With ageing, skeletal muscle atrophy in humans appears to be inevitable. A gradual loss of muscle fibres begins at approximately 50 years of age and continues such that by 80 years of age, approximately 50% of the fibres are lost from the limb muscles that have been studied. 
The loss of muscle mass with age (sarcopenia) is associated with a decline in muscle strength, muscle power, muscle quality, and physical function and increases in fat infiltration and mortality.
|Robert Oppenheimer did it.|
I love jumping. Been doing it as long as I can remember. Jumped long jump, high jump and triple jump when I was young. Played basketball, and preferred jumping for rebounds. Did martial arts and preferred jump kicks. Now I’ve quit all organized exercise and generally just jump around. Oh, and landing is also great.
But jumping is more than childish fun for me. When I worked as an exercise instructor for older people, most all of them told me that the ability that was most quickly reduced with aging, and one of the things they missed the most, was the explosiveness – the spring in their feet. But they also told me that they just stopped moving in this way as they got older. Not because jumping became more difficult in any way, but because they just did. Truth be told, you rarely see older people jumping. They usually prefer less explosive ways of moving, and I really think that is sad.
An important part of our health as we age is the ability to be self-sufficient and good physical function is paramount to our quality of life. Both strength training and power training are effective at increasing physical performance in older adults , although power training may yield similar results with less total work performed per training session . Power training has also been found to be superior to regular strength training in older adults . Even moderate plyometric training improves chair-rise performance in older adults  and all in all it seems that we should keep jumping as we age. It really matters.
A decline in muscle mass and strength with aging can be found in every living creature endowed with muscle tissue. Worms, flies, fish, frogs, mice, rats, dogs, and primates are among the many species that have been studied that show atrophic and structural degeneration in aging muscles often with the accumulation of abnormal degenerative proteins. 
As we age our muscles atrophy. We lose muscle cells and the reason seems to be that we lose motor neurons: the neurons that signal the muscles to move. Our body has no interest in having a lot of energy demanding muscle tissue hanging around if it’s not being used. Less signal to move means less muscle tissue. This first graph from Faulkner et al  show the relationship between the total number of fibres in the vastus lateralis muscles and the age of men between 18 to 82 years of age. The second graph shows the relationship between the number of motor units in the extensor digitorum brevis muscles (muscle in the foot that extends a couple of toes) and the age of men between 5 and 88 years of age.
Why we lose these motor neurons is not well understood. Some people will say that aging in itself is the cause, but the reason for believing this seem simply to be that older people have less motor neurons, as does other animals as they age. Exercise will slow this process, but by how much we cannot say. We really do not know how much off the neuron loss is caused by aging and how much is caused by inactivity or lack of challenge to skeletal muscles. Still, exercise helps, and particularly explosive exercise.
For most elderly people, the decrease in muscle mass is accompanied by at least an equal, but usually even greater, decrease in strength and power, as well as an increase in muscle weakness (the strength per unit of cross-sectional area of muscle) and fatigability. The sum total of these effects is that age-related changes in the musculoskeletal system have a significant impact on the everyday activities of the elderly.
It is also usually said that aging causes a preferential degradation of fast muscle fibers and this is has always made sense, given how old people actually move less explosively. New research however, indicate that there might also be quite a large loss of slow muscle fibers .
An important difference between regular resistance exercise and plyometric (high velocity strength) exercise is that while resistance exercise promotes more muscle hypertrophy, plyometric will more so improve neuron-muscle cooperation, increase strength per unit of cross-sectional area and also a preferential stimulation of fast twitch type II fibers. Plyometrics will improve the ability to generate a lot of power in a short time.
Consider the high jumper. The force generated to fling a 2 meter high man over a 2.4 meter obstacle is massive, and yet the high jumpers are not known for their large leg muscles. An incredible amount of force can be generated even by normal sized muscles. This video shows Swedish high jumper and Olympic gold medalist, Stefan Holm jumping over some hurdles (He is 1.81m tall and his personal best indoor is 2.40m.) I wonder how he will move when he is 60.
An important benefit of resistance exercise is prevention of osteoporosis, and all sorts of high power exercises are particularly effective at building bone mass. But we should remember that the main problem with osteoporosis is falling. If we don’t fall or injure ourselves, it does not matter much if our bones are a bit brittle. So we also need to focus on preventing falling and this is where speed may be important and where plyometrics could be more important than resistance exercise.
Now, I wasn’t joking about also liking landing. Landing is in itself a perfectly fine exercise. Remember that landing is great eccentric exercise. For obvious reasons we are stronger landing than we are jumping. This is good; as it will prevent us from jumping so high that the body cannot take the force of the landing. Further, it means that if you want to exercise using landing, you need to either go higher than you can jump, find some extra weights or land on one foot.
I use landing as an exercise form and often do several jumps per session down from hights that will really challenge my leg muscles.
Sarcopenia is a multifactorial consequence of aging that will affect many adults. Resistance training is the most effective and safe intervention to attenuate or recover some of the loss of muscle mass and strength that accompanies aging. 
I think the take home message here is this: we don’t have to become week; we don’t have to lose the spring in our steps. We don’t have to stop jumping if we don’t want to. What we need to do in order to live long independent lives is simply to keep moving and keep jumping.
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2. Hanson ED, Srivatsan SR, Agrawal S, Menon KS, Delmonico MJ, Wang MQ et al.: Effects of strength training on physical function: influence of power, strength, and body composition. J Strength Cond Res 2009, 23: 2627-2637.
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9. Purves-Smith FM, Solbak NM, Rowan SL, Hepple RT: Severe atrophy of slow myofibers in aging muscle is concealed by myosin heavy chain co-expression. Exp Gerontol 2012.