Strength Assessment

In many ways strength assessment represents the gold standard of sarcopenia research as it is the loss of strength and the consequences of this loss of strength that make sarcopenia an important topic of study. Most sarcopenia studies include measures of strength to test the effects of declines in muscle mass, effects of changes in muscle composition, or effects of specific treatments on muscle strength.

Strength is often assessed via measurement of the force of static or dynamic muscle contractions. Static muscle contractions are muscle contractions where the force is exerted on a stationary object and the involved joints do not move during the contraction. Isometric contractions are the most widely used means of measuring static muscle strength due to the safety and ease of isometric testing as well as the lower cost of equipment (Hunter et al., 1998). A variety of reliable and portable equipment, such as handgrip dynamometers, are available for testing.

Despite the ease of static testing, dynamic strength testing may be more relevant as most daily activities, such as walking, climbing stairs, and doing housework, are dynamic tasks (Hunter et al., 1998). A dynamic muscle contraction is a muscle contraction where the involved joints move while under a resistive load. Dynamic strength can be measured either via the one repetition maximum (1RM) approach or with a isokinetic dynamometer. The 1RM approach involves finding the heaviest weight that can be lifted through a complete range of motion. This can be accomplished easily by using commonly available gym weight machines (Hunter et al., 1998). Concerns have been raised about the safety of this approach especially in older subjects, but in carefully supervised settings, 1RM testing has been performed safely (Fiatarone et al., 1994; Fiatarone et al., 1990). Isokinetic dynamometers allow strength to be measured while the limb or joint moves at a defined speed. Most dynamometers are amenable to computer monitoring, so force data can be measured over the entire range of movement. Use of a dynamometer also has a safety advantage over the 1RM method as both the speed of movement and range of motion can be controlled (Hunter et al., 1998). A significant limitation of dynamometers is the cost of the equipment.

An important concern during strength testing is to be sure that differences in strength observed is due to differences in muscle contractile properties instead of differences in muscle activation. When older subjects, or inactive younger subjects, are first tested for strength, there is often incomplete recruitment or suboptimal firing of motor units (Hunter et al., 1998). Consequently, repeated testing less than a week later can show a significant improvement due to improvements in muscle motor unit activation as opposed to muscle hypertrophy (Hunter et al., 1998). This issue can be addressed by providing a practice session first and then performing the actual strength testing at a later second session. These retest measures have been shown to be reliable for both younger and older people (Hunter et al., 1998). A more rigorous approach is to use pulses of electrical stimulation during an isometric contraction that is believed to be maximal. The stimulation can either be delivered via electrodes over the motor nerve or the muscle itself. If the isometric contraction is maximal, then delivering the stimulus should not increase the force of contraction, but submaximal contractions are augmented by the pulses. The benefit of this approach is to provide feedback to both the observer and subject regarding the degree of muscle activation. Electrical stimulation also has been utilized in the study of dynamic muscle contractions through the use of tetanic stimuli (Hunter et al., 1998).

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