It is well known that many detriments occur when we age. That is, if we don’t do anything about it; like exercise! We lose muscle mass, strength, gain fat, etc.
To kick a dead horse when its down, lets add loss of efferent drive to that list!
What 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 (i.e. contacting muscle)
Remodeling of the nervous system occurs from age-related loss of muscle strength (1). The gradual loss of motoneurons contribute to the deleterious effects with age, 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.
These impairments compromise the efferent drive to the muscle and consequently the force production of contracting skeletal muscle.
Does strength training help?
Strength training in the elderly has demonstrated improvement in motoneuron recruitment and firing frequency. Research has shown that efferent drive, specifically motoneuron firing frequency, and likely also motoneuron recruitment, can be improved by strength training in the elderly.
Interestingly, even the strongest elderly individuals, with similar muscle strength as the young, exhibited substantial deficits in efferent drive. There may be some deficiency that occurs over a lifetime of not strength training that a short-term strength training program can’t overcome.
The 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?
Let’s look at a study that compared master athletes who were involved in long-term high intensity strength training with recreationally active and sedentary old individuals (5).
What does the research tell us?
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 indicated a higher efferent drive among the strength trained master athletes and suggests that high-intensity strength training in particular may be necessary to preserve efferent neural drive and that physical activity per se is not sufficient to induce any efferent drive maintenance.
In addition, none of the investigated characteristics of the neuromuscular system in the study displayed any differences between sedentary and recreationally active old participants. This is somewhat surprising but may suggest that high-intensity strength training in particular, and not physical activity per se, may be essential to preserve the neuromuscular function with age.
It may be that high-intensity strength training, targeting the fast twitch motor units, could be particularly beneficial for counteracting the age-related loss of neuromuscular function. The results from this study are in accordance with this assumption.
Take home message
This cross-sectional study shows that efferent drive to contracting muscle is compromised with age. Furthermore, it shows that old subjects involved in long-term strength training mitigate this decline in efferent drive. In contrast, no difference in efferent drive was observed between recreationally active and sedentary old subjects. This indicates that strength training in particular may be beneficial for counteracting the age-related loss of efferent drive.
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.