All about Muscle

This column will be somewhat academic or educational as I respond to many requests to clarify the structural and functional nature of muscle. The “micro-anatomy” and the physiology of muscle is a fascinating study. Animals, including humans use muscle to convert the stored chemical energy found in food into the molecule ATP that in turn is changed into mechanical work.

The skeletal muscle is the basic instrument of mobility and it is unique in many ways. Although most cells are 50 - 70% water, the dry weight (after removing all water) of a muscle cell is 75% amino acids (protein building blocks). Unlike other cells, those you have seen diagrammed in health education textbooks and magazines, the muscle cell also known as a fibre or fibril can vary in diameter from 10 microns to over 100 microns. (the diameter of an average human hair is 5 microns).

A single muscle is made of thousands of fibres. Muscle fibres can vary in length from a few millimetres to 30 centimetres (almost a foot) or longer. Muscles fibres are multi-nucleated cells (they have many nuclei whereas most cells have only one) and they contain numerous mitochondria (the oxygen using power-house where energy is transformed from the chemical storage form of ATP into the kinetic version for useful work) allowing a muscle to increase its oxidative capacity by 50 fold in a very short time.

Such variation in energy expenditure requires the “support organ systems” to not only supply enough oxygen and nutrients for this increased metabolic demand, but some of the systems must adapt to provide for heat dissipation and removal of waste products generated by this metabolism. Therefore muscle function depends on the support of the cardiovascular, respiratory, digestive, and excretory systems.

There are three different kinds of muscle in the body:

• Heart Muscle or cardiac muscle makes up the wall of the heart. Throughout life it contracts some 70+ times per minute and pumps about 5 litres of blood per minute. Obviously these numbers increase during exercise and stress. Control of the rate is due to a complicated series of bio-chemical reactions in the body.
• Smooth muscle is found in the walls of all the hollow organs of the body (except the heart). Its contraction reduces the size of these structures thus regulating blood flow, expelling wastes, moving food through the digestive tract. Contraction of smooth muscle is also controlled by the “autonomic” nervous system, which is not under conscious control.
• Skeletal muscle is the muscle(s) that give us locomotion, posture control structure and shape. Also called “striated muscle” due to the microscopic structures exhibited. Skeletal muscle is primarily under voluntary control (exceptions occur in reflex reactions) and obviously from the name is attached to the bones of the skeleton. The attachments occur via ligaments, which the muscle fibres attach to at both ends of the muscle. One end of the attachment is known as the “origin” and the other end is the ”insertion”. Most skeletal muscles stretch across a joint and contraction (shortening) of the muscle causes movement of the two bones that make up the joint with the effect being movement.

A skeletal muscle is made of many fibres and each fibre is attached to a single motor nerve (a nerve carrying a signal for action). Each fibre is anatomically separated from the neighbouring fibres by a thin membrane and that allows the nervous system to control how many fibres are stimulated for any given action. Without this separation the muscle would respond the same to lifting a cup of tea or a 100-pound weight.

In man it is estimated that the number of muscle fibres in a muscle group is finally established after the embryo has reached the age of 4 – 5 months. Increased strength and muscle mass comes about through an increase in the thickness of the individual fibres and increases in the amount of connective  tissue within the muscle. At birth the fibre is about twice a thick as in the 4th fetal month, but only one-fifth of the adult thickness.

Each muscle fibre (called a cell even though it is actually many cells fused together) contains many nuclei, many mitochondria, an elaborate collection of “organelles” within the liquid part of the cell known as the endoplasmic reticulum and a collection of “myofibrils” that are the functional units of contractions.

Myofibrils are stacked lengthwise and run the entire length of the fibre. Each myofibril is made up of arrays of parallel filaments.

• The thick filaments have a diameter of about 15 nano-microns. They are composed of the protein myosin.
• The thin filaments have a diameter of about 5 nm. They are composed chiefly of the protein actin along with smaller amounts of two other proteins: troponin and tropomyosin.

The act of contraction occurs when the nervous system sends a signal to the membrane of the muscle fibre that changes chemistry causing the actin and myosin filaments to move closer together (identified as the “sliding filament model of muscular contraction). If the stimulus is strong enough it affects a collection of filaments all at the same time causing a shortening of the muscle and movement.

The proteins making up the filaments are made of collections of amino acids. The majority of muscle filaments are made of amino acids of which 1/3 are known as the “Branched Chain Amino Acids” (BCAA’s) Leucine, Isoleucine and Valine.

They are called “branched chain” because of their molecular configuration. These BCAA’s are precursors to many metabolic functions in a working muscle and are also metabolized to produce energy.

Consequently muscle training requires an adequate supply of amino acids in the form of high quality protein containing a good profile of BCAA’s.

Therefore research has shown that for optimum muscle recovery (that is energy and avoidance of muscle soreness) requires the consumption of adequate BCAA’s immediately following heavy exercise.

To build more muscle (that is increase the diameter of the muscle fibres) the stimulus of increased intensity and the supply of adequate protein must be optimal. New muscle is not built quickly or by protein alone.

The combination of adequate, complete protein and progressive resistance exercise produce the optimum results. The trick is to match the protein intake to the training program.

A study from the Letterman Army Institute gave one group 1.4 gm protein per kg of body weight (0.63 gm/pound) and a second group 2.8 gm protein per kg of body weight (1.27 gm/pound) in 3600 Calories.

They trained to near exhaustion daily for 40 days. The lower protein group gained 1.21 kg (2.66 pounds) of lean mass whereas the higher protein group gained 3.28 kg (7.22 pounds) of lean mass.

The range of muscle growth in this extreme study was 1 ounce to 2.8 ounces of new muscle daily. The typical capacity for serious muscle training lies in the range of 1 ounce to 1.5 ounces per day or 22 to 35 pounds in one year. Don’t be fooled by advertising that makes bigger claims.

My personal recommendations regarding protein requirements are:

1. Class 1 athletes, where strength demand is first, speed is second and endurance is third, require 2.75 gm protein/kg of desirable weight per day (1.25 gm/pound).
2. Class 2 athletes, where speed is first, then strength, then endurance, require 2.2 gm protein / kg desirable (1.0gm / pound) and
3. Class 3 athletes, where endurance dominates, require 1.75 gm / kg desirable weight (0.8 gm / pound).

Further Reading

Muscle Recovery and Downhill Skiing - Yes, I finally “walked my talk”. I have understood the science for years but for some reason never thought about it as a way to end a full day of activity. I encourage you to try it.