Kevin Thomas

The effect of two plyometric training techniques on muscular power and agility in youth soccer players

Thomas, K, French, D, and Hayes, PR. The effect of two plyometric training techniques on muscular power and agility in youth soccer players. J Strength Cond Res 23(1): 332-335, 2009-The aim of this study was to compare the effects of two plyometric training techniques on power and agility in youth soccer players. Twelve males from a semiprofessional football clubs academy (age = 17.3 ± 0.4 years, stature = 177.9 ± 5.1 cm, mass = 68.7 ± 5.6 kg) were randomly assigned to 6 weeks of depth jump (DJ) or countermovement jump (CMJ) training twice weekly. Participants in the DJ group performed drop jumps with instructions to minimize ground-contact time while maximizing height. Participants in the CMJ group performed jumps from a standing start position with instructions to gain maximum jump height. Posttraining, both groups experienced improvements in vertical jump height (p < 0.05) and agility time (p < 0.05) and no change in sprint performance (p > 0.05). There were no differences between the treatment groups (p > 0.05). The study concludes that both DJ and CMJ plyometrics are worthwhile training activities for improving power and agility in youth soccer players.

Improved endurance capacity following chocolate milk consumption compared with 2 commercially available sport drinks.

This study examined the effects of 3 recovery drinks on endurance performance following glycogen-depleting exercise. Nine trained male cyclists performed 3 experimental trials, in a randomized counter-balanced order, consisting of a glycogen-depleting trial, a 4-h recovery period, and a cycle to exhaustion at 70% power at maximal oxygen uptake. At 0 and 2 h into the recovery period, participants consumed chocolate milk (CM), a carbohydrate replacement drink (CR), or a fluid replacement drink (FR). Participants cycled 51% and 43% longer after ingesting CM (32 +/- 11 min) than after ingesting CR (21 +/- 8 min) or FR (23 +/- 8 min). CM is an effective recovery aid after prolonged endurance exercise for subsequent exercise at low-moderate intensities.

Effects of Deception on Exercise Performance: Implications for Determinants of Fatigue in Humans

PURPOSE The aim of this study was to investigate whether it was possible to reduce the time taken to complete a 4000-m cycling time trial by misleading participants into believing they were racing against a previous trial, when, in fact, the power output was 2% greater. METHODS Nine trained male cyclists each completed four 4000-m time trials. The first trial was a habituation and the data from the second trial was used to form a baseline (BL). During trials 3 and 4, participants raced against an avatar, which they were informed represented their BL performance. However, whereas one of these trials was an accurate (ACC) representation of BL, the power output in the other trial was set at 102% of BL and formed the deception condition (DEC). Oxygen uptake and RER were measured continuously and used to determine aerobic and anaerobic contributions to power output. RESULTS There was a significant difference between trials for time to completion (F = 15.3, P = 0.00). Participants completed DEC more quickly than BL (90% CI = 2.1-10.1 s) and ACC (90% CI = 1.5-5.4 s) and completed ACC more quickly than BL (90% CI = 0.5-4.8 s). The difference in performance between DEC and ACC was attributable to a greater anaerobic contribution to power output at 90% of the total distance (F = 5.3, P = 0.02, 90% CI = 4-37 W). CONCLUSIONS The provision of surreptitiously augmented feedback derived from a previous performance reduces time taken for cyclists to accomplish a time trial of known duration. This suggests that cyclists operate with a metabolic reserve even during maximal time trials and that this reserve can be accessed after deception.

Central and peripheral fatigue in male cyclists after 4-, 20-, and 40-km time trials.

PURPOSE Few studies have assessed neuromuscular fatigue after self-paced locomotor exercise; moreover, none have assessed the degree of supraspinal fatigue. This study assessed central and peripheral fatigue after self-paced exercise of different durations. METHODS Thirteen well-trained male cyclists completed 4-, 20-, and 40-km simulated time trials (TTs). Pre- and immediately post-TT (<2.5 min), twitch responses from the knee extensors to electrical stimulation of the femoral nerve and transcranial magnetic stimulation of the motor cortex were recorded to assess neuromuscular and corticospinal function. RESULTS Time to complete 4-, 20-, and 40-km TTs was 6.0 ± 0.2, 31.8 ± 1.0, and 65.8 ± 2.2 min at average exercise intensities of 96%, 92%, and 87% of maximum oxygen uptake, respectively. Exercise resulted in significant reductions in maximum voluntary contraction, with no difference between TTs (-18%, -15%, and -16% for 4-, 20-, and 40-km TTs, respectively). Greater peripheral fatigue was evident after 4-km (40% reduction in potentiated twitch) compared with that after 20-km (31%) and 40-km TTs (29%). In contrast, longer TTs were characterized by more central fatigue, with greater reductions in voluntary activation measured by motor nerve (-11% and -10% for 20- and 40-km TTs vs -7% for 4-km TTs) and cortical stimulation (-12% and -10% for 20- and 40-km vs -6% for 4-km). CONCLUSIONS These data demonstrate that fatigue after self-paced exercise is task dependent, with a greater degree of peripheral fatigue after shorter higher-intensity (6 min) TTs and more central fatigue after longer lower-intensity TTs (>30 min).

Dietary glycemic index influences lipid oxidation but not muscle or liver glycogen oxidation during exercise.

The glycemic index (GI) of dietary carbohydrates influences glycogen storage in skeletal muscle and circulating nonesterified fatty acid (NEFA) concentrations. We hypothesized that diets differing only in GI would alter intramuscular lipid oxidation and glycogen usage in skeletal muscle and liver during subsequent exercise. Endurance-trained individuals (n = 9) cycled for 90 min at 70% Vo(2peak) and then consumed either high- or low-GI meals over the following 12 h. The following day after an overnight fast, the 90-min cycle was repeated. (1)H and (13)C magnetic resonance spectroscopy was used before and after exercise to assess intramuscular lipid and glycogen content of the vastus muscle group and liver. Blood and expired air samples were collected at 15-min intervals throughout exercise. NEFA availability was reduced during exercise in the high- compared with the low-GI trial (area under curve 44.5 +/- 6.0 vs. 38.4 +/- 7.30 mM/h, P < 0.05). Exercise elicited an approximately 55% greater reduction in intramyocellular triglyceride (IMCL) in the high- vs. low-GI trial (1.6 +/- 0.2 vs. 1.0 +/- 0.3 mmol/kg wet wt, P < 0.05). There was no difference in the exercise-induced reduction of the glycogen pool in skeletal muscle (76 +/- 8 vs. 68 +/- 5 mM) or in liver (65 +/- 8 vs. 71 +/- 4 mM) between the low- and high-GI trials, respectively. High-GI recovery diets reduce NEFA availability and increase reliance on IMCL during moderate-intensity exercise. Skeletal muscle and liver glycogen storage or usage are not affected by the GI of an acute recovery diet.

Neuromuscular Fatigability during Repeated-sprint Exercise in Male Athletes

PURPOSE This study aimed to determine the pattern of neuromuscular fatigability that manifests during repeated-sprint running exercise. METHODS Twelve male participants (mean ± SD: age, 25 ± 6 yr; stature, 180 ± 7 cm; body mass, 77 ± 7 kg), currently training and competing in intermittent sprint sports, performed a repeated maximal sprint running protocol (12 × 30 m, 30-s rest periods). Pre- and postexercise twitch responses to transcutaneous motor point stimulation and transcranial magnetic stimulation were obtained to assess knee extensor neuromuscular and corticospinal function, respectively. Throughout the protocol, during alternate rest periods, blood lactate samples were taken and a single knee extensor maximal voluntary contraction (MVC) of the knee extensors was performed, with motor point stimulation delivered during and 2 s after, to determine voluntary activation (VA) and peripheral fatigue. RESULTS The repeated-sprint protocol induced significant increases in sprint time and blood [lactate] from the third sprint onwards (P < 0.001). Furthermore, knee extensor MVC, resting twitch amplitude, and VA were all significantly reduced after two sprints and reached their nadir after sprint 10 (Δ12%, Δ24%, Δ8%, P < 0.01, respectively). In line with a reduction in motor point-derived VA, there was also a reduction in VA measured with transcranial magnetic stimulation (Δ9%, P < 0.05) immediately after exercise. CONCLUSIONS These data are the first to demonstrate the development of neuromuscular fatigability of the knee extensors during and immediately after repeated-sprint exercise. Peripheral and central factors contributing to muscle fatigability were evident after two maximal sprints, and over half of the drop in postexercise MVC was due to supraspinal fatigue. Thus, peripheral, central, and supraspinal factors all contribute to the performance decrement and fatigability of the knee extensors after maximal repeated-sprint activity.

Intensity-dependent Contribution of Neuromuscular Fatigue after Constant-Load Cycling

PURPOSE We tested the hypothesis that central and peripheral fatigue after constant-load cycling exercise would vary with exercise intensity and duration. METHODS Twelve well-trained male cyclists (V˙O2max, 4.49 ± 0.35 L·min) completed three constant-load cycling trials to the limit of tolerance in a randomized crossover design. Exercise intensities were set according to the respiratory responses to a preliminary ramp test to elicit cardiorespiratory and metabolic responses consistent with exercise in the severe and heavy exercise domains: 1) at power at V˙O2max (S+, 379 ± 31 W), 2) at 60% of the difference between gas exchange threshold and V˙O2max (S-, 305 ± 23 W), and 3) at the respiratory compensation point (RCP, 254 ± 26 W). Pre- and postexercise twitch responses from the quadriceps to the electrical stimulation of the femoral nerve and magnetic stimulation of the motor cortex were recorded to assess neuromuscular and corticospinal function, respectively. RESULTS Exercise time was 3.14 ± 0.59, 11.11 ± 1.86, and 42.14 ± 9.09 min for S+, S-, and RCP, respectively. All trials resulted in similar reductions in maximum voluntary force (P = 0.61). However, the degree of peripheral fatigue varied in an intensity-dependent manner, with greater reductions in potentiated twitch force after S+ (-33% ± 9%) compared with both S- (-16% ± 9%, P < 0.001) and RCP trials (-11% ± 9%, P < 0.001) and greater after S- compared with RCP (P < 0.05). For central fatigue, this trend was reversed, with smaller reductions in voluntary activation after S+ compared with RCP (-2.7% ± 2.2% vs -9.0% ± 4.7%, P < 0.01). CONCLUSION These data suggest the magnitude of peripheral and central fatigue after locomotor cycling exercise is exacerbated with exercise intensity and duration, respectively.

Etiology and Recovery of Neuromuscular Fatigue after Simulated Soccer Match Play

Purpose We profiled the etiology and recovery of neuromuscular fatigue after simulated soccer match play. Methods Fifteen semiprofessional players completed a 90-min simulated soccer match. Before, immediately after, and at 24, 48, and 72 h, participants completed a battery of neuromuscular, physical, and perceptual tests. Perceived fatigue and muscle soreness were assessed via visual analog scales. Maximum voluntary contraction (MVC) and twitch responses to electrical (femoral nerve) and magnetic (motor cortex) stimulation during isometric knee extensor contractions and at rest were measured to assess central (voluntary activation) and peripheral (quadriceps potentiated twitch force, Qtw,pot) fatigue, and responses to single and paired magnetic stimuli were assessed to quantify corticospinal excitability and short intracortical inhibition, respectively. Countermovement jump, reactive strength index, and sprint performance were assessed to profile the recovery of physical function. Results Simulated match play elicited decrements in MVC that remained unresolved at 72 h (P = 0.01). Central fatigue was prominent immediately postexercise (−9% reduction in voluntary activation) and remained depressed at 48 h (−2%, P = 0.03). Qtw,pot declined by 14% postexercise, remained similarly depressed at 24 h, and had not fully recovered 72 h after (−5%, P = 0.01). Corticospinal excitability was reduced at 24 h (P = 0.047) only, and no change in short intracortical inhibition was observed. Measures of jump performance and self-reported fatigue followed a similar time course recovery to neuromuscular fatigue. Conclusion Central processes contribute significantly to the neuromuscular fatigue experienced in the days after soccer match play, but the magnitude and slower recovery of peripheral fatigue indicates that it is the resolution of muscle function that primarily explains the recovery of neuromuscular fatigue after soccer match play.

Exploring the performance reserve: Effect of different magnitudes of power output deception on 4,000 m cycling time-trial performance

Purpose The aim of the present study was to investigate whether a magnitude of deception of 5% in power output would lead to a greater reduction in the amount of time taken for participants to complete a 4000 m cycling TT than a magnitude of deception of 2% in power output, which we have previously shown can lead to a small change in 4000 m cycling TT performance. Methods Ten trained male cyclists completed four, 4000 m cycling TTs. The first served as a habituation and the second as a baseline for future trials. During trials three and four participants raced against a pacer which was set, in a randomized order, at a mean power output equal to 2% (+2% TT) or 5% (+5% TT) higher than their baseline performance. However participants were misled into believing that the power output of the pacer was an accurate representation of their baseline performance on both occasions. Cardiorespiratory responses were recorded throughout each TT, and used to estimate energy contribution from aerobic and anaerobic metabolism. Results Participants were able to finish the +2% TT in a significantly shorter duration than at baseline (p = 0.01), with the difference in performance likely attributable to a greater anaerobic contribution to total power output (p = 0.06). There was no difference in performance between the +5% TT and +2% TT or baseline trials. Conclusions Results suggest that a performance reserve is conserved, involving anaerobic energy contribution, which can be utilised given a belief that the exercise will be sustainable however there is an upper limit to how much deception can be tolerated. These findings have implications for performance enhancement in athletes and for our understanding of the nature of fatigue during high-intensity exercise.

Mirror Training Augments the Cross-education of Strength and Affects Inhibitory Paths

PURPOSE Unilateral strength training strengthens not only the muscles on the trained side but also the homologous muscles on the untrained side; however, the magnitude of this interlimb cross-education is modest. We tested the hypothesis that heightened sensory feedback by mirror viewing the exercising hand would augment cross education by modulating neuronal excitability. METHODS Healthy adults were randomized into a mirror training group (MG, N = 11) and a no-mirror training group (NMG, N = 12) and performed 640 shortening muscle contractions of the right wrist flexors at 80% maximum voluntary contraction (MVC) during 15 sessions for 3 wk. Maximal strength and specific transcranial magnetic stimulation metrics of neuronal excitability, measured in the mirror and no-mirror setup at rest and during unilateral contractions at 60% MVC, were assessed before and after the strength intervention. RESULTS Trained wrist flexor MVC increased 72% across groups, whereas cross-education was higher for the MG (61%) than NMG (34%, P = 0.047). The MG showed a reduction (15%-16%) in the contralateral silent period duration measured from the contracting left-untrained flexor carpi radialis, whereas the NMG showed an increase (12%, P ≤ 0.030). Interhemispheric inhibition, measured from the trained to the untrained primary motor cortex, increased in the MG (11%) but decreased in the NMG (15%) when measured in the mirror setup at rest (P = 0.048). Other transcranial magnetic stimulation measures did not change. CONCLUSION Viewing the exercising hand in a mirror can augment the cross-education effect. The use of a mirror in future studies can potentially accelerate functional recovery from unilateral impairment due to stroke or upper limb fracture.