Increased spinal reflex excitability is not associated with neural plasticity underlying the cross-education effect

Abstract

The purpose of this study was to examine the effects of a 5-wk unilateral, isometric strength-training program on plasticity in the spinal Hoffmann (H-) reflex in both the trained and untrained legs. Sixteen participants, 22-42 yr old, were assigned to either a control (n = 6) or an exercise group (n = 10). Both groups were tested for plantar flexion maximal voluntary isometric contractions (MVIC) and soleus H-reflex amplitude in both limbs, at the beginning and at the end of a 5-wk interval. Participants in the exercise group showed significantly increased MVIC in both legs after training (P < 0.05), whereas strength was unchanged in the control group for either leg. Subjects in the exercise group displayed increased (P < 0.05) H-reflex amplitudes on the ascending limb of the recruitment curve (at an equivalent M wave of 5% of the maximal M wave, H(A)) only in the trained leg. Maximal H-reflex and M-wave remained unchanged with training. Increased amplitude of H(A) in the trained limb concurrent with increased strength suggests that spinal mechanisms may underlie the changes in strength, possibly because of increased alpha-motoneuronal excitability or reduced presynaptic inhibition. Despite a similar increase in strength in the contralateral limb of the exercise group, H(A) amplitude was unchanged. We conclude that the cross-education effect of strength training may be due to supraspinal to a greater extent than spinal mechanisms.

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).…”

“…We did not test for any spinal effects. Considering the inconsistent results from TMS (9,25,30) and peripheral nerve stimulation studies (1,4,13,15,23,24,32,35,38), a role for segmental effects cannot be dismissed. We intentionally designed the study to consist of only 10 sessions so that the nature of adaptation would primarily be neuronal (9); it is still possible that a portion of the strength gains was due to muscle hypertrophy.…”

The Effects of Self-induced and Therapist-assisted Lower-limb PNF Pattern Training on the Activation of Contralateral Muscles

“…Zhou (2000) analyzed previous studies to examine the effect of cross-training and argued that the muscular power of the limbs on the other side can be strengthened, depending on the intensity of the exercise 16) . Lagerquist et al (2006) suggested that the effect of crosstraining would come from the brain region of the central nervous system, not the spinal cord 17) . Munn et al (2005) explained that muscular training on one side of the body can enhance the muscular power of the limbs on the other side because it involves more motor units and the activation increases in the central nervous system 15) .…”

The H-reflex as an important indicator in kinesiology

“…The most commonly assessed muscles of the lower limbs that display the H-reflex are the soleus muscle [37][38][39][40] and the gastrocnemius muscle [5,41,42]. While the H-reflex is very difficult to observe in the upper limbs, some researchers have shown that it can be induced for the flexor carpi radialis muscle [43][44][45][46][47] and the extensor carpi radialis muscle [43,46].…”