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Written by Karel Nunnink   

It’s no secret that muscle size and strength drop rapidly after the age of 50. When we understand the physiology of muscle fibers, however, it becomes apparent that the big drop isn’t inevitable; it can be countered or reversed. Let’s take a look at aging muscle fibers and the nerves that activate them.

The loss of muscle size that occurs as we age is called sarcopenia, taken from Greek meaning “abnormal reduction” or “deficiency.” Sarcopenia occurs for two reasons. First, individual muscle cells (fibers) shrink and eventually die. Because muscle fibers are bundled together, when individual fibers shrink, the whole bundle loses size. In addition, every muscle fiber has a nerve, called a motor nerve, which innervates it. For efficiency, motor units branch out and control many muscle fibers (100 to 10,000). The motor nerve and the fibers it controls are called a motor unit.

“As we age, our motor nerves and their associated muscle fibers die off at an ever increasing rate. “A small percentage of these fibers are rescued by neighboring motor units, but the number of living fibers still drops exponentially…The bottom line is that as we age we not only have smaller but also fewer muscle fibers.”

Fiber type also plays a major role in muscle size. Slow-twitch fibers are small, and fast-twitch fibers are large. The slow-twitch fibers are the endurance fibers, which predominate in marathon runners and other endurance athletes. Like the Energizer Bunny, they don’t give out, they keep on contracting. They don’t generate much force, however. Fast-twitch fibers are the strength fibers, which rule the roost in sprinters, weight lifters, and other strength athletes. They are strong, but fatigue rapidly. Most of us are born with a roughly equal balance of slow/small and fast/large fibers.

“As we age, the motor units that we lose are mainly the fast-twitch variety. The slow-twitch fibers show practically no change. This phenomenon is due, in part, to the rescue process mentioned earlier. “During the rescue process, slow-twitch motor units rescue fast-twitch fibers and change them into slow-twitch fibers. So the cost of rescuing a few fibers is that the aging muscle shows a predominance of slow-twitch fibers. The curve of the slow-twitch fibers over time is no curve at all; it’s flat, showing essentially the same number of slow fibers at 60 and 90. On the other hand, the fast-fiber curve drops rapidly from 60 to 90.

In short, the loss of muscle size with age is virtually all due to shrinkage and death of fast-twitch fibers. Translated to the activities of everyday life, this means the untrained person becomes slower and weaker with age. Independence suffers over time.

It’s a dismal picture, but all is not lost. Far from it. We can keep our fast-twitch fibers alive and well with resistance training. How many people are affected by sarcopenia? . It has been estimated that from the age of 60 to 80 the prevalence of sarcopenia [2 standard deviations below the mean muscle mass of healthy young adults] in the general population progresses from 15% to 32% for men and 23% to 36% for women. After the age of 80 these values increase to 51% for women and 55% for men.

It’s a big problem. 

Reduced physical activity is the main culprit. Resistance training is the best antidote. Resistance training has been shown to positively affect neurological, hormonal, and mechanical factors associated with muscle maintenance and growth.  

The bottom line is that resistance training can have a positive effect on all aspects of the neuromuscular and biochemical decline that accompanies aging.

Front and center is the effect on the fast-twitch muscle fibers, the type 2 fibers, which are most dramatically affected by the aging process. Resistance training increases the size of the type 2 fibers and the ability of the motor nerves to recruit those fibers. Resistance training also increases the ability of the fast fibers to repair themselves, thus reaching higher levels of hypertrophy.

On the biochemical side, resistance training increases levels of testosterone and growth hormone. What’s more, it increases blood glucose utilization and ATP production and recycling.

Resistance training can be used to target different needs—muscular strength, endurance, power, hypertrophy, and maximal strength—by changing the nature of the training.  The key, is finding a training routine that the individual enjoys and wants to continue over the long term. The biggest job facing fitness professionals is not finding the best combination of sets and reps--it is motivating clients to keep training. As we’ve already seen, regular trainers reap the greatest benefits over a lifetime.)

Now, let’s talk cardiovascular fitness and intervals.

 

The most common measure of cardiovascular capacity is maximal oxygen uptake, or VO2max.  VO2max is a measure of the maximal rate at which your body can use oxygen, or simply your aerobic power. As in the case of neuromuscular function, an exponential drop in maximal aerobic power occurs with age. Men and women decline at essentially the same rate, with men having marginally higher VO2max throughout life. 

Again, it doesn’t have to be that way.  

Researchers who examine training protocols to increase aerobic capacity consistently report that high-intensity exercise produces greater improvement in VO2max than low- or moderate-intensity exercise produce. If improving aerobic power is the goal, intensity is the name of the game.

Interval training is the most effective and efficient form of high-intensity cardio training—for old and young alike. If increasing aerobic capacity, reducing high blood pressure, or weight loss (especially around the waistline) is the goal, then interval training is one of the most effective tools you possess to reach that goal.

Dr. Signorile PhD, Author of Bending The Aging Curve, who did most of the research in this article found,“One study compared moderate-intensity continuous training (70% peak HR) with aerobic interval training (95% peak HR) performed three times per week for 12 weeks,” Signorile writes. “The subjects were 27 postinfarction heart failure patients and the average age was 75.7 years. VO2 peak increased more than three times as much with the interval training than it did with the moderate continuous training.” (Emphasis mine) What’s more, only interval training improved “the filling and emptying capacity of the heart” and the ability of the arteries to accommodate increased blood flow.

What’s the most efficient work/rest ratio to improve the cardiovascular system? Signorile says, “The winner is…the old 2:1 standby composed of 20 seconds of work and 10 seconds of recovery.” (Sound familiar? It’s the original Tabata protocol reported here in 1997.)

That, however, doesn’t mean that Dr. Signorile recommends only one ratio. He wisely suggests a varied approach to interval training. “This is not to say that a work-recovery duty cycle of 20 seconds to 10 seconds is the panacea of cardiovascular training. In fact, the best idea is to use a diverse mix of work-recovery cycles…”

In addition to work-recovery ratio, other variables are the length of the work cycle, intensity, and number of work-recovery cycles. The options are almost limitless. Dr. Signorile lists several things to keep in mind. “Short work cycles (10 to 20s) allow the highest intensity, while short recovery periods (20 to 40s) limit recovery between reps. Moderately short work cycles (30s) allow for fairly high intensity, and somewhat longer recovery periods (60s) allow more complete recovery. Longer work cycles (60s) require the most difficult mix of work and recovery (60 to 120s), perhaps too difficult to be feasible for non-athletes. Finally, very long work (2 to 5 min) and recovery (4 to 10 min) cycles are impractical, and make it more beneficial to simply do steady state exercise.  

My suggestion is to make intervals as hard as you can tolerate without killing motivation—hard but not too hard.

 

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