Martyn G. Morris

Multi-modal characterization of rapid anterior hippocampal volume increase associated with aerobic exercise.

The hippocampus has been shown to demonstrate a remarkable degree of plasticity in response to a variety of tasks and experiences. For example, the size of the human hippocampus has been shown to increase in response to aerobic exercise. However, it is currently unknown what underlies these changes. Here we scanned sedentary, young to middle-aged human adults before and after a six-week exercise intervention using nine different neuroimaging measures of brain structure, vasculature, and diffusion. We then tested two different hypotheses regarding the nature of the underlying changes in the tissue. Surprisingly, we found no evidence of a vascular change as has been previously reported. Rather, the pattern of changes is better explained by an increase in myelination. Finally, we show that hippocampal volume increase is temporary, returning to baseline after an additional six weeks without aerobic exercise. This is the first demonstration of a change in hippocampal volume in early to middle adulthood suggesting that hippocampal volume is modulated by aerobic exercise throughout the lifespan rather than only in the presence of age related atrophy. It is also the first demonstration of hippocampal volume change over a period of only six weeks, suggesting that gross morphometric hippocampal plasticity occurs faster than previously thought.

Reliability of single and paired-pulse transcranial magnetic stimulation in the vastus lateralis muscle

Introduction: Transcranial magnetic stimulation (TMS) is an important tool to examine neurological pathologies, movement disorders, and central nervous system responses to exercise, fatigue, and training. The reliability has not been examined in a functional locomotor knee extensor muscle. Methods: Within‐ (n = 10) and between‐day (n = 16) reliability of single and paired‐paired pulse TMS was examined from the active vastus lateralis. Results: Motor evoked potential amplitude and cortical silent period duration showed good within‐ and between‐day reliability (intraclass correlation coefficient [ICC] ≥ 0.82). Short‐ and long‐interval intracortical inhibition (SICI and LICI, respectively) demonstrated good within‐day reliability (ICC ≥ 0.84). SICI had moderate to good between‐day reliability (ICC ≥ 0.67), but LICI was not repeatable (ICC = 0.47). Intracortical facilitation showed moderate to good within‐day reliability (ICC ≥ 0.73) but poor to moderate reliability between days (ICC ≥ 0.51). Conclusions: TMS can reliably assess cortical function in a knee extensor muscle. This may be useful to examine neurological disorders that affect locomotion. Muscle Nerve 52: 605–615, 2015

Central and peripheral fatigue following non‐exhaustive and exhaustive exercise of disparate metabolic demands

The development of fatigue after non‐exhaustive and exhaustive exercise eliciting differing metabolic demands is poorly understood. Sixteen active males completed five cycling trials. The first trial established the lactate threshold (LT) and maximal oxygen uptake (VO2max). Two of the remaining trials were completed at a severe intensity (halfway between LT and VO2max, SI) and two at a moderate intensity (90% LT, MI). Each trial involved two non‐exhaustive bouts matched for work between intensities before cycling to exhaustion. Responses to stimulation of the femoral nerve and motor cortex were determined after each bout to determine peripheral and central fatigue. Corticospinal excitability, cortical silent period (cSP), short‐interval intracortical inhibition (SICI), and intracortical facilitation (ICF) were also assessed. Non‐exhaustive cycling induced greater peripheral and central fatigue in the SI compared with the MI (P < 0.05). At exhaustion, there was no difference between intensities; however, peripheral fatigue tended to be greater in the SI vs MI (−31% vs −17%, respectively, P = 0.051). Exhaustive cycling increased SICI (24%, P < 0.001) and reduced the cSP (−14%, P < 0.001) in the SI, whereas ICF was reduced in the MI (−16%, P < 0.001). These findings demonstrate exercise‐induced metabolic stress accelerates the development of peripheral and central fatigue, and differentially influences intracortical excitability.

Endurance capacity and neuromuscular fatigue following high- vs moderate-intensity endurance training: A randomized trial

High‐intensity exercise induces significant central and peripheral fatigue; however, the effect of endurance training on these mechanisms of fatigue is poorly understood. We compared the effect of cycling endurance training of disparate intensities on high‐intensity exercise endurance capacity and the associated limiting central and peripheral fatigue mechanisms. Twenty adults were randomly assigned to 6 weeks of either high‐intensity interval training (HIIT, 6‐8×5 minutes at halfway between lactate threshold and maximal oxygen uptake [50%Δ]) or volume‐matched moderate‐intensity continuous training (CONT, ~60‐80 minutes at 90% lactate threshold). Two time to exhaustion (TTE) trials at 50%Δ were completed pre‐ and post‐training to assess endurance capacity; the two post‐training trials were completed at the pretraining 50%Δ (same absolute intensity) and the “new” post‐training 50%Δ (same relative intensity). Pre‐ and post‐exercise responses to femoral nerve and motor cortex stimulation were examined to determine peripheral and central fatigue, respectively. HIIT resulted in greater increases in TTE at the same absolute and relative intensities as pre‐training (148% and 43%, respectively) compared with CONT (38% and −4%, respectively) (P≤.019). Compared with pre‐training, HIIT increased the level of potentiated quadriceps twitch reduction (−34% vs −43%, respectively, P=.023) and attenuated the level of voluntary activation reduction (−7% vs −3%, respectively, P=.047) following the TTE trial at the same relative intensity. There were no other training effects on neuromuscular fatigue development. This suggests that central fatigue resistance contributes to enhanced high‐intensity exercise endurance capacity after HIIT by allowing greater performance to be extruded from the muscle.

Motor impairment and its relationship to fitness in children

Objectives The aim of this work was to explore the physiological and perceptual limits to exercise in children with varying degrees of motor impairment, and the relationships to measures of health. Design and methods In a group comparison design, 35 boys aged 12–15 years completed the Movement ABC test for the assessment of motor impairment, followed by an incremental cycle ergometer test to exhaustion for the assessment of maximal oxygen uptake (VO2peak), respiratory exchange ratio (RER), heart rate (HR) and rating of perceived exertion (RPE). Ten participants classified as having either high or no motor impairment also performed a maximal voluntary isometric contraction (MVIC) for the assessment of lower limb extensor strength. Results 18 boys were classified as having high motor impairment. There was a significant difference in peak (34.9 vs 48.5 mL kg/min), workload (12.5 vs 10.0 mL W), maximal HR (176 vs 188 bpm), maximal oxygen pulse (12.1 vs 15.9 mL beat) and MVIC (5.7 vs 9.1 Nm kg) between the high and non-motor impaired participants, respectively, (p<0.05). There was no significant difference in the RER or RPE between groups. Conclusions When performing cycling ergometry, perceived exertion was not a limiting factor in children with high motor impairment. The lower maximal HR, coupled with reduced movement efficiency and muscle strength reported in this group, suggests that exercise is limited by impairment at the muscular level. This finding was supported by high RER values despite low maximal HR values attained at exercise cessation and reduced maximal strength. Perception of effort is not heightened in children with high motor impairment and future-exercise interventions should be focused on improving muscular condition in these participants to enable them to be better prepared to engage in physical activity for health.

Impact of time and work: rest ratio matched sprint interval training programmes on performance: A randomised controlled trial

OBJECTIVES The aim of this study was to examine the effects of a short training intervention using two repeated sprint protocols matched for total sprint duration and work:rest ratio. DESIGN Randomised-controlled trial. METHODS Thirty physically active males were randomly allocated to one of two sprint training groups: a 6s group, a 30s group or a non-exercising control. The training groups were matched for work:rest ratio and total sprint time per session, and completed 6 training sessions over a 2-week period. Before and after the 2 week training period, participants completed a VO2max test and a 10km time trial on a cycle ergometer. RESULTS Time trial performance increased significantly by 5.1% in 6s (630±115s to 598±92s; p<0.05) and 6.2% in 30s (579±68s to 543±85s; p<0.05) from baseline testing, but there was no significant change in the control group (p>0.05), and no significant difference between exercise groups (p>0.05). The 6s group increased peak power output by 9.0% (from 1092±263W to 1181±248W; p<0.05) from sprint session 1 to 6, and the 30s group by 20.0% (1041±161W to 1237±159W; p<0.05). CONCLUSIONS This study indicates that both 6 and 30s bouts of repeated sprint exercise, matched for total sprint duration and W:R can improve athletic performance.

The impact of high and low-intensity exercise in adolescents with movement impairment

Five to six percent of young people have movement impairment (MI) associated with reduced exercise tolerance and physical activity levels which persist into adulthood. To better understand the exercise experience in MI, we determined the physiological and perceptual responses during and following a bout of exercise performed at different intensities typically experienced during sport in youth with MI. Thirty-eight adolescents (11–18 years) categorised on the Bruininks-Oseretsky Test of Motor Proficiency-2 Short-Form performed a peak oxygen uptake bike test (V˙O2peak) test at visit 1 (V1). At visits 2 (V2) and 3 (V3), participants were randomly assigned to both low-intensity (LI) 30min exercise at 50% peak power output (PPO50%) and high-intensity (HI) 30s cycling at PPO100%, interspersed with 30s rest, for 30min protocol (matched for total work). Heart rate (HR) and rating of perceived exertion (RPE) for legs, breathing and overall was measured before, during and at 1, 3 and 7-min post-exercise (P1, P3, P7). There was a significant difference in V˙O2peak between groups (MI:31.5±9.2 vs. NMI:40.0±9.5ml⋅kg-1⋅min-1, p<0.05). PPO was significantly lower in MI group (MI:157±61 vs. NMI:216±57 W)(p<0.05). HRavg during HI-cycling was reduced in MI (140±18 vs. 157±14bpm, p<0.05), but not LI (133±18 vs. 143±17bpm, p>0.05). Both groups experienced similar RPE for breathing and overall (MI:7.0±3.0 vs. NMI:6.0±2.0, p>0.05) at both intensities, but reported higher legs RPE towards the end (p<0.01). Significant differences were found in HRrecovery at P1 post-HI (MI:128±25.9 vs. NMI:154±20.2, p<0.05) but not for legs RPE. Perceived fatigue appears to limit exercise in youth with MI in both high and low-intensity exercise types. Our findings suggest interventions reducing perceived fatigue during exercise may improve exercise tolerance and positively impact on engagement in physical activities.