Kinesiology Muscular System

Muscle

Tissue made up of many muscle cells and associated connective tissue

3 main muscle types

• Skeletal

• Cardiac

• Smooth

Skeletal muscle cell (muscle fibre or myocyte)

An individual cell that when activated produces force that can lead to motion

Sarcomere

The fundamental unit of skeletal and cardiac muscle

Many sarcomeres are...

arranged in sequence within a single myofibril and many myofibrils make up a muscle cell

Myofilaments

Sarcomeres are composed of highly organized arrangement of myofilaments (composed mainly of actin & myosin) that interact with each other to generate force (slide across each other)

Motor neuron

A neuron that innervates a skeletal muscle

Motor unit

one motor neuron and ALL of the skeletal muscle cells it innervates

Neuromuscular junction

The synapse between motor neuron and skeletal muscle cell

Muscle action potential

An action potential that occurs on the sarcolemma (skeletal muscle cell membrane)

Cross bridge

The binding of actin to myosin myofilaments and change in the confirmation of myosin

The cross bridge cycle

The process involving attachment, conformational change and detachment (with ATP) that generates force

Sliding filament theory

Explains the mechanism of muscle contraction associated with the cross bridge cycling and the sliding of myofilaments past each other to generate force

Depolarization of a motor neuron (neuronal action potential)

Electrical

Neurotransmitter release at neuromuscular junction

Chemical

Depolarization of muscle fibre

Electrical

Cross-bridge formation and sarcomere shortening

Mechanical

Muscle fibres activate in...

an ALL or NONE manner

How can you control the amount of force a whole muscle generates?

1. Increase action potential frequency of the already active motor neurons

2. Recruit more motor units

What determines the maximum isometric for a whole muscle could generate?

Skeletal muscle, muscle cross-sectional area

Electromyography (EMG)

Measures electrical activity produced by muscles (measurement of action potential that occurs when a muscle is stimulated)

Surface EMG

Electrodes are placed on the skin's surface

Intramuscular EMG

A small needle is inserted into a muscle and the electrical activity is recorded directly

Advantages of Surface EMG

Less invasive than intramuscular

Disadvantages of Surface EMG

• There are multiple muscles in one area (e.g., forearm)

• Only able to measure superficial muscles

What is the y-axis on EMG tracing?

mV, V

What is the x-axis on EMG tracing?

time (timing of activation and relaxation)

Is there a relationship between EMG amplitude and muscle force

Yes!

You CANNOT compare EMG between...

• Different skeletal muscles

• Different individuals

Reaction time is dependent on:

• Stimulus intensity and modality

• The number of potential stimuli and responses

• Age

• Sex

• Neuromuscular disease/inquiry

Simple reaction time (SRT)

One stimulus and one response

Choice reaction time (CRT)

A number of different stimuli presented each requiring a different response

When the number of stimuli increase...

the response time increases

Dual-task interference

Simultaneous performance of two tasks often leads to performance deficits in the component tasks