In this bulletin we continue our super-feature on muscle and will discuss somekey concepts of muscle physiology. Sofar we’ve covered muscle anatomy, ul-trastructure and biochemistry, as well ascontrol of muscle tissue by the nervoussystem. In final three parts of this serieswe’ll discuss metabolic adaptations ofmuscle to exercise and how to design ef-fective training strategies to achieve yourgoals. Please refer back to your old is-sues of the Parrillo Performance Press forour articles about hormones and cellularenergy metabolism, as these tie in directlyto muscle metabolism and physiology.In Part 2 I introduced the concept ofthe motor unit: a lower motor neuron(nerve cell) in the spinal cord plus themuscle fibers that it controls. I said thata motor unit fires according to the all-or-nothing principle. That means it eitherfires at full power or not at all. There’sno such thing as partially contracting amuscle fiber, or it contracting at mediumintensity. What determines the strengthof a muscular contraction is then howmany motor units are recruited to fire(contract).
This month we will extendthis concept to the next level, and talkabout patterns of muscle fiber recruit-ment.Nerve impulses traveling down theaxon of a motor neuron (nerve fiber) to amuscle cell travel in discrete bundles calledaction potentials. An action potential isan electrical signal that is carried alongnerves that stimulates muscle fibers tocontract by triggering the release of cal-cium from the sarcoplasmic reticulum.Rather than being like the continuous flowof electricity that is delivered from a bat-tery, an action potential is a short burst orpulse of electricity, like flipping a switchon and then quickly off again. An ex-ample you may be familiar with is an EKGtracing of the heart – this is the actionpotential of the heart muscle.Each action potential results in a shortperiod of activation of the muscle fiber,and is referred to as a twitch (1). Thecalcium released during a twitch is suffi-cient to allow optimal activation of actinand myosin, and therefore maximal forcedevelopment by the muscle fiber (1).
However, as the contracting muscle fi-bers begin to pull on the tendons and “takeup the slack,” pumps begin pumping thecalcium back into the sarcoplasmicreticulum. Thus the muscle fibers beginto relax before the muscle has time togenerate maximal force on the tendons.So while a twitch stimulates a muscle fi-ber to contract maximally, it begins to re-lax before maximal force is generated bythe muscle (1). If a second action poten-tial (nerve impulse) arrives at the musclefiber and causes another twitch before thefiber has completely relaxed, the forcefrom the two twitches summates (addstogether) to generate a greater force thanfrom a single twitch (1). As we increasethe frequency of action potentials we willdecrease the rest period between twitches,and the summation of force increases (1).At a high enough frequency of stimula-tion the twitches fuse (that is, there’s notime for the fibers to relax), and forceproduction reaches a plateau called tetany.This is the highest force that a motor unitcan produce (1).A given muscle is composed of sev-eral different fiber types. There are manyclassification schemes for describing dif-ferent muscle fiber types. The first ap-proach is to classify muscle fibers accord-ing to twitch time.
You’ve probably heardof slow-twitch versus fast-twitch fibers.A fast twitch fiber (also termed type 2)develops force rapidly and has a shorttwitch time (1). A slow twitch fiber (alsotermed type 1) develops force slowly andhas a long twitch time (1). This results indifferent abilities to develop force and re-sist fatigue. Slow twitch fibers generallyare fatigue resistant and have a high ca-pacity for aerobic energy production (re-fer back to our series on cellular energymetabolism). This makes them ideal forlow energy activities that you need to beable to sustain for a long time, like walk-ing or standing or keeping the spine erectwhile sitting. Fast twitch fibers, in con-trast, are easily fatigued, are relativelypoor at aerobic energy production, but areable to generate tremendous forces veryrapidly. Fast twitch fibers are further sub-divided into two subtypes, 2a and 2b, ac-cording to differences in ATPase activity(2).Perhaps the oldest classificationscheme is based on gross appearance. Inthe early 1800’s it was noticed thatmuscles range in color from deep red topale white (2). This is most easily ob-served when looking at the muscles ofbirds, where the differences are the great-est. It is now understood that somemuscles are red because they containgreater capillary density and more myo-globin (an oxygen storing molecule likethe hemoglobin found in red blood cells)and mitochondria (the little furnaces in-side cells where food molecules areburned to produce energy).
These prop-erties (more capillaries, myoglobin, andmitochondria) make red muscle fibersbetter at aerobic energy production.White muscle fibers have less myoglobinand mitochondria but more stored glyco-gen, making them better at anaerobic en-ergy production. The difference is easilyseen when comparing a chicken breast toa chicken thigh. A chicken breast is whitemeat (white muscle fibers) and a thigh isdark meat (red muscle fibers). These dif-ferences make sense if you think about it.The breast of a bird is involved in beatingthe wings during flight, which requires ahigh level of force production. The thighs,however, are involved in weight supportand walking, requiring a lower level of force production.Another classification scheme is basedon metabolic and histochemical (micro-scopic staining) properties of musclecells. This is basically an extension ofthe fast twitch/slow twitch scheme, butalso takes into account fuel types preferredby different fiber types. According to thisscheme, fibers may be either slow oxida-tive (SO), fast glycolytic (FG), or fastoxidative glycolytic (FOG) (2). Thisscheme is based on microscopic analysisof fibers looking at various enzyme sub-types (such as subtypes of ATPases) andit gets real technical real fast.
For thoseof you interested in greater detail, the bestdiscussion of muscle fibers types is foundin Lieber, Skeletal Muscle Structure andFunction, pages 70-89 (2).In summary: Within any given muscledifferent fiber types exist for performingdifferent functions. There are many waysto classify the different fiber types. Theseinclude twitch time, muscle color, fuelsources, enzyme subtypes, fatigability, andcombinations of the above. A compari-son of different classification schemes ispresented in the table.In general, slow twitch (ST) fibershave a high level of aerobic endurance (3).ST fibers are thus very efficient at pro-ducing ATP from the oxidation of carbo-hydrate and fat (3). As long as oxygenand fuel are available, ST fibers can con-tinue to produce ATP and thus the energyto contract. ST fibers are therefore pref-erentially recruited for low intensity ac-tivities like walking, jogging, or biking.Fast twitch (FT) fibers, on the other hand,are better suited for anaerobic energy pro-duction. This is largely through the con-version of stored muscle glycogen to lac-tic acid via glycolysis.
FT motor unitscan generate considerably more force andpower (work per unit time) than ST mo-tor units because their rate of force pro-duction is not limited by the rate of oxy-gen delivery. Furthermore, FT motor unitsare generally larger (contain more musclefibers) than ST units. FT motor unitsfatigue easily however, because they ex-haust their fuel supply (and other inter-mediates) and build up lactic acid. Whenthe acid level builds up too high in the cell,this shunts down the cellular enzymes thatproduce energy, so contraction comes toa halt. FT fibers are thus best suited forbrief, high intensity activities such assprinting. During extremely high inten-sity exercise, such as weight lifting, bothST and FT units are recruited to maxi-mize force production.Remember that force production withina muscle is increased by increasing thefrequency of action potentials arriving atthe muscle, and thus the frequency oftwitches, and by increasing the numberof motor units participating in the con-traction. When only a little force isneeded, only a few motor units are re-cruited. Recall also that FT motor unitscontain more fibers than ST units. There-fore, when only a little force is needed,small motor units, which are primarily theST type, are recruited (3). As exerciseintensity increases, FT type 2a (corre-sponding roughly to FOG type fibers) arealso recruited. At maximal intensity, FTtype 2b (or FG) fibers are called in.This gets us back to the concept ofintensity threshold that we talked about inthe first article of our muscle series.
Ex-ercise must provide a high intensity stimu-lus in order to recruit all of the fibers. Ilike to refer to these as the “high thresh-old nerve pathways.” It is of great im-portance to realize that the fast twitch fi-bers are the ones with the greatest poten-tial for hypertrophy (growth). In orderto stimulate these fibers to grow we mustrecruit them to contract, and to do thatwe must apply a high intensity stimulus.This is why you have to lift big weightsto get big muscles. Curling 5 pounddumbbells all day will never give you bigbiceps.So for maximal muscular growth abodybuilder has to perform at least fourdistinct types of training: 1. Drop sets toensure that nearly 100% of muscle fibersare recruited. 2. Heavy sets around 1-3rep maximum to recruit the high thresh-old nerve pathways. 3. High intensityaerobics (around 30 minutes 3 times aweek) to increase capillary supply ofmuscles. 4. Standard “bodybuilding sets”carried to failure at 8-10 reps. I’ll explainmore about this in the future, but the ba-sic function of these is to induce local tis-sue trauma which serves as a stimulusfor inflammation and remodeling. In themedium rep work (8-10 rep range), spe-cial attention must be paid to going to fail-ure and to resisting the weight during theeccentric (lowering) phase of the contrac-tion.
1. Baechle TR. Essentials of StrengthTraining and Conditioning. Human Kinet-ics, Champaign, IL, 1994.
2. Lieber RL. Skeletal Muscle Struc-ture and Function. Williams and Wilkins,Baltimore, MD, 1992.
3. Wilmore JH and Costill DL. Physiol-ogy of Sport and Exercise. HumanKinetics, Champaign, IL, 1994.