Age-related differences in rapid muscle activation after rate of force development training of the elbow flexors

Abstract

In young adults, improvements in the rate of force development as a result of resistance training are accompanied by increases in neural drive in the very initial phase of muscle activation. The purpose of this experiment was to determine if older adults also exhibit similar adaptations in response to rate of force development (RFD) training. Eight young (21-35 years) and eight older (60-79 years) adults were assessed during the production of maximum rapid contractions, before and after four weeks of progressive resistance training for the elbow flexors. Young and older adults exhibited significant increases (P<0.01) in peak RFD, of 25.6% and 28.6% respectively. For both groups the increase in RFD was accompanied by an increase in the root mean square (RMS) amplitude and in the rate of rise (RER) in the electromyogram (EMG) throughout the initial 100 ms of activation. For older adults, however, this training response was only apparent in the brachialis and brachioradialis muscles. This response was not observed in surface EMG recorded from the biceps brachii muscle during either RFD testing or throughout training, nor was it observed in the pronator teres muscle. The minimal adaptations observed for older adults in the bifunctional muscles biceps brachii and pronator teres are considered to indicate a compromise of the neural adaptations older adults might experience in response to resistance training.

Load knowledge reduces rapid force production and muscle activation during maximal-effort concentric lifts

“…In line with this, Duchateau and Baudry (2014) presented extensive evidence that training-related increases in the rate of force production (i.e., RFD) during ballistic contractions primarily resulted from adaptations in motor unit discharge rate. These conclusions underscore the critical role of maximal motor unit discharge rate (i.e., muscle activation) on the ability to rapidly develop force, and are consistent with research showing that explosivetype training elicits increases in high-frequency motor unit discharges at the onset of muscular contraction (Van Cutsem et al 1998) and research showing strong relationships between changes in RFD and changes in the rate of EMG amplitude rise (RER; ∆EMG/∆time) after periods of resistance-type training (Aagaard et al 2002;Barry, Warman and Carson 2005;Blazevich et al 2008).…”

“…The greater MVIC trial was used to normalize the muscle activation during the bench press throws. EMG data were analyzed over the following time intervals: pre100-pre50 ms, pre50-0 ms (where 0 corresponds to the movement onset), 0-50 ms, 50-100 ms and 100-150 ms. Additionally, the normalized change in RMS EMG between the different time intervals was used as a measure of the rate of EMG rise (RER) (Aagaard et al 2002;Barry, Warman and Carson 2005;Blazevich et al 2008). All data were stored for offline analysis with custom-built software (LabView, National Instruments, Austin, Texas).…”

Age-related changes in the rate of muscle activation and rapid force characteristics

“…2). The rate of EMG rise (RER; % PEMG s -1 ) was used to examine the rate of muscle activation (Clark et al 2011(Clark et al , 2013Mitchell et al 2011) and was calculated as the linear slope of the normalized EMG-time curve at time intervals of 30 ms (RER30), 50 ms (RER50), and 75 ms (RER75) from onset similar to the procedures described by Aagaard et al (2002) and Barry et al (2005). The specific time intervals for determining RER (in particular the selection of 75 ms instead of 100 ms) were used because of a characteristic decrease in the EMG signal amplitude occurring at~80-100 ms following the signal onset (Aagaard et al 2002; Note: RER time intervals do not correspond directly to the RFD intervals due to the electromechanical delay.).…”

Changes in muscle force–length properties affect the early rise of force in vivo

“…Such an increase in the quantity of activated muscle can result from an increase in descending neural drive to the pool of spinal motoneurons. It is not surprising, therefore, that a higher RFD can be seen in individuals who have the highest rise in neuromuscular activity [largest rates of electromyographic (EMG) rise] at contraction onset, 12,13 that strength training leading to an increased and more rapid rate of neuromuscular activity (EMG) is accompanied by increases in contractile RFD, 1,5,8,26,29 and that the greater presence of doublet motor unit firing patterns induced by strength training is associated with greater RFD. 29 In a recent study we showed that increases in RFD resulting from longer-term (10 weeks) strength training were well related to increases in neuromuscular activity; however, we were unable to explain the increases in RFD after shorter-term (5 weeks) training.…”