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Zschorlich VR, Qi F, Schorer J, Büsch D. Sensory Stimulation of the Triceps Surae Muscle Complex Modulates Spinal Reflex Responses-A Comparison between Tapotement Massage and Repetitive Peripheral Magnetic Stimulation (rPMS). Brain Sci 2024; 14:119. [PMID: 38391694 PMCID: PMC10887412 DOI: 10.3390/brainsci14020119] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2023] [Revised: 01/18/2024] [Accepted: 01/23/2024] [Indexed: 02/24/2024] Open
Abstract
BACKGROUND The reduction of muscular hypertonia is important in the treatment of various diseases or rehabilitation. This study aims to test the efficacy of a 5 Hz mechanical muscle stimulation (tapotement massage) in comparison to a 5 Hz repetitive peripheral magnetic stimulation (rPMS) on the neuromuscular reflex response. METHODS In a randomized control trial, 15 healthy volunteers were administered with either 5 Hz rPMS, tapotement massage, or rPMS sham stimulation. The posterior tibial nerve was stimulated with rPMS and sham stimulation. The Achilles tendon was exposed to a mechanically applied high-amplitude 5 Hz repetitive tendon tapotement massage (rTTM). The tendon reflex (TR) was measured for the spinal response of the soleus muscle. RESULTS After rPMS, there was a reduction of the TR response (-9.8%, p ≤ 0.034) with no significant changes after sham stimulation. Likewise, TR decreased significantly (-17.4%, p ≤ 0.002) after Achilles tendon tapotement intervention. CONCLUSIONS These findings support the hypothesis that both afferent 5 Hz sensory stimulations contributed to a modulation within the spinal and/or supraspinal circuits, which resulted in a reduction of the spinal reflex excitability. The effects could be beneficial for patients with muscle hypertonia and could improve the functional results of rehabilitation programs.
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Affiliation(s)
- Volker R Zschorlich
- Institute of Sports Science, Faculty of Philosophy, University of Rostock, Ulmenstr. 69-House 2, 18057 Rostock, Germany
- Institute of Sport Science, School IV-School of Humanities and Social Sciences, Carl von Ossietzky Universität Oldenburg, Ammerländer Heerstraße 114-118, 26129 Oldenburg, Germany
- Department Aging of Individuals and Society, Faculty of Interdisciplinary Research, University of Rostock, Gehlsheimer Str. 20, 18051 Rostock, Germany
| | - Fengxue Qi
- Sports, Exercise and Brain Sciences Laboratory, Beijing Sport University, Beijing 100084, China
| | - Jörg Schorer
- Institute of Sport Science, School IV-School of Humanities and Social Sciences, Carl von Ossietzky Universität Oldenburg, Ammerländer Heerstraße 114-118, 26129 Oldenburg, Germany
| | - Dirk Büsch
- Institute of Sport Science, School IV-School of Humanities and Social Sciences, Carl von Ossietzky Universität Oldenburg, Ammerländer Heerstraße 114-118, 26129 Oldenburg, Germany
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Muanjai P, Haas C, Sies W, Mittag U, Zange J, Schönau E, Duran I, Kamandulis S, Rittweger J. Effect of Whole-body Vibration frequency on muscle tensile state during graded plantar flexor isometric contractions. J Exerc Sci Fit 2023; 21:405-415. [PMID: 37965131 PMCID: PMC10641229 DOI: 10.1016/j.jesf.2023.10.003] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2023] [Revised: 10/12/2023] [Accepted: 10/24/2023] [Indexed: 11/16/2023] Open
Abstract
Background Acute physiological and biomechanical alterations have been reported following whole-body vibration (WBV). Stiffening of muscles has only been anecdotally reported in response to WBV. Accordingly, this study investigated active plantar flexor muscle stiffness in response to a single WBV bout at four mechanical vibration frequencies. Methods Thirteen healthy adults (37.1 ± 14.4 years old) randomly received WBV in 4 different frequencies (6, 12, 24, and 0 Hz control) for 5 min. Shear wave speed (SWS) in longitudinal and transverse projections, architecture, and electric muscle activity were recorded in the medial gastrocnemius (MG) and soleus (SOL) muscle during graded plantar flexor contraction. Subjective rating of perceived muscle stiffness was assessed via Likert-scale. Results SWS of the MG at rest was enhanced in response to 5 min of 24 Hz WBV (p = 0.025), while a small reduction in SOL SWS was found during contraction (p = 0.005) in the longitudinal view. Subjective stiffness rating was increased following 12 Hz intervention. After 24 Hz WBV, pennation angle for MG was decreased (p = 0.011) during contraction. As a secondary finding, plantar flexor strength was significantly increased with each visit, which, however, did not affect the study's main outcome because of balanced sequence allocation. Conclusion SWS effects were solely limited to 24 Hz mechanical vibration and in the longitudinal projection. The observed effects are compatible with an interpretation by post-activation potentiation, warm-up, and force-distribution within the triceps surae muscles following 5 min WBV. The outcome may suggest SWS as a useful tool for assessing acute changes in muscle stiffness.
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Affiliation(s)
- Pornpimol Muanjai
- Department of Physical Therapy, Allied Health Sciences Faculty, Burapha University, Chonburi, Thailand
- Exercise and Nutrition Innovation and Sciences Research Unit, Burapha University, Chonburi, Thailand
| | - Chris Haas
- University of Texas Medical Branch, Galveston, TX, USA
| | - Wolfram Sies
- Institute of Aerospace Medicine, German Aerospace Center (DLR), Cologne, Germany
| | - Uwe Mittag
- Institute of Aerospace Medicine, German Aerospace Center (DLR), Cologne, Germany
| | - Jochen Zange
- Institute of Aerospace Medicine, German Aerospace Center (DLR), Cologne, Germany
| | - Eckhard Schönau
- Center of Prevention and Rehabilitation, Cologne University Hospital and Medical Faculty, Germany
- Department of Pediatrics and Adolescent Medicine, University of Cologne, Cologne, Germany
| | - Ibrahim Duran
- Center of Prevention and Rehabilitation, Cologne University Hospital and Medical Faculty, Germany
| | - Sigitas Kamandulis
- Institute of Sport Science and Innovations, Lithuanian Sports University, Kaunas, Lithuania
| | - Jörn Rittweger
- Institute of Aerospace Medicine, German Aerospace Center (DLR), Cologne, Germany
- Department of Pediatrics and Adolescent Medicine, University of Cologne, Cologne, Germany
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Boulard C, Gross R, Gautheron V, Lapole T. Room for improvement: metrological properties of passive muscle-tendon stiffness measures in children with cerebral palsy. Eur J Appl Physiol 2020; 120:1999-2000. [PMID: 32715392 DOI: 10.1007/s00421-020-04434-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2020] [Accepted: 07/10/2020] [Indexed: 11/30/2022]
Affiliation(s)
- Clément Boulard
- Laboratoire Interuniversitaire de Biologie de La Motricité, Univ Lyon, UJM Saint-Etienne, Bâtiment IRMIS, 10 rue de la Marandière, 42270, Saint Priest en Jarez, France. .,Service de Médecine Physique Et de Réadaptation Pédiatrique, CHU Saint-Étienne, Saint-Étienne, France.
| | - Raphaël Gross
- Movement-Interactions-Performance, CHU Nantes, MIP, Nantes Université, 4334, F-44000, Nantes, EA, France
| | - Vincent Gautheron
- Laboratoire Interuniversitaire de Biologie de La Motricité, Univ Lyon, UJM Saint-Etienne, Bâtiment IRMIS, 10 rue de la Marandière, 42270, Saint Priest en Jarez, France.,Service de Médecine Physique Et de Réadaptation Pédiatrique, CHU Saint-Étienne, Saint-Étienne, France
| | - Thomas Lapole
- Laboratoire Interuniversitaire de Biologie de La Motricité, Univ Lyon, UJM Saint-Etienne, Bâtiment IRMIS, 10 rue de la Marandière, 42270, Saint Priest en Jarez, France
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Pereira BM, Magalhães FA, Lacerda ACR, Andrade AGP, Peixoto GHDC, Chagas MH. Comparison of four local vibratory stimuli on mechanical and sensorial variables related to muscle‐tendon unit response. TRANSLATIONAL SPORTS MEDICINE 2020. [DOI: 10.1002/tsm2.150] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Affiliation(s)
| | | | | | | | | | - Mauro Heleno Chagas
- Department of Physical Education Federal University of Minas Gerais Belo Horizonte Brazil
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Souron R, Lapole T. Eight Weeks of Local Vibration Training Do Not Increase Tibialis Anterior Muscle Stiffness Evaluated by Supersonic Shear Imaging. Ing Rech Biomed 2018. [DOI: 10.1016/j.irbm.2018.09.003] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
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Schoenrock B, Zander V, Dern S, Limper U, Mulder E, Veraksitš A, Viir R, Kramer A, Stokes MJ, Salanova M, Peipsi A, Blottner D. Bed Rest, Exercise Countermeasure and Reconditioning Effects on the Human Resting Muscle Tone System. Front Physiol 2018; 9:810. [PMID: 30018567 PMCID: PMC6037768 DOI: 10.3389/fphys.2018.00810] [Citation(s) in RCA: 25] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2018] [Accepted: 06/08/2018] [Indexed: 01/11/2023] Open
Abstract
The human resting muscle tone (HRMT) system provides structural and functional support to skeletal muscle and associated myofascial structures (tendons, fascia) in normal life. Little information is available on changes to the HRMT in bed rest. A set of dynamic oscillation mechanosignals ([Hz], [N/m], log decrement, [ms]) collected and computed by a hand-held digital palpation device (MyotonPRO) were used to study changes in tone and in key biomechanical and viscoelastic properties in global and postural skeletal muscle tendons and fascia from a non-exercise control (CTR) and an exercise (JUMP) group performing reactive jumps on a customized sledge system during a 60 days head-down tilt bed rest (RSL Study 2015–2016). A set of baseline and differential natural oscillation signal patterns were identified as key determinants in resting muscle and myofascial structures from back, thigh, calf, patellar and Achilles tendon, and plantar fascia. The greatest changes were found in thigh and calf muscle and tendon, with little change in the shoulder muscles. Functional tests (one leg jumps, electromyography) showed only trends in relevant leg muscle groups. Increased anti-Collagen-I immunoreactivity found in CTR soleus biopsy cryosections was absent from JUMP. Results allow for a muscle health status definition after chronic disuse in bed rest without and with countermeasure, and following reconditioning. Findings improve our understanding of structural and functional responses of the HRMT to disuse and exercise, may help to guide treatment in various clinical settings (e.g., muscle tone disorders, neuro-rehabilitation), and promote monitoring of muscle health and training status in personalized sport and space medicine.
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Affiliation(s)
- Britt Schoenrock
- Vegetative Anatomy, Charité Universitätsmedizin Berlin, Berlin, Germany
| | - Vanja Zander
- Neuroscience Group, German Sports University, Cologne, Germany
| | - Sebastian Dern
- Neuroscience Group, German Sports University, Cologne, Germany
| | - Ulrich Limper
- Human Physiology, German Aerospace Center, Cologne, Germany
| | - Edwin Mulder
- Human Physiology, German Aerospace Center, Cologne, Germany
| | - Alar Veraksitš
- Institute of Biomedicine and Translational Medicine, Faculty of Medicine, University of Tartu, Tartu, Estonia
| | - Ragnar Viir
- Ragnar Viir, Limited Partnership, Helsinki, Finland
| | - Andreas Kramer
- Neuromechanics Research Group, Sport Sciences, University of Konstanz, Konstanz, Germany
| | - Maria J Stokes
- Faculty of Health Sciences, University of Southampton, Southampton, United Kingdom
| | - Michele Salanova
- Vegetative Anatomy, Charité Universitätsmedizin Berlin, Berlin, Germany.,Center of Space Medicine Berlin, Berlin, Germany
| | | | - Dieter Blottner
- Vegetative Anatomy, Charité Universitätsmedizin Berlin, Berlin, Germany.,Center of Space Medicine Berlin, Berlin, Germany
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Constantino C, Galuppo L, Romiti D. Efficacy of mechano-acoustic vibration on strength, pain, and function in poststroke rehabilitation: a pilot study. Top Stroke Rehabil 2018; 21:391-9. [PMID: 25341384 DOI: 10.1310/tsr2105-391] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
BACKGROUND Vibration therapy may be used to help cortical reorganization after stroke as it can cause different adaptive metabolic and mechanical effects. OBJECTIVE This study examined whether the application of mechano-acoustic vibration on upper limb muscles could induce changes in range of motion (ROM), function, pain, and grip strength in individuals with chronic stroke. METHODS Out of 52 individuals post stroke with upper limb spasticity who were eligible,16 received mechano-acoustic vibration therapy (ViSS device) 3 times weekly for 12 sessions. The frequency of vibration was set to 300 Hz for 30 minutes. The treated muscles were the extensor carpi radialis longus and brevis and triceps brachii during voluntary contraction. All participants were evaluated in both upper limbs before (T0) and at the end (T1) of treatment with a dynamometer (hand grip strength), Modified Ashworth Scale, QuickDASH, FIM score, Fugl-Meyer scale, Verbal Numerical Rating Scale of pain, and Jebsen-Taylor Hand Function Test. RESULTS After 4 weeks, hand grip power had improved and pain and spasticity had decreased. Improvements were recorded for all parameters and were considered statistically significant. CONCLUSIONS Application of vibratory stimuli to a muscle can increase the motor-evoked potential recorded from the muscle, suggesting an enhancement of corticospinal excitability. Low amplitude, high-frequency vibration treatment (300 Hz) can significantly decrease tone and pain and improve strength in upper limb of hemiplegic individuals, when applied for 30 minutes, 3 times a week over 4 weeks.
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Affiliation(s)
- Cosimo Constantino
- Department of Biomedical, Biotechnological and Translational Sciences, University of Parma, Parma, Italy
| | - Laura Galuppo
- Department of Biomedical, Biotechnological and Translational Sciences, University of Parma, Parma, Italy
| | - Davide Romiti
- Department of Biomedical, Biotechnological and Translational Sciences, University of Parma, Parma, Italy
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A Study on the Improvement of Walking Characteristics of the Elderly with Vibration Stimuli Applied to the Tibialis Anterior Tendon. BIOMED RESEARCH INTERNATIONAL 2018; 2017:5342485. [PMID: 29318151 PMCID: PMC5727664 DOI: 10.1155/2017/5342485] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/26/2017] [Accepted: 10/19/2017] [Indexed: 11/17/2022]
Abstract
The purpose of this study was to identify the gait pattern of the elderly with aging and to analyze the elderly's gait changes by the focal tendon vibratory stimulation. A total of 10 elderly males and 15 young adult males participated in this study. Using 3D motion analysis, we analyzed that difference between the elderly gait and young adults gait and the changes of the elderly gait by applying focal vibratory stimuli. As a result, specifically in the early stance, the elderly's gait was more flexed and the lower extremity extensors of the elderly worked harder. When the focal vibratory stimuli were applied, joint angle of the elderly was induced to that of the young adults. There was a reduction in demands for supporting bodies and progressing gait in the stance phase. This means that focal vibratory stimuli affect the gait of the elderly. Also, the changes of the gait of the elderly varied according to the characteristics of the focal vibratory stimuli. This implies that the activity of the motor may be dependent on vibratory stimuli characteristics.
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Souron R, Bordat F, Farabet A, Belli A, Feasson L, Nordez A, Lapole T. Sex differences in active tibialis anterior stiffness evaluated using supersonic shear imaging. J Biomech 2016; 49:3534-3537. [DOI: 10.1016/j.jbiomech.2016.08.008] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2016] [Revised: 06/23/2016] [Accepted: 08/02/2016] [Indexed: 11/26/2022]
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10
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Lapole T, Tindel J, Galy R, Nordez A. Contracting biceps brachii elastic properties can be reliably characterized using supersonic shear imaging. Eur J Appl Physiol 2014; 115:497-505. [DOI: 10.1007/s00421-014-3037-0] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2014] [Accepted: 10/23/2014] [Indexed: 10/24/2022]
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Lapole T, Canon F, Pérot C. Ipsi- and contralateral H-reflexes and V-waves after unilateral chronic Achilles tendon vibration. Eur J Appl Physiol 2013; 113:2223-31. [PMID: 23652708 DOI: 10.1007/s00421-013-2651-6] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2013] [Accepted: 04/19/2013] [Indexed: 12/01/2022]
Abstract
Chronic Achilles tendon vibration has previously shown its effectiveness in improving plantar flexor's strength and activation capacities. The present study investigated the related neural mechanisms by analyzing H-reflexes and V-waves of the soleus (SOL) and gastrocnemii (GM gastrocnemius medialis; GL gastrocnemius lateralis) muscles under maximal isometric plantar flexion. Moreover, recordings were conducted bilaterally to address potential crossed effects. 11 subjects were engaged in this study. Maximal voluntary contraction and superimposed H-reflexes and V-waves were quantified in both legs at baseline (PRE) and 2 weeks later to verify repeatability of data (CON). Then, subjects were retested after 14 days of daily unilateral Achilles tendon vibration (VIB; 1 h per day; frequency: 50 Hz). No changes were reported between PRE and CON data. In the VIB condition, there was an increase in MVC for both the vibrated (+9.1 %; p = 0.016) and non-vibrated (+10.2 %; p = 0.009) legs. The H-reflex increased by a mean 25 % in the vibrated SOL (p < 0.001), while it remained unchanged for the contralateral side (p = 0.531). The SOL V-wave also increased in the vibrated limb (+43.3 %; p < 0.001), as well as in the non-vibrated one (+41.9 %; p = 0.006). Furthermore, the GM V-wave increased by 37.8 % (p = 0.081) in the vibrated side and by 39.4 % (p = 0.03) in the non-vibrated side. However, no changes were reported for the GL muscles. While the present study confirmed the strength gains induced by chronic Achilles tendon vibration, the results indicated a cross-education phenomenon with differences in neural adaptations between the vibrated leg and non-vibrated leg.
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Affiliation(s)
- Thomas Lapole
- CNRS UMR 7338 Biomécanique et Bioingénierie, Université de Technologie de Compiègne, 60205 Compiègne Cedex, France
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Lapole T, Deroussen F, Pérot C, Petitjean M. Acute effects of Achilles tendon vibration on soleus and tibialis anterior spinal and cortical excitability. Appl Physiol Nutr Metab 2012; 37:657-63. [DOI: 10.1139/h2012-032] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Prolonged vibration is known to alter muscle performance. Attenuation of Ia afferent efficacy is the main mechanism suggested. However, changes in motor cortex excitability could also be hypothesized. The purpose of the present study was therefore to analyze the acute and outlasting effects of 1 h of Achilles tendon vibration (frequency, 50 Hz) on the soleus (SOL) and tibialis anterior (TA) neuromuscular excitability. Spinal excitability was investigated by means of H-reflexes and F-waves while cortical excitability was characterized by motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation. Twelve subjects performed the experimental procedures 3 times: at the beginning of the testing session (PRE), immediately after 1 h of Achilles tendon vibration (POST), and 1 h after the end of vibration (POST-1H). Prolonged vibration led to acute reduced H-reflex amplitudes for SOL only (46.9% ± 7.7% vs. 32.8% ± 7%; p = 0.006). Mainly presynaptic inhibition mechanisms were thought to be involved because of unchanged F-wave persistence and amplitude mean values, suggesting unaffected motoneuronal excitability. While no acute effects were reported for SOL and TA cortical excitability, both muscles were characterized by an outlasting increase in their MEP amplitude (0.64 ± 0.2 mV vs. 0.43 ± 0.18 mV and 2.17 ± 0.56 mV vs. 1.26 ± 0.36 mV, respectively; p < 0.05). The high modulation of Ia afferent input by vibration led to changes in motor cortex excitability that could contribute to the enhancement in muscular activation capacities reported after chronic use of tendon vibration.
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Affiliation(s)
- Thomas Lapole
- EA 3300 Adaptations Physiologiques à l’Exercice et Réadaptation à l’Effort, UFR STAPS Amiens, Université de Picardie Jules Verne, Amiens, France
- Université de Technologie, Compiègne CNRS UMR 7338, Biomécanique et Bioingénierie, F-60205 Compiègne CEDEX, France
| | - François Deroussen
- Service de Chirurgie Orthopédique Pédiatrique, Centre Hospitalier Universitaire d’Amiens, France
| | - Chantal Pérot
- Université de Technologie, Compiègne CNRS UMR 7338, Biomécanique et Bioingénierie, F-60205 Compiègne CEDEX, France
| | - Michel Petitjean
- EA 2931 Centre de Recherche sur le Sport et le Mouvement, Université Paris Ouest, Nanterre, France
- Service d’Explorations Fonctionnelles du Système Nerveux, Centre Hospitalier Universitaire d’Amiens, France
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Lapole T, Pérot C. Hoffmann reflex is increased after 14 days of daily repeated Achilles tendon vibration for the soleus but not for the gastrocnemii muscles. Appl Physiol Nutr Metab 2011; 37:14-20. [PMID: 22148919 DOI: 10.1139/h11-129] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
In a previous study, Achilles tendon vibrations were enough to improve the triceps surae (TS) activation capacities and also to slightly increase TS Hoffmann reflex (H-reflex) obtained by summing up soleus (Sol) and gastrocnemii (GM and GL) EMGs. The purpose of the present study was to analyze separately Sol and GM or GL reflexes to account for different effects of the vibrations on the reflex excitability of the slow soleus and of the gastrocnemii muscles. A control group (n = 13) and a vibration group (n = 16) were tested in pre-test and post-test conditions. The Achilles tendon vibration program consisted of 1 h of daily vibration (frequency: 50 Hz) applied during 14 days. Maximal Sol, GM and GL H-reflexes, and M-waves were recorded, and their H(max)/M(max) ratios gave the index of reflex excitability. After the vibration protocol, only Sol H(max)/M(max) was enhanced (p < 0.001). The enhanced Sol reflex excitability after vibration is in favor of a decrease in the pre-synaptic inhibition due to the repeated vibrations and the high solicitation of the reflex pathway. Those results of a short period of vibration applied at rest may be limited to the soleus because of its high density in muscle spindles and slow motor units, both structures being very sensitive to vibrations.
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