Effects of unilateral, isometric resistance training on strength development and the Hoffmann-reflex response in the trained and untrained limb

The purpose of this study was to determine the effects of a 5-week unilateral, isometric strength-training program on presynaptic inhibition (PSI) and alterations in the H-reflex in both the trained and untrained limbs. Thirteen subjects, aged 23--42 years old, were assigned to either a control group (n = 6) or an exercise group (n = 7). Both groups were tested at the beginning and end of a 5-week interval on both limbs for maximal voluntary isometric contractions (MVIC) of the plantar flexors as well as three different conditioning protocols of the Soleus Hoffmann (H) reflex. Experimental group participants significantly increased MVIC in both legs following training (p < 0.05) while control group participants showed no increase for either leg. Experimental subjects displayed increased normalized H-reflex values at M = 5% (HA) (p < 0.05) in the trained leg only. Adaptations in HA for the trained limb in the presence of a substantial strength increase suggests that spinal mechanisms may partly explain the increase in strength, possibly due to increased alpha-motoneuronal excitability. However, the lack of HA increase in contralateral limb in the presence of a substantial strength increase, points to different neural mechanisms responsible for the cross education effect.