It is well known in the exercise science literature that older adults have less ability to produce powerful movements, like a standing vertical jump from a bent-knee position. Power is force times speed, so you could lose power from a loss of strength (thus less ability to produce force) or from your muscles getting slower (unable to contract as fast), or a combination of both. It has been controversial which of these is the culprit. An interesting new study did an experiment that seemed to place the blame more on loss of strength. The authors recruited younger subjects (age ranging from 20-30), and older subjects (age ranging from 65-86). They first tested the subjects’ static leg strength in a knee extension machine. It was found that older subjects had lower static strength on average. So when they are trying to do a jump, which requires accelerating their bodyweight, they are having to move a higher percentage of their maximum leg strength. Muscles can contract faster against less resistance, which is why you can throw a baseball farther than a shotput. Next, they came up with a way to make subjects effectively lighter or heavier as they did the jump. To make them heavier they just wore a weighted vest. To make them lighter, they wore a vest attached to a cable that went over a pulley, with weight on the other side, so the cable was pulling them up. Now they had the subjects do the jump against normal weight, weight reduced by 15%, and weight increased by 15%. From the results they were able to show the older subjects were able to produce the same take off velocity as the younger subjects, as long as they were jumping against the same percentage of their max strength as the younger subjects. The authors noted: “The main observation of this study was that the lower power generating capacity of older than young people was primarily due to a loss of force generating capacity, rather than a slowing of the contractile properties of the muscle“.
The authors also measured performance in a movement that is a typical activity of daily living called the “timed up and go” (TUG) test: The subjects had to get up out of a chair, walk a short distance around a cone, then sit back down and the sequence was timed. It was found that all subjects, young and old, took about the same amount of time to do this movement, as long as their ability to generate power, as measured in the jump test, was above a certain critical value compared to their bodyweight (this value turned out to be 23.7 watts per kilogram of bodyweight). But for subjects whose ability to generate power fell below this critical value, performance in the TUG test declined significantly.
All of this should be motivation to keep doing strength training as we age. Movements that are activities of daily living, like getting out of a chair or going up and down stairs, will otherwise become increasingly difficult and slower, as shown in the TUG test results above. This new study implies that it is more important to maintain strength, and power will take care of itself, so leg presses might be more important than box jumps. But I like to cover my bases by working on both.