Dominic Stone
Voluntary muscle activation (VA) is a measure of the central drive to the motor neurons during a voluntary effort that is translated into force generation. In other words, it quantifies the muscle's maximum force-generating capacity under given task constraints. The interpolated twitch technique (ITT) is a non-invasive method for assessing the completeness of muscle activation in clinical settings. The simultaneous measurement of VA and the evoked V-wave amplitude may provide greater and more precise insights into neural drive to skeletal muscle and how this drive results in motor-unit recruitment and torque production. This is particularly important in the context of physical therapy for elderly individuals, as well as for children with cerebral palsy, where interventions for increasing force production are necessary.
| Characteristics | Values |
|---|---|
| Definition | Voluntary muscle activation (VA) is the amount of central drive to the motor neurons during a voluntary effort that is translated into force generation. |
| Use | VA is used to quantify a proportion of the muscle’s maximum force-generating capacity under given task constraints. |
| Limitations | VA does not provide information about the magnitude of central drive reaching the spinal motor neurons, whether they are firing at their maximum rates, or the multitude of factors that determine the net excitability of the motor neuron pool. |
| V-wave | The first volitional wave (V-wave) is evoked through the supramaximal stimulation of a peripheral mixed nerve during a voluntary effort. |
| V-wave and H-reflex | The evoked V-wave response involves similar spinal circuitry as the Hoffman reflex (H-reflex), which has been described as the electrical analogue to the stretch reflex. |
| V-wave and MVC | V-wave amplitude increased linearly from 20% to 100% MVC while VA increased in a linear manner at lower force levels, but plateaued by 80% of MVC. |
| Incomplete VA | Incomplete voluntary muscle activation may contribute to impaired muscle mechanical function and physical function in older adults. |
| Power training | Exercise interventions, such as power training, have been shown to increase voluntary muscle activation in older adults. |
| Cerebral palsy | Cerebral palsy (CP) can lead to profound weakness in affected portions of the body, and children with CP have been found to have lower agonist voluntary muscle activation. |
| Non-linear relationship | There is a non-linear relationship between muscle voluntary activation level and voluntary force, which has been measured using the interpolated twitch technique (ITT). |
| Transcranial magnetic stimulation | Transcranial magnetic stimulation (TMS) may underestimate VA compared to motor point stimulation. |
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What You'll Learn
Voluntary muscle activation and evoked volitional-wave responses
Voluntary muscle activation (VA) is a measure of the amount of central drive to the motor neurons during a voluntary effort that is translated into force generation. It quantifies the muscle's maximum force-generating capacity under given task constraints. The interpolated twitch technique (ITT) is a non-invasive method for assessing the completeness of muscle activation in clinical settings.
The first volitional wave (V-wave) is evoked through the supramaximal stimulation of a peripheral mixed nerve during a voluntary effort. The evoked V-wave response involves similar spinal circuitry as the Hoffman reflex (H-reflex), which has been described as the electrical analogue to the stretch reflex.
The simultaneous measurement of VA and the evoked V-wave amplitude may provide greater and more precise insights into neural drive to skeletal muscle and how this drive results in motor-unit recruitment and torque production. A study on college-aged men and women found that V-wave amplitude increased linearly from 20% to 100% MVC while VA increased linearly at lower force levels, plateauing by 80% MVC.
The relationship between muscle voluntary activation level and voluntary force has been studied using the interpolated twitch technique (ITT). This technique involves applying a supramaximal electrical stimulation to the tibia nerve at rest and during contractions. The voluntary activation level is calculated as the interpolated twitch torque relative to the control twitch torque.
Transcranial magnetic stimulation (TMS) has also been used to estimate VA by expressing a TMS-evoked twitch force during a contraction as a percentage of the estimated resting PT force. However, VA was found to be substantially underestimated when using TMS compared to motor point stimulation, especially in non-fatigued muscles.
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Voluntary activation levels (VA)
Voluntary activation (VA) is a measure of the amount of central drive to the motor neurons during a voluntary effort that is translated into force generation. In other words, VA quantifies the proportion of a muscle's maximum force-generating capacity under given task constraints.
The interpolated twitch technique (ITT) is a non-invasive method to study human muscle activation and assess the completeness of muscle activation in clinical settings. The voluntary activation level (VA) is calculated by comparing the amplitude of the SIT force with the actual or predicted PT force. The formula for this is: VA% = (1−SITforce/PTforce) x 100. The relationship between VA and true voluntary contraction force has not yet been formally confirmed, and the predictive ability of VA has only been speculated.
The ITT has been used in studies to examine the effects of power training on voluntary muscle activation in older adults. These studies found that voluntary muscle activation improves with power training and is associated with changes in gait speed. In addition, high-intensity progressive resistance/power training has been shown to partially reverse incomplete voluntary muscle activation in older adults.
VA can also be estimated by expressing a TMS-evoked twitch force during a contraction as a percentage of the estimated PT at rest. However, VA has been found to be substantially underestimated when using TMS compared to motor point stimulation in non-fatigued and fatigued elbow flexors.
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Incomplete voluntary muscle activation
Voluntary muscle activation (VA) is the amount of central drive to the motor neurons during a voluntary effort that is translated into force generation. In other words, VA quantifies the proportion of a muscle's maximum force-generating capacity under given task constraints.
Exercise interventions have been shown to increase voluntary muscle activation in older adults. Specifically, high-intensity progressive resistance and power training have been found to partially reverse incomplete voluntary muscle activation. For instance, a study found that 12 weeks of progressive high-load power training improved voluntary muscle activation in mobility-limited older adults.
Furthermore, eccentric contractions, where activated muscles are forcibly lengthened, may also result in incomplete muscle activation. It is challenging to achieve maximum force during eccentric contractions, even with voluntary commands.
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Voluntary muscle activation in children with cerebral palsy
Voluntary muscle activation (VA) is the amount of central drive to the motor neurons during a voluntary effort that is translated into force generation. It quantifies a proportion of the muscle's maximum force-generating capacity under given task constraints.
Children with cerebral palsy (CP) often experience profound weakness in the affected portions of their bodies, including the trunk and extremities. This is caused by low force production, which is attributed to either incomplete recruitment or decreased motor unit discharge rates during maximum voluntary contractions. This means that children with CP have lower agonist voluntary muscle activation and greater antagonist coactivation.
A study by Stackhouse et al. (2005) compared 12 children with CP (aged 7-13) to 10 unaffected children (controls, aged 8-12) and found that children with CP were significantly weaker and had lower agonist voluntary muscle activation. The study also examined antagonist coactivation during agonist MVICs, contractile properties, and fatigability using electrically elicited tests.
The results of this study and others like it can help to design interventions to increase force production in children with CP. For example, the use of enhanced feedback and neuromuscular electrical stimulation may be helpful for strengthening muscles that cannot be sufficiently activated through voluntary effort alone. Additionally, high-intensity strength training has been shown to improve force production, walking velocity, and gross motor function in children with CP.
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Voluntary muscle activation and physical activity level
Voluntary muscle activation (VA) is a measure of the amount of central drive to the motor neurons during a voluntary effort, which is translated into force generation. It is a quantification of the muscle's maximum force-generating capacity under given task constraints. The interpolated twitch technique (ITT) is a non-invasive method for assessing the completeness of muscle activation in clinical settings.
The relationship between VA and voluntary force is non-linear, and this relationship needs to be clarified for correct use in clinical settings. The ITT measures the amplitude of the twitch force and contraction force, and the voluntary activation level should be 100% if no twitch force can be evoked. The ITT can also be used to study human muscle activation and the central nervous system's ability to fully activate the contracting muscle.
Voluntary muscle activation is influenced by factors such as physical activity level, functional status, and lifestyle. Exercise interventions, particularly high-intensity progressive resistance or power training, can increase voluntary muscle activation in older adults, improving gait speed and muscle strength. In sedentary older individuals, submaximal levels of voluntary muscle activation have been observed, indicating that low functioning and sedentary lifestyles may contribute to incomplete knee extensor voluntary muscle activation.
Incomplete voluntary muscle activation is a key mechanism influencing muscle mechanical function and gait speed in older adults. The deterioration of the neural system with age contributes to impairments in lower limb muscle function and physical function. Thus, voluntary muscle activation and physical activity levels are closely related, as physical activity and exercise interventions can improve voluntary muscle activation, particularly in older adults.
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Frequently asked questions
Voluntary muscle activation (VA) is the amount of central drive to the motor neurons during a voluntary effort that is translated into force generation.
The relationship between muscle voluntary activation level and voluntary force is non-linear. The interpolated twitch technique (ITT) is a non-invasive method to study this relationship.
Age impacts voluntary muscle activation, with deterioration of the neural system contributing to impaired muscle function and physical function in older adults.
Voluntary muscle activation can be improved through progressive power training, which can increase activation levels and improve physical function.
