Fatigue, Muscle Soreness, and Muscle Cramps

Chapter 6

Ideas regarding what causes fatigue examples?

decr. glycogen, lactic acid, adenosine, phosphates, hyperthermia

Catastrophe theory

once fatigue begins it causes a chain reaction of system failures — hard to pinpoint where fatigue begins

fatigue definition

inability to maintain requires power output to continue muscular work at a given intensity

factors that influence fatigue…

• type/intensity of exercise

• muscle fiber type

• training status, diet

Testing muscle fatigue: removing muscle from body…

removes psychological component of fatigue

Muscle fatigue

lower rate of force development (means slower relaxation too)

need to find optimal muscle length first (length/tension relationship)

Central fatigue comes from..

the brain

examples of central fatigue

• reduced motor cortex excitability

• strength of signal from CNS

• serotonin decreases willpower

Peripheral fatigue examples

• local muscle

• NMJ (not enough ACh at the NMJ)

the 4 major causes of fatigue

1. inadequate energy delivery/metabolism

2. accumulation of metabolic by-products

3. failure of muscle contractile mechanism

4. altered neural control of muscle contraction

PCr depletion coincides with fatigue

• PCr depletes faster than ATP (short, high intensity effort)

• operating like drained molecular battery

• P_i accumulation may be potential cause

Glycogen depletion correlated with fatigue

• specifically muscle glycogen

• glycogen depletes faster with high intensity and in first few minutes of exercise

• depletion depends on fiber type + recruitment (recruited first and often = deplete fastest)

Blood glucose depletion correlated with fatigue

muscle glycogen depletion + hypoglycemia = fatigue

Muscle glycogen is _______ for prolonged exercise

insufficient

_______ glycogen resynthesizes glucose into blood

Liver

as muscle glycogen declines, liver glycogenolysis increases

Type I vs Type II muscle fibers influences fatigue

% type I vs type II demonstrates which activities are best

How are fiber types tested?

muscle biopsy, can be estimated from anaerobic tests

ATPase staining

• Alkaline = Dark —> Type II

• Acid = Light —> Type I

Other muscle stains…

• H&E = morphology

• SDH = Oxidative

• GADPH= glycolysis

as we age we generally become more type ____ dominant

type I dominant

Metabolic by-products that potentially lead to fatigue

• P_i: from rapid breakdown of PCr, ATP

• Heat: retained by body, core temp high

• Lactic Acid: product of anaerobic glycolysis

• H⁺ accumulation: causes muscle acidosis (lower pH)

Brief, high intensity exercise =

lactic acid

H⁺ + lactic acid —> lactate + H⁺

accumulation of H⁺ causes muscle acidosis

buffers help maintain pH (7.1 to 6.5))

at a pH less than 6.9, __________ inhibited

glycolytic enzymes

at a pH of 6.4…

glycogen breakdown prevented

Reactive oxygen species accumulation (ROS)

• impaired mitochondrial function

• slower rate of PCr recovery

• reduction in oxidative ATP production

PCr is broken down by…

creatine kinase

Neural Transmission - failure at the NMJ

• reduced ACh synthesis & release

• altered ACh breakdown in synapse

• increase in muscle fiber stimulus threshold

  • altered muscle resting membrane potential

fatigue may inhibit _____ release from the SR

calcium (Ca²⁺)

could lead to Mg accumulation in smaller amounts

Conscious aspects of fiber recruitment (CNS fatigue)

• need to fully commit

• pain tolerance

• turn down inhibition (mothers lifting cars off their children)

Examples of psychobiological aspects…

• conscious decision to terminate activity

  • motivation

  • muscle

  • motivational speech

  • pain tolerance

  • self-talk

  • screaming

Examples of heat altering metabolic rate

• increased carbohydrate utilization

• hastens glycogen depletion

• impairs muscle function

• muscle precooling prolongs exercise

Critical power

highest intensity exercise that can be sustained

greatest exercise intensity is maintained by…

oxidative metabolism

increases with endurance and high intensity training

Muscle soreness results from …

exhaustive or high intensity exercise

acute soreness

• during/immediately after exercise

• accumulation of metabolic by-products (H⁺)

• tissue edema (acute muscle swelling, plasma fluid into interstitial space)

• disappears in minutes to hours

Delayed onset soreness

• 1-2 days later

• stiff/restrictive pain

• major cause is eccentric contractions

  • damage to muscle fiber

  • downhill running

  • eccentric contractions

damage to muscle from DOMS is indicated by…

muscle enzymes in blood

increase 2-10x after training

Creatine Kinase- elevated levels associated with DOMS

Z-disks streaming

myofilament damage seen after eccentric work

Muscle damage induces …

hypertrophy

Inflammation & soreness are connected…WBC count _____ with soreness

increases

Tissue healing: ______ released initiate inflammation

cytokines

Tissue healing: what comes with inflammation

WBCs, swelling, pain, blood clotting

Phases of tissue healing

1. inflammation

2. proliferation

3. maturation/remodeling of structures

DOMS reduces muscle force generation… loss of strength results from

physical disruption of muscle

failure in excitation coupling

loss of contractile protein

Strategies to reduce DOMS for effective training

• minimize eccentric work

• gradual increase in exercise intensity

• also: KT tape, ice bath, stretching, sauna, RICE, NSAIDs

Exercise-induced muscle cramps

localized to overworked muscle due to…

• lack of conditioning

• improper training

• depletion of muscle energy stores

Exercise-induced muscle cramps are treated with…

stretching — changes excitatory properties of the motor neuron

Neuromuscular control theory

altered control of motor neuron and muscle

1. central origin- cramps develop from hyperexcitable motor neurons

2. peripheral origin- cramps develop from spontaneous discharges of motor nerves

increased risk with age, history, exercise intensity/duration, and lack of fitness

Electrolyte depletion theory

heat cramps — large sweat & electrolye losses (Na + Cl) + dehydration

treatment for heat cramps

high sodium solution, ice, massage