In Part 3 of our series about musclewe are going to take a microscopic lookat exactly what happens inside a musclecell when you lift weights. First, let’s re-view some basics about how muscles arecontrolled by the central nervous system.The first thing that happens is thatyou decide to lift a weight. This happensin the frontal lobe of the brain, where con-scious thought occurs. The frontal lobesends a signal to the prefrontal gyrus, ormotor strip, of the brain. You see, eachmuscle cell is controlled by a chain of twoneurons, or nerve cells. The first one isin the brain, in the motor strip, and is calledthe upper motor neuron. The second isin the spinal cord, and is called the lowermotor neuron. The upper mo-tor neuron is a very long cell,and sends a cellular process (along extension) called an axoninto the spinal cord. There itmakes a contact called a syn-apse with the lower motor neu-ron.
The upper motor neuronreleases a chemical called a neu-rotransmitter into the synapticspace, which then binds to a re-ceptor on the lower motor neu-ron. Neurotransmitters can ei-ther be inhibitory or excitatory.The balance of inhibitory andexcitatory neurotransmitters is what de-termines if the lower motor neuron firesor not. If the lower motor neuron re-ceives the signal to fire from the brain, itin turn sends a signal out to the musclecell. Lower motor neurons are also verylong cells—sometimes over three feetlong! The body of the lower motor neu-ron is in the spinal cord, and its axon iscalled a peripheral nerve. Muscles in yourfeet are thus controlled by a chain of twonerve cells, one that has its body in thebrain and sends its axon all the way to thelower spinal cord, and the second whichhas its body in the lower spinal cord andsends its axon all the way to the foot.Before a message is sent out from themotor strip in the brain, other parts of thebrain are also contacted to help performthe computations necessary for goodmuscular control.
The motor strip con-tacts the basal ganglia, which helps themuscle contract in a smooth, controlledfashion. This part of the brain helps getjust the right balance of excitatory andinhibitory nerve impulses, so the weightmoves smoothly and under control. With-out the basal ganglia, the muscle wouldsometimes contract too hard and some-times not hard enough, and the weightwould jerk up and down. People withParkinson’s disease have a problem in thebasal ganglia, and have tremors when theyuse their muscles. The frontal gyrus alsocontacts the cerebellum, which helpsmaintain balance and coordination. With-out the cerebellum, you would lose yourbalance and fall over if you picked up a50 pound dumbbell with one hand. Soafter you decide to lift a weight in the fron-tal cortex, the motor strip has a telecon-ference with the basal ganglia and the cer-ebellum to help do the calculations so thatthe weight is lifted under control in asmooth fashion and you don’t lose yourbalance. After all the computations aredone, which happens almost instanta-neously, the motor strip sends its signalto the lower motor neuron.One lower motor neuron can controlanywhere from one to several hundredmuscle cells. Some muscles, like thosecontrolling the fingers and the eye, areunder very fine control, so that each lowermotor cell controls only one or a fewmuscle cells.
Other muscles, like thosein the quadriceps or glutes, don’t requiresuch fine control and each lower motorcell may control hundreds of these musclecells. A lower motor neuron and themuscle cells under its control is called aMOTOR UNIT. This is a very importantconcept, not just for muscle physiologistsbut for bodybuilders too. When a motorunit fires, it is an all or nothing phenom-enon. This means that either all or themuscle cells controlled by thatmotor neuron fire, or none ofthem do, depending on the bal-ance of excitatory and inhibi-tory impulses arriving at thatnerve cell. There’s no suchthing as a muscle cell partlycontracting, or contracting atmoderate intensity. It eithercontracts completely, at fullpower, or not at all.You will recall that a givenmuscle, like the biceps of thearm for example, is made ofhundreds of individual musclefibers, or muscle cells. The strength of amuscle is defined by the maximum weightyou can lift for one repetition, the one repmaximum (1RM). The strength of yourbiceps is determined by a combination offour general parameters: 1. The numberof muscle fibers in the muscle. 2. Thesize of each individual muscle fiber. 3. Thenumber of muscle fibers you can stimu-late to fire (contract) at once.
4. Lever-age factors, such as the length of yourbones and the points of insertion of themuscle tendons onto the bones. You can’tdo anything about the leverage factors,this is purely genetics. All other thingsbeing equal, someone with better lever age factors will be stronger. Genetics isvery important in athletics, includingpowerlifting, for this reason.You can, however, address the otherthree factors by using specific trainingtechniques. The strength of your bicepsis determined not only by the size of themuscle itself, but by how many of themuscle fibers you can make contract atthe same time. Remember, each musclefiber either contracts completely or not atall. Let’s say for example thatyour single rep maximum in thedumbbell curl is 50 pounds. Nowyou pick up a 5 pound dumbbelland begin curling. Only about10% of the muscle fibers in yourbiceps are contracting, and theother 90% are just along for theride. If you keep curling longenough, the 10% of the fibersyou started with will eventuallyfatigue and no longer be able tocarry on. Then a different set ofmuscle fibers will take over thework while the first set rests. Bythe time you go through all of themuscle fibers, the first set is wellrested and is ready to go again.This is why you can maintain lowintensity work for a very longtime. Now let’s pick up a 25pound dumbbell and do a set ofcurls. Here, we have to fire about50% of the fibers to lift the weight. Aftera few reps these fibers are tired, and theothers take over.
After about 15 or 20reps all of the fibers are tired and you can’tget any more reps. The first set of fibersto fire didn’t get enough time to rest andaren’t ready to go again yet. This is whyyou can’t get as many reps as you couldwith the lighter weight. Now let’s con-sider curling a 50 pound dumbbell, yoursingle rep maximum in this example. Yourecruit 100% of the fibers to fire, so theyall get “spent” after one rep. There areno other fibers left to recruit, which wouldallow the tired ones to rest, so the set isdone after one rep. Read on, becausehere’s where it gets interesting.You should realize that the above ex-ample is not quite accurate. Here’s why:very few people, if any, actually have theability to contract all of their muscle fi-bers to fire at once. The best estimatesare that a typical person only has the abil-ity to fire about 50% of his muscle fibersat once, and that with training this mayincrease to about 70%. This is kind of asafety mechanism to make sure you al-ways have some strength left, even if it’sonly a little. (It also helps prevent youfrom ripping the tendons off of the bones!)Everyone knows after going to failure ona heavy set it’s still possible to crank outa few more reps with a lighter weight.This means that there must be a few fi-bers left.
This is the physiologic basisfor drop sets, or strip sets. Start with aheavy weight where you can get about 5reps and go to failure. Then pick up amoderate weight that will allow you to getabout 10 more reps and go to failure again.Finally pick up a very light weight and goto failure at about 15-20 reps. By the timeyou’ve finished a triple drop set you willhave recruited virtually 100% of the fi-bers in that muscle. There’s really noother way to do it. Don’t rest at all be-tween sets, because that would allow thefibers to recover. The point here is toNOT allow the first set of fibers to re-cover, which FORCES the muscle to re-cruit the other fibers that haven’t fired yet.Drop sets are a very effective way to in-crease both size and strength and shouldbe a part of every bodybuilder’s andpowerlifter’s program.You see, when you lift weights you’renot only training your muscles, you’re alsotraining your nervous system. With prac-tice you can learn to recruit more musclefibers to fire at once, thus increasingstrength. This is a key differ-ence between bodybuilding andpowerlifting. As we mentionedbefore, bodybuilders generallyhave bigger muscles butpowerlifters can usually liftmore weight. Powerlifters aregenerally stronger because theywere born with better leveragefactors and because they havetrained their nervous systems’to recruit more motor units tofire simultaneously. The wayyou do this is by practice. Lift-ing very heavy weights, in the1-3 rep maximum range, forcesyour body to fire more musclefibers at once.
This makes youstronger. And this is why lowrep work forms the basis forpowerlifting-style training.Powerlifters train with explosivemovements using very heavyweights, and often don’t care about thenegative portion of the exercise. (Youmight see a powerlifter virtually throw theweight on the floor after completing aheavy snatch, for example.) This is notthe most effective training style for in-creasing muscular size however. To un-derstand the basis for training to increasemuscle size, you need to know a little moreabout muscle physiology first.You will recall from a previous ar-ticle that each muscle fiber is made up ofhundreds to thousands of smaller unitscalled myofibrils. These are the contrac-tile units of skeletal muscle (1). Myo-fibrils are long chains of still smaller sub-units called sarcomeres. Sarcomeres havea striped, or striated, appearance when vi-sualized in a microscope, and this is why skeletal muscle is sometimes called stri-ated muscle. Sarcomeres are made of al-ternating light regions, called I bands, anddark regions, called A bands. In the middleof each I band is a dark line called the Zdisk (1). A sarcomere spans from Z diskto Z disk. A myofibril is thus a long chainof sarcomeres joined end to end atthe Z disks.
As we discussed in aprevious article, the two main pro-teins in muscle are actin and myo-sin. Actin is a thin protein filament,while myosin is a thick protein fi-ber. The light I band is the regionof the sarcomere that contains onlythin actin filaments. The A band,in the middle of the sarcomere, isa region containing both thin andthick filaments organized in anoverlapping arrangement. Whenthe muscle is fully relaxed, a re-gion in the middle of the A bandcalled the H zone becomes appar-ent, called that because it containsonly heavy, thick filaments(1,2,3,4).Each actin molecule has oneend anchored in the Z disk, and theother end extending toward themiddle of the sarcomere where itinterdigitates with the thick myo-sin molecules. Each myosin mol-ecule is composed of two proteinstrands twisted around each other(1,2,3,4). One end of the myosin mol-ecule forms a globular structure called themyosin head, which is attached to themyosin chain by a cross bridge. Eachmyosin molecule contains several heads,which protrude from the surface of themyosin fiber to interact with special siteson the actin molecules. Each actin fila-ment is actually composed of three pro-teins: actin, tropomyosin, and troponin(1,2,3,4).The basic idea of what’s happeninghere is that muscle is mainly composedof two types of protein strands, actin andmyosin. These strands are lying parallelto each other and are overlapping. Whena muscle contracts, the fibers slide pasteach other to make the muscle shorter.Each sarcomere acts as a single unit sowhen it contracts its fibers slide past eachother to pull the Z disks closer together.Each sarcomere is only microscopic insize, so when it contracts it only gets alittle shorter.
But since each myofibril ismade up of thousands of sarcomeresjoined end to end, when they contract thewhole muscle gets shorter. How does thishappen?When a nerve impulse arrives at amuscle, the electrical signal is spreadacross the muscle cell membrane and isconducted to the interior of the cell bythe T tubule system (discussed last time).The T tubule system carries the impulseto the sarcoplasmic reticulum (SR) andcauses the SR to release a bunch of cal-cium ions. In a muscle’s resting state,tropomyosin molecules lie on top of theactive sites of actin, blocking their inter-action with myosin (1). When calcium isreleased, it binds to troponin, causing theprotein molecule to change shape (1).This in turn pushes tropomyosin off ofthe active site of actin. Actin is now freeto bind to the myosin head groups. Whena myosin cross bridge attaches to an ac-tin chain, it undergoes a conformationalchange (a change in molecular shape)which causes the two filaments to slidepast one another. This is referred to asthe power stroke (1).
Immediately afterthe myosin head tilts, it breaks away fromactin, rotates back to its original position,and attaches to a new active site onactin. Repeated cycles of attachmentsand power strokes cause the filamentsto slide past each other in ratchet-likefashion, giving rise to the term “slidingfilament theory” (1,2,3,4).You know from pervious articlesthat the energy that drives this processcomes from ATP. So, how does ATPtie in here? Each myosin head grouphas an enzyme called an ATPase, whichcan break down ATP to release its en-ergy. The energy released from ATP isused to bind the myosin head to the actinfilament (1,2,3,4). The muscle is thus“primed” and ready to contract. It’sjust waiting for calcium to bind tropo-nin and push tropomyosin out of theway. So the energy comes from ATP,the immediate signal to contract comesfrom calcium, and the calcium releaseis triggered by neurotransmitters re-leased from the peripheral nerve.Simple, eh?
1. Wilmore JH and Costill DL. Physi-ology of Sport and Exercise. Human Ki-netics, Champaign, IL, 1994.
2. McArdle WD, Katch FI, and KatchVL. Exercise Physiology – Energy, Nutri-tion, and Human Performance. Lea &Febiger, Malvern, PA, 1991.
3. Lieber RL. Skeletal Muscle Struc-ture and Function. Williams and Wilkins,Baltimore, MD, 1992.
4. Baechle TR. Essentials of StrengthTraining and Conditioning. Human Kinet-ics, Champaign, IL, 1994.