* Muscle Fiber Action Potential * Muscle fiber must receive an electrical impulse from a motor neuron

Transcription

1 Chapter 8

2 Muscle Fiber Action Potential Muscle fiber must receive an electrical impulse from a motor neuron Starts in the brain, travels down the central and peripheral nervous systems, and ends at the motor endplate Action Potential: the ability to contract

3 Muscle Fiber Action Potential Resting Membrane Potential (RMP) The electrical activity in a resting muscle Approximately -90mV Different concentrations of sodium, potassium, and chloride ions across the sarcolemma of the muscle fiber Slow-twitch fibers have a RMP about 9 to 15mV more positive than fast-twitch fibers RMP is not a fixed value as it can be altered by exercise training

4 Muscle Fiber Action Potential Action Potentials Neural messenger for activating every segment in a muscle fiber Starts with a change in the muscle fiber s permeability to sodium This leads to a change (reversal) of the muscle fiber membrane from -90mV to +30mV

5 Muscle Fiber Action Potential Action Potentials (cont.) This reversal increases the the muscle fiber s permeability to potassium Leads to a second reversal from +30mV back to -90mV The AP moves (propagates) in both directions from the motor endplate As the AP propagates, the membrane potential at each subsequent muscle fiber section changes from negative to positive and back to negative as each adjacent muscle fiber area is successively activated

6 Muscle Fiber Action Potential Muscle Fiber Conduction Velocity AP generation is an ionic process The velocity that the AP travels is dependent on the permeability of the muscle fiber membrane Fast-twitch fibers have faster conduction velocities than slow-twitch fibers (permeability differences) Atrophied fibers have slower conduction velocities Increases in muscle length tend to decrease conduction velocities

7 Motor Unit Action Potential (MUAP) Generally, a motor neuron innervates several hundred muscle fibers Depends on the size of the muscle Can vary from around 10 to as many as several thousand The motor unit is one motor neuron and all of the muscle fibers it innervates The MUAP is the sum of all electrical activity of all muscle fibers activated in the motor unit

8 Motor Unit Action Potential (MUAP) Motor Unit Activation Muscular force is created by activating an increasing number of motor units Smaller motor units are recruited first Larger motor units are recruited as the force requirement increases As the motor unit fires at faster rates, it produces an increasing amount of muscle force

9 Motor Unit Action Potential (MUAP) Motor Unit Activation (cont.) Other ways to increase muscular force When considerable force is required, motor unit may be fired in two short bursts before firing at a regular rate. These twin burst are known as doublets. Two or more motor units can be fire simultaneously to increase muscle force. This process is known as synchronization. Simultaneous activation of other muscles that can perform the same task. Example: 3 elbow flexors. This process is known as muscle synergy.

10 Motor Unit Action Potential (MUAP) Motor Unit Action Potential and the EMG Signal The EMG signal is a composite sum of all active motor units A large peak in the EMG signal might be the result of the activation of multiple motor units The relationship between EMG amplitude and muscle force is non-linear One cannot assume that increases in EMG activity are indicators of a parallel increase in muscle force

11 Electrode Types Surface Electrodes The most common type of EMG electrode Two conducting surfaces (one positive and one negative) in contact with the skin Signal Distortion (next slide)

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14 Electrode Types Fine-Wire Electrodes Two fine wires separate by insulation within a hollow needle One wire is the positive electrode & the other wire is the negative electrode Needle Electrodes Small wire placed in the middle of a hollow needle Small wire is the positive electrode; hollow needle is the negative electrode

15 EMG Signal Processing For human movement, little or no relevant EMG signal is contained at frequencies below 10 Hz or above 1000 Hz. For surface electrodes the upper frequency limit drops from 1000 Hz to 400 Hz The Nyquist limit states that the EMG sampling rate must be at least twice the highest frequency component in the signal Therefore, surface EMG recordings generally require a sampling rate of at least 1000 samples per second

16 Influences on the EMG Signal Electrodes Type: Surface vs. In-Dwelling Electrode-Skin Contact Affects the magnitude of the recorded EMG signal Can result in movement artifact noise Distance from skin surface to muscle Subcutaneous fat is an issue Reduces the magnitude of the recorded EMG signal

17 Influences on the EMG Signal Tissue Influences Skin, tendon, and subcutaneous fat can alter the high frequency content of the EMG signal The deeper the muscle is below the skin surface, the greater the distortion Muscle Length Muscles stretched to longer lengths demonstrate (1) decreased conduction velocity, (2) decreased AP amplitude, and (3) reduced high frequency signal content.

18 Influences on the EMG Signal Muscle Depth AP magnitude decays rapidly with depth to the muscle fibers Surface electrodes are limited to superficial muscles Crosstalk Any contracted muscle near the surface contact with the electrodes will contribute to the recorded EMG signal. Thus, isolating the EMG signal to a specific muscle is impossible In-Dwelling electrodes can be used if targeting a specific muscle is required.

19 Average Rectified Amplitude The raw EMG signal is an alternating current (AC) signal Alternating positive and negative values are recorded In a correctly collected EMG signal, the average of this alternating signal is zero Consequently, the mean value of the Raw EMG signal is not a valid indicator of EMG size To compute a valid mean value of the EMG signal, the raw signal must be rectified (see next slide)

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21 Linear Envelope The first step in determining the volume of muscular activity that was performed. The linear envelope is created by passing a low-pass filter through the full-wave rectified EMG signal (see previous slide) The linear envelope is a type of moving average amplitude of the EMG signal Intergrated EMG A measure of the volume of muscular activity that was performed It is the amount of EMG activity performed for a specific period of muscle activation, the area under the curve (see previous slide)

22 Normalization of the EMG Signal EMG data from different subjects, muscles, and days cannot be directly compared. Some type of normalization process is required Most frequently, subjects are asked to perform a maximal isometric contraction The magnitude of the EMG signal of interest is divided by the peak EMG value of maximal isometric contraction to give the normalized EMG value