It is commonly accepted regular physical activity and exercise has many positive benefits to the physiological, muscular and neurological systems. Therefore, it follows that researchers have considered the effects of physical activity and exercise on proprioception. Following other research into proprioception, outcome variables include balance ability using centre of pressure measures to record proprioception acuity. For example, Shim et al., (2010) and Gauchard et al., (1999) considered the effects of a range of activities, namely Tai Chi (a traditional Chinese mind-body exercise), Yoga, soft gymnastics, running, cycling and swimming, on balance ability. Results confirmed participants who were regularly active had better balance than sedentary controls. Furthermore, those people participating in more position focussed sports such as Tai Chi, Yoga and soft gymnastics, had better balance ability compared to aerobic
based sports; however the exact mechanism for this increase in balance was not confirmed. The majority of participants considered in proprioception and physical activity are elderly (on average above 60 years) and as previously stated age may have a detrimental effect on proprioceptive ability, this can lead to an increased risk of falls and degenerative joint diseases (Van Heuvelen et al., 1998).
As stated elderly populations are also most commonly used in studies considering JPS and TTDPM outcome measures and Tai Chi is a popular activity of interest. Tsang and Hui-Chan (2003, 2004) found that elderly Tai Chi practitioners (>60years old) had better knee joint position sense when compared to aged matched sedentary controls. Elderly Tai Chi practitioners and golfers also may have comparable knee JPS to young healthy controls (Tsang and Hui-Chan, 2004) when JPS is collected using an IKD and passive-passive angle replication. Regular Tai Chi practice can also improve TTDPM in the ankle and knee joint (Li
et al., 2008a, Xu et al., 2003). Elderly (>65years) Tai Chi practitioners had the lowest TTDPM in the ankle compared to aged matched elderly swimmers and runners and sedentary controls (Li et al., 2009, Xu et al., 2003). However, the same group was only better in knee flexion TTDPM, not knee extension when compared to the other groups. The authors attributed this to the most common position used in Tai-Chi, which has the knees flexed. Aside from Tai Chi, regular aerobic exercise programmes three times a week or more appear to improve weight bearing knee joint position sense (Petrella et al., 1997). Active adults aged 60 years or over had significantly better knee joint position sense compared to aged matched sedentary controls. The active elderly also had comparable ability to young sedentary people. However, the young active group had the best knee JPS out of the three groups. It should also be noted again the measurement protocol (fully weight-bearing) was too demanding for the two elderly groups, they were not able to weight-bear unaided during knee flexion and extensions. Therefore, knee JPS was collected differently in these groups; this is an obvious limitation of the study and hence application of the findings. Petrella’s et al., (1997) study results are supported by Ribeiro and Oliveira’s (2010) open kinetic chain knee JPS study on elderly and young groups. Again, exercising (aerobic, flexibility and strength training) older people had significantly better JPS ability to non-exercised older people and comparable results to non-exercised young. The evidence strongly suggests regular physical activity and exercise can attenuate the age related decline of knee joint proprioception.
Further supporting evidence shows kinaesthetic sense may improve after a period of physical activity and exercise training (Jacobson et al., 1997, Li et al., 2008a, 2008b). Li et al., (2008a)
reported a significant improvement in knee TTDPM into flexion and extension in an elderly population (>60years) following 16 weeks of Tai Chi training. However, ankle TTDPM was not improved; this may be explained again by the most common Tai Chi position that already focusses on knee flexion positions. Furthermore, Jacobson et al., (1997) provided evidence shoulder TTDPM in a younger sample group (aged 20-45 years) can be improved following 12 weeks of Tai Chi training. Moreover, six weeks of strength training, both loaded and using body weight, can improve both dynamic (TTDPM) and static (JPS) knee proprioception in older women (>65years) (Thompson et al., 2003). In this instance, proprioception was measured using a self-built device for TTDPM and electrogoniometer for JPS. More acute effects of training on knee proprioception in a young adult group were considered by Ju et al., (2011). Repetitive passive knee flexion and extension movements were applied to each participant 30 times at three different angular velocities (2°/s, 90°/s and 150°/s). At the two higher velocities knee JPS (passive-active repositioning) and TTDPM improved. This has implications on rehabilitation programmes as continuous passive movement is a common form of intervention. Overall, evidence suggests elderly and younger populations can modify their knee proprioceptive ability following physical activity training.
The majority of evidence indicates moderate and regular physical activity can improve knee proprioception. However, we know the total number of mechanoreceptors does not increase with exercise (Ashton-Miller et al., 2001) so other peripheral adaptations must occur. Morphological changes may take place in the muscle spindles, specifically reduction in the latency and increase in the amplitude of stretch reflexes (Hutton and Atwater, 1992). It may also be muscle spindle sensitivity is increased following the repetition of motor skills which increases reliance of afferent information during performance of a skill (Thompson et al., 2003, Ju et al., 2011). Muscle strength improves with regular physical activity and exercise; Petrella
et al., (1997), Tsang and Hui-Chan (2003) and Thompson et al., (2003) postulate this increase causes more neuromuscular control over the movement and hence more efficient proprioception. However, improvements in muscular strength would only explain active measurements of proprioception.
It is theorised central adaptations also occur following participation in regular physical activity and exercise. It is accepted the brain has plasticity and as such adapts to regular experiences such as physical activity (Ju et al., 2011). The muscle spindle gain (the receptor output firing rate / magnitude of the input stimulus) is modulated via the gamma motor neuron route during repetitive movements of physical activity (Ribeiro and Oliveira, 2010, Tsang and Hui-Chan,
2003). Specifically, the muscle spindle output is increased during repetitive movements facilitating plastic changes in the central nervous system which over time increases the strength of the synaptic connection and modifies the organisation and number of connections among neurons in central pathways (Ribeiro and Oliveira, 2010, 2007, Tsang and Hui-Chan, 2003). This results in modification of the cortical maps and hence cortical representation of the joints and thus an improvement in proprioception.
It is important to state proprioceptive adaptations as a result of regular physical activity and exercise only occur in muscle mechanoreceptors as these are the only receptors that are centrally modulated (Thompson et al., 2003). However, cutaneous receptors do respond to changes in temperature (Green, 1977, Inman and Peruzzi, 1961). As skin temperature increases excitation thresholds decrease and hence the reaction latency decreases (Inman and Peruzzi, 1961, Gescheider et al., 1997). The amplitude of the impulse from cutaneous receptors may also increase with temperature increases (Inman and Peruzzi, 1961). However, these relationships are curvilinear, specifically an inverted U shape; the optimum condition for sensitivity of pacinian corpuscles found in the skin occurs at approximately 37°C (Green, 1977). Therefore, during physical activity and exercise in which temperature increases cutaneous receptors will be more sensitive. The only adaptation that may exist in the articular receptors following regular physical activity and exercise in an increase in joint laxity but this is more likely to occur as a result of longitudinal participation in elite level sport such as ballet (Grahame and Jenkins, 1972).
In conclusion, evidence indicates a positive effect of regular physical activity and exercise on knee proprioception. Whether this adaptation occurs peripherally or centrally or in combination is not yet known (Ashton-Miller et al., 2001). However, as the muscle spindle sensitivity is dependent on both the input stimulus and the central efferent signals it would be intuitive to suggest it is most likely a combination of components that can improve proprioception.