Muscle Performance 1: Force Production, Motor Control, and Fatigue in Neurologic Populations

what is strength?

the ability of a muscle to produce force sufficient for the intended movement, the ability to initiate, accelerate, or decelerate movement

what is power?

the ability to control torque at varying speed, product of strength and speed

what is endurance?

the ability of a muscle to generate force over long periods of time

what does producing muscle force involve?

at the muscular level involves structural factors like muscle size, density of fibers, and muscle fiber types, as well as mechanical factors like length-tension relationship; force velocity relationship. at CNS level there are neural factors that are affected by number, type, and frequency of motor units recruited during muscle contraction; synergistic muscle activation

as strength does not fully predict function, what are the other factors that affect functional performance?

• magnitude of force

• rate of force development

• movement speed

• power output

what are some methods for assessing muscle performance?

• MMT

• hand held dynamometry (jamar and microfet)

• observation during functional activities

• technology assisted analysis

• physical capacity tests

what is a central activation problem?

when lower motor neurons can’t receive signals from upper motor neurons, reduced neural traffic from brain to spinal segments results in disruption in muscle force production

what might occur if the muscles can't receive signals from LMN?

might see disuse atrophy, denervation and changes in muscle fiber composition

what is muscle weakness defined as?

reduced force production during max voluntary contractions, inability to generate sufficient levels of torque

what is weakness following brain injury characterized by?

• impaired force production/force magnitude

• slowness to produce force

• rapid onset of fatigue

• difficulty producing and maintaining force effectively within context of the task

what is motor control?

the CNS ability to regulate and direct neuromuscular system in purposeful movement by selectively controlling muscles across appropriate joints, ability to regulate and direct the mechanisms essential to movement, organization of individual muscles and joints into coordinated movement

what are the components of motor control?

• normal muscle tone

• postural control mechanisms

• selective movements

• coordination

what do motor control deficits manifest as?

• muscle weakness

• abnormal movement patterns and impaired coordination

• decreased consistency of force production

how might hypertonia or spasticity affect motor control?

this may impede agonist and antagonist torque production and coordination

what are the clinical issues with assessing force production in patients with UMN lesions?

abnormal reflex activity and abnormal muscle tone can restrict ability to produce movement, recruitment problems also an issue: altered timing of force generation, abnormal synergistic muscle activity. all these things can make MMT testing difficulty or inaccurate

what are the clinical issues with assessing force production in patients with LMN lesions?

• muscle atrophy

• fatiguable weakness

• proximal weakness

• distal weakness

• associated sensory deficits

(ex is myasthenia gravis: strength decreases with repeated activity)

what is fatigue?

failure to maintain expected force production during sustained or repetitive activity. clinically may be described as a sense of exhaustion, lack of energy, tiredness. can be a lack of physical or mental energy interfering with the desired activity

what is peripheral fatigue?

• originates within the muscle or motor unit

• involves mechanical and cellular changes

  • metabolic changes at muscle level- depleted Ca2+ stores

what is central fatigue?

• originates within and involves changes in CNS (ie neurotransmitters)

• changes can occur at neural/axonal/myoneuronal junction

• influenced by cognition, motivation, and disease process

• exercise induced decrease in muscle activation

what are potential contributors to fatigue
?

1. motor cortex activation

2. CNS drive to motor neurons

3. motor unit function

4. NM transmission

5. excitation-contraction coupling

6. metabolic substrate availability

7. intracellular milieu

8. contractile apparatus

9. muscle blood flow

what is a clinical indicator of fatigue?

decline in maximal voluntary contraction, low intensity fatigue is often central, high intensity fatigue is often peripheral

what is important to be used to assess fatigue in neurological clients?

RPE scales since patients with muscle weakness requires increase applied effort to perform the same activities, leading to higher perceived exertion to complete activities

what are diseases/disorders that cause peripheral fatigue?

• guillain barre’s syndrome (disorder of peripheral nerves)

• myasthenia gravis (disorder of neuromuscular junction)

• ALS (disorder inv alpha motor neuron and motor units)

  • involves mechanical and cellular changes in the muscular system (metabolic changes within muscle)

what are the associated mechanisms of central fatigue?

• neurotransmitter changes

• demyelination

• altered metabolic function

• reduced muscle activation

additional contributors: motivation and cognitive fatigue

how do we examine fatigue?

should examine the source of fatigue, frequency and severity, fatigue threshold, activity intensity and duration effects, onset and recovery pattern (and consider factors like heat sensitivity, stress, and overall health status)

what are tests and measures for fatigue?

• modified fatigue impact scale (MFIS)

• repetitive functional testing

• repeated strength testign

• isokinetic dynamometry

• EMG

• 6-Minute Walk Test

• Fatigue Severity Scale (FSS)