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1 Department of Neurophysiology, Institute of Medical Physiology and 2 Anatomy Department C, Panum Institute, University of Copenhagen; 3 Copenhagen Muscle Research Centre, Rigshospitalet; 4 Team Danmark Test Centre, Sports Medicine Research Unit, Bispebjerg Hospital, DK-2200 Copenhagen, Denmark
Combined V-wave and
Hoffmann (H) reflex measurements were performed during maximal muscle
contraction to examine the neural adaptation mechanisms induced by
resistance training. The H-reflex can be used to assess the
excitability of spinal 
-motoneurons, while also reflecting
transmission efficiency (i.e., presynaptic inhibition) in Ia afferent
synapses. Furthermore, the V-wave reflects the overall magnitude of
efferent motor output from the -motoneuron pool because of
activation from descending central pathways. Fourteen male subjects
participated in 14 wk of resistance training that involved heavy
weight-lifting exercises for the muscles of the leg. Evoked V-wave,
H-reflex, and maximal M-wave (Mmax) responses were recorded
before and after training in the soleus muscle during maximal isometric
ramp contractions. Maximal isometric, concentric, and eccentric muscle
strength was measured by use of isokinetic dynamometry. V-wave
amplitude increased ~50% with training (P < 0.01)
from 3.19 ± 0.43 to 4.86 ± 0.43 mV, or from 0.308 ± 0.048 to 0.478 ± 0.034 when expressed relative to
Mmax (± SE). H-reflex amplitude increased ~20%
(P < 0.05) from 5.37 ± 0.41 to 6.24 ± 0.49 mV, or from 0.514 ± 0.032 to 0.609 ± 0.025 when normalized to Mmax. In contrast, resting H-reflex amplitude remained
unchanged with training (0.503 ± 0.059 vs. 0.499 ± 0.063).
Likewise, no change occurred in Mmax (10.78 ± 0.86 vs. 10.21 ± 0.66 mV). Maximal muscle strength increased
23-30% (P < 0.05). In conclusion, increases in
evoked V-wave and H-reflex responses were observed during maximal muscle contraction after resistance training. Collectively, the present
data suggest that the increase in motoneuronal output induced by
resistance training may comprise both supraspinal and spinal adaptation
mechanisms (i.e., increased central motor drive, elevated motoneuron
excitability, reduced presynaptic inhibition).
M-wave; -motoneurons; skeletal muscle
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