A Question

Last year in Pt school we had a lecture series on the physiology of aging accompanied by the Online Curriculum in Geriatrics and Gerontology hosted by the UCSF Academic Geriatric Resource Center. I highly recommend the online course as it was both informative and entertaining (gerigero-onlinecourse.ucsf.edu) but the experience left me wondering if there was any physiological parameter in the human body that improved with age. I joked with my classmates that I would be able to pass the test without studying if I just picked the answer that stated the greatest lost in capacity with aging. Maximal muscle strength decreases significantly with age and sarcopenia is an average loss of muscle mass of 40% in men from age 20 to 80. So what about muscular endurance, does this also decrease with age?

Some Data

Muscular endurance (resistance to muscular fatigue) is distinguished for cardiovascular endurance by the way it is tested in the lab, for example the former might be tested by asking someone to do bicep curls to failure, and the second by running on a treadmill to exhaustion. I found a review addressing the former and aging titled “Systematic Review and Meta-Analysis of Skeletal Muscle Fatigue in Old Age” by Anita Christie, Erin Snook and Jane Kent-Braun in the April 2011 volume of Medicine & Science in Sports & Exercise. This meta analysis pooled the results from 37 studies using the standardized differences in means. The average age of the young subjects ranged from 21 to 36 and that of older subjects ranged from 61 to 84 years.

To my surprise the older individuals had greater endurance overall but analysis conducted by categorizing the included studies according to testing methodology revealed two exceptions. However, before discussing these exceptions, let’s give the older individuals credit for their performance in the following testing conditions: better endurance in both sustained and intermittent contractions, better endurance at maximal and sub maximal contraction, able to maintain a specific percent of their max for a longer duration, able to produce a greater percentage of their max over a specified time period, greater endurance in both voluntary and electrically stimulated contractions, greater endurance for all muscle groups tested and this greater endurance was seen in both sexes. As a side note, the same relationship between age and muscle endurance has been seen in non-human animals. Despite their stellar performance in the above categories, older individuals demonstrated less endurance than their younger counterparts in dynamic contractions and when power was used as the outcome measure.

An isometric contraction is one in which a muscle contracts but does not change length, a practical example of an isometric contraction of the biceps muscle is holding a tray still at waist height.  Dynamic muscle contractions are characterized by a change in muscle length, practical examples include picking  a tray up from the ground (concentric contraction) and lowering it back to the ground (eccentric contraction).  During dynamic contractions the older individuals has less endurance.

The second condition in which the older individuals had less endurance was when power output was measured. The formula for mechanical power is force times velocity. Since velocity is the change in displacement over the change in time, power is the ability to create a lot of force and move an object far in a short amount of time. The ability to generate a high power output is the key to performance in many athletic endeavors from sprinting and jumping to rowing and cycling. The measure of power is watts and next time you are on a stationary bike, treadmill, elliptical or rowing ergometer, flip through the settings and you will likely find a setting that allows you to measure the power of your activity in watts.  In the studies included in this review, even when older adults were able to generate comparable dynamic force they took increasingly longer to do it and thus their power output fatigued sooner.

My Application

After reading these results I immediately wanted to know the mechanism for these observed differences. Unfortunately, mechanisms are only speculative at this point. One potential mechanism for the greater endurance and concomitant power loss is the observed loss in type two and subsequently greater percentage of fatigue resistant type one fibers that occurs with age.

The decreased ability to maintain power output combined with less muscle strength overall might make more demanding activities of daily living more difficult to accomplish at the end of the day. On the other end, the increased ability of older individuals to maintain a constant force over time might be considered when designing devices that interact with this demographic or be a consideration in planning activities.

The big question on the intervention side of things is whether muscle strength and power loss with age can be remediated or reversed with training. Because baseline muscle power is highly predictive of future morbidity and is related to measures of physical function a further understanding of how instantaneous power and muscular power over time can be improved in the elderly may improve quality of life significantly for these individuals.

Caution

Just because systematic reviews with meta-analysis are at the top of the evidence pyramid, doesn’t mean their results are impervious to bias. Fewer studies tested dynamic contractions than isometric and the not many studies used decrease in power as a decrease in strength so these exceptional results may be due to investigation bias. More prospective investigation of muscular endurance using dynamic contractions and power is needed to conclusively answer this question.

References

1. Christie A, Snook EM, Kent-Braun JA. Systematic Review and Meta-Analysis of Skeletal Muscle Fatigue in Old Age. Medicine & Science in Sports & Exercise 2011; 43(4):568-577

2.  Metter EJ, Talbot LA, Schrager M, Conwit RA. Arm-cranking muscle power and arm isometric muscle strength are independent predictors of all-cause mortality in men. J Appl Physiol. 2004; 96(2):814–21.