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The force generated during a maximal voluntary contraction (MVC) is known to increase by resistance training. Although this increase cannot be solely attributed to changes in the muscle itself, many studies examining muscle activation at peak force failed to detect neural adaptations with resistance training. However, the activation prior to peak force can have an impact on maximal force generation. This study aims at investigating the role of rate of force development (RFD) on maximal force during resistance training. Fourteen subjects carried out 5 days of isometric resistance training with dorsiflexion of the ankle with the instruction to generate maximal force. In a second experiment, 18 subjects performed the same task with the verbal instruction to generate maximal force (instruction I) and to generate force as fast and forcefully as possible (instruction II). The main findings were that RFD increased twice as much as the 16% increase in maximal force with training, with a positive association between RFD and force within the last session of training and between training sessions. Instruction II generated a higher RFD than instruction I, with no difference in maximal force. These findings suggest that the positive association between RFD and maximal force is not causal, but is mediated by a third factor. In the discussion, we argue for the third factor to be physiological changes affecting both aspects of a MVC or different processes affecting RFD and maximal force separately, rather than a voluntary strategic change of both aspects of MVC.

The amplitude of a surface electromyogram is dependent on the number of active motor units, their size and the relative position of the recording electrode. It is not possible to interpret the surface electromyogram quantitatively without disentangling these different aspects. In this study the decline of different components of the motor unit potential with increasing radial distance from the motor unit is quantified. Fifty-two motor units in the biceps brachii muscle were studied using 36-channel surface electromyography combined with intramuscular scanning electromyography. Scanning electromyography was used to locate precisely the motor unit. The dependence of the surface motor unit potential magnitude on the radial distance between the motor unit and the recording electrodes can be described fairly well by an inverse power function. The steepness of this function depends on the chosen motor unit potential parameter and the interelectrode distance, but also varies between motor units. The change of the negative peak amplitude of the motor unit potential over the skin surface can be used to give a fairly accurate estimate of the location of the motor unit under the skin surface. We found that for all practical purposes the depth of a motor unit in the biceps brachii muscle can be estimated as 20% of the distance over the skin surface where motor unit potentials can be recorded with higher amplitudes than 50% of the maximal amplitude recorded at the skin surface caused by activity of the same motor unit.

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Parallel increases in strength and rate of force development (RFD) are well-known outcomes from the initial phase of resistance training. However, it is unknown whether neural adaptations with training contribute to improvements of both factors. The aim of this study was to examine whether changes in H-reflex amplitude with resistance training can explain the gain in strength or rather be associated with RFD. Twelve subjects carried out 3 weeks of isometric maximal plantarflexion training, whereas 12 subjects functioned as controls. H-reflexes were elicited in the soleus muscle during rest and sub-maximal contractions at 20 and 60% of maximal voluntary contraction (MVC). In addition, surface electromyography (sEMG) was recorded from the soleus, gastrocnemius and tibialis anterior muscles during MVC. The resistance training provided increases in maximal force of 18%, RFD of 28% and H-reflex amplitude during voluntary contractions of 17 and 15% while no changes occurred in the control group. In contrast, the maximal M-wave, the maximal H-reflex to maximal M-wave ratio during rest and sEMG during MVC did not change with training. There was a positive correlation between percentage changes in H-reflex amplitude and RFD with training (r = 0.59), while significant association between percentage changes in H-reflex amplitude and maximal force was not found. These findings indicate the occurrence of changed motoneuron excitability or presynaptic inhibition during the initial phase of resistance training. This is the first study to document that increased RFD with resistance training is associated with changes in reflex excitability.