Chapter 1 Introduction

Muscle fibers are necessary but not sufficient causes for human movements. Movements include an intricate combination of active and passive tension that is created by muscle fibers. However, muscle fibers cannot create tension for movement in isolation. First, muscle fibers must come together as a muscle and combine their ability to create tension to create movement. They create tension with interaction with attachments, mainly, to bones¹¹1Mostly to bones because sometimes muscles attach to something other than bones such as the eye muscles or facial muscles that attach to skin to create facial expressions, or muscles that attach to themselves. that form joints that are capable of movement. The process of coming together, creating tension and transmitting tension to the bones is covered in the next two chapters (Chapter 2 on Fundamentals & Chapter 3 on Tension). Other aspects of movement such as the mechanics and kinematics of the musculoskeletal system, or the coordination, control, and learning of the neuromotor and behavioral systems are not covered in this book. To create tension, muscle fibers need to have the ability to be excited (Chapter 4 in Excitation), be regulated to create the right amount of tension (Chapter 5 in Regulation), and convert energy (Chapter 6 in Energetics). It is important to note that these capabilities are characteristics of muscle fibers. They exist in muscles because they exist in muscle fibers.

The capability to create tension requires support. Support is received from the surroundings of the muscle fiber, the extra cellular fluid (ECF). ECF is supported and tightly regulated through the activity of several physiological systems. In this sense, as depicted in Figure 1.1, the ECF supports muscle tension.²²2It is admittedly much more complicated than this and the use of the term causes can be challenged, but the assertion is that without the proper ECF muscle active tension cannot occur. It is true that muscle passive tension can occur with a passive stretch of the muscle but such a passive stretch is typically the product of another system putting energy into the muscle to create the passive stretch and that other system putting energy into the muscle often relies in some way on ECF. When a muscle fiber creates tension it utilizes and exchanges resources with the ECF, thus muscle cells thus rely on and contribute to ECF. These systems that refresh and regulate the ECF are the focus of several chapters in Part III on Muscle Support.

Question: Questions

What are some substances that enter a muscle fiber from the ECF for it to do what it needs to do as a muscle fiber? What are some substances that leave the muscle fiber and enters the ECF while it is doing what it does?

The ECF contains and sustains the correct materials for supporting and supplying muscle fibers. The ultimate source of materials in the ECF is movement (Figure 1.1), meaning, movement supports the ECF. It may be the movement of the heart, the movement of the arterioles that regulate blood flow, the movement of the breathing muscles that move air into and out of the lungs, the movement of limbs to get food and water, the movement of the jaw and esophagus and gastric tube to digest and move food and water for absorption, the movement of the colon for waste products. These have all been examples of movements caused by muscle tension in support of the ECF that is ultimately supporting the muscle fibers causing the muscle tension. Conversely, the ECF directly supports movement beyond bathing muscle fibers in a supportive environment by providing the environment for all tissues to work together for coordinated movement of the entire body.

Figure 1.1 constitutes a model. Models are simplified abstractions of reality. This model does not include all the details. It is not supposed to include all the details. If it included all the details it would be overwhelming. Hashing out just some of the detail requires the rest of this book. Hashing out more of the details includes most of your education. Hashing out all of the details is not currently possible and requires more than one lifetime.³³3The lifetime of many scientists has already been spent on hashing out the details that we currently know. Some may say that all the details is computationally intractable considering the depth of mechanisms at the molecular level. Hashing out the details we continue to create and consider additional models, over and over again, models of models of models of models as we successively approximate something useful to understand and knowledge to base practice.

There are numerous models throughout this book. Every figure provided is a model. With each model we consider a quote by George Box: “All models are wrong some models are useful.” The muscle centered approach is our first step. Three variables and interrelationships provide a model of clinical physiology as a guide to reasoning for physical therapists in the context of injuries, conditions, and diseases. This is a useful model. There are certainly going to be situations that don’t fit the model since not all details are contained in the model. But for the task of learning clinical physiology to practice physical therapy, it’s useful.

Question: Questions

What are some models that have been useful to your learning up to this point? Can you think of any useful models in physical therapy practice? Can you think of some reasons that models are useful for clinical reasoning?

Clinical physiology from a muscle centered approach starts with a simple model. Based on this model if a PT considers muscles as a cause of movement but doesn’t consider the factors that influence the ECF that is critical to the excitation, regulation, and energetics⁴⁴4The convention we will utilize is that word ”energetic” is an adjective, something is energetic, marked by vigor or effect, related to energy. However, adding an ’s’ to the end of energetic, energetics changes it to a verb, the process of converting energy and should not be confused as a plural form of the adjective energetic. of muscle fibers then they are ignoring a critical component. If a PT considers how the ECF influences muscle fibers and thus movement and does not consider how continued movement influences the ECF they may miss the opportunity to stop repeated hospital admissions.⁵⁵5Such as readmissions due to repeated breakdowns in a person that goes home, cannot move enough to sustain hydration or nutrition or ventilation and returns to the hospital dehydrated, malnourished and with pneumonia [collins_heart_2015] Considering these relationships is a first step to thinking about clinical physiology with a muscle centered approach for the purpose of practicing physical therapy.

Living organisms have causal mechanisms whose functions are explainable by a description of cause-effect relationships. Identifying and specifying causal relationships and models (to understand mechanisms) are an important part of physical therapy practice [collins_synthesis_2018]. Causal mechanisms are learned through rigorous manipulation and observation (experiment) or systematic observations (epidemiology) and subjecting these observations to statistical inference. There is also an assumption that underlying any effect we observe there is a cause, or a set of inter-related causes. This is an essential aspect of causality in physiology.

Genes (DNA) code for the synthesis of proteins (muscle filaments and enzymes are proteins). This is not all the genome⁶⁶6All the genes together or the epigenome⁷⁷7Above the genome, the meta genome, does, but it is an important part of muscle cell differentiation, specialization and myostasis. Genes in a particular cell that are expressed determine the functions of that cell (e.g., make it a muscle fiber and not, say, a liver cell), and additionally can make it a particular type of muscle fiber (e.g., a fast or slow twitching muscle fiber).

The life of the organism requires the constant expenditure of energy. The acquisition, transformation, and transportation of energy are essential functions. Experimental approaches to understand muscle are focused on muscle fibers as energy conversion machines, taking chemical energy and converting it into mechanical energy [woledge_energetic_1985]. The earliest of these studies measured work and heat based on the physical law of the conservation of energy. Energy is neither created or destroyed. Energy is transformed from one form to another. When a muscle fiber breaks down adenosine triphosphate (ATP) to generate tension, as discussed in Chapter 6 on Muscle Energetics), the energy of that chemical bond is transformed to mechanical work (force x distance) or as heat [hill_heat_1938].

The quantity of ”stuff” in any system, or in a compartment of a system, is determined by the inputs into the system and the outputs from that system or compartment. This concept is based on the law of conversation of mass. When someone consumes 50 grams of carbohydrate (CHO) those 50 grams are then part of them and will be processed (digested, metabolized and either eliminated, stored or transformed).

The transport of “stuff” (ions, molecules, nutrients, blood, and gas) is a central process at all levels of organization in the organism. A Ohm’s Law is a simple model that describes such transport.

Ohm’s Law describes the relationship between current, voltage and resistance:

For fluid transport, such as blood through vessels and air through the airways we consider:

For diffusion across membranes we consider:

Background: Important Concept

Current, flow and diffusion are proportional to the voltage, pressure or concentration gradient (numerator), and inversely proportional to resistance (denominator). You must understand these questions before proceeding. Ohm’s Law can be expanded for additional context and explanation. For example, considering the factors that influence resistance of blood or air flow (expanding the denominator) provides additional context and explanation for the regulation of cardiac output. Ohm’s Law is an often utilized equation throughout the book. You should practice manipulating it. Above it is solved for current. Manipulate it to solve for voltage. Then manipulate it to solve for resistance.

Tip Equations and laws are an extremely helpful way to consolidate and summarize your understanding about concepts. Concepts have universal applicability. When you are reasoning in practice you are situated at one time, in one place, with one situation (a particular situation). The more universal concepts you can relate to a particular situation the more you understand that situation (even when you have never before been in that situation). It is recommended that write down the equations (separate from the book) so you can refer back to it easily while reading.

The internal environment of an organism is actively maintained ”relatively constant” by the feedback function of cells, tissues, and organs. This is organized into primarily negative feedback systems.
Important points about homeostasis:

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  Not everything is regulated (i.e. heart rate)

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  Homeostasis is not an on/off switch

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  ”Relatively constant” – meaning there can be acceptable variation

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  Set points can change

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  There is a hierarchy of homeostatic regulation

The hierarchy of homeostatic regulation includes the fact that cells manage their internal environment (homeostasis), which includes taking and giving molecules and water to the ECF, the ECF is kept in homeostasis via exchange with the blood volume (intra-vascular ECF) and homeostasis of the blood volume (water, cells, electrolytes, nutrients, etc) is exchanging with organ systems (lungs, liver, pancreas, gut). A simple graphic model of glucose homeostasis is shown in Figure 1.2 and shows the interactive relationship between glucose, glucagon and insulin.

Figure 1.3 below provides a more complete model of glucose regulation. It includes multiple mechanisms that glucose can be regulated and the role of metabolic organs such as the liver and the endocrine pancreas.

Cells, tissues, organs, organ systems interact with one another (are dependent on the function of one another) to sustain life. Interdependence includes hierarchies of homeostatic regulation. The function and capacity of the whole system is related to the interdependent relationship between the parts, capabilities and attributes of each of the multiple systems.

The function of a cell, tissue, or organ is determined by its form. Structure and function (from the molecular level to the organ system level) are intrinsically related to each other. Structure influences function (right now) and function influences structure (eventually). Part II is how muscle structure influences function right now. Part IV includes how function influences structure eventually (hypertrophy and atrophy).

A hierarchy of adaptation and adaptability works within and around physiological systems. Adaptation refers to changes that occur to the system so that it optimizes function. A muscle fiber creates tension. Adaptation refers to changes that occur so a muscle fiber can attain, sustain and maintain the ability to create tension. Adaptability refers to the ability to adapt. The hierarchy includes genetic, epigenetic, anatomic, physiologic, behavioral and cultural changes. Cultural changes being a mechanism of passing behavioral adaptation from one generation to another. This entire hierarchy is involved through the life span and has relevance to physiological adaptation. Physiological adaptation includes genetic, epigenetic, anatomic, physiologic mechanisms and include changes to structure and function. Behaviors are enabled by our physiology, but then also influence our physiology (our physiology allows us to eat sugar, and then our eating of sugar influences our physiology). And the culture around us gives rise to acceptable, and not acceptable, behaviors.