Introduction
There seems to be an almost limitless selection of exercise equipment on the market that is designed for resistance training. The equipment ranges from simple to complex, compact to space-consuming, and inexpensive to expensive. An assortment of simple but versatile handheld and cuff weights or elastic resistance products is useful in clinical and home settings, whereas multiple pieces of variable resistance equipment may be useful for advanced-level resistance training. Sources of information about new products on the market are the literature distributed by manufacturers, product demonstrations at professional meetings, and studies of these products reported in the research literature.

Although most equipment is load resisting (augments the resistance of gravity), a few pieces of equipment can be adapted to be load assisting (eliminates or diminishes the resistance of gravity) to improve the strength of weak muscles. Equipment can be used for static or dynamic, concentric or eccentric, and open-chain or closed-chain exercises to improve muscular strength, power, or endurance, neuromuscular stability or control, as well as cardiopulmonary fitness.

In the final analysis, the choice of equipment depends primarily on the individual needs, abilities, and goals of the person using the equipment. Other factors that influence the choice of equipment are the availability; the cost of purchase or maintenance by a facility or a patient; the ease of use (application or setup) of the equipment; the versatility of the equipment; and the space requirements of the equipment. Once the appropriate equipment has been selected, its safe and effective use is the highest priority. General principles for use of equipment are listed below.

General Principles for the Selection and Use of Equipment
• Base the selection of equipment on a comprehensive examination and evaluation of the patient.
• Determine when in the exercise program the use of equipment should be introduced and when it should be altered or discontinued.
• Determine if the equipment could or should be set up and used independently by a patient.
• Teach appropriate exercise form before adding resistance with the equipment.
• Teach and supervise the application and use of the equipment before allowing a patient to use the equipment independently.
• Adhere to all safety precautions when applying and using the equipment.
• Be sure all attachments, cuffs, collars, and straps are securely fastened and that the equipment is appropriately adjusted to the individual patient prior to the exercise.
• Apply padding for comfort, if necessary, especially over bony prominences.
• Stabilize or support appropriate structures to prevent unwanted movement and to prevent undue stress on body parts.
• If exercise machines are used independently, be certain that set-up and safety instructions are clearly illustrated and affixed directly to the equipment.
• If compatible with the selected equipment, use range-limiting attachments if ROM must be restricted to protect healing tissues or unstable structures.
• If the patient is using the equipment in a home program, give explicit instructions on how, when, and to what extent to change or adapt the equipment to provide a progressive overload.
• When making a transition from use of one type of resistance equipment to another, be certain that the newly selected equipment and method of set-up initially provides a similar level of torque production to the equipment previously employed to avoid insufficient or excessive loads.
• When the exercise has been completed:
• Disengage the equipment and leave it in proper condition for future use.
• Never leave broken or potentially hazardous equipment for future use.
• Set up a regular routine of maintenance, replacement, or safety checks for all equipment.

Free Weights and Simple Weight-Pulley Systems
Types of Free Weights
Free weights are graduated weights that are handheld or applied to the upper and lower extremities or trunk. They include commercially available dumbbells, barbells, weighted balls, cuff weights, weighted vests, and even sandbags. Free weights can also be fashioned for a home exercise program from readily available materials and objects found around the home.

Simple Weight-Pulley Systems
Free-standing or wall-mounted simple weight-pulley systems with weight-plates are commonly used for resisted upper and lower extremity or trunk exercises. Permanent or interchangeable weights are available. Permanent weights are usually stacked with individual weight plates of 5- to 10-pound increments that can be easily adjusted by changing the placement of a single weight key.

NOTE: The simple weight-pulley systems described here are those that impose a relatively constant (fixed) load. Variable resistance weight machines, some of which incorporate pulleys into their designs, are discussed later in this section.

Characteristics of Free Weights and Simple Weight-Pulley Systems
Free weights and weight-pulley systems are resistance equipment that impose a fixed (constant) load. The weight selected, therefore, maximally challenges the contacting muscle at only one portion of the ROM when a patient is in a particular position. The weight that is lifted or lowered can be no greater than what the muscle can control at the point in the ROM where the load provides the maximum torque. In addition, there is no accommodation for a painful arc.

When using free weights, it is possible to vary the point in the ROM at which the maximum resistance load is experienced by changing the patient’s position with respect to gravity or the direction of the resistance load. For example, shoulder flexion may be resisted with the patient standing or supine and holding a weight in the hand.
• Patient position: standing: Maximum resistance is experienced and maximum torque is produced when the shoulder is at 90° of flexion. Zero torque is produced when the shoulder is at 0° of flexion. Torque again decreases as the patient lifts the weight from 90°to 180° of flexion. In addition, when the weight is at the side (in the 0° position of the shoulder), it causes traction force on the humerus; and when overhead, it causes compression force through the upper extremity.
• Patient position: supine: Maximum resistance is experienced and maximum torque is produced when the shoulder is at 0° of flexion. Zero torque is produced at 90° of shoulder flexion. In this position the entire load creates a compression force. The shoulder flexors are not active between 90° and 180° of shoulder flexion. Instead, the shoulder extensors must contract eccentrically to control the descent of the arm and weight.

The therapist must determine at which portion of the patient’s ROM maximum strength is needed and must choose the optimum position in which the exercise should be performed to gain maximum benefit from the exercise.

Simple weight-pulley systems provide maximum resistance when the angle of the pulley is at right angles to the moving bone. As the angle of the pulley becomes more acute, the load creates more compression through the moving bones and joints and less effective resistance.

Unlike many weight machines, neither free weights nor pulleys provide external stabilization to guide the moving segment or restrict ROM. When a patient lifts or lowers a weight to an overhead position, muscles of the scapula and shoulder abductors, adductors, and rotators must synergistically contract to stabilize the arm and keep it aligned in the correct plane of motion. The need for concurrent contraction of adjacent stabilizing muscle groups can be viewed as an advantage or disadvantage. Because muscular stabilization is necessary to control the plane or pattern of movement, less resistance can be controlled with free weights than with a weight machine during the same movement pattern.

Advantages and Disadvantages of Free Weights and Simple Weight-Pulley Systems
• Exercises can be set up in many positions, such as supine, side-lying, or prone in bed or on a cart, sitting in a chair or on a bench, or standing. Many muscle groups in the extremities and trunk can be strengthened by simply repositioning the patient.
• Free weights and simple weight-pulley systems typically are used for dynamic, non-weight-bearing exercises but also can be set up for isometric exercise and resisted weight-bearing activities.
• Stabilizing muscle groups are recruited; however, because there is no external source of stabilization and movements must be controlled entirely by the patient, it may take more time for the patient to learn correct alignment and movement patterns.
• A variety of movement patterns is possible, incorporating single plane or multiplanar motions. An exercise can be highly specific to one muscle or generalized to several muscle groups. Movement patterns that replicate functional activities can be resisted.
• If a large enough assortment of graduated free weights is available, resistance can be increased by very small increments, as little as 1 pound at a time. The weight plates of pulley systems have larger increments of resistance, usually a minimum of 5 pounds per plate.
• Most exercises with free weights and weight-pulley systems must be performed slowly to minimize acceleration and momentum and prevent uncontrolled, end-range movements that could compromise patient safety. It is thought that the use of exclusively slow movements during strengthening activities has less carryover to many daily living activities than the incorporation of slow- and fast-velocity exercises into a rehabilitation program. However, a weighted ball can be used with catching and throwing exercises as part of plyometric training during the advanced phase of upper extremity rehabilitation. • Free weights with interchangeable disks, such as a barbell, are versatile and can be used for patients with many different levels of strength, but they require patient or personnel time for proper assembly.
• Bilateral lifting exercises with barbell weights often require the assistance of a spotter to ensure patient safety, thus increasing personnel time.

Variable-Resistance Machines
Variable-resistance exercise equipment falls into two broad categories: specially designed weight-cable (weight-pulley) machines and hydraulic and pneumatic units. Both categories of equipment impose a variable load on the contracting muscles consistent with the changing torque-producing capabilities of the muscles throughout the available ROM.

Variable Resistance Weight-Cable Systems
Variable-resistance weight-cable machines use a cam in their design. The cam (an elliptical or kidney-shaped disk) in the weight-cable system is designed to vary the load (torque) applied to the contracting muscle even though the weight selected remains the same. In theory, the cam is configured to replicate the length-tension relationship and resultant torque curve of the contracting muscle with the greatest amount of resistance applied in the mid-range. This system varies the external load imposed on the contracting muscle based on the physical dimensions of the “average” individual. How effectively this design provides truly accommodating resistance throughout the full ROM is debatable.

With each repetition of an exercise, the same muscle group contracts and is resisted concentrically and eccentrically. As with simple weight-pulley systems and free weights, exercises must be performed at relatively slow velocities, thus compromising carryover to many functional activities.

Hydraulic and Pneumatic Variable-Resistance Units
Other variable-resistance machines employ hydraulic or pressurized pneumatic resistance to vary the resistance throughout the ROM. These units allow concentric, reciprocal muscle work to agonist and antagonist muscle groups but no eccentric work. Patients can safely exercise at fast velocities with these units. These units also allow a patient to accommodate for a pain-free arc.

Advantages and Disadvantages of Variable-Resistance Machines
• The obvious advantage of these machines compared to constant load equipment is that the effective resistance is adjusted, at least to some extent, to a muscle’s tension-generating capabilities throughout the ROM. The contracting muscle is loaded maximally at multiple points in the ROM, rather than just one small portion of the range.
• Most pieces of equipment are designed to isolate and exercise a specific muscle group. For example, resisted squats are performed on one machine and hamstring curls on another. Some units, such as a leg press or shoulder press, strengthen multiple muscle groups simultaneously.
• Unlike functional movement, most machines allow only single-plane movements, although some newer units now offer a dual-axis design allowing multiplanar motions that strengthen multiple muscle groups and more closely resemble functional movement patterns.
• The equipment is adjustable to a certain extent to allow individuals of varying heights to perform each exercise in a well aligned position.
• Each unit provides substantial external stabilization to guide or limit movements. This makes it easier for the patient to learn how to perform the exercise correctly and safely and helps the patient maintain appropriate alignment without assistance or supervision.
• One of the main disadvantages of weight machines is the initial expense and ongoing maintenance costs. Multiple machines, usually 8 to 10 or more, must be purchased to target multiple major muscle groups. Multiple machines also require a large amount of space in a facility.

Elastic Resistance Bands and Tubing
The use of elastic resistance products in therapeutic exercise programs has become widespread in rehabilitation and has been shown to be an effective method of providing resistance and improving muscle strength.146 Despite the popularity of these products, not until the past 10 years has quantitative information been reported on the actual or relative resistance supplied by elastic products or the level of muscle activation during use. These studies suggest that the effective use of elastic products for resistance training requires not only the application of biomechanical principles but also an understanding of the physical properties of elastic resistance material.

Types of Elastic Resistance
Elastic resistance products, specifically designed for use during exercise, fall into two broad categories: elastic bands and elastic tubing. Elastic bands and tubing are produced by several manufacturers under different product names, the most familiar of which is Thera-Band® Elastic Resistance Bands and Tubing (Hygenic Corp., Akron, OH). Elastic bands are available in an assortment of grades or thicknesses. Tubing comes in graduated diameters and wall thickness that provide progressive levels of resistance. Color-coding denotes the thickness of the product and grades of resistance.

Properties of Elastic Resistance—Implications for Exercise
A number of studies describing the physical characteristics of elastic resistance have provided quantitative information about its material properties. Knowledge of this information enables a therapist to use elastic resistance more effectively for therapeutic exercise programs.
Effect of elongation of elastic material. Elastic resistance provides a form of variable resistance because the force generated changes as the material is elongated. Specifically, as it is stretched, the amount of resistance (force) produced by an elastic band or tubing increases depending on the relative change in the length of the material (percentage of elongation/deformation) from the start to the end of elongation. There is a relatively predictable and linear relationship between the percentage of elongation and the tensile force of the material.

To determine the percentage of elongation, the stretched length must be compared to the resting length of the elastic material. The resting length of a band or tubing is its length when it is laid out flat and there is no stretch applied. The actual length of the material before it is stretched has no effect on the force imparted. Rather, it is the percentage of elongation that affects the tensile forces.

The formula for calculating the percentage of elongation/deformation is:

Percent of elongation = (stretched length - resting length) ÷ resting length × 100

Using this formula, if a 2-foot length of red tubing, for example, is stretched to 4 feet, the percentage of elongation is 100%. With this in mind, it is understandable why a 1-foot length of the same color tubing stretched to 2 feet (100% elongation) generates the same force as a 2-foot length stretched to 4 feet.

Furthermore, the rate at which elastic material is stretched does not seem to have a significant effect on the amount of resistance encountered. Consequently, when a patient is performing a particular exercise, so long as the percentage of elongation of the tubing is the same from one repetition to the next, the resistance encountered is the same regardless of whether the exercise is performed at slow or fast velocity.

Determination and quantification of resistance. In order to make decisions based on evidence, rather than solely on clinical judgment, about the grade (color) of elastic material to select for a patient’s exercise program, a number of studies have been done to quantify the resistance imparted by elastic bands or tubing. These studies measured and compared the tensile forces generated by various grades of elastic bands or tubing in relation to the percentage of elongation of the material. The forces expected at specific percentages of elongation of each grade of tubing or bands can be calculated by means of linear regression equations. Detailed specifications about the material properties of one brand of elastic resistance products, Thera-Band,® are available at www.thera-bandacademy.com/.

During exercise, the percentage of deformation and resulting resistance (force) from the material is not the only factor that must be considered. The amount of torque (force × distance) imposed by the elastic on the bony lever is also an important consideration. Just because the tension produced by an elastic band or tubing increases as it is stretched, it does not mean that the imposed torque necessarily increases from the beginning to the end of an exercise. In addition to the resistance (force) imposed by the elastic material as it is stretched, the factor of the changing length of the moment arm as the angle of the elastic changes with respect to the moving limb affects the torque imparted by the elastic material. Studies have indicated that bell-shaped torque curves occur, with the peak torque near mid-range during exercises with elastic material. Careful scrutiny of these studies is necessary to determine if the elastic resistance is at a 90° angle to the moving limb at mid-range. As in all forms of dynamic resistance exercise, the length-tension relationship of the contracting muscle also affects its ability to respond to the changing load.

Fatigue characteristics. It has been suggested that elastic resistance products tend to fatigue over time, which causes the material to lose some of its force-generating property. That being said, the extent of material fatigue is dependent on the number of times the elastic band or tubing has been stretched (number of stretch cycles) and the percentage of deformation with each stretch.

Studies have shown that the decrease in tensile force is significant but small, with much of the decrease occurring within the first 20 or 50 stretch cycles. However, in the former study, investigators found that after this small initial decrease in tensile force occurred there was no appreciable decrease in the force-generating potential of the tubing after more than 5000 cycles of stretch. In other words, a patient could perform 10 repetitions each of four different exercises, three times a day on a daily basis for 6 weeks with the same piece of tubing before needing to replace it.

Elastic materials also display a property called viscoelastic creep. If a constant load is placed on elastic material, in time it becomes brittle and eventually ruptures. Environmental conditions, such as heat and humidity, also affect the force-generating potential of elastic bands and tubing.

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