Therapists providing intervention to children with movement challenges have traditionally considered and have perhaps treated “muscle tone” specifically as an impairment impeding motor performance in daily living function. In our hands, we can “feel” the resistance, heaviness, responsiveness and changes of each child’s body with whom we interface in therapy. Our own motor system learns and remembers how each child’s limbs and body feel in the various positions we place them in and during the movement experiences we provide them in therapy. We know in our hands how quickly the child is able to bring their muscles on line and how much help they need from our cues in order to be able to activate, hold or change the subtle movements in their bodies. We store this data in our own kinesthetic memory; the force production, the speed of movement and the directional cues provided through our motor system as we interact with a client’s movement; we remember how that feels each session. It is like learning to dance with a partner, moving as one! The challenge of course lies in our capacity to measure and to accurately describe with words in a quantifiable and qualifiable ways indicating change in functional performance.
We also know from experience that changing the “feel” of the child’s body does not always translate into an automatic change in movement performance. There are many motor control components that contribute to what we know as “action”. Muscle activation, coordination, postural control and voluntary movement are all components of movement performance. In our practice, many of us see children with many different varieties of muscle tone occurring within a broad spectrum of possibilities. Floppy throughout their bodies, floppy in certain muscle groups, fluctuating in floppiness, stiffness, stiff in some muscles while floppy in other muscles and so on. We also know that the complex neural nets involved in producing tone or this state of readiness to move are influenced by a multitude of factors such as motivation, cognition, mood, arousal, hunger, illness, etc. further contributing to the complexity of this matter. As we influence this state of readiness to move with our intervention, we must consider the many other components of motor control.
Science has attempted to investigate this issue of tone as it contributes to motor control. To date we do not have an appropriate animal model that sufficiently mimics the tone changes we see in human neurological impairments. As a result, we don’t yet have a complete understanding of how tone is generated in the neuro-musclar system.
Early on in motor control research, it was believed that the brain worked like a step ladder, with each subsequent step working properly for the next step to function. Now, we know that the brain works more like a factory with many parts of the brain working together simultaneously and synchronistically to produce a single action. Our brain is an eloquently designed structure, built for efficiency! Many systems and parts working together to create the whole. This means in therapy that our ability to discover exactly which part of the nervous system isn’t working will be tricky! It also means that we have more than one road to the same end.
There are both neural and non-neural aspects involved in getting the body ready to move. Studies investigating the “neural part” of tone have examined muscle spindles or the sensory receptors located in the muscles that register stretch of muscle fibers. Continuous neural information from the brain seems to change the channel on the receptivity of the muscle spindle, increasing its reactivity to sensory input. A motor contraction is easily stimulated in the presence of small stretch stimulus. It remains unresolved to date whether this augemented drive of “tone” in spastic muscle is caused solely by an increase in excitation from the neural structures in the brain or whether there is a loss of inhibitory influences in the brain. Is it too much information or not enough inhibition? The jury is out.
In addition to the nervous system, we also have a musculoskeletal system that is affected by tone. This includes muscles, tendons, connective tissue and ligaments. We can examine muscles with ultrasound and with electromyography to determine how they are contracting and the timing of their activation. We can also do biopsies of muscle fibers and tissue to examine the cellular structure in different subjects with motor impairments. Studies conducted in the 1980′s emphasized changes in muscle fibers, emphasizing changes in muscle fiber types in response to spastic neural drive. These studies indicated that muscle fibers converted in structure, adapting to the continuous neural messages sent from the brain to the motor units in muscles. It appeared that the constant “on” message to a muscle changed the muscle fiber’s responsiveness to neural information. Muscles that were traditionally used for moving a limb, for example seemed to change in structure and function, altering the fiber’s potential for movement to a fiber responsible for posture. Muscles designed to move become muscles designed to hold on against gravity. With increasing sophistication in technology, science is now discovering that this conversion of fiber typing is not consistently observed in neuromotor dysfunction. Tone changes do not equal muscle fiber changes.
What we are finding in current research however, is that spasticity does change the cellular structure of muscle fibers rather than just the phenotype of a muscle fiber. Changes in sarcomere stucture, intracellular proteins and extracellular matrix substance have been observed in muscles identified as spastic in nature. Changes in the amount of connective tissue and collagen types were also consistently noted in the spastic muscles. Increased connective tissue or collagen where muscle fibers should be would suggest a definite change in a muscles potential to do it’s job properly!
Not only must we consider muscles that are stiff as problematic in motor performance, but also consider the challenges of children who are floppy or hypotonic. This floppy state of readiness to move is even more prevalent in children with neuro-motor challenges than the state of stiffness. Despite the frequency of a continuum of hypotonia, the etiology of this phenomenon is even less understood than hypertonia. Floppy tone is associated with a wide variety of pathophysiologies of the neurological, metabolic, genetic and musculoskeletal systems.
In children with hypotonia we typically observe a situation where musculoskeletal structures take on the role of maintaining the stability of posture against gravity; children will bend their joints into end range or lean into their joint surfaces, locking them in order to stay upright against the forces of gravity. These motor strategies help the child compensate for muscles that have a hard time “coming online”. It may be difficult for these children to use their muscles appropriately in concert to produce a movement, either holding on too tightly or with insufficient strength to produce smooth and efficient movement patterns. This interaction between the brain and the body is an integrated dance of inter-communication.
Although we are not yet be able to scientifically understand or measure this phenomenon of “tone”, we as therapists intuitively and functionally know that the state of readiness to move is a foundation for motor performance. Through sensory cues, environmental adaptations and the rehearsal of motor skills, this important substrate for movement continues to be both an implicit and an explicit aspect of our therapeutic intervention.