Muscular Strength - the greatest amount of force that a muscle or muscle group can produce in a single maximal effort.  (units:  Kg)

Muscular Endurance – ability of a muscle group to perform repeated contractions against a light load for an extended period of time.  E.g. Push ups

Muscular Power – ability to produce force quickly. (units: Watts, Kg m min^-1) Eg. Vertical jump

Muscular power can be calculated from force of muscle contraction multiplied by the speed of muscle contraction.

OR

From measuring the work done (force x distance) over time.

BENEFITS OF STRENGTH TRAINING

•    STRENGTH PROVIDES A FOUNDATION FOR OTHER COMPONENTS OF PHYSICAL FITNESS SUCH AS CARDIORESPIRATORY CAPACITY AND MUSCULAR ENDURANCE

•    SLOWS DOWN THE MUSCLE LOSS THAT NORMALLY ACCOMPANIES THE AGING PROCESS --> INCREASES FUNCTIONAL MOBILITY SO THAT DAILY ACTIVITIES ARE MADE BOTH POSSIBLE AND EASIER

•    INCREASE THE SIZE AND STRENGTH OF MUSCLE FIBERS RESULTING IN A GREATER PHYSICAL CAPACITY TO PERFORM WORK

•    INCREASED TENDON, LIGAMENT AND BONE TENSILE STRENGTH

•    STRONGER MUSCLES BETTER PROTECT THE JOINTS THAT THEY CROSS

•    IMPROVED PHYSICAL APPEARANCE

•    BETTER TONE OF THE MUSCLES OF THE TRUNK HELPS TO PREVENT COMMON POSTURAL PROBLEMS

•    STRONGER MUSCLES ARE LESS LIKELY TO BE STRAINED AND INJURED

·     IMPROVEMENTS IN SELF-CONCEPT AND SELF-ESTEEM FOR BOTH ATHLETIC AND PATIENT POPULATIONS

A. Types of Muscular Contraction

1. Isotonic (dynamic) contraction – iso (same) tonic (tone / force / weight) tension is the same throughout the range of motion.¹

Concentric contraction - the muscle shortens with varying tension as it lifts a constant load.¹

Eccentric contraction - the muscle lengthens while developing force as the external resistance (weight) exceeds the muscle force.  Eccentric contractions are usually used in resisting gravity.¹                                                                       

2. Isometric contraction – iso (same) metric (length) static contraction.  Tension is developed but there is no change in the angle of the joint or the length of the muscle.¹

3. Isokinetic contraction – iso (same) kinetic (velocity) the muscle is shortened at a constant velocity determined by computer instrumentation which allows a person to exert maximal force throughout the full range of motion.¹

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B. Purpose of Strength Assessment

1. Assess muscular fitness

2. Identify specific areas of weakness

3. Monitor progress in a rehabilitation program

4. Measure effectiveness of a resistance training program

5. Motivation for training

C. Strength Assessment Techniques

1. a. One Repetition Maximum (1 RM) - maximum amount of weight lifted once.  Use free weights or machines that allow dynamic muscle contractions.

   Appropriate for those who are very experienced with strength training.

1.b. Variable Repetition Maximum – estimation of one repetition maximum from repetitions to failure with free weights or machines

   Safer and more appropriate than using 1 RM for novices and recreational athletes

   1 RM = (weight lifted) / [1.0278 – (n X 0.0278)]

   n = repetitions to failure (not to exceed 10)

   e.g.  bench press 40 kg 8 times before failing

   1 RM = (40) / [1.0278 – (8 X .0278)] = 50 kg

2. Dynamometer Techniques - a strength testing dynamometer usually consists of a spring of some type which is deformed a certain amount when a specific force is applied to it. (eg) - hand grip dynamometer, Jackson Strength Evaluation System

- measures isometric strength

- relatively inexpensive, high reliability if body position is carefully standardized for each trial of the test

3. Computer-Assisted, Isokinetic Methods - equipment such as Biodex, and Kin-Com.

  - isokinetic machine, but many of these machines can also test strength in isometric, concentric, and eccentric modes

- has a computer which can be programmed at any initial and final force, angle, velocity, or number of repetitions.

- measure peak torque using a force transducer and joint angle using an electrogoniometer.

- extensive manipulations can be performed on collected data using computer software

- accurate and reliable, but expensive

- used for research and elite athlete assessment

D. Strength Assessment Considerations

In addition to validity, reliability, etc.,

1.   Standardized instructions should be given prior to testing.

2.   If a warm-up is given, it should be of uniform duration and intensity.

3.   Ensure that the angle of measurement on the limb or test device is consistent among subjects. (recall length-tension relationship and angle of muscle pull discussed in skeletal muscle lecture).

4.   Consider individual differences in body size and proportion when evaluating strength scores between individuals and groups. (see below)

5.   Test and training mode specificity are important for optimal expression of true strength gains.  Training using one mode of muscle activity (concentric, eccentric, isometric, isokinetic) should typically be assessed with the same type of muscle activity.

6.   Safety is an important consideration when performing strength evaluations.  Ensure that all equipment is in proper working order and that standard weight room safety procedures are being followed.

7.   Test administrators should be well trained and have a thorough understanding of all testing procedures and protocols.

E. Effect of Various Factors on Strength

1. Muscle Cross Sectional Area - there is a strong positive relationship between muscle CSA and strength.  Muscles increase in strength by increasing their size and by enhancing the recruitment and firing rates of their motor units.

Hypertrophy - an increase in size of a cell such as a muscle fiber

Hyperplasia - an increase in number of cells such as muscle fibers.

Current research indicates that muscle fiber hypertrophy accounts for most, if not all, of the increase in muscle cross-sectional area due to an overload training program.  There is evidence that muscle fiber hyperplasia occurs in animals, but there is presently no direct evidence that it occurs in humans.

2. Body Size - there is a positive correlation between body size or mass and absolute strength.

There is a negative correlation between body mass and the strength/mass ratio.  The strength to mass ratio directly reflects an athlete’s ability to accelerate his or her body.  All else being equal, smaller athletes are stronger pound for pound than larger athletes.

In comparing performances of Olympic weight-lifters in different weight categories, the most widely used formula is to divide the weight lifted by body weight to the 2/3 power.

Large athletes dominate sport events which require a high level of absolute strength (e.g., throwing events in track and field) while smaller athletes dominate sport events which require a high strength/body mass ratio (e.g., gymnastics).

Relationship Between Body Size and Strength

Olympic Champion A

Height = 4 feet, 10 inches = L = 1.0

Muscle CSA = L² = 1.0

Weight = 132 lbs = L³ = 1.0

Clean and Jerk = 410 lbs = 3.2 X body weight

Olympic Champion B

Height = 6 feet, 2 inches = L = 1.28

Muscle CSA = L² = 1.63

Weight = 290 lbs = L³ = 2.08

Clean and Jerk = 560 lbs = 1.9 X body weight

3. Muscle Fiber Composition - for a given size of muscle, there is a positive correlation between percentage of fast twitch fibers and strength.

4. Mechanical Factors - the force exerted by a muscle is affected by:

   a) the initial length of the muscle fibers

   b) the angle of pull of the muscle on the bony skeleton

   c) the speed of shortening

5. Muscle Strength and Sex - after age 16, the average female is about 2/3 as strong as the average male when the measure of strength is the absolute amount of force exerted or weight lifted.  Prior to puberty the strength of boys is only slightly greater than the strength of girls. 

Sex differences in strength in adults are greater in the arms and shoulders than in the legs.  On average, the female's upper body is 50% to 60% as strong as the male's upper body and the female's lower body is 70% to 80% as strong as the male's lower body.

It is important to note here that we are talking about the "average" male and the "average" female.  However, individuals are not average and the differences between two individuals of the same sex are often greater than differences between statistical averages of each of the sexes.

The reasons why males are stronger are as follows:

• The average male is physically larger (height and weight) than the average female.

• Differences in body composition - the average male has more muscle and less fat due to the male sex hormone, testosterone, which stimulates muscle growth.  Because most human muscles can produce approximately 16 to 30 newtons of force per square centimeter cross-sectional area, larger muscles are stronger muscles.

• Body proportion differences - during adolescence, the skeletal proportions change: boy's shoulders broaden relative to their hips, and girl's hips broaden relative to their waists and shoulders.  The broader shoulders of the adult male allow more muscle to be packed onto the skeletal frame and create a mechanical advantage for muscles acting on the shoulder.

• Cultural factors - less emphasis on strength activities for females, although weight training has become much more popular and socially acceptable for females.

Females are equally as strong as males when strength is expressed per unit cross sectional area of muscle.  There is no qualitative difference between male and female muscle.  You cannot distinguish male muscle from female muscle under a microscope.  Female muscle tissue does not differ, unit for unit, in potential force output from male muscle tissue.  This indicates that the training potential and methods of training for men and women should be similar.

After strength training on the same routine, men have greater absolute increases in both strength and muscle hypertrophy than women.  However, women achieve similar percentage increases in strength as men.  Women display less muscle hypertrophy since they lack testosterone.

Research has revealed that men and women of all ages can increase their muscle size and strength as a result of progressive strength training.

There is a great deal of variability in the responsiveness of muscles to strength training within sexes and age groups - individual differences.

6. Muscle Strength and Age

Muscle strength in children:

• Muscle strength progressively improves as children age and mature, principally as a result of increasing muscle size.

• Hormonal influences at puberty (testosterone) are responsible for the dramatic increase in muscle bulk and strength in males.  During this time period the increase in muscle mass in both sexes is due to hypertrophy of individual muscle fibers and not hyperplasia.

• Muscle strength can be improved by resistance training before puberty in both boys and girls.  Resistance training is safe and effective if children have good motor skills, follow instruction, focus on functional exercises and do not work to failure. Strength improvements occur through neuromuscular adaptation rather than significant increases in muscle size.

• Consider chronological age versus biological age when designing individualized weight training programs for children.

• During the time period surrounding peak height velocity (age 11.5 in girls and age 13.5 in boys), young athletes may be at increased risk for injury.

Maximum strength of men and women is generally achieved between the ages of 20 and 30 years, at the time when muscle cross sectional area is usually the greatest.  Thereafter, there is a progressive decrease in strength for most muscle groups due primarily to a reduced muscle mass which is brought about by a decrease in the total number of muscle fibers in a given muscle and a decrease in individual fiber size.  These changes are more pronounced in the fast-twitch fibers.  These changes in muscle volume are due to a combination of decreased physical activity patterns and aging.

Indirect evidence (cross sectional studies) indicates that habitual physical activity slows down the strength decrements with aging.

Muscle rehabilitation programs for well, older populations have shown significant increases in muscle strength, muscle volume, and other parameters of muscle structure and function. Studies have documented that, given an adequate training stimulus, older men and women show similar or greater percentage strength gains compared to young individuals after a properly designed strength training program.

For the older person (age 65-85 years), muscle strength is a major component of successful performance in almost every activity of daily living. It is vital to the maintenance of upright posture, walking, going up and down stairs, and the accomplishment of simple tasks such as eating and dressing.

Training able to reduce impacts of Sarcopenia (see skeletal muscle lecture), and return some patients to activities of daily living.

Summary of Adaptations to Aging and Resistance Training*

*This table was taken from page 182 in “Essentials of Strength Training and Conditioning” by Thomas Baechle and Roger Earle, Human Kinetics Publishers, 2000.