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SCIENTIFIC EXPLANATION OF ELASTIC RESISTANCE
Elastic resistance exercise has been used for almost a century. It began as a fitness device, and then progressed to a rehabilitation device. The following chapters provide the scientific basis for the use of elastic resistance exercise.
SCIENTIFIC BASIS OF RESISTANCE TRAINING
Muscle strengthening methods commonly fall under the category of isotonic, isokinetic, or isometric exercise. Operational definitions are associated with each form of training. Isotonic exercise requires that an individual lift or move a constant weight through a range of movement. Free weights are the most common and popular form of isotonic exercise. Isokinetic exercise requires a dynamometer to ensure that segment motion occurs at a preset and controlled velocity. With isokinetic exercise, a varied and accommodating resistance is provided in direct relationship to changing levels of torque output that are evident throughout the range of motion to allow a constant velocity when the load is lifted. Isometric exercise does not require the use of special equipment.
Various research studies have compared strength gains resulting from various methods of training but have not conclusively determined the superiority of one particular method. According to the principle of specific adaptations to imposed demands (SAID), human tissue including skeletal muscle incurs specific adaptations in response to the stimulus imposed.
Therefore, it seems logical to infer that different methods may be suitable for different training goals. Surprisingly, elastic resistance exercise does not display typical characteristics of any of the three previously mentioned exercise classifications (isotonic, isokinetic, isometric). Resistance training with elastic is unique. The variability in force when elastic tubing is stretched negates classifying elastic exercise as a form of isotonic training and the accompanying change in stretch rate (velocity) prohibits classification as isokinetic exercise.
Resistance exercise and human motion in general are also categorized based on the mechanics of muscle contraction. Concentric, eccentric, and isometric contractions are described commonly in the literature.
A concentric muscle effort is defined as an increase in muscle tension that results in muscle shortening and, consequently, limb movement. This type of contraction is highly dependent on the leverage changes between the resistance or load provided and the net effort from agonist muscles. Torque or moment of force represents the net effect of the forces acting through the lever system that cause rotation about the joint axis. When the net moment or torque from muscle contraction exceeds the moment generated by an external resistive force, a concentric action takes place.
When the moment generated by muscles attempting to rotate a skeletal segment is less that the moment generated by an external resistive force, an eccentric contraction occurs. Eccentric contractions occur when the muscle lengthens during contraction and the limb segment moves in a direction opposite of the muscle effort. Eccentric contractions commonly are generated in opposition to inertial or gravitational forces.
When the net moment from internal (muscle contraction) and external (weight, gravity) forces acting on a skeletal segment is zero, the moment generated by the muscles acting on the segment is equal to the opposing resistive moment. This event characterizes an isometric condition. An isometric condition occurs when there is no joint movement although forces and moments are acting on the segment. Training with elastic tubing can include all three mechanical conditions of muscle contraction (concentric, eccentric, isometric), similar to isotonic exercise.
Regardless of the method of training, all forms of resistance exercise must adhere to the overload principle to increase strength. If contractions are strong and prolonged and if they are repeated regularly, then adaptive changes will take place, which allow the muscle to handle greater amounts of work. A muscle must be overloaded to a certain threshold before it will respond and adapt to training.
PROPERTIES OF FREESTYLER ELASTIC TUBING
The specific characteristic of elastic material is that the resistance provided changes as the material is elongated or stretched. The change in length as a function of the applied force, the modulus of elasticity of the material, and the cross-sectional area all dictate the level of resistance and the amount of potential energy stored by elastic. (Ozkaya and Nordin 1991).
Although elastic resistance has been accepted as a standard mode of resistance training, some may not understand the biomechanical principles of elastic resistance.
FORCE – ELONGATION CHARACTERISTICS
Knowing the actual variation of force as a function of length is important when prescribing exercise that uses elastic resistance. Researchers have attempted to quantify the amount of force generated during stretching of elastic material. Different types of elastic material from different manufacturers may provide different amounts of force. Regardless of the form of the elastic material, the physical characteristics of force – elongation will remain the same. The resistance provided by elastic tubes is based on the amount of elastic material: thicker tubes provide more resistance than thinner ones. The force of elastic resistance depends on percentage of elongation, regardless of initial length. For example, if a 40 cm length of tubing is elongated to 80 cm (100 % elongation), the force exerted would be the same as if an 80 cm length of tubing was elongated to 160 cm.
The slope of the resistance curve varies with changes in the stiffness and diameter of the material. Because elastic resistance normally is prescribed categorically only by color, one must not assume that changes in resistance are of equal increment when moving from one tubing to another. There is a considerable difference in the slope of the curve representing tube 1 strength versus the tube 4 strength. There is generally a 20 % to 30 % increase in force between each strength of the Freestyler tubing.
Typical clinical elongations rarely exceed 200 %. Hughes et al. (1999) found tubing length changes of 18 % to 160 % from starting length during abduction exercise. Researchers in the Freestyler scientific team also have developed linear regression equations to predict force at different stages of elongation. The resistance has to be sufficient and accommodative within the range of motion to maximize training gains in accordance with the overload principle and SAID principle. Therefore the resting length of the tubing can be adjusted to meet the amount of elongation required to complete the range of motion of various exercises.
THE ROLE OF TORQUE GENERATION IN RESISTANCE EXERCISE
Skeletal mechanics state that strength must be evaluated in relation to torque to understand the true strength requirements and effect of the exercise on the musculoskeletal system. Resistance training must include variables that identify magnitude of force, moment arm and angle between the moving segment and the direction in which the resistance is acting. If the culmination of these variables is considered, then load effects can be displayed graphically with a torque versus joint angle curve. A torque versus joint angle curve is otherwise known as a strength curve.
Three common patterns of strength curves are associated with single joint exercises: ascending, descending and ascending – descending. An ascending curve portrays strength or torque increasing as the joint angle of movement increases. A descending curve shows an inverse relationship between strength and joint angle. An ascending – descending curve is a pattern of effort that first shows an increase in strength as joint angle increases and then displays strength decreases with further increases in joint angle.
An ascending– descending strength curve is the most common pattern exhibited for a majority of joints. Factors such as the mechanics of contraction (concentric, eccentric, or isometric), type of contraction, speed of contraction, and body position can influence the form of strength curve presented. Ultimately, muscle force output will vary throughout the joint range of motion and depends on training mode, interaction of resistance provided and the changing mechanical advantage of the musculoskeletal system. With elastic resistance training, as with other types of resistance training, clinicians should consider the applied and resistive torque generated throughout the range of motion as a factor in determining whether an exercise is appropriate and effectively overloads the muscle.
Elastic resistance offers some of the advantages of isotonic resistance and minimizes its disadvantages.
Similar to isotonic resistance, elastic resistance provides a constant load, requiring the muscle to recruit more motor units to complete the motion. The use of tubing is not influenced by the considerable inertia of the some outside resistance tool (like weights) and is free to move outside a predetermined range of motion. This allows for exercise patterns without concern for gravity, such as diagonal patterns. This attribute also may be beneficial for using elastic tubing resistance in polymetrics.
Although gravity does not significantly affect the resistive characteristics of elastic products, other factors need to be considered. The rate of stretch and overall elongation of the elastic tubing, the point of application, the distance of the resistance from the joint axis and the orientation (angle) of the elastic tubing to the moving limb all affect the torque required by the muscles to move the limb segment. With Freestyler elastic tubing, it is possible to have a linear change in resistance throughout the range of movement as well as a variable resistive torque pattern, because of the changing angle of pull. Extensive research has revealed a linear resistance pattern as the tubing length increased (Hughes et al. 1999).
Freestyler elastic resistance training that loads muscle to its limits throughout the range of motion should result in maximal muscle activation and greater strength gains. Furthermore, the attachment points of the band or tubing to the moving body segment will influence the resistive torque curve. This point of attachment can be referred to as the resistance arm angle (RAA). The RAA is the angle formed by the interaction of the resistive device and lever arm.
As the RAA changes, the effective torque on the exercised joint changes as well. This is based on the following formula, where joint torque equals the product of the sine of the RAA, the force of the resistance, and the length of the lever arm:
Torque = sin (RAA) x force x lever arm
Recently, the torque production of elastic resistance was compared with pulley system resistance. Page and Labbe (2000) studied the torque produced by a weight and pulley system and elastic tubing. The researchers used an isokinetic dynamometer in the continuous passive mode to determine torque production throughout 180o of motion. Torque data were collected during both lengthening (concentric phase) and shortening (eccentric phase) of the tubing. The researchers found that elastic tubing demonstrated symmetrical concentric and eccentric torque curves with peak torque occurring near the middle range of motion.
The pulley resistance demonstrated asymmetrical concentric/eccentric torque curves with the eccentric torque values approximately half of the concentric values. Additionally, the peak torque provided by pulleys occurred earlier in the motion, and torque tended to decrease toward the end of range. Speed did no affect the peak torque values of either resistive mode, but speed did affect the overall pulley curve characteristics, demonstrating more rapid and variable torque production early in the concentric motion. Tubing and pulley systems represent different forms of
resistance exercise patterns.
ELECTROMYOGRAPHIC RESEARCH ON ELASTIC RESISTANCE EXERCISE
Several researchers have investigated electromyographic activity in muscles during elastic resistance exercise. (Cordova, Jutte and Hopkins 1999; Hintermeister, Bey, et al. 1998…) Willet et al. (1998) found that closed – chain terminal knee extensions against elastic resistance were superior to no resistance in activating the vastus medialis oblique and vastus lateralis. Electromyographic activity for the vastus medialis oblique and vastus lateralis ranged from 44 to 48 % maximal voluntary isometric contraction.
Therefore elastic tubing provides suitable progressive resistance in the lower extremity muscles for patients beginning a typical rehabilitation programs. In another study, Hintermeister, Lange et al. (1998) found that exercises that used elastic resistance could effectively target the rotator cuff muscles for postinjury and postoperative patients. Unfortunately, none of the studies cited compared the electromyographic activity of elastic resistance with other forms of resistance.
An interesting observation can be made when comparing isotonic exercise with elastic exercise for shoulder rehabilitation. In study by Hughes and McBride (2000), 12 non-insured subjects compared shoulder rehabilitation exercises that used free weights and elastic tubing. Surface electromyographic activity was collected for the shoulder and scapular muscles while four different exercises were performed with both elastic tubing and dumbbells. The researchers noted distinct differences for muscle activation that were dependent on load and method of resistance. As an example, figure xx shows almost twice the amount of peak muscle activation in the posterior deltoid during the scaption exercise when strength 3, 4 and 5 of the Freestyler tubing was used compared with isotonic loads of 1, 3 and 5 lb with a dumbbell.
In addition, infraspinatus activity was significantly greater when tubing was used versus free (isotonic) weights during the external rotation exercise in side – lying (free weights) and standing (tubing) positions. Factors relating to these differences included postural stabilization, force – elongation changes, direction of resistance application (gravity dependent vs. non gravity dependent), load equivalencies and changes in resistive torque.