protective adaptions mentioned earlier renders the nervous system vulnerable to edema, ischemia, fibrosis, and hypoxia.¹¹^,¹²

Testing for ANT can be made a part of the clinical evaluation based on the patient’s presentation. The best known test is the straight-leg raise (SLR). However, evaluations for many other peripheral nerves are possible and should be considered when a patient presents with limb pain or paresthesia with movement, or restricted ROM. The nervous system’s sensitivity to movement can be considered as a proxy of its physiology, including blood flow, ion channel activity, and inflammation.¹³ Therefore, the aim of a neural tension evaluation is to test the mechanics and physiology of the nervous system.¹³ Findings of ANT may suggest the need for neural mobilization, which aims to facilitate nerve gliding, reduce nerve adherence, disperse noxious fluids, increase nerve vascularity, and improve axoplasmic flow.¹¹

Shacklock⁷ has elegantly summarized the connection between the musculoskeletal and nervous systems: Very simply, the musculoskeletal system is felt to be the mechanical interface to the nervous system. This interface occurs at both central and peripheral levels. Centrally, the mechanical interface consists of the cranial and spinal canals, which contain the central nervous system, cranial nerves, and spinal nerve roots. Peripherally, the nervous system interfaces with bone, muscle, joints, and other tissues. The critical concept for the clinician to recollect when evaluating pain is that as the body moves, the mechanical interface between the musculoskeletal and nervous systems changes dimensions, placing force and deformation on nerve structures, which may potentially generate pain or other symptoms.⁷

The nervous system must be able to adapt to mechanical loads and may undergo a variety of adaptations in order to do so, such as nerve elongation, sliding, cross-sectional change, angulation, and compression.¹¹ As an example, the SLR is a commonly used provocation test that has been shown to displace not only the lumbar nerve roots¹⁴^,¹⁵ but more proximal structures such as the conus medullaris as well.¹⁶^,¹⁷^,¹⁸ During elbow flexion, the ulnar nerve will lengthen while the median and radial nerves will shorten.¹² Additionally, sliding movements are also possible. The median nerve has a mean displacement of 2.09 mm with fist motion of healthy volunteers.¹⁹ Median nerve displacement is reduced in patients with carpal tunnel syndrome (CTS) when compared to normal volunteers, with smaller amounts of median nerve motion seen when comparing mild to severe CTS.²⁰

Several physical examination maneuvers are available to assist the clinician in determining whether ANT is present and related to the patient’s presenting complaints. Commonly used neural tension tests are detailed in the following text (images courtesy of Michael Schmidt, PT, DPT, and Preston Roundy, PT, DPT).

Lower Limb

Straight-leg raise: The SLR is among the most commonly performed evaluations of patients with neuropathic leg pain. Performance is straightforward: The patient is positioned supine. The knee of the symptomatic leg is kept extended, and the leg is flexed at the hip (Fig. 89.1).

A positive response will provoke the patient’s typical leg pain, frequently cited as being between 30 and 70 degrees.¹³

FIGURE 89.1 Straight-leg raise.

FIGURE 89.2 Median nerve A.

FIGURE 89.3 Median nerve B.

Upper Limb

Neurodynamic testing of the upper limb can be likened to straight-leg raising of the arm. The median, ulnar, and radial nerves can all be evaluated. Although passive positioning of the patient is possible, patients actively positioning themselves is most straightforward. If provocation of symptoms is not achieved with limb positioning alone, additional stress may be incorporated through neck or shoulder positioning.

Median nerve: Neurodynamic testing of the median nerve may be considered when upper limb pain is speculated to be neuropathic and possibly localized to the median nerve pathways, provoked with forearm pronation or supination, or nerve conduction testing suggests median nerve injury.¹³ The patient can be asked to look at the palm, extend the elbow, and then extend the arm until it comes level to the head. A positive test would provoke the patient’s typical arm symptoms at any step of this test (Figs. 89.2 and 89.3).

Radial nerve: Neurodynamic evaluation of the radial nerve may be considered when the patient presents with lateral elbow pain or a diagnosis of lateral epicondylitis. A very straightforward way to perform this test is for the patient to actively perform it. The patient can be instructed to hold the arm to the side, flex the wrist, internally rotate the arm, and look at the palm over the shoulder. Subsequently, the patient should be asked to depress the shoulder girdle (by “pushing the wrist to the floor”) and to laterally flex the neck (by “looking away”). A positive test would provoke typical lateral arm symptoms (Fig. 89.4).¹³

Ulnar nerve: Neurodynamic testing of the ulnar nerve is best performed with the elbow in flexion. The patients can be asked to look at their hand and hold it up as if they were carrying a tray of food at their shoulder. Additional provocation may be added by asking the patient to look away, add more elbow flexion, or depress the shoulder girdle (Figs. 89.5 and 89.6).¹³

NEUROMUSCULAR CONTROL

Motor control is often overlooked in musculoskeletal rehabilitation. A muscle or muscle group may be quite strong, but if it does not fire at the appropriate time (within a synergistic movement pattern), the proper movement pattern is already forfeited. The muscle might as well not fire at all. Neuromuscular control is a combination of sensory feedback from the body part, premotor planning, and motor execution.²¹^,²² Impairments in any of these pathways can lead to altered mechanics of movement with a resultant musculoskeletal dysfunction and pain.

This is commonly seen in persistent pain after ankle inversion injuries as well as patellofemoral pain. Normal neuromuscular control can be lost through injury or disuse and but can be regained through appropriate retraining.²³ In addition, the effects of neuromuscular training can be seen in those without pain, in the setting of sports performance enhancement independent of strength gains.²⁴

Proprioceptive neuromuscular facilitation uses predictable patterns to facilitate efficient muscle movement. These exercises are often prescribed as balance or proprioceptive in nature. Proper movement patterns require properly timed muscle activation. For example, when lifting an object, a person must first fire the foot and ankle muscles to stabilize the feet on the ground. Then, one must fire the thigh muscles to stabilize the
knee. Next, the hip girdle muscles must stabilize the pelvis before the spine extensors can elevate the torso. If the gluteus maximus fails to fire before the erector spinae fires, the pelvis will remain anteriorly tilted and abnormal motion will occur in the lumbar spine.

FIGURE 89.4 Radial nerve.

FIGURE 89.5 Ulnar nerve A.

FIGURE 89.6 Ulnar nerve B.

Corrective technique focuses on the pattern of movement rather than the overall strength gains, which can explain why traditional gym exercises do not necessarily correlate to improvement in pain. This is a key concept in explaining why use of physical and occupational therapy for pain syndromes can be helpful even in the active population.

BIOMECHANICAL CONSIDERATIONS IN THE SETTING OF COMMON PHYSICAL EXAMINATION TECHNIQUES

In the setting of pain, the goal of any proper patient examination is to not only elucidate the painful complaint but also hypothesize causation of pain. In doing so, one must consider the biomechanical nature of musculoskeletal pain syndromes. Basic tenets such as inspection, palpation, and ROM may be highly informative and help achieve a more comprehensive interventional plan.

Inspection: Evaluation of the patient begins with visual inspection. Pattern of shoe wear, assistive device usage, brace needs, general posture, and habitus as well as gross movement patterns during standing, walking, and transfers are significant considerations. These observations provide an overall view of how the patient moves about his or her environment and help the clinician to discern what may be gross mechanical abnormalities.

Palpation: Palpation provides the clinician with information about generalized tenderness, potential targets for injection, tissue texture, and also pain patterns. Kinetic chain abnormalities will often present with palpable pain patterns. For example, patellofemoral pain has been associated with iliotibial band, hamstring, and gastrocsoleus restrictions as well as leg length discrepancies, hip muscle dysfunction, overpronation, patellar malalignment, and patellar hypermobility.²⁵ Given these biomechanical correlations, a patient with patellofemoral pain has the potential to demonstrate tenderness about the ipsilateral SI joint, greater trochanter, iliotibial band, pes anserine, or medial tibial structures as well. The clinician is thus cued to treat distant areas of dysfunction in order to affect pain at the site of concern.

Range of motion: Lack of adequate ROM may be the most common biomechanical deficit seen and one of the easiest to address. Deficits in ROM can cause pain directly or may lead to pain elsewhere. The musculoskeletal clinician must have knowledge of normative values, employ adequate examination skills to identify major ROM limitations, decide which of these contribute most significantly to the patient’s symptoms, and implement appropriate therapies to correct the deficits. For example, during gait, an ankle plantar flexion contracture can cause excessive knee extension and resultant increased hip flexion and forward trunk lean. According to kinetic chain theory, one must address ankle mechanics to normalize the dysfunction at the knee, hip, and spine. This may be required to facilitate adequate pain intervention.²⁶ ROM restrictions can be due to shortening of muscle-tendon units, restrictions in joint capsule distensibility, ANT, or bone-on-bone contact at joint interfaces. The first three conditions are almost always amenable to specific stretching techniques. ROM deficits due to bone-on-bone contact are generally not improved with therapeutic exercise or physical modalities but at times can be addressed by compensatory strategies.

In addition, quality of ROM can be assessed in the setting of functional movements. For example, functional forward bending is a combination of hip, pelvis, and a spinal movement. This is often clinically evaluated by having a patient bend forward and measuring the distance between the patient’s fingertips and the floor. A subject with restricted hamstrings and gluteal muscles will present with increased lumbar and thoracic flexion to reach as low as a subject with highly flexible hamstrings and gluteal muscles. This second subject may exhibit minimal motion through the spine as he or she is able to achieve the required amount of “bend” using predominantly hip flexion.

Strength: Strength assessment is another part of comprehensive patient examination. Whether it is through manual muscle testing (MMT), or a more formal dynamometric assessment, the goal of strength testing is threefold: (1) determining etiology of strength deficits (i.e., deconditioning, pain inhibition, poor neuromuscular control; these often improve with repeated cueing) or neural deficit, (2) painting a clinical picture of overall strength, and (3) demonstration/determination of kinesiophobia (an important factor in chronic pain). As a general rule, the patient should be examined with regard to muscles surrounding the region that is painful and in the regions immediately proximal and distal to the painful region. For example, in a patient with elbow pain, the clinician should evaluate strength in the shoulder, elbow, and wrist. Given that strength deficits are relative, contralateral strength values should be used as an internal control.

Additionally, because many pain complaints are concerning for an underlying spine etiology (e.g., a radiculopathy), it is often useful to examine the key myotomes in the affected region. Using the same example of elbow pain, the clinician would evaluate muscle strength of the C5-T1 myotomes bilaterally to assess for focal weakness. These have been covered earlier in this book.

Admittedly, MMT is a faster method than more formal strength measurement such as dynamometry or manual sphygmomanometry. However, one must acknowledge that the MMT grading scale of 0 to 5 consists only of ordinal values. These values represent directional relationship only and should not be used as replacement for interval or ratio measures, such as torque, force, or percentage of muscle activity. In fact, Beasley²⁷ noted that a knee extensor strength loss of 50% is needed before clinicians reported a grade change within MMT.

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