Strength training-induced responses in older adults: attenuation of descending neural drive with age

Although reductions in resting H-reflex responses and maximal firing frequency suggest that reduced efferent drive may limit muscle strength in elderly, there are currently no reports of V-wave measurements in elderly, reflecting the magnitude of efferent output to the muscle during maximal contraction. Furthermore, it is uncertain whether potential age-related neural deficiencies can be restored by resistance training. We assessed evoked reflex recordings in the triceps surae muscles during rest and maximal voluntary contraction (MVC), rate of force development (RFD), and muscle mass in seven elderly (74 +/- 6 years) males before and after 8 weeks of heavy resistance training, contrasted by seven young (24 +/- 4 years) male controls. At baseline, m. soleus (SOL) V/M ratio (0.124 +/- 0.082 vs. 0.465 +/- 0.197, p<0.05) and H/M ratio (0.379 +/- 0.044 vs. 0.486 +/- 0.101 p=0.07) were attenuated in elderly compared to young. Also, SOL H-reflex latency (33.29 +/- 2.41 vs. 30.29 +/- 0.67 ms, p<0.05) was longer in elderly. The reduced neural drive was, despite similar leg muscle mass (10.7 +/- 1.2 vs. 11.5 +/- 1.4 kg), mirrored by lower MVC (158 +/- 48 vs. 240 +/- 54 Nm, p<0.05) and RFD (294 +/- 126 vs. 533 +/- 123 Nm s(-1), p<0.05) in elderly. In response to training SOL V/M ratio (0.184 +/- 0.092, p<0.05) increased in the elderly, yet only to a level similar to 40 % of the young. This was accompanied by increased MVC (190 +/- 70 Nm, p<0.05) and RFD (401 +/- 147 Nm . s(-1), p<0.05) to levels of similar to 80 % and similar to 75 % of the young. H/M ratio remained unchanged. These findings suggest that changes in supraspinal activation play a significant role in the age-related changes in muscle strength. Furthermore, this motor system impairment can to some extent be improved by heavy resistance training.