“a condition in which there is a loss in the capacity for developing force and velocity of a muscle resulting from muscle activity under load which is reversible by rest”
Fatigue!! Important part is that it is reversible by rest
Name at least 5 factors that influence development of muscular strength? (there are 11)
Training Load Speed of contrac. Metabolic changes Training to failure Type of contrac. Biomech. Adap. Rest Neural Adap. Endocr. Influ. Muscle Size Training Vol.
Fatigue can occur during prolonged contractions of submaximal intensity without an apparent decrement in the ____ ____ targeted force
targeted force
Fatigue processes begin when?
immediately with the start of PA
What causes the temporary reduction in neuromuscular performance?
Peripheral (or muscle) factors or Central (or neural) factors
“mechanisms associated with excitation-contraction coupling as well as metabolic accumulation”
peripheral or muscle factors of fatigue
“Mechanisms including areas proximal (upstream) to the neuromuscular junction”
central or neural factors of fatigue
Fatigue depends on what 4 things?
- Type of exercise performed
- Fiber type of muscle involved
- Fitness status
- Nutritional status
What are the 4 potential sites of fatigue development? Are they central or peripheral?
Central: 1. Brain 2. Spinal Cord Peripheral: 3. Neuromuscular Junction 4. Muscle cells
Fatigue may be due to alterations in what 9 things?
- Activation of the PMC
- Propagation of command from CNS to MN
- Activation of MU and muscles
- Neuromuscular propagation (and NMJ)
- Excitation-contraction coupling
- Availability of metabolic substrates
- State of the intracellular medium
- Performance of the contractile apparatus
- Blood flow
Why would alterations in blood flow cause fatigue?
occluding blood flow means less O2 to the muscles and less clearing of metabolites
Three local factors of fatigue?
- Fiber Composition (FT vs ST) –> fuel availability
- Failure @ NMJ –> more common in type II
- Failure of contractile mechanism –> depletion of P stores, inc. acidity and temperature, and degradation of contractile proteins
8 muscle factors in development of local (peripheral) fatigue?
- Sarcolemma permeability
- Sarcolemma excitability/transmission of AP
- T-tubule propagation
- Ca++ release from SR
- Binding of Ca++ TnC
- Interaction of A+M (x-bridge)
- Metabolite accumulation (H+, Pi, ADP, heat)
- Substrate depletion (muscle glycogen, Cr, ATP)
What is the depletion hypothesis?
Fatigue within the muscle is due to a decrease in metabolites –> ATP, PC, and glycogen
What is the Accumulation hypothesis?
Fatigue within the muscle is due to the accumulation of metabolites known to impaire force generation –> lactate, H+ ions, NH3 and Pi
When H+ and lactate run amok, it causes what 5 bad things that lead ot fatigue?
- Decrease amt of Ca++ release
- Interferes with Ca++/Troponin binding
- Disrupts Na/K pump
- Inhibits PFK
- Interferes with actin/myosin interaction
What are the two Neural (CNS) types of central fatigue?
Supraspinal and spinal
What 3 neurotransmitters are involved in central fatigue?
Serotonin
Norepi
Dopamine
What 4 neural factors inhibit force production?
- Supraspinal fatigue causes decreased cortical excitability
- Decreased MN discharge rate
- Inhibition from Renshaw cells
- Inhibition from muscle and tendinous afferents
What is the TRY-5HT theory of central fatigue?
increases in free tryptophan –> 5-HT —> Serotonin
= relaxation, sleep, fatigue
Exercise = inc. FFA release = competition for albumin = bumping off tryptophan = inc free tryptophan = 5-HT synthesis = inc serotonin?
What declines in motor neuronal output lead to central fatigue? (3)
- Alpha MN affected/inhibited
- Cutaneous fiber info (sensory output from periphery?)
- Renshaw Cells activity enhanced (inhibitory interneurons)
How does less action potentials lead to decreased force and therefore fatigue?
Less AP on sarcolemma = less Calcium released cause less ryanodine receptor action = less calcium binding to calcium = less troponin being pulled away = less contraction occurring, decreased force
DECREASE IN AP COMING DOWN MOTOR NEURON FROM THE INHIBITORY EFFECTS OF THE RENSHAW CELLS
- Excitatory input to cortex
- descending drive from cortex
- motor neuron excitability (altered threshold, ability to transmit repetitive signals)
- antagonist activity (thinking about fatigue)
- NMJ transmission
What do all of these things cause?
-Reduced firing frequency and recruitment
-ultimately fewer x-bridges formed
Therefore - imparted force production and fatigue due to failing central (neural) factors
Fatigue is not all bad! What can fatigue contribute to in a positive way?
Initiates muscular adaptations in metabolic and structural areas
