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Energy Systems and Fatigue

Muscular Strength & Endurance

(Testing)

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Outline

• Muscle strength versus endurance• Energy systems and fatigue

– what limits muscle strength and endurance• Reasons for strength and endurance

evaluation• Types of muscle contraction and testing

equipment• Problems with muscular fitness testing

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Muscle Activity Definitions• Muscular Strength

the amount of force that a muscle, or muscle group, can produce in a single maximal effort. One repetition max, dynamometer

• Muscular Endurance the ability of a muscle group to perform repeated

contractions against a light load for an extended period of time. Partial curl ups, push ups, repetitions of a % of body

weight• Muscular Power

the ability to produce force quickly. Work (force * distance) over time, or force *

velocity Vertical jump

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Power = Force x Velocity

Maximum power occurs at about 1/3 maximal velocity of shortening and about 1/3 maximum concentric force?

Power

Force

Velocity

Force

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Fatigue• Fatigue- inability to maintain a given exercise intensity

– rarely completely fatigued - can maintain lower intensity output

– Observe reduction in force and velocity and a prolonged relaxation time

• Causes of muscle fatigue have been classified into central and peripheral

• Central - includes CNS, motivation and psychological factors– restoration of force with external stimulation of muscle -

indicates central fatigue– NH3, hypoglycemia, reticular formation

• Peripheral - PNS to muscle - EC coupling, energy supply and force generation– CP depletion, Glycogen depletion, proton buildup

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Energy Systems• Maintenance of ATP levels is required for muscle

contraction to continue• ATP can be maintained through a variety of pathways

determined by factors such as hormone levels, internal cellular environment (O2 levels, fuel, acidity,

calcium) and demand• All pathways are operational, there is a shift in

dominance and ATP output as a result of the influence of these factors

• If ATP production or muscle contraction can not be maintained, the muscle is said to be fatigued

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NSCA Essential of Strength Training and Conditioning, 2008

8NSCA Essential of Strength Training and Conditioning, 2008

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Immediate Energy System• intracellular ATP and Creatine Phosphate

(CP) stores• CP + ADP --> C +ATP

– enzyme for reaction is Creatine Kinase

• Very high power output but low capacity• Exhausted in ~10 seconds of maximal effort

– Weight lifting, shot put, vertical jump

• Fatigue due to CP and ATP depletion– Eg. End of a weight training exercise set

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Anaerobic Glycolysis• Breakdown of glucose (2 ATP) or glycogen (3

ATP) to pyruvate– Pyruvate then converted to lactate to allow glycolysis

to continue anaerobically• Fairly fast, moderate yield

– Predominates for all out activity that can last between 30 seconds and 2-3 minutes

– Ideal for test of anaerobic power to be maximal and end within this time frame

• Fatigue due to H+ (acidity) from glycolysis– Inhibits PFK (enzyme in glycolysis)– Inhibits ATPase and ATP hydrolysis– Inhibits Calcium binding to troponin C– Inhibits SR ATPase - reducing calcium reuptake and release– Increased threshold of free Calcium required for contraction

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Aerobic systems• Fats, carbohydrates and some protein

oxidized to CO2 and H2O in mitochondria– Slow production - fats theoretically unlimited ATP

production - glycogen has limited stores• Predominant for activity lasting > 2-3 min• Power (VO2 max)limited by oxygen delivery to

working muscle– Cardiac output - Hb content

• Performance limited by respiratory capacity of muscle - mitochondrial content

• Fatigue can result from – glycogen depletion (1.5 to 2 hours high intensity)– Uncoupling of mitochondria (calcium buildup)– CNS involvement (central fatigue)

General Characteristics of the Energy Systems

ATP/PC System Lactic Acid System Aerobic System

Anaerobic Anaerobic Aerobic (oxygen)

Very rapid Rapid slow

PC and ATP utilized Glycogen/glucose used Glycogen, fats, and protein used

Very limited production Limited production of ATP Unlimited production of ATP

Limited stores in muscle - fatigue with depletion

By product - lactate and acidity leads to fatigue

No fatiguing by products

Used in sprint and high-power short duration activity

Activities that are maximal for 1-3 minutes

Long duration activities, 5 minutes plus

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Capacity and Power of the Three Energy Systems

For ATP Production(Untrained Male Subjects)

Energy System Capacity (total Moles) Power (moles / min)

Phosphagens (ATP /PC) 0.6 3.6

Anaerobic glycolysis 1.2 1.6

Aerobic Unlimited (if including fats)

1.0

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Endurance time decreases with increasing force

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Copyright ©2008 American Physiological Society

Jones, A. M. et al. Am J Physiol Regul Integr Comp Physiol 294: R585-R593 2008;doi:10.1152/ajpregu.00731.2007

Fig. 1. Estimation of critical power (CP) in a representative subject

Critical Power - highest constant work ratethat can be maintained without fatigue

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Skeletal muscle Fiber types• Fiber types have different capacities for each

energy system - specialized for different types of activity

• Variability in size of fibers and motor units, speed and strength of contraction, capillarization, enzyme and fuel content

• Fibers recruited based on demand– Type I - light activity– Type I and II a - moderate activity– All fibers types - high intensity demand - high

power output - contribution of II b will be high

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Fiber Recruitment• CNS driven stimulation of number of motor

units required for contraction• Motor unit - motor neuron and all muscle

fibers that it stimulates - all fibers within a motor unit are of the same type– Variable threshold for stimulus of different motor

units - type I, IIa, IIb • Orderly recruitment and size principles

• Asynchronous recruitment - endurance activity, alternate recruitment of motor units to allow recovery and sustained low % contractions

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Muscular Strength Testing• Purpose

– Assess muscular fitness– Identify specific areas of weakness– Monitor progress in a rehabilitation program– Measure effectiveness of a resistance training

program• Correlation between Endurance and Strength

is greater than .90– These measures are clearly related– We know, however,that training programs can

result in specific improvements in strength or endurance depending on design

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Strength Training Benefits

• Increased muscle fiber size (hypertrophy).• Reduced percent body fat• Increased muscle contractile strength.• Increased bone mineral density and tensile

strength.• Increased ligament tensile strength.• Improved physical functioning with old age• These changes improve our physical

capacity, physical appearance, metabolic function and injury risk.

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Types of Muscular Contraction

• Isometric Contraction (static)• Concentric isotonic Contraction (shortening)• Eccentric isotonic Contraction (lengthening)• Isokinetic Contraction (joint angular velocity

remains constant)

Muscle ActionPrime moversAntagonistStabilizers

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Static Strength Tests

• Hand grip dynamometers• Leg and back strength dynamometers• Cable tensiometers

Advantages -- good reliability, good objectivity, norms available.

Disadvantages - angle specific, boring, increase SBP, validity ?

Force-Length Curve of Muscle

Lo

Tension

Length of Contractile element

Physiological Range

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NSCA fig 4.7 - change in moment arm and mechanical advantageduring a bicep curl

27Fig 4.9 NSCA - impact of insertion on moment arm and torque

Torque advantageRotational velocity advantage

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Dynamic Strength Testing

• One-repetition maximum tests.• Batteries of tests have been developed. bench

press, squat, overhead press and others. • Variable-rep maximum tests.

1 RM = (weight lifted) / [1.0278 - (n X 0.0278)]n = repetitions to failure (not to exceed 10)

Safer than using 1-RM, especially with beginners.• Fixed weight tests: YMCA bench press test

(Females 35-lbs, males 80-lbs)• Elderly tests of ADL - activities of Daily Living• Alternate RM estimations using 45% of body mass

Women 60-70 yrs 1RM = (.92 * wt(kg)) + (.79 * reps) - 3.73

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Isokinetic Strength Tests• Expensive and therefore less practical.

Strength -- 30o- 60o per secondEndurance -- 120o - 180o per secondPower -- 120o - 300o per secondEg: Canadian Ski Team cybex knee extension

test is performed at 100o/s.

• Many isokinetic systems (e.g. Omnitron) have their own protocols.

• Disadvantages• humans do not move at constant velocity• isokinetic tests use isolated joint movements • may not be reflective of performance ability

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Problems Associated With Muscular Fitness Testing

• Strength & endurance are specific to the muscle group, the type and speed of contraction, and the angle being tested (how applicable are isometric tests?).

• Caution should be used in selecting test items to measure muscular strength (often really measuring endurance!).

• Most tests of strength & endurance require a maximal effort (motivation?).

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Problems Associated With Muscular Fitness Testing (cont.)

• Power tests often require skill• Because strength is related to lean body

weight of the individual, the tests should be expressed in relative terms.

• Performance on some endurance tests (e.g. chin-ups and push-ups) is highly dependent on the strength and strength to weight ratio of the individual.

• There is a lack of up-to-date endurance norms for men and women (especially >25).

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Strength vs Age & Gender

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Musculoskeletal Health inOlder Adults

• Muscle function is reduced in older adults as a result of aging and inactivity

• Largest impact on lower limb and trunk extensors• lower limb flexors and upper limb muscles are also

affected• There is a selective loss of Type II muscle fibers

and a proportionately larger decrease in power and strength

• Strength to mass ratio also declines• Ability to adapt to a resistance training program is

not affected by aging

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Muscular Strength and Endurance Lab

• Preparation Determine % of body weight requirements for weight

room portion of lab prior to start of lab sessionsee lab manual

Bring proper clothing to perform strength and endurance assessments in both the weight room and lab room.

Be prepared to travel between these facilities quickly during lab session (Burnaby only).

• Instructions for weight room in lab manual• Instructions for lab room in CSEP-PATH manual• Enter all results in log book