During the last decade, proprioceptive training has been described as one of the most important aspects in a rehabilitation or training program. Training facilities often use the term “proprioceptive training” to describe balance exercises with the premise that it will prevent injuries to occur. Rehabilitation professionals also use “proprioceptive training” exercises as a mean to improve and recover the sensorimotor function.
Proprioception and balance are two different concepts. However, balance tests are often used in order to assess proprioceptive function. Even for testing the efficacy of a training program on proprioceptive function, researchers often use balance measures or tests. So, this leads to an important question: are we really improving proprioception, or are we just improving balance?
First, it’s important to understand the definitions of proprioception and balance.
1. What is proprioception?
Proprioception involves awareness of joint position (joint position sense), movement (kinesthesia), and force, heaviness, and effort (force sense) (Martin and Jessell, 1991; Riemann and Lephart, 2002).
Proprioception is the product of sensory information supplied by specialized nerve endings termed mechanoreceptors, i.e., transducers converting mechanical stimuli to action potentials for transmission to the CNS (Martin and Jessell, 1991; Yahia et al., 1992).
According to this, proprioception is an unconsciously perception, an afferent information from the proprioceptors.
Mechanoreceptors specifically contributing to proprioception are also termed proprioceptors, and can be found in muscle, tendon, joint, fascia and skin. (see table 1)
Table 1. Mechanoreceptors of the human body
The muscle spindles, found in all skeletal muscles in parallel with the extrafusal muscle fibers (Peck et al., 1984; Kulkarni et al., 2001; Banks, 2006) are considered the most important source of proprioception (Gordon and Ghez, 1991; Proske and Gandevia, 2012). They are highly sensitive and their density varies widely throughout the body, reflecting different functional demands. The sub-occipital muscles of the neck have an exceptionally high density of muscle spindles, thought to reflect the cervical spine’s unique role in head and eye movement control (Liu et al., 2003). Importantly the sensitivity of the muscle spindles can be adjusted via innervation of the polar ends of the intrafusal muscle fibers by gamma motorneurons (Gordon and Ghez, 1991).
Joint proprioceptors have historically been considered “limit detectors”, stimulated at the extremes of joint range-of-motion (ROM) (Burgess and Clark, 1969). However it is now known that joint proprioceptors provide input throughout a joint’s entire ROM under both low and high load conditions stimulating strong discharges from the muscle spindle and are thus vital for joint stability (Sojka et al., 1989; Johansson et al., 1990; Needle et al., 2013).
This is why mobility is so important to perception. If mobility is reduced, afferent information from the proprioceptors is likely to be negatively affected, resulting in less awareness.
2. What about balance?
Balance is a mechanical term describing the state of an object when the resultant loads acting upon it are zero (Pollock et al., 2000). An object is in balance when its centre of gravity (COG) falls within its base of support (BOS), preventing falling.
Human balance, or postural control, is the capacity of maintaining stability by changing line of gravity (LOG) within BOS, or by changing BOS in order to prevent falling (Pollock et al., 2000). The main difference between objects and humans is that among humans, even in the case that LOG falls out their BOS, that doesn’t mean falling, because we can recover balance through stepping or swaying, for example. Additionally, we also need LOG falling out their BOS, in order to accelerate, changing direction, and other sort of dynamic tasks.
Also, we can further define human balance such as maintenance of a static posture, movement between different postures and reaction to an external stimulous. To achieve these goals, one needs voluntary but also reflexive control of movement. (Horak, 2006)
Therefore, human balance relates to the conscious and reflexive capacities to control their COG, LOG and BOS positions, relative to intrinsic and/or extrinsic disturbances.
3. What is proprioceptive training?
Proprioceptive training is often described as single leg drills using different implements, such as unstable surfaces of various types. Unstable surfaces was assumed to create a proprioceptively enriched environment that progressively challenges the proprioceptors and nervous system. But is this an absolute truth?
Analyzing the definition of proprioception stated in the beginning of this text, we can conclude that proprioceptive training refers to any type of intervention that promote proprioceptive feedback and afferent information to the CNS. So, even simple mobility drills are proprioceptive.
Additionally, it is assumed that by doing exercises which stimulate proprioceptors, proprioception will improve, and that this improvement leads to improved balance. However, there is no evidence that proprioception can be improved by training.
In order to improve proprioception, at least one of the following must be improved: acquisition of mechanical stimulus, its conversion to neural signal, followed by its transmission to the CNS. It is known that the velocity conversion of stimulus into neural signal and its transmission to the CNS is not variable, being its value fixed. (Lepart and Fu, 2000)
So, the only possible way left to improve proprioception is through improved proprioceptors sensivity, such as muscle spindles. However, it has only been seen in research an increase in muscle spindles fusimotor firing rate, which may be due to increased alpha-gamma coactivation during voluntary stiffening of a muscle. So, it does not necessarily mean that proprioception was improved, and more studies on this are required. (Granit, 1970)
4. Practical applications
Balance improvements involves not only neuromuscular or musculoskeletal factors, but is also dependent on motor learning and CNS function.
Closed-loop activities (where afferent information assumes an important role) and slow to moderately fast conscious and reactive movements, such as static single leg balance, require an important contribution of proprioceptive feedback. However, in order to get this feedback it takes time, around 100 miliseconds. This period of time exceeds the time available to avoid injuries during time-critical tasks such as running, where ground reaction force reaches to the injurious level in less than 50 miliseconds. So, proprioceptive feedback is not effective enough to avoid sports injuries. (Magill, 2010; Milia et al., 1998)
Instead, open-loop system (mainly efferent information from CNS to musculoskeletal system) assumes a very important role of preventing injuries to occur, being the CNS crucial to this antecipatory mechanism.
Putting this, acquisition and practice of motor skills under different contexts such as non-reactive and reactive circunstances, challenging decision-making, single or sequenced patterns, low and high intensities are extremely important in order to achieve motor learning, and subsequent injury prevention and performance enhancement.
5. Final thoughts
Until today, it has not been proved that proprioception can be improved by exercise. Despite that, proprioceptive information is only important during slow closed-loop activities, such as static single leg balance. However, being the vast majority of sports played with high speed, and injuries often occuring during high speed movements, proprioception is not effectively used under this circunstances. On the other hand, CNS has a very important role, so emphasis should be given to motor learning strategies in order to enhance performance and reduce the likelihood of injury.
(based and adapted from Kim, Van Ryssegem & Hong, 2011)
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