Investigations of the Neuromuscular Response During and Following Elite Maximum Strength and Power Type Resistance Exercise

Abstract

The thesis aimed to analyse the acute neuromuscular (NM) response during and following maximum strength and power training methods.

The primary aim of study one was to establish the reliability of biomechanical and surface electromyographic (sEMG) measurements during barbell squat exercise. This would enable the subsequent studies to precisely assess muscle activity and mechanical power during barbell resistance exercise sessions. Nine male well-trained subjects performed squat exercise on three separate trial days. Each trial comprised one set of squat at 50, 75 and 100% of 3RM load. Synchronous recordings of knee joint kinematics from a flexible electrogoniometer, barbell displacement from a single linear position transducer and quadriceps sEMG amplitude were made. The mean maximum knee angle during squat was recorded at each load, and the overall inter-trial coefficient of variation (CV) was 5.5%. Mean concentric repetition power was processed from displacement data and derived into force and velocity values. The overall inter-trial CV for mean power was found to be 8.4%. The raw sEMG signal was processed into root mean square (RMS) amplitude and normalised to values taken from pre-trial knee extension maximum voluntary contractions (MVC). RMS amplitude was processed for the whole concentric phase and a 200 ms time interval at a knee angle of 70°, which matched the knee angle used during MVC. Inter-trial CV for RMS amplitude from the concentric phase and 70° knee angle were 7.2% and 16.4% respectively. There were no differences in RMS amplitude, maximum knee angle or mean power across trial days. It was concluded there was acceptable reliability for all three measurements (CV < 10%), if RMS amplitude was processed from the concentric phase. Based upon the measurement reliability, the analysis system was considered suitable for monitoring power and sEMG during barbell exercise. The second study aimed to establish the reliability of muscle fibre conduction velocity (MFCV) measurements during barbell squat. This was of interest, as MFCV may provide useful information of NM recruitment and fatigue processes during resistance exercise. The study was also used as a preliminary investigation of MFCV response, in comparison to RMS amplitude, to increasing fatigue and load during squat exercise. Nine well-trained male subjects performed a series of exercises on two separate trial days. Each trial comprised isometric knee extensions at 50, 75 and 100% of MVC force, followed by barbell squats at 50, 75 and 100% of 3RM, and then a maximal bout of squat jumps at 50% 3RM load, performed until failure. sEMG measurements were recorded from a four-electrode array, secured upon the vastus lateralis. Normalised RMS amplitude was processed as above, and MFCV was processed from the inter-electrode distance and time delay between two double differentiated and correlated signals, using bespoke software. The overall value of MFCV during squat was 5.8 m.s-1. The inter-trial CV for MFCV was 9.6% during squat and 12.1% during squat jump. Based upon acceptable reliability of 10%, MFCV measurements from barbell squats were considered reliable. As expected, MFCV significantly increased with each knee extension force level (4.7 ± 1.4, 5.6 ± 1.5 and 6.2 ± 1.8 m.s-1) (p<0.01), along with RMS amplitude (p<0.0001). No differences in MFCV were found between squat loads, whilst RMS amplitude significantly increased with load (p<0.0001). Power (1920 ± 143 versus 1407 ± 254 W) and MFCV (5.7 ± 1.4 versus 4.6 ± 1.0 m.s-1) significant decreased (p<0.001) from the start to the end of the squat jump trial, with RMS amplitude unchanged. Therefore, MFCV altered with increasing fatigue, but not load, during dynamic squat exercise. It was concluded that MFCV provides useful and reliable data for acute fatigue investigations of barbell resistance exercise, in addition to sEMG amplitude measures.

The following three investigations compared NM responses during and following maximum strength and power type resistance exercise sessions with different exercises, loads and movement speeds. The sessions were designed to represent elite athlete training practices, to help inform the optimisation of resistance exercise programmes. The first of these studies aimed to compare NM response to a typical maximum strength session performed with barbell squat or deadlift exercise. The purpose was to assess if technical differences between the exercises, influenced the acute NM response. Nine elite trained weightlifters performed the trial sessions of five sets of five repetitions on separate days. Normalised RMS amplitude, MFCV and power was continually measured during exercise repetitions, using the methods established above. NM function was assessed pre- and post- sessions using MVC force, central activation ratio (CAR) from superimposed stimulation during MVC, and jump performance (CMJ). The exercises were performed with subjectively matched load levels, corresponding to active muscle RPE = 17 (Borg scale), and also with controlled lifting speed. However, the squat load was lowered and raised upon the lifter’s back, whilst deadlift load was grasped in the hands, raised from the floor and then dropped. Repetition mean power was unchanged within and across sets of both sessions. Repetition RMS amplitude significantly increased (p<0.001) within sets of squat and deadlift, whilst a significant interaction between sessions and set (p<0.001) demonstrated RMS increased more during squat. Furthermore, a significant reduction in repetition MFCV was found within sets of squat (p = 0.034), but not deadlift. This suggests that motor unit activation increased during both exercises, as a response to the task of maintaining power during repetitions of whole body lifting. However, acute fatigue within squat sets led to additional increased activation as a NM compensation strategy. No pre- versus post- session differences were found for MVC, CAR or CMJ; suggesting minimal change in NM function occurred following five sets of maximum strength type resistance exercise, in well-trained subjects. The primary aim of the second study was to compare NM response and 24-hour recovery following barbell exercise maximum strength and power type sessions. The purpose was to specifically establish the degree and nature of NM response, as previous findings were unclear and barbell exercise sessions of this type have not been compared. 10 elite sprint athletes performed sessions comprising squat, split squat and push press, with four sets x repetitions per exercise. The maximum strength session exercises involved loads corresponding to active muscle RPE = 17 (Borg scale) and metronome controlled movements. The power session exercises used 30% of the maximum strength barbell load, performed as fast as possible. Repetition sEMG and power was monitored throughout each session, as above. NM function was assessed, pre-, post- and 24-hour post- each session, using the same tests as above. However, evoked peak twitch force (Pt) was also included to the pre- and post- assessments. Overall, the maximum strength session involved greater total work (p = 0.008), but lower mean power during exercise repetitions (p<0.001) in comparison to the power session. MVC and Pt force values both significantly decreased (p<0.05) pre- versus post- both sessions. However, MVC reduced more following maximum strength session (p<0.01). CAR and CMJ were unchanged post-both sessions and no differences were found between pre and 24-hour post session NM tests. The decreased Pt but not CAR findings, suggest peripheral fatigue explains the reduced force generation capacity following maximum strength and power sessions, contrary to previous resistance exercise session findings. Up to 24-hours may be required to recover force generation capacity following this volume of resistance exercise. Additional analysis suggested strength levels influenced the degree of fatigue following the power session. This was because barbell exercises involve lifting body mass and bar mass. Therefore, stronger subjects lifted relatively lighter loads during a barbell power session using 30% of bar mass. This supports the use of system mass loads to determine relative load levels during power type sessions. The aim of the final study was to compare NM and hormonal response following high intensity ‘explosive’ squat at three load levels. This training method is specific to elite athletes and has not been previously assessed. The purpose was to further understanding of the load level of explosive exercise that provides the most effective training stimulus. 15 elite power athletes, from track and field and rugby, completed 10 sets of high intensity squat exercise on three separate days. The heavy session involved loads corresponding to active muscle RPE = 17 (Borg scale), as above. The moderate and light sessions were 75% and 50% system mass of heavy session load, respectively. The execution of every repetition was maximal in all three sessions. Methods followed previous studies with the addition of isometric knee extension rate of force development (RFD) and loaded squat jump (SJ) power to the NM function tests. Saliva samples were taken at baseline, mid-, and post- session for testosterone (T) and cortisol (C) assay analysis. Heavy session involved greatest repetition impulse in comparison to moderate and light sessions (p<0.001), whilst light session involved highest repetition power (p<0.001). Total work performed in each session was similar. MVC, RFD and Pt force values were significantly reduced post- sessions (p<0.01). However, MVC and RFD reduced most following heavy, then moderate and then light sessions. This corresponded to significantly reduced repetition power during sets of the heavy session only (p<0.001). Repetition RMS amplitude also increased most during sets of heavy session (p<0.001), followed by moderate, with no change during light session. These findings suggest NM response was greatest during heavy session, providing effective training stimulus, but so was acute NM fatigue. Moderate load explosive exercise may also provide sufficient NM stimulus, however with less fatigue. Decrement in RFD was significantly greater than MVC force (p<0.001), and was reduced mid- as well as post- session. This suggests high intensity squat training affects NM mechanisms related to RFD capacity. No significant changes in CAR, CMJ or loaded SJ were found. Significant reductions in C relative to baseline (p<0.001) occurred mid- and post all three sessions, as expected following circadian rhythms. A significant interaction between session and time (p<0.01) was found, where T was maintained relative to baseline following moderate and heavy sessions, but reduced following the light session. This also suggests heavy and moderate high intensity sessions may provide more effective training stimulus than light load.

The findings of this thesis show that the NM response during maximum strength and power type resistance exercise sessions involves increased motor unit activation within exercise sets. This may occur without fatigue during exercise repetitions and indicates the NM stimulus for adaptation. The nature of NM fatigue following maximum strength and power training, in terms of reduced force generation, involves peripheral, and not central, mechanisms, contrary to previous conclusions and general belief amongst sports coaches. Importantly, stimulus may not be directly related to the degree of post-session NM fatigue, but instead the NM activation during exercise repetitions. The data implies certain exercises (e.g. deadlift and explosive moderate load squats) provide sufficient stimulus for adaptation, with a limited NM fatigue response. This informs training programme design for elite athletes completing diverse and concurrent training activities.