If we don’t exercise as we get older, it’s well known both anecdotally and from research-based evidence that it can be detrimental to our health.
By choosing to not exercise, not only do we lose muscle mass, strength, functionality, health, but we also gain fat which increases chances at developing many diseases.
Recent evidence also shows that we also lose our efferent drive too.
What exactly is efferent drive?
Efferent neurons are neurons that send impulses from the central nervous system to your limbs and organs.
So, efferent drive is our ability to excite those impulses from our central nervous system to our organ and limbs and enable us to move and contract our skeletal muscle. As a result, remodeling of the nervous system (e.g. loss of motor neurons) occurs as we lose muscle from aging (1).
Motor neurons are neuronal cells located in the central nervous system
They control a variety of downstream targets such as muscle contraction, and the gradual loss of motoneurons contributes to the deleterious effects of aging, which results in reduced motoneuron firing frequency (2), increased presynaptic inhibition (4), and slower nerve conduction velocity (3).
Basically, this describes what happens when our efferent drive slows down, which is essentially a slower impulse from start to finish. These impairments compromise the efferent drive to the muscle and consequently the force production of contracting skeletal muscle.
Does high-intensity strength training help?
Strength training in the elderly improves motoneuron recruitment and firing frequency. Research has shown that efferent drive, specifically motoneuron firing frequency and motoneuron recruitment, can be improved by strength training as we age.
Interestingly, even the strongest elderly individuals, with similar muscle strength as their younger counterparts; exhibited substantial deficits in efferent drive. This tells us that there may be some decrements that occurs over a lifetime of not strength training that a short-term strength training program can’t overcome.
The very pertinent question is: does a lack of strength training over many decades lead to irrefutable neuronal loss, which might not be restored if it is first lost?
A recent study compared master athletes who were involved in long-term high intensity strength training with recreationally active and sedentary elderly individuals (5).
Main findings from this research
The main finding was that the strength-trained master athletes had twofold higher maximal voluntary contractions compared with the sedentary and recreationally active age-matched individuals. This demonstrated that these master athletes had a higher efferent drive suggesting that high-intensity strength training in particular may be necessary to preserve efferent neural drive.
The research indicates that physical activity alone without adequate intensity doesn’t enhance or potentially maintain efferent drive.
In support of this notion, there weren’t any characteristics of the neuromuscular system that showed any differences between sedentary and recreationally active elderly individuals. Although this is surprising, it supports the idea that high-intensity strength training in particular, and not physical activity per se, may be essential to preserve the neuromuscular function with age.
It's plausible that high-intensity strength training which targets fast twitch motor units, could be particularly beneficial for counteracting the age-related loss of neuromuscular function. The results from this study agree with this concept.
Take home message
Evidence shows that efferent drive declines as we age. Elderly people involved in long-term strength training prevent this decrement in efferent drive. When comparing recreationally active and sedentary elderly individuals; there were now differences seen in efferent drive. This indicates that high-intensity strength training may be beneficial for counteracting the age-related loss of efferent drive.
References
1. Aagaard P, Suetta C, Caserotti P, Magnusson SP, and Kjaer M. Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a countermeasure. Scandinavian Journal of Medicine & Science in Sports 20: 49-64, 2010.
2. Klass M, Baudry S, and Duchateau J. Voluntary activation during maximal contraction with advancing age: a brief review. European journal of applied physiology 100: 543-551, 2007.
3. Metter EJ, Conwit R, Metter B, Pacheco T, and Tobin J. The relationship of peripheral motor nerve conduction velocity to age-associated loss of grip strength. Aging (Milano) 10: 471-478, 1998.
4. Morita H, Shindo M, Yanagawa S, Yoshida T, Momoi H, and Yanagisawa N. Progressive decrease in heteronymous monosynaptic Ia facilitation with human ageing. Experimental brain research 104: 167-170, 1995.
5. Unhjem R, Nygard M, van den Hoven LT, Sidhu SK, Hoff J, and Wang E. Lifelong strength training mitigates the age-related decline in efferent drive. Journal of applied physiology 121: 415-423, 2016.
