CHAPTER 1
Nick D. Panagiotacopulos and Francisco Padilla
- Electromyography
Electromyography (EMG) is a technique used for measuring, recording and evaluating the physiological, biochemical, and electrical activity of skeletal muscles during contraction. The EMG signal reflects the effect of an external excitation on a skeletal muscle. If it is done properly and interpreted correctly (based on muscle physiology), the outcome from the study of S-EMG signal can be very useful in the fields of biomechanics, ergonomics and kinesiology. More specifically, S-EMG results can be useful in the evaluation of muscle activity and its performance [87].
- The Central Nervous System (CNS)
Figure 1 shows a simplified functional diagram of the central nervous system and how an EMG signal is acquired.
Fig 1. Central Nervous System and a typical EMG acquisition system (Curtsey of Dr. Nick P.)
- Anatomical structure of Skeletal Muscle
The muscle is surrounded by a connective tissue called epimysium and is divided by fascicles by perimysium which contain several muscle fibers. The structural unit of skeletal muscle is the muscle fiber, or cell (Fig. 2). A muscle cell is a thin structure ranging from 10 to 100 microns in diameter and from a few millimeters to 30 cm in length [5]. The muscle fibers do not extend the entire length of the muscle. Instead, the cells are attached to either the origin or insertion tendon at one end and connective tissue septa at the other end [87]. The muscle fiber is further subdivided into myofibrils where thick and thin filaments are arranged longitudinally. A muscle fiber produces force by contracting.
Fig 2. The organization of skeletal muscle
A muscle fiber is surrounded by the sarcolemma. The sarcolemma is a thin semi permeable membrane composed of a lipid bilayer that has channels by which certain ions can move between the intracellular and the extra cellular fluid [37, 71].
There are two types of skeletal muscle fibers: Type I and Type II fibers. Type I are slow twitch fibers with high endurance (long-endurance such as distance running,), while Type II are fast twitch fibers with lower endurance (these muscles fatigue faster but are used in powerful bursts of movements like sprinting).
Muscle fibers are always grouped together sharing the same nerve fiber. This nerve fiber (axon), refer to Fig 1 is the transmission line of a nerve cell and runs from the spinal cord to the peripheral muscle. A nerve cell which innervates a group of muscle fibers is known as motor-neuron. Every cell is located close to the spinal cord.
Muscle fatigue is the inability of a muscle to generate force. It can be a result of vigorous exercise, but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction.
- Action Potential
There is a difference in the composition of the extra cellular fluid and intra cellular fluid in resting state [87]. The resting membrane potential is the voltage difference across the plasma membrane induced by the electrochemical potential difference. The major ions that maintain the resting potential are Na+ K+, Cl–, etc. When substantially large stimulus is applied, an action potential is triggered. This takes place only when the depolarization is sufficient to reach a threshold value (Fig 3).
Electrical muscle activity, electromyography (EMG), is the result of an external excitation of the muscle fiber (cell) which usually runs along the whole muscle from tendon to tendon. In its resting state, there is a potential of approximately -90mV across the cell membrane (the fiber wall) with zero reference on the outside. The fiber is an ‘on-off device’ in the sense that mechanical work is produced in twitches only, which have durations ranging from 35 to 75 ms (1).
Fig 3. Action potential
An action potential is a rapid change in the membrane potential followed by a return to the resting membrane potential. An action potential is propagated with the same shape and size along the whole length of a nerve or muscle cell [9].
- Discussion on Motor Unit (MU) Action Potential (AP).
The smallest functional unit of muscle contraction is the motor unit (Figs 1 and 4). A motor unit consists of a group of muscle fibers innervate by a single motor neuron (in other words, a motor unit consists of one motor neuron and all the muscle fibers it stimulates). Within a single motor unit, individual muscle fibers discharge nearly at the same time.
Fig 4. Motor Unit
CHANGE the line names to motor neurons
Motor unit recruitment (motor unit activatin) refers to the activation of additional motor units to accomplish an increase in contractile strength in a muscle. The higher the recruitment the stronger the muscle contraction will be. In physiology, an action potential occurs when the membrane potential of a specific cell location rapidly rises and falls: this depolarization then causes adjacent locations to similarly depolarize.
When an action potential is evoked in a motor nerve by a critical depolarization, an endplate, or the neuromuscular junction, potential is produced by acetylcholine release. Acetylcholine stimulates the membrane of the muscle fiber, or sarcolemma. This triggers rapid depolarization (about +20mV) and repolarization of the muscle fiber (Fig.3). The action potential is propagated along the sarcolemma and into the muscle fiber through the transverse tubules. The generation of an action potential takes place in all the muscle cells in the motor unit and is followed by synchronous contractions of all the muscle cells in the motor unit [71]. A motor-neuron together with its muscle fibers is called motor unit (MU). The number of muscle fibers coupled in one MU range from 3 (in small muscles) up to 1000 (in large muscles). All fibers belonging to the same MU are triggered simultaneously. The AP seen in by an electrode recording is the superposition of all the APs of the muscle fibers belonging to the triggered MU (MU-AP). The MU is usually activated repeatedly with a firing frequency of 10-50 Hz (3). A whole muscle usually consists of several hundreds of MUs.
The force development of the muscle is modulated by the CNS by recruitment (activation) of an appropriate number of MUs and by varying the firing frequencies of the recruited MUs (3). The MUs seem to be recruited in a fixed order according to the reversed size order of their motor-neuron (4).
The depolarization generates an electric field near the muscle fibers which can be detected by a skin surface electrode located near this field. The resulting signal is called the muscle fiber action potential. The combination of the muscle fiber action potential from all the muscle fibers of a single motor unit is the motor unit action potential.
When an electrical impulse originated from the central nervous system (CNS) reaches a point on the muscle (a point at which a nerve fiber comes in contact with a muscle fiber called innervation point) of the muscle fiber, a twitch is triggered via a biochemical transmission process. The biochemical process of transferring chemical energy into mechanical energy is elicited on both sides of the innervation point and spreads in both directions towards the ends of the muscle fiber (2). The biochemical process is accompanied by a transient drop of the membrane potential called depolarization. The electrical activity of the depolarization is detected with a electrode in the vicinity of the fiber. This signal is referred to as an action potential (AP). The AP resembles the transient membrane potential in Fig. 3.
- EMG Electrodes
In general, EMG can be sensed by two types of apparatus. One is a needle electrode that has high selectivity in deep muscles and the other is a surface electrode that is useful in detecting the integrated behavior of large muscle groups.
- Needle Electrodes
The action potential recorded during voluntary muscle contraction depends to a great extent on the type of recording electrode [16, 33]. Isolated motor unit action potentials can be recorded with the use of a needle electrode. The standard and bipolar concentric needle electrodes (Fig 5) as well as monopolar needle electrodes are commonly used in routine electromyography. Less commonly used special purpose electrodes include the multielectrode and flexible wire electrode. A flexible wire electrode [20] is used for kinesiology examination. The wires permit freedom of movement while they stay within the muscle.
Fig 5. Needle Electrodes
- Surface Electrodes
Surface EMG ( S-EMG) is a non-invasive technique that measures signals containing certain temporal characteristics useful for understanding the muscle’s response. Surface electrodes (Fig 6), place over the muscle, and register summated activities from many motor units [50]. S- EMG can be obtained by using electrodes affixed to the surface of the skin. With the development of very sophisticated electrodes, S-EMG is now being used in many areas of ergonomics, sports medicine and even in clinical applications. S-EMG is a very useful tool in detecting integrated muscular behavior.
Fig 6. Surface Electrodes
One advantage of the S-EMG technique is the convenience in terms of relative ease of application and lessened discomfort of the subject. On the other hand, there are few disadvantages in using the S-EMG technique. The surface electrodes do not have selectivity to a specific muscle fiber because of the wide pickup are from the muscle as compared to fine wire electrodes. The signals detected from the pickup area may originate in a deep muscle, or even worse, from different muscle groups.
Therefore, there are concerns about the validity of a recording. Although efforts have been made to quantify and determine the effect of cross talk, there is no established or easy way for S- EMG to eliminate this problem. In general, however, S-EMG is satisfactory for the analysis of temporal, force, or fatigue relationships [87]. The main advantage of surface electrode can be most effectively utilized when the simultaneous activity of many muscles is being studied in a large group of muscles.
In summary, the EMG signal can be detected either by intramuscular electrodes (needle or wire) or by surface electrodes attached to the skin over the studied muscle. In both cases the EMG is the sum of the contributions from a large number of MU-APs in the muscle. The MU-AP is attenuated substantially on its way through the tissue. Hence, with intramuscular electrodes, MU-APs adjacent to the electrode, strongly dominate over contributions from more distant MU-APs. With surface electrodes, a larger number of MUs contribute. Since the amplitude of the MU-AP rapidly decreases with distance, the main contribution to a S-EMG comes from the superficial part of the muscle (2). Hence, S-EMG is restricted to superficial muscles.
The electrically conducting part of a surface electrode usually consists of a silver-covered surface (10-50 mm2) which is taped or glued to the skin. The impedance between electrode and skin is reduced by applying some kind of electrode paste. The standard procedure is to use two electrodes, 10-20 mm apart, placed over the belly of the muscle and aligned in the muscle fiber direction. The two electrodes are connected to a differential amplifier with a third electrode placed somewhere else on the body as a reference. The input impedance and common mode rejection should be as high as possible.
- Features of EMG
Considerable progress has been made when EMG analysis has been used in ergonomics to investigate topics like musculoskeletal injury, low-back pain, and muscle fatigue from overexertion. Although modern instrumentation has greatly facilitated the acquisition of EMG data, many issues remain unresolved in the interpretation of EMG signals [87]. The basic features of EMG signals are first discussed in this section followed by the problems regarding each feature.
The information obtainable from EMG analysis can be generally divided into the following three categories. (1) The relationship between temporal aspects of EMG and anatomically associated movement. (2) The relationship between EMG and production of force. (3) The relationship between EMG and muscle fatigue.
- Temporal Information
The most basic information obtainable from an EMG recording is whether the muscle was on or off during an activity or at a particular time. In order for a muscle to be considered on, the EMG recording must exceed a certain threshold, whether defined by an arbitrarily of statistically predetermined level or by the noise level of the equipment responsible for the measurement. It often is more difficult to determine that a muscle is off because a muscle may infrequently be in a state of total relaxation.
Some researchers describe the threshold in an absolute value. This measure simply relates how active the muscle was during the experimental conditions. The measure is not an indication of muscle force, but simply a function of muscle usage. The signal can be quantified in several ways. Quantification may include peak activity, mean activity, activity as a function of a given position or posture, and rate of muscle activity onset [87].
To determine the on-off state, force, or fatigue present within a muscle, some form of EMG signal treatment usually is recommended and often required. If the raw or processed signal exhibited activity, the muscle was in use during the exertion [87]. Differences do exist, however, between the temporal characteristics of the EMG and the produced tension. The most apparent is the pure time delay. Initial tension levels in the muscle also influence the delay times seen at onset.
Most studies that now investigate the on-off state of the muscle are interested in the phasing of the EMG activities under various experimental conditions. A quantitative evaluation of muscle on-off state was performed by Marras and Reilly [65], using statistical analysis of muscle event time’s derived form processed EMG. They were interested in how the patterns of trunk muscle activation changed as the angular velocity of the trunk increased during controlled simulated lifting motion.
- EMG-Force Information
An EMG-force measurement seeks to quantify the average number and firing rate of motor units contributing to a particular muscle contraction, and to relate the quantity to the actual force produced. With increasing intensity of a contraction, more and more units will be recruited, and the unit firing frequency will also be increased. The observed motor unit activity reflects these changes as the resulting interference pattern becomes denser and of greater amplitude. The signal can be processed to estimate a numerical value (usually a percentage of a maximum voluntary contraction) to the level of EMG activity associated with the generation of a corresponding force.
However, a great deal of confusion exists regarding the relationship between processed EMG and produced force. Researchers have long suggested that EMG could be used to represent the active control input of the muscle, and that some relationship must exist. Some researchers have presented EMG as a direct indication of muscle force while others have presented very complex models using these signals to predict force. Many researchers have attempted to investigate the force of a muscle by simply observing the rectified and averaged (in some cases integrated) EMG signal in terms of the absolute number of micro volts generated and associated with a particular activity [87]. A direct comparison of the relationship between the EMG signal and force output has been reported by Lawrence and De Luca [56]. One aspect of this study investigated was whether or not the normalized surface EMG signal amplitude versus normalized force relationship was dependent on exercise level. The RMS value of the signal amplitude was used as the variant parameter because it is the parameter that more completely reflects the physiological correlates of the motor unit behavior during a muscle contraction. (This result showed almost linear relationship between normalized RMS and force) [7].
- EMG-Fatigue Information
Very little is known about general physical fatigue that is experienced after heavy work. Metabolic changes, such as an increased blood lactate concentration and a fall in pH, may contribute to the perception of fatigue but cannot fully explain the phenomenon [71].
Fatigue can be experienced slowly or rapidly depending on the type of work performed by a muscle. When the workload is relatively small, the slow motor units are initially excited. If the task requires fast and forceful contractions, fast motor units are also recruited for maximum output. Rapid muscle fatigue can be observed in the latter case.
- Localized Muscle Fatigue
We are interested in fatigue on a local level within the body. This is true particularly in musculoskeletal exertions. Typical externally visible symptoms of fatigue are loss of force production capabilities and localized discomfort and pain. This type of fatigue has become known as localized muscle fatigue (LMF) [17]. Muscle exertion levels do not necessarily need to be high to cause LMF. Isometric contractions of as low as 10% of maximum voluntary contraction have shown signs of LMF [87].
- Spectral Charges in EMG during Fatigue
During LMF, changes occur in the surface recorded EMG signal. Two of the most commonly cited changes are a shift in the frequency content of the signal toward the low end [17] and an increase in the amplitude. Many researchers have applied spectrum analysis methods to show the presence of fatigue [38]. Lindstrom [60] and DeLuca [27] contend that the spectrum shift and amplitude increase is related. They state that tissue acts as low pass filter. As the frequency content of the original signal shifts to the lower frequencies, more energy is transferred through the tissues to the electrodes. This energy transfer, in turn, increases the amplitude of the recorded signal.
A group of investigators have demonstrated a decrease of power density in the high frequency region of the EMG signal and increase in the low frequency region during fatigue contractions [46, 47, 51, and 54]. Many researchers have proposed physiologic explanations for the changes in amplitude and spectral characteristic [87]. Lindstrom et al. has demonstrated that the frequency shifts were almost entirely dependent on the propagation velocity of the action potentials [60]. The reduced propagation velocities have been linked to the production and accumulation of acid metabolites [70].
Some investigators, however, showed that the use of spectral analysis does not provide significant differences with fatigue [66]. They may be related to the fact that the frequency of the EMG signal can be altered by many factors, for example, by the change in load [32] or by the change in muscle length [8].
- Muscle Fatigue through microvolt
As described above, the EMG signal will increase its amplitude while the muscle is exerting a given amount of force in an isometric contraction. This may be due to the need to recruit more motor units to perform the same amount of work as the muscle fiber fatigue. Thus, by observing the processed EMG signal of a given portion of the muscle under constant force conditions, a quantitative indicator of the degree of muscle fatigue can be established. It is also important to note that this trend is evident only with surface electrodes.
- Limitations in Fatigue Analysis
Although the use of EMG in measuring localized muscle fatigue is well established and frequently used, the technique has limitations. It is important to understand some of these limitations before undertaking an EMG analysis in the field of ergonomics or biomechanics. The first problem is in the basis definition of fatigue. Because there is no universal definition of fatigue, agreement on the validity and meaning of EMG measures will be questioned. Other factors in LMF such as pain tolerance, motivation, and synergistic accommodation are not included in EMG analysis and yet have been argued to be important. Additionally, spectral shifts have been used for short-term contraction fatigue, but the use of EMG in long-term fatigue is questioned. For muscle fatigue that occurs over a longer period of time, possibly hours, the use of EMG has not been well established. In addition, shifts in the various EMG indices have been shown to decrease rapidly during the initial stages of a contraction, but do not decay as rapidly toward the end of a long session of work [87].
For more detailed information on the EMG background see (2).
References:
- Astrand P.O., Rodahl K, Textbook of work physiology, Second Edition. McGraw-Hill, New York, 1977.
- Basmajian J., DeLuca C.J., Muscles Alive, Fifth Edition, Williams& wilkins, Baltimore, 1985.
- Milne-Brown H. S. Stein R. B. , YemmR., Changes in firingrate of human motor units during linearly changing voluntary contraction. J. Physiol 230 (1973) 371-390.
- Haenneman E., Somjen G.,Carpenter D. O., Excitability and ihitabitability of motorneurons of different sizes. J. Neurophysiology 28 (1965) 599-620.
TAKE CARE OF THE REFERENCES
CHAPTER 2
