The compex and globus machines recruit the muscle indirectly through the stimulation of nerve endings.
ELECTROSTIMULATION-INDUCED CONTRACTION
Electrostimulation-Induced Contraction
Voluntary skeletal muscle contractions result from impulses commanded by the Central Nervous System (CNS) and transmitted through the nerves as electrical signals and eventually recruiting the desired muscles. The same can be achieved starting from an external electric impulse replacing the voluntary signal.
Tissue Sensitive to Induced Electrical Stimuli
Electrical impulses activating nerves are similar to electrical impulses activating muscles. Therefore one can decide whether to stimulate nerves and indirectly stimulate the muscles, or directly stimulate the muscles. However, there are differences between the two.
Direct Stimulation through Muscle Fibers
Direct stimulation of the muscle fibers bypasses all the rest of the CNS. However, this choice, even though possible, activates the muscles as if in a lab setting, by themselves and a bit out of of context, which is less conducive to training.
Difference between Rest and Threshold Potentials in Muscle
The other important factor is that the difference between rest potential and threshold of muscle fibers, i.e. the difference between -90 mV, and -55 mV, is 35 mV. In other words the electrostimulator will have to overcome this difference to stimulate muscle fibers.
Difference between Rest and Threshold Potentials in Nervous Fiber
The membrane potential of nervous fibers at rest is -70 mV, and the threshold to trigger an action potential is -55 mV. Therefore to excite nervous fibers it is necessary to change the potential of the membrane by only 15 mV. Comparing this value to the 35 mV calculated for muscle fibers, the difference is huge: one will need 57% less potential to stimulate nervous fibers.
Applying Electrostimulation on Nervous Motor Units Fibers
The diameter of motor nervous fibers is larger than other nervous fibers’. They are also characterized by an insulating liner that allows for faster transmission of action potentials, insulation from outside impulses and a very precise selection of which fibers are going to fire. This insulation effect tends to insulate nervous fibers even from the stimulus of an electrostimulator. Fortunately there is a gap just before the nervous fiber reaches the muscle, and from here it is possible to send an external electrical impulse to the muscle. In addition the neuromuscular plate is situated on the muscle surface, closer to the outer skin. Therefore the electrical signals to stimulate the muscle do not need to be to strong, and it is possible to limit side effects.
Source: (EMS DIGEST) http://docs.google.com/Doc?id=dgw665wn_33f8wb6ff9
CURRENTS
To obtain the desired contraction effect of electrostimulation, the current level will have to reach a compromise between a high enough current level to generate a strong contraction, and limited enough to exclude undesirable effects.
The current will have to be high enough. The excitation of the muscular tissue will be maximum when the electric current suddenly changes from zero to a certain value, and also when it suddenly changes from that value to zero.
Muscle fibers also have adaption capability, which means that they will tend to adapt to a certain current level. This means that if the current increase is too gradual muscle tissue will adapt to it, and the current will not elicit any contraction. Therefore the change of current will have to be sudden.
The type of current that reflects the characteristics just listed is a rectangular waveform, for which the current increase is practically instantaneous, which also has the following advantages:
Limited polarization effect
Limited nervous fiber adaptation
Good recruiting of nervous fibers
Low current level
Excitation Mechanism and Necessary Impulse
To excite the nervous tissue the following conditions are necessary
Enough current through the targeted tissue
Adequate duration of the stimulus for the muscle group
The next figure shows the relationship between current intensity and duration, which also changes for different muscle groups.
Chronaxie and Rheobase
The relationship between current intensity and duration has been determined by Lapique. As duration increases, the current intensity necessary to trigger a contraction decreases.
Another characteristic of body tissue is that of accomodation, which means that any tissue gets used to a particular stimulus and consequently needs next time a stronger stimulus to trigger a reaction. Lapique defined two parameters as reference points to characterize and compare the effects of electrical stimuli: rheobase and chronaxie.
Rheobase is defined as the minimum current intensity necessary to trigger an excitation (action potential), no matter how long the duration of the stimulus is.
Chronaxie is defined as the duration necessary to trigger a reaction, when current intensity is twice the rheobase. This value is an excellent compromise to trigger a good contraction in a reasonably short time, without generating any accommodation, and without causing any of the negative side effects.
Chronaxie in Various Muscular Groups
Chronaxie is an important parameter for electrostimulation, because it determines the duration of each impulse. Therefore the duration of each impulse has to change depending on the muscular group. Generally there are 6 different areas to stimulate, with 6 different chronaxie values and therefore 6 different impulse durations. Average values are the following.
Lower Leg, 430 microseconds;
Upper Leg, 380 microseconds;
Lower Torso, 330 microseconds;
Upper Torso, 280 microseconds;
Arm, 200 microseconds;
Forearm, 230 microseconds.
Source: (EMS DIGEST) http://docs.google.com/Doc?id=dgw665wn_33f8wb6ff9
It’s important for those who need to deeply understand the mechanisms of EMS, i.e. coaches who need to customize a training program to target specific goals, to highlight a very important and misunderstood feature of EMS: recruitment order.
Voluntary muscle activation activates muscle fibers in a very specific order, from smallest Slow-Twitch (ST) fibers, to largest Fast-Twitch (FT) fibers. Early research seemed to point out that this was reversed in EMS exercise: FT fibers were activated more easily than ST fibers. However, very recent research has demonstrates that EMS does not reverse the natural recruitment order, and it rather activates fibers indiscriminately, and onlywhich based on position relative to the electrode pads.
Voluntary recruitment order, as quoted from Zatsiorsky’s & Kraemer’s - Science and Practice of Strength Training; Human Kinetics, Champaign, IL:
The orderly recruitment of MUs (Motor Units) is controlled by the size of motorneurons (Hennemann’s size principle): Small motorneurons are recruited first, and requirements for higher forces are met by the activation of the large motoneurones that innerfate fast MUs.
Early studies that experimented on single fibers in a lab setting, found that FT larger fibers were easier to excite: see Gorman and Mortimer, The effect of stimulus parameters on the recruitment characteristics of direct nerve stimulation. However, those studies were performed on anesthetized animals, attaching electrodes directly to muscle fibers by cutting through the skin. Since this were the only available findings at the time, the literature on EMS started using this as a self fulfilling benchmark, which gained acceptance.
SOURE: (EMS DIGEST) http://docs.google.com/Doc?id=dgw665wn_33f8wb6ff9