Victor Forge
Muscle redundancy is a long-standing problem in motor neuroscience and biomechanics, referring to the fact that the human body has more muscles than mechanical degrees of freedom. This means that there are an infinite number of muscle activation patterns that can be used to achieve a single task goal. Muscle redundancy has implications for the robustness of the body to muscle dysfunction, as well as for motor control and learning. The optimization hypothesis and the muscle synergy hypothesis have been proposed to explain how the central nervous system coordinates redundant muscles, but the relationship between them is not yet well understood.
| Characteristics | Values |
|---|---|
| Definition | Muscle redundancy is having more muscles than mechanical degrees-of-freedom (DOFs) |
| Problem | Muscle redundancy has been a central problem in biomechanics and neural control |
| Challenge | The central nervous system (CNS) has to select muscle coordination patterns from a theoretically infinite set of possibilities |
| Lack of Attention | Little attention has been given to the clinical question of whether muscle redundancy grants the body robustness to dysfunction of a single muscle |
| Muscle Redundancy and Robustness | Muscle redundancy does not imply robustness to muscle dysfunction |
| Muscle Loss | The loss of certain muscles compromises force production significantly more than others |
| Lack of Robustness | Tendon-driven biomechanical systems exhibit surprisingly little robustness to dysfunction of even one muscle |
| Neck Musculature | The neck musculoskeletal system is characterized by complex anatomy and apparent muscle redundancy |
| Large Variable Movement Strategies | Having large variable movement strategies is extremely important to function |
| Muscle Synergy Theory | The nervous system builds the muscle activation commands by combining a few sets of activation (called modules, muscle synergies, or motor primitives) |
| Optimization Hypothesis | The brain can select the muscle activation pattern that minimizes the motor effort cost from among many solutions that satisfy the task requirements |
| Muscle-Synergy Hypothesis | Neurally established functional groupings of muscles alleviate the computational burden associated with motor control and learning |
| Redundant Structure | The human musculoskeletal system has a redundant structure, with more degrees of freedom (DoFs) than required to perform a task |
| Mathematical Tools | Mathematical tools for studying redundant systems exist but are not widely adopted in musculoskeletal simulation |
| Traditional Solutions | Traditional solutions to the muscle redundancy problem, such as the minimum effort criterion, are very popular but can hinder the validity of results |
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What You'll Learn
Muscle redundancy and robustness to muscle dysfunction
Muscle redundancy is a well-known phenomenon in biomechanics and neural control, where an individual has more muscles than mechanical degrees of freedom (DoFs). This presents the central nervous system (CNS) with an infinite set of options to perform a task, raising the critical clinical question of whether muscle redundancy grants the body robustness against the dysfunction of a single muscle.
The CNS can theoretically select from an infinite number of muscle activation patterns to perform a task, but the range of valid activation levels is limited and has significant implications for robustness. For instance, if a muscle can produce a force within a specific range, the implications for robustness will depend on whether the CNS can find valid coordination patterns if that muscle is lost or weakened.
Research on human fingers and legs has shown that tendon-driven biomechanical systems have surprisingly low robustness to the dysfunction of even one muscle. Further modelling of a multi-joint, multi-muscle leg demonstrated that this lack of robustness generalizes to whole limbs. These findings provide a biomechanical basis for understanding why redundant motor systems can be vulnerable to even mild neuromuscular issues.
However, muscle redundancy does offer some advantages. For example, individuals with neck muscle redundancy can exhibit a large variation in neck muscle activation strategies for the same task, allowing them to substitute movement with unaffected musculature and complete daily tasks even with a stiff neck. This ability to substitute movement helps injured structures recover, which is an important function of the cervical skeletal muscle system.
While muscle redundancy does not imply robustness to muscle dysfunction, it does provide the CNS with numerous options for performing tasks, and further research is needed to clarify the relationship between muscle redundancy and robustness to muscle dysfunction.
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The central nervous system and muscle coordination
The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system. The CNS includes the brain and spinal cord, while the peripheral nervous system consists of all other nerves in the body. The CNS is responsible for receiving, processing, and responding to sensory information. The brain, an organ of nervous tissue, is responsible for responses, sensation, movement, emotions, communication, thought processing, and memory. It is the brain that controls how we think, learn, move, and feel. The spinal cord carries messages back and forth between the brain and the nerves that run throughout the body.
The cerebellum, a part of the CNS, helps coordinate and fine-tune movement and balance. It uses Purkinje cells and cerebellar peduncles to communicate with other parts of the brain. The superior cerebellar peduncle connects the cerebellum to the midbrain, facilitating arm and leg coordination. The inferior cerebellar peduncle connects the medulla and cerebellum, maintaining balance and posture. The middle cerebellar peduncle is a one-way communication method from the pons to the cerebellum, alerting it about voluntary motor actions. The cerebellum constantly communicates with the cerebral cortex, taking higher-level instructions from the brain, processing them, and then sending messages to the cerebral motor cortex to make voluntary muscle contractions.
The basal ganglia, located deep within the cerebral white matter, are responsible for muscle movements and coordination. The thalamus, meanwhile, is the brain's relay center, receiving afferent impulses from sensory receptors throughout the body and processing the information for distribution to the appropriate cortical area. The hypothalamus, though small, is vital for maintaining homeostasis and connecting the CNS to the endocrine system. It regulates heart rate, blood pressure, appetite, thirst, temperature, and the release of various hormones.
The nervous system links thoughts and actions by sending messages (as electrical impulses) from the brain to other parts of the body. Nerves and muscles work together in the neuromuscular system to make the body move as desired and manage important functions like breathing. Messages are carried from the brain to the muscles through the spinal cord, activating the muscles. The neurons that make up these pathways are called motor neurons. Incoming messages are sent from the senses back to the spinal cord and brain along sensory pathways, via sensory neurons. Each motor neuron ending sits very close to a muscle fibre, releasing a chemical that signals the muscle fibre to contract, resulting in muscle movement.
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Muscle synergy theory
The theory has been applied in neurorehabilitation, robotics, and sports. In neurorehabilitation, muscle synergy patterns have been proposed as physiological markers of motor cortical damage, helping to guide rehabilitation approaches for patients with stroke or trauma. In robotics, muscle synergy theory has been explored for implementing control systems for upper-limb exoskeletons. In sports, the theory has been used to understand muscle synergy organisation during specific technical actions in badminton and rhythmic gymnastics, as well as the motor control underlying throwing movements.
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The optimization hypothesis
Muscle redundancy is a well-known phenomenon in which the central nervous system (CNS) has numerous options to perform a task. This is because there are more muscles than mechanical degrees of freedom, resulting in an infinite number of muscle activation patterns to achieve a goal.
The hypothesis posits that the brain will choose the muscle activation pattern that minimizes the motor effort required to complete a task. This is a purely mathematical operation, but it provides insight into how our redundant muscles are coordinated. For example, when producing a two-dimensional torque vector in the shoulder, the brain may select a muscle activation pattern that reduces the overall effort needed.
Furthermore, the hypothesis has been applied to understanding the cervical muscular system. In this context, muscle redundancy allows individuals to exhibit large variations in neck muscle activation strategies for the same task, which is important for overall function. For instance, if someone wakes up with a stiff neck, they can still substitute movement with unaffected musculature and complete their daily tasks, allowing strained tissues to recover.
However, the optimization hypothesis does not address the question of robustness to muscle dysfunction. While muscle redundancy theoretically provides infinite activation levels for a given task, the loss of certain muscles can significantly impact force production. This suggests that muscle redundancy may not provide sufficient robustness to compensate for the loss or dysfunction of even a single muscle.
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The cervical muscular system and muscle redundancy
Muscle redundancy is a well-known phenomenon in biomechanics and neural control, where the body has more muscles than mechanical degrees of freedom. This presents a problem for the central nervous system (CNS) as it must select muscle coordination patterns from an infinite set of possibilities.
The cervical muscular system, or the neck musculoskeletal system, is characterised by complex anatomy and apparent muscle redundancy. In this system, there is more neck musculature than there are directions in which independent motion can occur. This leads to individuals exhibiting large variations in neck muscle activation strategies for the same task. For example, waking up with a stiff neck could be caused by a variety of factors such as overuse, sleeping awkwardly, or tension. Despite strained neck muscles and difficult head movement, we can substitute movement with unaffected musculature and complete daily tasks. This ability to substitute movement with unaffected fibres allows injured structures to recover, an important function of our cervical skeletal muscle system.
The neck muscles support and enable the movement of the head and neck. The musculature of the neck can be divided into anterior, lateral, and posterior groups based on their position in the neck. They can be further divided into more specific groups based on depth, precise location, and function. Sternocleidomastoid is a key landmark as it divides the neck into the anterior region and vertebral regions.
The cervical spine functions as bony protection of the spinal cord as it exits the cranium. Despite the presence of seven cervical vertebrae, there are eight pairs of cervical nerves, termed C1 to C8. C1 through C7 exit the spine cranially to its associated vertebrae, while C8 exits caudally to C7. The cervical nerves C1, C2, and C3 control forward, backward, and side head and neck movements.
When exercising the deep cervical flexors, it is important to target these muscles specifically. For example, when using the Pendulum 4 or 5 Way Head and Neck Machine, one should use holes 12, 13, 14, or 15 on the cam to target these muscles and elicit the most effective training response.
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Frequently asked questions
Muscle redundancy is when there are more muscles than mechanical degrees of freedom (DoFs). In other words, there are more muscles than are required to perform a certain task.
Muscle redundancy is a natural occurrence in the human body. It is caused by the over-availability of muscles in the musculoskeletal system.
Muscle redundancy grants the central nervous system (CNS) numerous options to perform a task. This can lead to individuals exhibiting large variations in muscle activation strategies for the same task.
No, muscle redundancy does not imply robustness to muscle dysfunction. While it theoretically allows muscles infinite activation levels for a given submaximal task, the range of these activation levels has critical implications for robustness.
