Joint Proprioception

Ruffini and Pacinian receptors have been identified in canine cruciate ligaments and are innervated by axons penetrating from the peripheral synovium surrounding the ligament.³ The proximal third of the ligament contains a greater number of receptors.⁴ The cranial cruciate ligament is capable of sending afferent input to the central nervous system, and it is important to the biomechanical and proprioceptive functions of the stifle joint, including characteristics of movements and position-related stretches of these ligaments.⁵ The effects of the gamma-muscle-spindle system in the muscles around the knee are so potent that stretching of the cruciate ligaments at relatively moderate loads may induce major changes in responses of the muscle spindle afferents.⁵ These changes may result in changes in the muscular stiffness around the knee joint, helping to affect functional stability of the knee joint.

Mechanoreceptors have been studied in torn and reconstructed anterior cruciate ligaments (ACL) in animals and humans.⁶ In humans with untreated ACL lesions, normal mechanoreceptors remain in the ligament 3 months after injury. The number then gradually decreases, and by the ninth month, few free nerve endings are present.

The correlation between the number of mechanoreceptors in ACL remnants and joint position sense just before an ACL reconstruction in human patients has been studied.⁷ A positive correlation between the number of mechanoreceptors and accuracy of the joint position sense was found, suggesting that proprioceptive function of the ACL is related to the number of mechanoreceptors. Therefore consideration should be given to preserving cruciate ligament remnants during stabilization surgery if the ligament is relatively normal. However, a study in cats that evaluated joint capsule afferent neuron stimulation before and after transection of the cranial cruciate ligament suggested that capsule afferents continue to behave normally after ligament transection, despite the increased joint laxity.⁸

Proprioceptive ability declines with aging in humans.⁹ A randomized clinical trial evaluated the effects of an 8-week balance program on proprioception compared with only a fall-prevention program in healthy older adults (mean age = 73.9).¹⁰ Each group was assessed before and after the study, and a control group also underwent follow-up testing 8 weeks after intervention. Additionally, this study compared the intervention group to a younger, healthy population who were only assessed once for comparison. Interventions included typical balance and proprioceptive exercises such as activities on foam, foam balance beams, wobble board activities, and others. Subjects underwent 1-hour sessions three times per week, for a total of 8 weeks. The balance group showed significant improvement within the group in velocity discrimination from before and after intervention. In addition, there were significant differences after intervention between both treatment groups. In fact, the balance group showed no significant differences in velocity values after training when compared with younger adults. However, no other differences in joint position sense or threshold to perception of movement were encountered. Because velocity, regulated mainly by muscle spindles, is considered vital to postural stability, this study suggests that proprioceptive training could decrease the risk for falls in older adult humans. However, these results were not maintained long term, suggesting that continued training is necessary.¹⁰

Proprioception is further impaired in older adult patients with knee osteoarthritis (OA), which may contribute to additional functional impairment.1,9,11–13 On the other hand, the pain of osteoarthritis does not seem to have good association with disability and joint proprioception.¹⁴ Although patients with arthritis have decreased joint position awareness, physical therapy may improve joint position sense and proprioception through participation in a carefully constructed sensorimotor rehabilitation program in which pain, effusion, and fatigue are minimized.

In healthy individuals, muscle receptors are the primary determinants of joint position sense, and capsular receptors have a secondary role. Studies have been performed in humans to test position sense of the knee joint before and after fatigue.15,16 With fatigue, there appears to be a change in mechanism of appreciation of joint position, possibly because of an increased role of capsular receptors regarding joint position sensitivity with muscle fatigue–induced laxity.

Appreciation of joint position was also found to be significantly more sensitive after warming up by jogging and stretching for 4 minutes, demonstrating adequate warm-up prior to activity may be beneficial.¹⁷ Conversely, to test whether cryotherapy affects position sense of the knee, a cooling pad was applied to knees of humans.¹⁸ Cooling for 15 minutes resulted in increased knee joint stiffness and decreased sensitivity of joint position sense. However, another study indicated that cryotherapy of the shoulder does not affect joint position awareness.¹⁹ It may be possible that differences exist in different joints, with more superficially located joints affected more by cryotherapy. These findings should be considered in therapeutic programs that involve exercise immediately after applying cryotherapy.

Long-term effusions and the nature of the inflammatory fluid may be responsible for the seeming loss of proprioception observed in some clinical conditions with chronic joint effusion. However, infusion of saline to simulate acute joint effusion apparently does not change proprioceptive ability.²⁰ An elastic bandage placed on the knee of humans had a positive effect on joint proprioception, and may be recommended in those patients with joint proprioception problems.^21,22 Patella taping techniques improved joint position sense in patients with patellofemoral pain syndrome that had poor proprioception to begin with, but did not help those with good proprioception.²³

Neurologic Conditions

Proprioception is commonly affected by neurologic conditions in animals and people. Proprioception and rehabilitation have been studied in people following a stroke. There are many approaches to physical therapy following stroke injury in people. One method of treatment, known as supported treadmill training, appears to have beneficial effects on proprioceptive function in people after a stroke.24,25 A 4-week period of body weight–supported treadmill training with up to 40% of the patient’s body weight supported resulted in significantly improved clinical outcomes, including functional balance, motor recovery, overground walking speed, and overground walking endurance.²⁶ The effects lasted for at least 3 months after training. The subjects with greater gait impairments benefited the most from training with body-weight support.

Another study evaluated the delayed effects of a balance training program compared with no training on patients following a hemiplegic stroke.²⁷ Dynamic balance function of patients in the visual feedback training group had significant improvements when compared with the control group. Activities of daily living also had significant improvements at 6 months of follow up in the trained group. A study of older humans indicated that a short individualized exercise program improved functional balance. This improvement was maintained for at least 1 month, but there was no effect 1 year after training ceased, suggesting that continued training may be necessary.

Strength training should also be considered as part of the rehabilitation program for patients. In one study, chronic human stroke patients underwent a progressive resistance strength training program to evaluate changes in muscle strength, gait, and balance.²⁸ Subjects participated in a 12-week resistance training program. Lower limb strength improved 68% on the affected side. Repeated chair stand time improved 21%, motor performance improved 9%, and static and dynamic balance improved 12%. Other studies have confirmed the benefits of strength training in human stroke patients.²⁹ These results suggest that strength training should be considered even in patients with chronic deficits.

Proprioception and Activity

In the past decade, research in humans has suggested that stretching prior to activity may reduce acute proprioception, which might lead to injury. A recent study showed significant differences in detecting knee movement after undergoing a hold-relax proprioceptive neuromuscular facilitation stretch routine on the hamstrings and quadriceps compared with a control group.³⁰ The treated group had a significantly slower reaction time from pretest to posttest, whereas the control group did not. It is important to note that this study suggests stretching in general may promote decreased proprioception in certain joints immediately after stretching. This could hold implications for dogs in the late phases of rehabilitation, especially for sporting dogs, in which stretching prior to training could alter coordination and motor function, considering that proprioception is essential for skilled movements.

Proprioceptive training may help reduce injuries in athletes. In a recent randomized controlled trial of highly competitive basketball players, a treated group (n = 81), who received a proprioceptive training regimen along with normal workouts, was compared with athletes undergoing a normal training routine (n = 91).³¹ Proprioceptive training was performed once a week and consisted of six stations, each performed twice each for 45 seconds, followed by 30 seconds of rest as they switched stations. All exercises were clearly illustrated and showed proper progression. Over the course of an entire season, 21 injuries occurred in the control group compared with 7 in the treated group. The treated group also showed a more stable single limb stance in the frontal plane and overall sway compared with the control group. Although this study was performed on humans, it suggests that proprioceptive training may prevent injuries, and this might also be applicable to animal rehabilitation.

Finally, in a recent review to help guide clinical decisions on prevention of ACL injury with the use of proprioceptive and balance training, the authors concluded that there is a lack of solid evidence to determine if this type of training has beneficial effects. However, this was mainly a result of poor experimental design of the studies (such as poor description of training regimens and high drop-out rates), which at least determined no detrimental effects of proprioceptive training.³² Others have agreed, stating that many articles combine proprioceptive exercises with plyometrics, agility, and sport-specific exercises, making the specific effects of each individual treatment difficult to ascertain.³³