ORIGINAL RESEARCH 97 Medicina Sportiva, Vol. 11 (4): 97-101, 2007 DOI: 10.2478/v10036-007-0018-y Copyright © 2007 Medicina Sportiva EFFECT OF THREE MONTHS OF SERIAL MECHANICAL PROPRIOCEPTIVE STIMULATION ON PARAMETERS OF BALANCE IN OLDER WOMEN Erika Zemková(A,B,C,D,E,F), Dušan Hamar(A,D,E,G), Ľubica Bőhmerová(B) Faculty of Physical Education and Sport, Comenius University, Bratislava, Slovakia Abstract Zemková E, Hamar D, Bőhmerová L. Effect of three months of serial mechanical proprioceptive stimulation on parameters of balance in older women. Med Sport 2007, 11(4): 97-101. Introduction: Although several studies were carried out on effects of vibration exposure on strength, there are scarce of those concerning its influence on balance. Aim of the study: The study examines changes of postural sway in older women after 3-month of proprioceptive stimulation training applied to lower extremities. Methods: Altogether thirty-two older women, randomly divided into experimental and control group, participated in the study. They underwent two different forms of exercise during vibrations, either stood with slightly flexed knees or performed semi-squats. The intensity of exercise in the first case increased by lengthening the time of vibrations, whereas in the second by additional load. Vibrations were applied by means of special strength exercise device producing short-term counter movements at the frequency of 10 Hz and amplitude 3 mm. Stabilographic parameter (mean velocity of the center of pressure) was registered at 100 Hz by means of the stabilography system FiTRO Sway check based on dynamometric platform. Results: Results showed a significant (p < 0.01) decrease in velocity of the center of pressure from 11.2 ± 2.5 to 8.9 ± 2.0 mm/s. In addition, such an improvement of postural stability was more efficient after training in which semi-squats were performed as compared to those when subjects stood in semi-squat position on the vibrating platform (28.8 and 27.0 %, respectively). On the other hand, no significant changes in control group were observed (12.6 ± 2.4 and 11.9 ± 1.8 mm/s, respectively). Conclusion: It may be concluded that long-term proprioceptive stimulation training applied to lower extremities improve postural stability in older people. Such a positive change may be ascribed to the enhancement of neuroregulatory functions, namely increased rate of motoneuron firing and better synchronisation of motor units activation. Key words: elderly, postural sway, propropceptive stimulation, training Introduction Accidents related to falls have been recognized as a primary risk factor of injuries in elderly population (1). These consequently lead to fear of further falling, known as a “postfall syndrome”, activity restriction, loss of independence and decreasing quality of life. A major contributor to falls is an impaired capacity for proper neuromuscular response in controlling postural sway, a function substantially depending on muscle proprioceptors (e.g., 2, 3). Recently developed method of serial mechanical stimulation has been found to influence proprioceptive feedback mechanisms and specific neural components with subsequent enhancement of neuromuscular performance (4). Up to date longitudinal studies have documented an increase of the height and mechanical power in 5-s continuous jumping test in well-trained handball players (5), the isometric and dynamic knee-extensor strength and counter-movement jump height in untrained females (6), jump performance and isome- tric limb extension strength in sedentary subjects (7), rate of the force development in physical education students (8), and 18 % gain in chair-rising time in elderly (9) after vibrating training. On the other hand, there are only scarce reports (7, 10) studying the adaptive changes in postural stability due to systematic proprioceptive stimulation training, namely in elderly population. As decreased balance goes hand in hand with aging and potential risk of falls, its improvement by such novel, more efficient and lower-risk means than traditional aerobic and strength are, should be considered as an integral part of healthy orientated exercise program. Although results of preliminary studies related to utilization of vibrations for this purpose (11-13) seem to be promising, there is still lack of information whether such a positive effect should not be even more enhanced by adding resistance exercise while exposure vibrations. Indeed, a significant increase in strength has been proved when squats, deep squats, wide-stance squats, MECHANICAL PROPRIOCEPTIVE STIMULATION AND BODY BALANCE IN OLDER WOMEN 98 Fig. 1. Schematic representation of the experimental protocol. The arrows indicate the control tests one-legged squats, and lunge were performing during vibrations (6) whereas no changes has been found when subjects only stand on the vibrating platform with their knees angle flexed at 110° (14). In order to ascertain influence such standing with slightly flexed knees and performing semi-squats on the vibrating platform also on the parameters of balance, a 3-month of proprioceptive stimulation training has been carried out in elderly women. Material and Methods Subjects Altogether thirty-two older women, randomly divided into experimental group (n =16, mean age 61.2 ± 6.3 years, height 163.4 ± 5.8 cm, weight 72.1 ± 12.0 kg) and control group (n =16, mean age 61.8 ± 5.2 years, height 163.1 ± 5.9 cm, weight 72.1 ± 11.7 kg), volunteered to participate in the study. All of them were informed of the procedures and of the main purpose of the study. Study setting The training was applied twice a week during three - month period (Fig. 1). In the first month, to become familiar with proprioceptive stimulation training, subjects stood on vibrating platform while duration its exposure was increased by 5 sec every week (5 sets of 15, 20, 25, and 30 sec, respectively). Afterwards, subjects were randomized into two groups, in which different types of exercise during vibration were set up. One group stood barefoot with slightly flexed knees at an angle of 110° on the vibrating platform. The intensity of exercise was gradually increased by lengthening the time of vibrations (6 sets of 30 sec in the second, and 45 sec in the third month with 2 minutes of rest in-between). Second group performed semi-squats from full extension to a knee angle of 110°. An additional load (dumbbells in subject hands) was used (6 sets of 10 semi-squats without and with additional loads of 20 % body´s weight, also each session separated by 2 minutes of rest). In experimental groups control tests were carried out at baseline, every month, and at the end of session, in the control group only prior to and after three month period. Training and diagnostic equipments In order to avoid sinusoidal oscillations with potential problems related to resonance frequencies of particular body tissues vibrations were applied by means of special strength exercise device producing short-term counter movements (at the frequency of 10 Hz and amplitude of 3 mm eliciting force peaks of 3 g within 3 ms corresponding to the force gradient of 300 N/ms) developed at our department (Fig. 2). Fig. 2. Special strength exercise device producing vertical counter shocks MECHANICAL PROPRIOCEPTIVE STIMULATION AND BODY BALANCE IN OLDER WOMEN 99 Stabilographic parameter of the velocity of the center of pressure (mean and in antero-posterior and medio-lateral directions) was registered at 100 Hz by means of the posturography system FiTRO Sway check based on dynamometric platform (Fig. 3). The average of 2 min measurement was used as the test score. Subjects were asked to stand still as quiet as possible. However, after third month further changes in velocity of the center of presssure in group standing 45 sec in semi-squat position were not observed (8.9 ± 1.8 mm/s), mostly due to not enough intensive exercise. On the other hand, its values even decreased in group performing semi-squats with additional load (8.9 ± 2.2 mm/s). To sum up, 3-month of proprioceptive stimulation training led to a significant (p < 0.01) improvement of static balance in older women. This effect was more evident in group performing light squats than in those standing with slightly flexed knees (28.8 and 27.0 %, respectively). In contrast, in control group no differences in the velocity of the center of pressure in mentioned period were found (12.6 ± 2.4 and 11.9 ± 1.8 mm/s, respectively). In addition, in all groups a predominant shift of the velocity of the center of pressure in medio-lateral than antero-posterior direction was registered (Table 1). However, in the end of training its values showed a slight tendency to decrease from more than 60 % to about 55 % in X direction and vice versa in Y. Fig. 3. A computer based posturography system FiTRO Sway check Statistical analysis A paired Wilcoxon test was employed to determine the statistical significance of differences between velocity of the center of pressure prior to and after first, second, and third month, p < 0.05 was considered significant. Values (mean ± SD) obtained in the last test were expressed as percentages of the baseline value and presented for control and both experimental groups. Results The results showed (Fig. 4) that after first month of the training during which all subjects were standing in semi-squat position on the vibrating platform the velocity of the center of pressure decreased from 12.4 ± 2.5 to 10.0 ± 2.4 mm/s. Afterwards, they were randomly divided into two groups, in which different training program was carried out. Following second month the velocity of the center of pressure decreased more profoundly in group standing 30 sec on vibrating platform as compared to those performing semi-squats (9.1 ± 1.5 and 9.7 ± 2.4 mm/s, respectively). Light squats in average duration of 15 sec likely provided insufficient stimulus and therefore their benefit was neglected. Fig. 4. Changes in velocity of the center of pressure during 3-month of proprioceptive stimulation training applied to lower extremities in two different conditions, either standing with slightly flexed knees or performing semi-squats, in comparison with no training Table 1. Changes in velocity of the center of pressure during 3-month of proprioceptive stimulation training applied to lower extremities in antero-posterior (Y) and medio-lateral (X) directions in all groups examined PreSway variables Post-training (months) -training Sway velocity - x (mm/s) 1 2 3 – – 7.1 ± 0.9 Control group 7.4 ± 1.2 Experimental group 1 7.2 ± 0.8 6.3 ± 1.3 5.7 ± 0.6 5.2 ± 0.6 Experimental group 2 7.6 ± 1.4 6.4 ± 0.9 5.1 ± 0.5 4.9 ± 0.6 Sway velocity - y (mm/s) 1 2 3 – – 4.9 ± 0.8 Control group 4.9 ± 0.6 Experimental group 1 4.6 ± 0.8 3.9 ± 0.5 3.7 ± 0.7 3.9 ± 0.4 Experimental group 2 4.5 ± 0.6 3.7 ± 0.6 4.0 ± 0.9 3.9 ± 0.5 MECHANICAL PROPRIOCEPTIVE STIMULATION AND BODY BALANCE IN OLDER WOMEN 100 Discussion Improvement of postural stability in older women following 3-month of proprioceptive stimulation training applied to lower extremities may be ascribed to the improvement of neuromuscular functions, namely increased rate of motoneuron firing and better synchronization of motor unit activation. As close correlation between strength and balance performance has been found (15), such a positive changes in postural sway are very probably similar and occur via similar pathways as those by resistance training induced. It is known that within an initial two to three months such a training the specific neural components and their proprioceptive feedback mechanisms are influenced. During standing on the vibrating platform proprioceptive afferent inflow from the ankle region and afferent impulses from the sole are strongly stimulated. The vibratory stimulus activates the sensory receptors that results in reflexive muscle contraction. Long-term extensive sensory stimulation might thus lead to more efficient use of the positive proprioceptive feedback loop in the adjustment of postural sway. It is particularly important for elderly people. They tend to shift the corrective movements necessary for properly postural adjustments from the ankle joints to the knees or the hips. A reflex contraction of the ankle dorsiflexors is, however, more appropriate in relation to the overall goal of maintaining stability. Hence, improvement the ability to produce required output at the ankle joint and enhancement the efficiency of proprioceptive pathways is of special importance in these people. Besides this, tonic vibration reflex elicited by vibrations has been suggested to cause an increase in recruitment of the motor units (16). It supports also recent view, according which the vibration training has been found to increase the number of active motor units and their firing frequency (17). Since a decreased ability to develop force rapidly in elderly is associated with a lower capacity for neuromuscular response in controlling postural sway (15), an increase of force production at the onset muscle contraction may contribute to the speed of balance adjustment, e.g., on an unexpectedly moving surface. In addition, vibrations are thought to inhibit the activation of antagonist muscles via Ia inhibitory interneurons (18), thus increasing the co-contraction of synergists. An important consequence of such a muscle groups arrangement is an improvement of age-impaired coordination. In light of the above reports is evident that exercise performed under influence of proprioceptive stimulation represents strong stimulus to the sensory-motor pathways resulting in more rapid compensatory responses to stabilize posture and to prevent falling. It is related to mainly those known to be delay in elderly people, such as an immediate muscle stiffness, activation of a long-latency stretch reflex, vestibular activation, and myotatic stretch reflex. Mechanism of such an adaptation depends also on the body position and target muscles stimulated as well as the specificity of training. Escamilla et al. (19) has found the semi-squat position on the vibrating platform as an optimal for triggering vastus lateralis stimulation. It may be ascribed to the fact that sensitivity to vibrations increases when the target muscles are stretched (18, 20). Recent evidence also suggests that vibrations are more effective on improving forcegenerating capacity when applied during concentric rather than isometric or isokinetic activations (21). Based on these findings it may be assumed that as standing in semi-squat position as performing lower limbs resistance exercise on the vibrating platform may be considered as an effective and suitable means for enhancement of neuromuscular performance with subsequent improvement of balance in elderly population. Moreover, an enormous increase in neural activity during vibration, up to more than twice the baseline values (21), indicate that this type of treatment is able to stimulate the neuromuscular system more than others used to improve neuromuscular performance. The potential risk of vibration exercises is for elderly negligible. It may be corroborated by study of Rittweger et al. (23) who has shown the mild effect of exercises performed on vibrating platform to cardiovascular system. It is also in agreement with results of the present study and experience gained. Proprioceptive stimulation training has been found not to be exhausting for previously untrained subjects and the effect can be achieved within a relative short time. Hence, if carried out properly the exercises performed under influence of vibrations of appropriate frequency and amplitude may be safe physical activity with great potential also in a therapeutic context for patient who are not able to perform standard training programs. Conclusion It may be concluded that 3-month of proprioceptive stimulation training applied to lower extremities improve postural stability in older people. Its effect is more evident when during vibrations the resistance exercises, e.i., semi-squats are performed. Such a positive change may be ascribed to the improvement of neuroregulatory functions, namely increased rate of motoneuron firing and better synchronisation of motor units activation. This, together with improvement of proprioceptive functions and neuromuscular coordination enables more rapid postural sway adjustments in altered surface conditions. MECHANICAL PROPRIOCEPTIVE STIMULATION AND BODY BALANCE IN OLDER WOMEN 101 It has a practical impact on decrease a risk of falling with subsequent reduction of complications affecting namely elderly population. Acknowledgment This project was supported through a Scientific Grant Agency of the Ministry of Education of Slovak Republic and the Slovak Academy of Sciences (No. 1/9192/02). References 1. Runge M. Gehstörungen, Stürze, Hüftfrakturen. Darmstadt: Steinkopff-Verlag, 1998. 2. Pyykkö I, Aalto H, Hytönen M, et al. Effect of age on postural control. In: Amblard B, Berthoz A, Clarac F, ed. Posture and Gait: Development, Adaptation and Modulation. Amsterdam: Elsevier, 1988: 95-104. 3. Di Fabio RP, Badke MB, McEvoy A, Breunig A. Influence of local sensory afference in the calibration of human balance responses. 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Effects of preceding movements and contractions on the tonic vibration reflex of human finger extensor muscles. Acta Physiol Scand 1996; 156(4): 435-40. 21. Warman G, Humphries B, Purton J. The effects of timing and application of vibrations on muscular conractions. Aviat Space Environ Med 2002; 73: 119-27. 22. Bosco C, Cardinale M, Tsarpela O. Influence of vibration on mechanical power and electromyogram activity in human arm flexor muscles. Eur J Appl Physiol 1999, 79(4): 306-11. 23. Rittweger J, Beller G, Felsenberg D. Acute physiological effects of exhaustive whole-body vibration exercise in man. Clin Physiol 2000; 20(2): 134-42. Received: September 12, 2007 Accepted: November 16, 2007 Published: December 12, 2007 Address for correspodence: Erika Zemková, Ph.D. Faculty of Physical Education and Sport Comenius University Svobodovo nábrežie 9 814 69 Bratislava Slovakia Tel: 00421-2-54411624 E-mail: zemkova@yahoo.com