Isometric force-time parameters in monitoring of strength training : with special reference to acute responses to different loading resistances

Abstract

The aim of the present series of studies was to investigate acute neuromuscular responses to (1) different strength training loadings and using (2) different external resistances. In addition, chronic adaptations and dynamic performances were compared to (3) the changes in isometric force-time parameters at the group level, and to (4) the individual timing of the improvement in the rate of force development (RFD) due to hypertrophic or maximum strength followed by power strength training periods. The latter one was achieved by systematic, repeated monitoring. Sixty-nine physically active men (20-35 yrs), but not experienced in resistance training, took part in the present series of studies. Cross-sectional study designs included hypertrophic (5 sets of 10 repetition maximum), power (5 sets of 5 repetitions at 40% of one repetition maximum) and maximum strength (15 sets of one repetition maximum) loadings using pneumatic and weight- stack devices with and without additional elastic resistances for the knee extensor muscles. During single explosive contractions, pneumatic resistance allowed greater power production at lower resistance levels due to higher velocities compared to weight-stack resistance. Conversely, weight-stack with additional elastic resistance increased power production due to greater torque towards the end of the movement compared to “pure” weight-stack resistance. However, during power strength loadings with explosive contractions, weight-stack resistance targeted the initial force production and, consequently, induced greater neuromuscular fatigue indicated by changes in muscle activity (EMG) during the first 100ms compared to pneumatic and weight-stack with elastic resistances. Nevertheless, greater resistance during weight-stack with elastic resistance induced greater central fatigue compared to “pure” weight-stack. Weight-stack resistance followed more closely maximal force production of muscles inducing greater peripheral fatigue during hypertrophic and maximum strength loadings compared to pneumatic resistance. Repeated maximal repetitions during maximum strength loadings also led to central fatigue during weight-stack resistance. During the longitudinal study, time to reach peak RFD was identified as a potential parameter to differentiate adaptation between “peripheral and central focused” strength training, while the steepest phase of RFD may identify adaptations particularly during maximal strength/power training. Monitored isometric RFD parameters seemed to more sensitively and systematically reflect short-term responses from different training stimuli compared to peak isometric MVC. Nevertheless, MVC correlated strongly with the long-term changes in 1RM due to strength training. From the individual trainee’s perspective, the timing of the improvement in monitored RFD was related to baseline CSA and training-induced changes in anabolic and catabolic hormonal regulation. Based on these individual differences, RFD improved in one-third of the trainees following the maximum strength training period, one-third following the power strength training period, and the remaining one-third did not respond to either of the aforementioned training periods. Regularly repeated isometric monitoring during strength training could assist in tailoring training programs and selecting durations of the periodization cycles for each athlete, individually.