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The Princeton Longevity Center Medical News

The Anatomy of Resistance Training

 By: Christopher Volgraf, ACSM HFI©, NSCA C-PT© Certified

Muscle Anatomy

            Muscles are made up of thousands of cylindrical muscle fibers that lie parallel to each other.  There are two contractile proteins that slide past each other to produce a contraction of the muscle.  A muscle shortening contraction is called a concentric contraction, a muscle lengthening contraction is called an eccentric contraction, and a muscle contraction in which the muscle does not change in length (remains stationary) is called an isometric contraction.

            Genetics plays a big role in personal muscle make-up.  Have you ever noticed that some people can run faster, jump high and perform explosive movements with relative ease?  This is due to their genetic make-up of predominantly fast twitch muscle fibers as apposed to slow twitch muscle fibers.

            Fast twitch muscle fibers respond rapidly, but fatigue easily.  These properties make them suitable for performing activities that require intense responses over a short period of time. 
            Slow twitch muscle
fibers are more aerobically suited to perform activities of longer duration thanks in part to their rich supply of blood (to aid in O2 & CO2 transport). 

            Usually, humans are born with a relatively equal distribution of both slow and fast twitch muscle fibers.  But there are always exceptions to the rule…professional athletes all around the world continue to show what “good genes” can do for ones performance. A resistance-training program can improve the ratio of slow to fast twitch fibers and overall hypertrophy (increase in cross-sectional size), but nowhere near a genetically blessed individual!

 Bodily Responses to Resistance Training

            The initial adaptations to resistance training occur in the first weeks of training.  The neuromuscular system is the major player in these adaptations through increases in the number of neuromuscular junctions or motor units.  A motor unit is a number of muscles innervated by the same motor neuron or nerve.  Consistent resistance training causes an increase in the total number of motor units by recruiting muscles that may have been “dormant” and connecting them with a motor neuron.  Strength increases are a direct result of the increased number of muscle fibers performing the work.

            Hypertrophy or an increase in cross-sectional diameter of the muscle fibers occurs 4-8 weeks from the initiation of the training program due to an accumulation of proteins.  The “visual results” as some call them are due to an increase in muscle mass, not an addition of muscle.  Be careful though…after results begin to show, atrophy, or the decrease in muscle size and mass, occurs 3 times faster than hypertrophy.  In other words, don’t take too much time off!

Different types of training for different results

            There are different types of training that will elicit better results for individuals who wish to focus on

  • muscular strength

  • muscular volume

  • muscular power, or

  • muscular endurance. 

Each category uses a different number of sets and repetitions, differing rest periods, and the velocity of the exercise.

           Muscular strength results from the proper activation of specific muscle fibers, which are recruited in response to the demands of the resistance-training program.  This category results in muscle tone, hypertrophy, and overall strength.  The ACSM now recommends that new and intermediate lifters use 60-70 percent of their repetition maximum capacity for one lift, and that they progress at a rate of two to ten percent increase, depending on the muscle group involved. 

Muscular volume, which is the product of load and number of repetitions, should be low for beginners and increase systematically (but not dramatically) to best increase muscle strength.  Choice of exercise should include single joint (bicep curls) and multiple joint (squat-hamstrings, hip flexors and quadriceps using the hip, knee and ankle joints), with emphasis o the multiple joint movements.  The ACSM recommends that a combination of free weights and machines be used in a beginning weight-training program.  Emphasis on free weights is advised for advanced strength training.  The recommended training sequence of muscles is large before small, multiple joint before single joint, and higher before lower intensity exercises.

            Muscular power is important to both sports and everyday functional ability.  Power increases when the muscle produces the same amount of work in a shorter time or more work in the same time.  To increase power, the ACSM recommends predominantly multiple joint exercises in the same sequence as for strength training.

            Muscular endurance, or the ability of a muscle to produce a maximum number of repetitions with a specific training load, is enhanced by long duration sets and shorter recovery time between sets.  The ACSM recommends that both single and multiple sets be included, targeting specific muscle groups.  Keep the weights light, the repetitions high and the rest periods between sets short.

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