Adaptations in the activation of human skeletal muscle induced by short-term isometric resistance training

Abstract

This study employed longitudinal measures of evoked spinal reflex responses (Hoffman reflex, V wave) to investigate changes in the activation of muscle and to determine if there are "linked" neural adaptations in the motor pathway following isometric resistance training. Twenty healthy, sedentary males were randomly assigned to either the trained (n = 10) or control group (n = 10). The training protocol consisted of 12 sessions of isometric resistance training of the plantar flexor muscles over a 4-wk period. All subjects were tested prior to and after the 4-wk period. To estimate changes in spinal excitability, soleus Hoffman (H) reflex and M wave recruitment curves were produced at rest and during submaximal contractions. Recruitment curves were analyzed using the slope method (Hslp/Mslp). Modulation of efferent neural drive was assessed through evoked V wave responses (V/Mmax) at 50, 75, and 100% maximal voluntary contraction (MVC). After 4 weeks, MVC torque increased 20.0 +/- 13.9% (mean +/- SD) in the trained group. The increase in MVC was accompanied by significant increases in the rate of torque development (42.5 +/- 13.3%), the soleus surface electromyogram (60.7 +/- 30.8%), voluntary activation (2.8 +/- 0.1%), and the rate of activation (48.7 +/- 24.3%). Hslp/Mslp was not altered by training; however, V/Mmax increased 57.3 +/- 34.2% during MVC. These results suggest that increases in MVC observed in the first few days of isometric resistance training can be accounted for by an increase in the rate of activation at the onset of muscle contraction. Augmentation of muscle activation may be due to increased volitional drive from supraspinal centers.

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 facilitatory effect of duloxetine combined with pelvic floor muscle training on the excitability of urethral sphincter motor neurons

“…Regarding the facilitatory neuromodulative effects of muscle training, it could be demonstrated in several studies that even short-term muscle training increases the excitability in supraspinal and also in spinal centres [17,18]. This is one of the first mechanisms together with adaptations in motor unit recruitment which is involved in the very early changes after starting muscle training [18].…”

“…This is one of the first mechanisms together with adaptations in motor unit recruitment which is involved in the very early changes after starting muscle training [18]. The supraspinal regions involved in EUS control and coordination of pelvic floor muscles including the EUS have been revealed in several imaging studies using PET and fMRI [19].…”

Reflex Inhibition in Human Biceps Brachii Decreases With Practice of a Fatiguing Contraction

“…Practice-related adaptations in the nervous system can include changes in corticospinal excitability (Jensen et al 2005;Perez et al 2007), somatosensory feedback to the motor neurons (Geertsen et al 2007;Zehr 2006), and the output of the motor neuron pool to muscle (Del Balso and Cafarelli 2007;Duchateau et al 2006). The sites of plasticity in the nervous system are specific to the task performed and can differ, for instance, between motor skill training and strength training (Jensen et al 2005;Semmler and Nordstrom 1998).…”