A Test of Nerves

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April 2003

By Michael E. Powers, PhD, ATC, CSCS, and Geoffrey C. Dover, MS, ATC, CAT

Heat and cold modalities are commonly used in the management of musculoskeletal injury. These modalities are also incorporated into rehabilitation protocols and are often used before and after practices and games once an athlete has returned to competition. One of the physiological effects of cooling or heating involves peripheral nerve function, as changes in tissue temperature have been shown to alter nerve conduction.^1-4 In fact, tissue temperature and nerve conduction demonstrate a direct relationship, as nerve conduction velocity slows with decreasing tissue temperatures.^1-4

Because of this, it is possible that cold may potentially impair neuromuscular function if the delivery of the afferent signal from the periphery or motor response has been slowed. In contrast, heating may actually improve neuromuscular function. A review of the studies involving neuromuscular control and tissue temperature changes can provide information to clinicians who incorporate these modalities into their treatment protocols.

NEUROMUSCULAR CONTROL

Neuromuscular control is the interaction of the nervous and muscular systems to create coordinated movement. The action patterns used to restore homeostasis are defined as mechanisms.⁵ A mechanism comprises multiple processes that, when completed individually, ultimately lead to the mechanism effect.⁵ These processes can include afferent sensory information, neural transmission, integration of the central nervous system, and an efferent response.⁵ This provides protection to the joint structures when under an excessive amount of stress or perturbation.⁶ The motor response is usually the product of sensory information received from the periphery via afferent nerves, many of which are classified as mechanoreceptors.⁵

MECHANORECEPTORS

Michael E. Powers, PhD, ATC, CSCS

Mechanoreceptors have been identified in the capsules and ligaments throughout the body.⁷ These receptors are responsible for relaying both the sense of joint motion (kinesthesia) and joint position sense (proprioception), as they provide information regarding internal and external forces by increasing the rate of the afferent signal and/or the number of receptors activated.⁶ Each type of mechanoreceptor possesses some degree of specificity with regard to its activating stimulus; however, they work together in providing information that can be used in somatosensory feedback. It is theorized that the afferent fibers in the capsule and ligaments play a major role in joint stability. For example, decreases in balance have been observed when comparing involved and uninvolved ankles of individuals suffering from chronic instability.⁸ Likewise, increases in postural sway and peroneal reaction time have been observed in subjects experiencing functional ankle instability.⁹ More recently, it has been shown that unstable ankles were less efficient at controlling acceleration during an inversion perturbation.¹⁰ These results suggest that trauma to the mechanoreceptors in the capsule and ligaments are also involved in the pathology of joint injury. This is not limited to the ankle, however, as increased hamstring latencies have been observed in anterior cruciate ligament deficient knees.¹¹ In each of these studies, the authors suggest that the differences observed in the motor response were due to changes in the afferent signal from the mechanoreceptors. Likewise, it has been hypothesized that changes in tissue temperature would alter the motor response because of changes in the afferent signal.

COLD AND HEAT THERAPIES

Temperature has a significant effect on the peripheral nervous system. This is understandable, as most biological and chemical processes that control the nervous system are sensitive to temperature. These processes include voltage-gated ion channels, acetylcholinesterase, and the contractile properties of skeletal muscle.¹² Temperature fluctuation can be problematic as dependent variables can change with an increase or decrease of only a few degrees. Researchers go to great lengths to monitor tissue temperature during experiments to ensure the accuracy of the results and conclusions. Surface probes generally work well, as the temperature of the skin surface and near nerve temperature are within two degrees at room temperature.^2,13

Although the slowing of nerve conduction is commonly a desired effect of cryotherapy, it may be undesirable before therapeutic exercise or training. If neuromuscular control is impaired following treatment, it may decrease therapeutic benefit or predispose patients to reinjury. Like cold treatments, heat treatments are often used before therapeutic exercise and before participating in training sessions and games. It is possible that this might provide the benefit of enhanced nerve function and improved neuromuscular control.

NERVE CONDUCTION

Significant decreases in median and ulnar nerve conduction velocity have been observed previously following cold water immersion.¹ In that study, a strong positive correlation was observed between the decrease in skin surface temperature and the decrease in nerve conduction velocity. Others have also observed significant decreases in nerve conduction velocity, as well as significant increases in sensory and Hoffmann reflex (H-reflex) latency following tissue cooling.⁴ The H-reflex is a neurophysiological response that has long been used to measure central nervous system function. It represents the efferent response to percutaneous electrical stimulation of type Ia afferent nerve fibers. In that study, it was determined that nerve conduction velocity decreased 1.7 m/sec for every 1°C decrease in surface temperature.⁴

Cold applications have been used for some time to reduce muscle spasticity and the effects of these treatments on stiffness have been attri-buted to a reduced stretch reflex response. This is supported as the rate of muscle spindle discharge has been shown to decrease with muscle cooling.³ In fact, results from previous research suggest a linear relationship between the rate of muscle spindle discharge and muscle temperature. This is important, as any change in the afferent signal can in turn lead to a modification of the motor response. This was evident as the monosynaptic reflex almost completely disappeared with 10°C of cooling.³

Similar to changes following tissue cooling, significant increases in median and ulnar nerve conduction velocity have been observed previously following warm water immersion.¹ In that study, skin surface temperature increased 5.9°C following treatment, while intramuscular temperature increased only 1.6°C. It is possible that this would result in an enhanced motor response.

NEUROMUSCULAR RESEARCH

The effects of tissue temperature on neuromuscular function have been examined using measures such as strength, balance, and proprioception.^14-16 Unfortunately, no single conclusion regarding the effects of temperature on neuromuscular function can be made from the results of these studies. For example, some of the studies investigating the effects of cryotherapy on strength have reported increases,^17-19 while others have reported decreases,15,20 or no change at all.^21,22 Likewise, functional measures of muscle activity have either decreased¹⁵ or remained unchanged²³ following cryotherapy. In one study, a loss of both eccentric and concentric strength was observed following 25 minutes of ice treatment.¹⁵ However, it could not be determined if this loss was due to neurological changes. In another study, a direct relationship was observed between tissue temperature and isometric grip strength. Cooling resulted in a decreased grip strength while increased temperatures improved isometric grip strength.²⁰ A similar relationship has also been observed between muscle temperature and dynamic knee extension strength.²⁴

More consistent findings have been presented regarding measures of proprioception. A 15-minute ice immersion treatment 5 cm above the medial malleolus was shown to have no effect on joint position sense.¹⁴ Likewise, 20 minutes of ice application to the knee failed to impair joint position sense.¹⁶ The authors suggest that the receptors in the joint were being compensated for by a different mechanism of feedback. Finally, ground reaction forces were unaffected by cryotherapy treatments to the ankle, knee, or ankle and knee. The authors concluded that returning an athlete to activity immediately following treatment does not predispose an athlete to injury. However, results from a similar study led the authors to recommend an athlete wait approximately 15 minutes before engaging in activities that require the production of weight-bearing explosive strength or power.²⁵

CONCLUSION

While isolated studies of nerve function and tissue temperature consistently show a direct relationship between the two, the more applied studies are not as consistent. Thus, although nerve function appears to be impaired by tissue cooling, this does not necessarily mean that joint and muscle function will be impaired. Likewise, it does not necessarily mean that neuromuscular function will improve when heating the tissue. It would be of benefit for the clinician to observe and possibly test the effects of these treatments on each individual patient. In this way, safe and effective protocols for rehabilitation can be established and administered.

Michael E. Powers, PhD, ATC, CSCS is director of athletic training education and assistant professor, Department of Exercise and Sport Sciences, and Geoffrey C. Dover, MS, ATC, CAT is a doctoral student, Sports Medicine, at the University of Florida, Gainesville.

REFERENCES

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De Jesus PV, Hausmanowa-Petrusewicz I, Barchi RL. The effect of cold on nerve conduction of human slow and fast nerve fibers. Neurology. 1973;23:1182-1189.
Eldred E, Buchwald JS, Lindsley DF. The effect of cooling on mammalian muscle spindles. Experimental Neurology. 1960;2:144-157.
Halar EM, DeLisa JA, Brozovich FV. Nerve conduction velocity: relationship of skin, subcutaneous and intramuscular temperatures. Arch Phys Med Rehabil. 1980;61(5):199-203.
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McGowan HL. Effect of cold application on maximal isometric contraction. Physical Therapy. 1967;42:185-192.
Oliver RA, Johnson DJ. The effect of cold water baths on posttreatment leg strength. Physician and Sports Medicine. 1976;2:67-69.
Barnes WS, Larson MR. Effects of localized hyperthermia and hypothermia on maximal isometric grip strength. Am J Phys Med. 1985;64(6):305-314.
Cornwall MW. Effect of temperature on muscle force and rate of muscle force production in men and women. J Orthop Sports Phys Ther. 1994;20(2):74-80.
Kimura IF, Gulick DT, Thompson GT. The effect of cryotherapy on eccentric plantar flexion peak torque and endurance. Journal of Athletic Training. 1997;32(2):124-126.
Evans TA, Ingersoll CD, Knight KL, Worrell T. Agility following the application of cold therapy. Journal of Athletic Training. 1995;30:231-234.
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Kinzey SJ, Cordova ML, Gallen KJ, Smith JC, Moore JB. The effects of cryotherapy on ground-reaction forces produced during a functional task. Journal of Sport Rehabilitation. 2000;9(1):3-14.

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