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Brazilian Journal of Physical Therapy

Print version ISSN 1413-3555

Rev. bras. fisioter. vol.13 no.6 São Carlos Nov./Dec. 2009

http://dx.doi.org/10.1590/S1413-35552009000600009 

ORIGINAL ARTICLE

 

Mechanical evaluation of the resistance of elastic bands

 

 

Gilmar M. SantosI; Graziela M. S. TavaresII; Graziela de GasperiII; Giseli R. BauIII

IDepartment of Physical Therapy, Universidade do Estado de Santa Catarina (UDESC), Florianópolis (SC), Brazil
IIGraduate Program in Human Movement Sciences, UDESC
IIIPhysical Therapy Course, UDESC

Correspondence

 

 


ABSTRACT

BACKGROUND: Elastic bands are frequently used for resistance training, however the selection of the bands to progress through the levels of elastic resistance is done in a subjective manner. This is due to the lack of quantitative data on the value of the material's resistance as a function of its tension.
OBJECTIVES: To investigate the elastic resistance generated by each of the eight color-coded resistance levels of elastic bands, using 100% elongation, and to quantify the resistance variation from one level to the next.
METHODS: Tensile testing was performed in compliance with ASTM Standard D412-06a. The sample consisted of 80 die-cut test specimens (Die C) taken from the eight color-coded resistance levels. The sample was submitted to tensile testing in the universal testing machine EMIC DL-3000. Each of the tension cycles was performed at a speed of 500mm/sec. Statistical analysis was done using one-way ANOVA, with a significance level of p<0.05.
RESULTS: The sample showed a significant difference between all levels of resistance, except for the yellow (thin) and red (medium) elastic bands. The variation in resistance between the bands shows that the gold (max) band offers 5.13 times more resistance than the tan (extra thin) band, and that the greatest variation in progressive resistance is between the black and the silver bands. In addition, Young's modulus showed linear behavior between the different colors.
CONCLUSIONS: The results showed that the elastic resistance and stiffness of the material exhibits a linear and progressive variation. In addition, the data suggested the possibility of progressing from the tan band to the red band, skipping the yellow band, when prescribing resistance exercises.

Key words: elastic bands; elastic resistance; tensile testing.


 

 

Introduction

Resistive elastic materials, such as bands and tubes, are low-cost and highly versatile tools often used in therapeutic exercise programs1-3. They are also convenient for patients to train at home using a variety of movement, including eccentric and concentric contractions4. According to Pereira and Gomes5, training against resistance is an important tool in the prevention and maintenance of quality of life regarding health, and recommendations suggest that it should be part of exercise programs for young adults and the elderly.

To improve the prescription of muscle reeducation programs (dynamic strength training), physical therapists must learn the correct use of elastic bands for therapeutic purposes. Elastic bands (Thera-BandTM) emerged in rehabilitation centers of the United States in the 1980s6. In Brazil, latex bands made from polymerized synthetic rubber, known chemically as styrene-butadiene, are used on a large-scale7. However, the elastic bands produced by the Hygenic Corporation (Thera-BandTM) contain natural rubber latex that provides better elasticity and lower susceptibility to rupture8. Elastic band and tube manufacturers provide a range of products with different resistance levels usually distinguished by color. In theory, this spectrum makes it possible to offer to the patient the elastic band that best corresponds to the resistance level that is appropriate for his/her rehabilitation process, i.e. the force needed to elongate the material.

Hostler et al.9 analyzed the effectiveness of a short-term training program using elastic resistance in young adults and found skeletal muscle adaptations, such as: an increase in the percentage of type IIab fibers for both men and women; a decrease in the percentage of type IIb fibers in men; and minor changes in fiber type composition (IIb IIab), fiber size and capillarization. However, one disadvantage of using elastic resistance is the progressively greater force required with the elongation of the material. Near the end of the range of motion, an individual may not be able to perform the movement as desired, due to the weakness of the muscles undergoing rehabilitation or because the muscle is in a shortened position at a point where the resistance of the material is greater4.

It is argued that the misuse of the material can be harmful, because the use of force, torque or too much pressure may cause inflammation, keloid scarring or even deformities4. Another disadvantage of elastic resistance lies in the fact that not all elastic materials provide similar tension4. Therefore, it is essential to understand the mechanical properties of the material, which would imply the need to define the proportion between the different colors for the implementation of a suitable clinical treatment4, allowing the physical therapist to have greater control over the load offered to the patient.

Some studies1,2,10,11 have been conducted on the properties of elastic bands. These studies had different objectives, such as: establishing the stress-strain relationship of 12 samples of elastic bands1; investigating the relationship between force and percentage of change in length for six different colors of elastic tubes, by direct measurement of force and motion analysis during shoulder abduction exercise2; measuring the resistance as a function of elongation in seven colors of elastic bands10, and quantifying the resistance (force as a function of elongation) of six colors of elastic bands, allowing the construction of an index for color selection11. However, none of these studies1,4,6,10 followed the technical standards necessary to carry out the tensile tests under controlled conditions (temperature, humidity, etc.) and thus determine the curves which show the correlation between the exerted force and the deformation caused in the material.

It is argued that the search for objective data may better evaluate and direct the client's progress, thus allowing evidence-based physical therapy treatment. Additionally, the previously cited authors1,4,6,10 suggest further research to study the biomechanics of the elastic material in mechanically controlled trials in order to determine the real resistance change as a function of elongation. Therefore, the present study aimed to investigate the elastic resistance, i.e. force as a function of the elongation generated in each of the eight color-coded resistance levels of the elastic bands at 100% elongation and quantify the resistance change between each level.

 

Methods

This experimental study12 was conducted at the Instrumentation Laboratory (LABIN) of the Health and Sports Sciences Center (CEFID) during April and May 2008. The sample consisted of 80 die-cut test specimens, taken from the eight resistance levels represented, in order of increasing tensile resistance, by the colors tan (extra thin), yellow (thin), red (medium), green (heavy), blue (extra heavy), black (especially heavy), silver (super heavy) and gold color (max). Ten samples were taken from each resistance level.

Instruments

For the tensile test, we used the microprocessor-based electromechanical universal testing machine EMIC DL-3000 with force and displacement data acquisition through Virmaq (version 3.04) and Tesc (version 3.04) software13. The load cell used in this trial was the PLA model (Líder Balanças-SP) on a rigid base with a nominal capacity of 20 kgf. The load cell was placed between the metal clip, used to attach the elastic material to the rigid base, and the rigid base itself, used to measure the tension produced by the elastic material. Data were collected at a sampling frequency of 30 Hz by 12-bit A/O system and then stored in a computer.

In order to obtain the data proposed by this study, a tensile test was performed in compliance with ASTM Standard D412-06a14. The die-cut test specimens were taken from the eight color-coded resistance levels commercialized by MERCUR SA®, the Brazilian representative of Thera-BandTM. To prepare the die-cut test specimens, a model AC5 09 cutting die, equivalent to Die C as described in the standard, was used. The choice of elastic bands was simply based on the intention to test a range that crossed all levels of resistance that can be offered to patients.

The die-cut test specimens were standardized and measured for length, width and thickness with a Mitutoyo Digital Caliper®. The mean of the 80 samples was 115 mm in length, with 20 mm required at each end to set the band safely in the metal clamps. The width (=6.5 mm ± 0.11) and thickness (=0.26 mm ± 0.18) of the die-cut test specimens were also measured. The actual test area was standardized as 75 mm. Three points were standardized on the base, through measurements and calculation of the corresponding mean, to measure the width and thickness of the area to be deformed during the test for each specimen.

The elongation speed was set at 500 mm/min, which corresponds to an exercise carried out at a very low speed, simulating the optimal use of the elastic band in clinical practice1. Each die-cut test specimens was elongated once for 5 seconds on average. From the elongation, it was possible to determine the 80 curves of force versus percentage of deformation of the material in a 100% elongation, conventionally known as the maximum apparent deformation performed by a patient in clinical use.

Data collection

The die-cut test specimens were submitted to controlled temperature and humidity, according to ASTM Standard D412-06a14 for at least three hours prior to and throughout the trials. To conduct the trial, each specimen was attached to the test apparatus with clamps, so that the material was at resting length. During testing, each die-cut test specimen was subjected to a cycle of elongations until the material ruptured or became detached from the clamps or until elongation without rupture or detachment followed by the return to the starting position.

The tension of the elastic material (resistance to traction) was measured throughout the trial. The variation in elastic resistance between the different levels (colors) of the material was also measured. In this study, we chose to present the results of the resistance in Newtons, in a 100% deformation. The choice of this percentage of deformation was based on a test with an individual simulating shoulder and knee joint movements with elastic band resistance. The tests showed that, in the movements, the individual did not exceed 100% deformation.

Treatment of data

For statistical analysis between the different levels of resistance at 100% elongation, one-way ANOVA and post-hoc Tukey tests were used. The significance level was set at p<0.05.

 

Results

The results of this study (Table 1) showed a statistically significant difference between the levels of resistance, except between the levels represented by the yellow and red bands (p=0.107). Table 1 shows the mean force values and their standard deviations, at 100% elongation, for each resistance level of the elastic bands. The forces are presented in Newtons (N) and represent progressive elastic resistance from the tan to the gold band. Young's modulus showed progressive, linear behavior in the stiffness of the material between the different colors (Table 2). This table also shows the Pearson's product-moment correlation (r) and coefficient of determination (r2) for the color-coded resistance levels.

 

 

 

 

In the comparison of stress-elongation curves between the eight color-coded resistance levels (Figure 1), there is a lack of overlap between them and a progressive behavior in the force necessary to deform each band in the resistance levels offered by the manufacturer.

 

 

Figure 2 shows the percentage change in elastic resistance in the range of resistance levels tested at 100% elongation. The results showed that the gold band (12.4 N) offers 5.13 times more resistance than the tan band (2.0 N).

 

 

Discussion

The results showed that the force-deformation and stress-elongation relationship of the elastic bands commonly used in therapeutic exercise showed a strong linear relationship up to 100% deformation, i.e. the increase in length corresponded to a progressive increase in the elastic resistance and stiffness of the material in the color sequence: tan, yellow, red, green, blue, black, silver and gold. Furthermore, the results showed a mean value of r2 equal to 0.95, in agreement with Hughes et al.2 and Page, Labbe and Topp10 who also found a linear force-deformation relationship with mean values of r2 of 0.93 and 0.97, respectively. In this linear behavior, there was a 53.8% variation from the least resistant band (tan) to the most resistant band (gold).

These results showed a variable slope of the curve in the stress-elongation relationship for each color of elastic band. For each change in elastic band, there is a non-uniform change in recoil force. At the same level of deformation, there is a 20 to 67% change in the level of recoil force between successive elastic bands. Thus, the smallest increment in force, i.e. the least progression of difficulty, occurs between the yellow and red bands, while the greatest increase in force, i.e. the greatest difficulty in progression occurs between the black and silver elastic bands.

The percentage difference of 13.1% in elastic resistance between the yellow and red elastic bands was not significant, suggesting that, in clinical practice, the progression between the bands can be made directly from tan to red because of this small increase in resistance. Additionally, the data show that extra care must be taken when the physical therapist progresses from the black to the silver band, because this progression showed the highest value (67%) of change in elastic resistance.

Brien et al.15 and Mikesky et al.3 were the first to mention a formal method or a mechanical procedure for determining elastic resistance as a function of elongation before starting a training program. However, the studies1,4,6,10 on the properties of elastic bands and tubes were performed in clinical settings, which hinders a more accurate comparison to the data obtained in the present trial. Despite this difficulty, it must be highlighted that the trial was based on international standards for tensile trials in which control criteria must be followed to obtain die-cut test specimens of standardized shapes and sizes so that the results can be compared or, if necessary, reproduced.

The Hygenic Corporation (Thera-BandTM)8, through the manufacturer's manual for elastic bands and tubes, provides a range of forces required to elongate the material at different percentages. These data come from a study by Page, Labbe and Topp10 that measured resistance as a function of elongation by fixing one end of the elastic band to an exercise handle and the other end to a plastic clip attached to a strain gauge. The gauge quantified the force generated to the nearest 0.5 kg, according to a protocol with samples of the seven colors (the tan band was not tested) cut into different lengths. Each elastic band was manually elongated at a rate of 1 inch/second, with elongations of 25% to 250% recorded from the first length of the sample. The authors' conclusion indicates that the elastic bands provide a consistent and predictable increase in force as a function of the elongation at all levels of resistance, as found in the present study.

However, it was not possible to compare the present data with data supplied by the manufacturer because the study by Page et al.10 did not mention any technical standard and because the instruments and especially the dimensions of the die-cut test specimens were quite distinct. The main limitation is due to the width of the samples, ranging from 6.5 mm on average in the present study to 100 mm in the referred study10, a difference that directly influence the force needed to elongate the material. The difference in length does not affect the comparison of the forces because the elongation percentage is the same16.

Based on the variations of resistance between the colors of elastic bands in the present trial, it is possible to quantify the overload offered to the patient when progressing from one resistance level to the next. It is also possible to establish more appropriate level transitions, offering a progression of resistance according to the need and purpose of treatment, thus providing safety, efficiency and objectivity when choosing an elastic resistance treatment program. Moreover, through tensile trials that measure the properties of elastic materials, elastic resistance exercise protocols may be validated.

 

Conclusions

After the tensile trial, the results showed that the elastic bands have a progressive and linear behavior in elastic resistance and stiffness of the material between the different colors at 100% elongation, corroborating that the order of colors (tan, yellow, red, blue, green, black, silver and gold) indicates least to greatest resistance. The analysis of elastic resistance between bands showed that the greatest variation occurs between the black and silver bands, suggesting that the physical therapist must take particular care when deciding to transition from one to the other. Furthermore, the results suggest that it is possible to advance from the tan to the red band, eliminating the need for the yellow band, when prescribing a strengthening program. Finally, the results suggest that the transition in elastic resistance between tan, yellow, red, blue, green and black bands can be safely used during physical therapy treatment.

 

Acknowledgments

This study had the support of Caribor Tecnologia da Borracha Ltd in the manufacture of the die-cut test specimens and Mercur SA in the donation of the elastic bands. We are also grateful for the support of the Instrumentation Laboratory (LABIN) at UDESC, especially professors Noé Gomes Borges Júnior and Susana Domenech.

 

References

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2. Hughes CJ, Hurd K, Jones A, Sprigle S. Resistance properties of thera-band tubing during shoulder abduction exercise. J Orthop Sports Phys Ther. 1999;29(7):413-20.         [ Links ]

3. Mikesky AE, Topp R, Wigglesworth JK, Harsha DM, Edwards JE. Efficacy of home based training program for older adults using elastic tubing. Eur J Appl Physiol Occup Physiol. 1994;69(4):316-20.         [ Links ]

4. Patterson RM, Stegink Jansen CW, Hogan HA, Nassif MD. Material properties of thera-band tubing. Phys Ther. 2001;81(8):1437-45.         [ Links ]

5. Pereira MIR, Gomes PSC. Efeito do treinamento contra-resistência isotônico com duas velocidades de movimento sobre os ganhos de força. Rev Bras Med Esporte. 2007;13(2):91-6.         [ Links ]

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8. Resistance band & tubing: instruction manual. Akron: The Hygenic Corporation; 2006. Acesso em 08 de agosto de 2008. Disponível em: http://www.uwex.edu/ces/flp/conference/pdfs/06_current_concepts_Graf_manual.pdf.         [ Links ]

9. Hostler D, Ischwirian CI, Campos G, Toma K, Crill MT, Hagerman GR, et al. Skeletal muscle adaptations in elastic resistance-trained young men and women. Eur J Appl Physiol. 2001;86(2):112-8.         [ Links ]

10. Page PA, Labbe A, Topp RV. Clinical force production of thera-band elastic bands. J Orthop Sports Phys Ther. 2000;30(1):47-8.         [ Links ]

11. Sakanoue N, Katayama K. The resistance quantity in knee extension movement of exercise bands (Thera-Band®). J Phys Ther Sci. 2007;19(4):287-91.         [ Links ]

12. Rudio FV. Introdução ao projeto de pesquisa cientifica. 19ª ed. Petrópolis: Vozes; 1995.         [ Links ]

13. EMIC Equipamentos e Sistemas de Ensaio LTDA [homepage na Internet]. Catálogos e folhetos em formato eletrônico. Máquinas universais de ensaios eletromecânicas e microprocessadas digital line. São José dos Pinhas (PR). 5ªed; [atualizada em 26 Jun 2007; acesso em 30 Mar 2008]. Disponível em: http://www.emic.com.br/.         [ Links ]

14. American Society for Testing and Materials. ASTM D 412-06ae2: Standard test methods for vulcanized rubber and thermoplastic elastomers-tension. United States; 2006.         [ Links ]

15. O'Brien JM. lsokinetic evaluation of the posterior rotator cuff musculature following a strengthening program utilizing rubber tubing. Springfield: Springfield College; 1992.         [ Links ]

16. Hibbeler RC. Resistência dos materiais. 5ª ed. São Paulo: Prentice Hall; 2004.         [ Links ]

 

 

Correspondence:
Gilmar Moraes Santos
Rua Professor Bayer Filho, 125, Coqueiros
CEP 88080-300, Florianópolis (SC), Brazil
e-mail: d2gms@udesc.br

Received: 10/10/2008
Revised: 19/12/2008
Accepted: 14/04/2009

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