Plantar flexor activation capacity and H reflex in older adults: adaptations to strength training

Abstract

The purpose of this study was to investigate whether the voluntary neural drive and the excitability of the reflex arc could be modulated by training, even in old age. To this aim, the effects of a 16-wk strengthening program on plantar flexor voluntary activation (VA) and on the maximum Hoffman reflex (H(max))-to-maximum M wave (M(max)) ratio were investigated in 14 elderly men (65-80 yr). After training, isometric maximum voluntary contraction (MVC) increased by 18% (P < 0.05) and weight-lifting ability by 24% (P < 0.001). Twitch contraction time decreased by 8% (P < 0.01), but no changes in half relaxation time and in peak twitch torque were observed. The VA, assessed by twitch interpolation, increased from 95 to 98% (P < 0.05). Pretraining VA, also evaluated from the expected MVC for total twitch occlusion, was 7% higher (P < 0.01) than MVC. This discrepancy persisted after training. The interpolated twitch torque-voluntary torque relationship was fitted by a nonlinear model and was found to deviate from linearity for torque levels >65% MVC. Compared with younger men (24-35 yr), the H(max)- to M(max) ratio and nerve conduction velocity (H index) of the older group were significantly lower (42%, P < 0.05; and 29%, P < 0.001, respectively) and were not modulated by training. In conclusion, older men seem to preserve a high VA of plantar flexors. However, the impaired functionality of the reflex pathway with aging and the lack of modulation with exercise suggest that the decrease in the H(max)- to M(max) ratio and H index may be related to degenerative phenomena.

Control of support limb muscles in recovery after tripping in young and older subjects

“…However, Porter et al (1995) question the generalization of such neural training effects across tasks. In addition, Scaglioni et al (2002) found increased voluntary activation of the plantar flexors in elderly after strength training but no increase in motor neuron excitability as evidenced by Hreflex amplitudes. It is therefore questionable whether the control of responses can be trained.…”

“…Motor neuron excitability appears reduced in the elderly, as evidenced by lower H-reflex amplitudes as compared to young adults (Scaglioni et al 2002). Pavol et al (2001) ascribed falls after tripping in a group of elderly subjects to a slower execution of the recovery strategy.…”

Chronic low-frequency rTMS of primary motor cortex diminishes exercise training-induced gains in maximal voluntary force in humans

“…However, resistance training of the tibialis anterior was associated with a 32% increase in MEP amplitude produced by TMS during low-level contractions without changes in M-wave amplitude, indicating a role for spinal, corticospinal neurons, possibly M1 (25). In addition, several other studies showed that adaptations to acute and chronic voluntary and electrical stimulation-evoked muscle contractions, without a skill component, increased volitional drive from supraspinal centers (4, 13, 15, 24, 31, 32) without concomitant changes in H reflex, measured at rest or during mild voluntary contraction (1,10,13,15,23,24,35,38,50,55). However, the interpretation of these studies must be viewed carefully because they do not provide direct evidence for M1's involvement in strength gains and some studies did find increases in H reflex after resistance training (1,4,15,32).…”

Assessing Voluntary Muscle Activation with the Twitch Interpolation Technique

“…Numerous studies have shown that the relationship is concave upward at medium forces and asymptotic at high voluntary forces [eg.1,18,28,29] . Some studies also indicate an upward convexity in the relationship at low voluntary forces [1,30] , although this is not always the case [15,26,31] .…”

“…It has been suggested that twitch interpolation involving a single superimposed stimulus may be incapable of detecting sub-optimal motor unit firing rates [42,43] and that the evoked-voluntary force relationship becomes less steep or even asymptotic at forces at which recruitment is complete and rated coding becomes the sole means of enhancing force [31] . This view is contradicted by two observations.…”