Benjamin K. Barry

Concurrent strength and endurance training: A review

AbstractConcurrent strength and endurance training appears to inhibit strength development when compared with strength training alone. Our understanding of the nature of this inhibition and the mechanisms responsible for it is limited at present. This is due to the difficulties associated with comparing results of studies which differ markedly in a number of design factors, including the mode, frequency, duration and intensity of training, training history of participants, scheduling of training sessions and dependent variable selection. Despite these difficulties, both chronic and acute hypotheses have been proposed to explain the phenomenon of strength inhibition during concurrent training. The chronic hypothesis contends that skeletal muscle cannot adapt metabolically or morphologically to both strength and endurance training simultaneously. This is because many adaptations at the muscle level observed in response to strength training are different from those observed after endurance training. The observation that changes in muscle fibre type and size after concurrent training are different from those observed after strength training provide some support for the chronic hypothesis. The acute hypothesis contends that residual fatigue from the endurance component of concurrent training compromises the ability to develop tension during the strength element of concurrent training. It is proposed that repeated acute reductions in the quality of strength training sessions then lead to a reduction in strength development over time. Peripheral fatigue factors such as muscle damage and glycogen depletion have been implicated as possible fatigue mechanisms associated with the acute hypothesis. Further systematic research is necessary to quantify the inhibitory effects of concurrent training on strength development and to identify different training approaches that may overcome any negative effects of concurrent training.

The neurobiology of muscle fatigue: 15 years later

This brief review summarizes progress that has been made in the study of muscle fatigue since a review published 15 years ago (Enoka RM, Stuart DG. 1992. Neurobiology of muscle fatigue. J Appl Physiol 72:1631-48.). The present review first discusses progress on the four themes identified in the 1992 review and then describes a new approach that can be used to identify the functionally significant physiological adjustments that occur during fatiguing contractions. As described in the previous review, it is currently not possible to develop a comprehensive model of muscle fatigue because the prevailing mechanism that impairs performance varies with the characteristics of the task that is being performed. An alternative approach is to focus on the mechanisms that cause failure to complete the task. This task-failure approach involves comparing two performances and identifying the adjustments that limit the rate for the more difficult condition. With this approach, initial studies have demonstrated that the time to failure of a sustained contraction can be influenced by such variables as the type of load supported by the limb, the posture of the limb, and the group of muscles involved in the task. The challenge is to identify the mechanisms that enable these different variables influence the time to task failure.

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

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.

Resistance training enhances the stability of sensorimotor coordination

Strategies for the control of human movement are constrained by the neuroanatomical characteristics of the motor system. In particular, there is evidence that the capacity of muscles for producing force has a strong influence on the stability of coordination in certain movement tasks. In the present experiment, our aim was to determine whether physiological adaptations that cause relatively long–lasting changes in the ability of muscles to produce force can influence the stability of coordination in a systematic manner. We assessed the effects of resistance training on the performance of a difficult coordination task that required participants to synchronize or syncopate movements of their index finger with an auditory metronome. Our results revealed that training that increased isometric finger strength also enhanced the stability of movement coordination. These changes were accompanied by alterations in muscle recruitment patterns. In particular, the trained muscles were recruited in a more consistent fashion following the programme of resistance training. These results indicate that resistance training produces functional adaptations of the neuroanatomical constraints that underlie the control of voluntary movement.

Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

OBJECTIVE This study compared reflex responsiveness of the first dorsal interosseus muscle during two tasks that employ different strategies to stabilize the finger while exerting the same net muscle torque. METHODS Healthy human subjects performed two motor tasks that involved either pushing up against a rigid restraint to exert a constant isometric force equal to 20% of maximum or maintaining a constant angle at the metacarpophalangeal joint while supporting an equivalent inertial load. Each task consisted of six 40-s contractions during which electrical and mechanical stimuli were delivered. RESULTS The amplitude of short and long latency reflex responses to mechanical stretch did not differ significantly between tasks. In contrast, reflexes evoked by electrical stimulation were significantly greater when supporting the inertial load. CONCLUSIONS Agonist motor neurons exhibited heightened reflex responsiveness to synaptic input from heteronymous afferents when controlling the position of an inertial load. Task differences in the reflex response to electrical stimulation were not reflected in the response to mechanical perturbation, indicating a difference in the efficacy of the pathways that mediate these effects. SIGNIFICANCE Results from this study suggest that modulation of spinal reflex pathways may contribute to differences in the control of force and position during isometric contractions of the first dorsal interosseus muscle.

Aerobic training increases pain tolerance in healthy individuals

UNLABELLED The hypoalgesic effects of acute exercise are well documented. However, the effect of chronic exercise training on pain sensitivity is largely unknown. PURPOSE To examine the effect of aerobic exercise training on pain sensitivity in healthy individuals. METHODS Pressure pain threshold, ischemic pain tolerance and pain ratings during ischemia were assessed in 24 participants before and after 6 wk of structured aerobic exercise training (n = 12) or after 6 wk of usual physical activity (n = 12). The exercise training regimen consisted of cycling three times per week for 30 min at 75% of maximal oxygen consumption reserve. RESULTS Significant increases in aerobic fitness (P = 0.004) and ischemic pain tolerance (P = 0.036) were seen in the exercise group after training, whereas pressure pain threshold and pain ratings during ischemia were unchanged (P > 0.2). No change in aerobic fitness (P > 0.1) or pain sensitivity (P > 0.1) was observed in the control group. CONCLUSION Moderate- to vigorous-intensity aerobic exercise training increases ischemic pain tolerance in healthy individuals.

Acute resistance exercise and pressure pain sensitivity in knee osteoarthritis: a randomised crossover trial

OBJECTIVE To determine whether a single bout of resistance exercise produces an analgesic effect in individuals with knee osteoarthritis (OA). DESIGN Eleven participants with knee OA (65.9 ± 10.4 yrs), and 11 old (61.3 ± 8.2 yrs) and 11 young (25.0 ± 4.9 yrs) healthy adults performed separate bouts of upper and lower body resistance exercise. Baseline and post-exercise pressure pain thresholds were measured at eight sites across the body and pressure pain tolerance was measured at the knee. RESULTS Pressure pain thresholds increased following exercise for all three groups, indicating reduced pain sensitivity. For the young and old healthy groups this exercise-induced analgesia (EIA) occurred following upper or lower body resistance exercise. In contrast, only upper body exercise significantly raised pain thresholds in the knee OA group, with variable non-significant effects following lower body exercise. Pressure pain tolerance was unchanged in all groups following either upper or lower body exercise. CONCLUSION An acute bout of upper or lower body exercise evoked a systemic decrease in pain sensitivity in healthy individuals irrespective of age. The decreased pain sensitivity following resistance exercise can be attributed to changes in pain thresholds, not pain tolerance. While individuals with knee OA experienced EIA, a systemic decrease in pain sensitivity was only evident following upper body exercise.

Spontaneous transitions in the coordination of a whole body task.

This paper describes an example of spontaneous transitions between qualitatively different coordination patterns during a cyclic lifting and lowering task. Eleven participants performed 12 trials of repetitive lifting and lowering in a ramp protocol in which the height of the lower shelf was raised or lowered 1 cm per cycle between 10 and 50 cm. Two distinct patterns of coordination were evident: a squat technique in which moderate range of hip, knee and ankle movement was utilised and ankle plantar-flexion occurred simultaneously with knee and hip extension; and a stoop technique in which the range of knee movement was reduced and knee and hip extension was accompanied by simultaneous ankle dorsi-flexion. Abrupt transitions from stoop to squat techniques were observed during descending trials, and from squat to stoop during ascending trials. Indications of hysteresis was observed in that transitions were more frequently observed during descending trials, and the average shelf height at the transition was 5 cm higher during ascending trials. The transitions may be a consequence of a trade-off between the biomechanical advantages of each technique and the influence of the lift height on this trade-off.

Acute Strength Training Increases Responses to Stimulation of Corticospinal Axons.

PURPOSE Acute strength training of forearm muscles increases resting twitch forces from motor cortex stimulation. It is unclear if such effects are spinal in origin and if they also occur with training of larger muscles. With the use of subcortical stimulation of corticospinal axons, the current study examined if one session of strength training of the elbow flexor muscles leads to spinal cord changes and if the type of training is important. METHODS In experiment 1, 10 subjects completed ballistic isometric training, ballistic concentric training, and no training (control) on separate days. In experiment 2, 13 subjects completed ballistic isometric training and slow-ramp isometric training. Before and after training, transcranial magnetic stimulation over the contralateral motor cortex elicited motor-evoked potentials (MEPs) in the resting biceps brachii, and electrical stimulation of corticospinal tract axons at the cervicomedullary junction elicited cervicomedullary motor-evoked potentials (CMEPs). Motor-evoked potential and CMEP twitch forces were also measured. RESULTS In experiment 1, CMEPs and CMEP twitch forces were significantly facilitated after ballistic isometric training compared to control. In experiment 2, MEPs, MEP twitch forces, CMEPs, and CMEP twitch forces increased for 15 to 25 min after ballistic and slow-ramp isometric training. CONCLUSION Via processes within the spinal cord, one session of strength training of the elbow flexors increases net output from motoneurons projecting to the trained muscles. Likely mechanisms include increased efficacy of corticospinal-motoneuronal synapses or increased motoneuron excitability. However, the rate of force generation during training is not important for inducing these changes. A concomitant increase in motor cortical excitability is likely. These short-term changes may represent initial neural adaptations to strength training.

Aerobic exercise intervention in young people with schizophrenia spectrum disorders; improved fitness with no change in hippocampal volume

To the Editors There is growing interest in the role of exercise and concomitant improved aerobic exercise capacity as a stimulus for hippocampal neurogenesis (van Praag, 2009). Pajonk et al. reported hippocampal plasticity in response to a 12-week aerobic exercise intervention in patients with established schizophrenia (Pajonk et al., 2010). Participants completed three 30-min sessions of stationary cycling per week at a pre-specified intensity that led to relative hippocampal volume increases of 12% among the eight subjects included in the final analysis. In addition, positive correlations were observed between change in relative hippocampal volume, and change in aerobic exercise capacity (r1⁄40.71, p1⁄40.003). Given these promising findings, we conducted a pilot study in a group of young peoplewith schizophrenia spectrum disorders who were experiencing a first episode psychosis (FEP). The South Eastern Sydney Local Health District (Northern Division) Human Research Ethics Committee approved the experimental protocol, and written informed consent was obtained from all participants. Patients (aged 15–25 years) who met DSM-IV criteria for schizophrenia, schizoaffective disorder or schizophreniform disorder were recruited from a community-based early psychosis treatment program. The intervention consisted of 12 weeks of twice-weekly, 45min sessions on a stationary exercise bike at a heart rate (710 beats/ min) corresponding to 65% of VO2 peak as determined by baseline exercise testing. Baseline assessments included aerobic exercise capacity (VO2 peak, assessed via a 3-min ramp protocol on a cycle ergometer and direct gas analysis), cognitive testing (short-term memory assessed with the Rey Auditory-Verbal Learning Test (RAVLT) and the spatial span subtest of theWechsler Memory Scale-III (WMS)), symptom ratings (Scale for the Assessment of Positive Symptoms (SAPS) and Scale for the Assessment of Negative Symptoms (SANS)), and self-rated quality of life (WHOQOL-BREF). Three-dimensional structural magnetic resonance imaging was performed (1.5T Phillips Achieva),and hippocampal volumes were extracted using an automated segmentation routine based on the principles of the Active Shape and Appearance Models within a Bayesian framework, as implemented by “FIRST” in the FSL software package (Patenaude et al., 2011). Before segmentation, the skull was stripped from structural images and the brain tissue extracted. FIRST was then applied to separately estimate left and right hippocampal volumes. Baseline and follow-up data were available for five male patients (20.274.2 years), whose duration of illness was o3 years at baseline assessment. Mean VO2 peak as assessed via a maximal exercise test was well below average ( 10th percentile) for the given age group. At follow-up, no significant change in hippocampal volume was observed (mean increase 0.4%; baseline mean: 8664.8 mm (SD 837.7), follow-up mean: 8697.8 mm (SD 897.6), mean change 33.0 mm (95% CI 161.5 to 227.5), t1⁄40.5, df1⁄44, ns) despite a statistically and clinically significant 20.1% mean increase in VO2 peak (baseline mean: 31.8 ml/kg/min (SD 9.5), follow-up mean 38.2 ml/kg/ min (SD 12.6), mean change 6.4 ml/kg/min (95% CI 2.0–10.8), t1⁄44.0, df1⁄44, p1⁄40.02). No statistically significant change was observed in short-term verbal (RAVLT Delayed Recall; 17%, baseline mean: 10.4 (SD 3.4), follow-up mean 8.6 (SD 5.9), mean change 1.8 (95% CI 6.6 to 3.0), t1⁄4-1.1, df1⁄44, ns) or spatial (WMS Spatial Span; þ15%; baseline mean: 16.8 (SD 3.4), follow-up mean 19.4 (SD 2.4), mean change 2.6 (95% CI 1.1 to 6.3), t1⁄42.0, df1⁄44, ns) memory. No significant changes in symptom or quality of life ratings were observed (data not shown). The intervention was effective, as evidenced by all participants demonstrating an increase in aerobic exercise capacity (mean 6.4 ml/ kg/min, p1⁄40.02). Increases in exercise capacity of approximately half this magnitude (3.5 ml/kg/min) have been associated with a 13% decrease in risk of all-cause mortality (Kodama et al., 2009), highlighting the clinical significance of the V02 peak increase we observed. Despite this improvement in exercise capacity, there was no significant hippocampal volume increase in our sample. Whilst the sample size (n1⁄45) is a limitation of the current study, the original study of Pajonk et al. found a highly statistically significant increase in hippocampal volume in eight patients who completed the exercise intervention. The mean hippocampal volume increase they reported (12%) was 30 times greater than that found in the sample of FEP patients we examined (0.4%). For the modest increase in volume we observed to be statistically significant, a sample size more than 40 times greater would be needed (n1⁄4206, for an effect size of 0.21 with a paired correlation of 0.986, for α1⁄40.05 and power of 0.85). The absence of significant changes in hippocampal volume despite the significant increase in VO2 may reflect a number of methodological differences between our protocol and that of Pajonk et al. Firstly, patients examined in this study were substantially younger than those who participated in the Pajonk et al. study (mean age 20.2 vs. 32.9 years) and had a shorter duration of illness (o3 years vs. mean duration of illness1⁄48.4 years). This difference may have led to different baseline aerobic exercise capacity in the two samples. Secondly, we used an automated method of measuring hippocampal volume as opposed to manual tracing. The lack of change in hippocampal volumes observed in this study is consistent with the negative findings of Scheewe et al. (2013) in a group of patients with schizophrenia with established illness; however, their intervention (2 h/week of combined aerobic and resistance exercise) also failed to demonstrate a statistically significant change in VO2 peak, probably reflecting the relatively low number of patients who completed at least 50% attendance over the 6-month intervention period. Recruitment for the current study was terminated due to implementation of a comprehensive lifestyle intervention, including individualized exercise prescription comprising both aerobic and resistance-based elements, that is offered to all new referrals to the