Browsing by Author "MacIntosh, Brian R."

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    Additional in-series compliance does not affect the length dependence of activation in rat medial gastrocnemius
    (2020-03-30) MacDougall, Keenan Barry; MacIntosh, Brian R.; Herzog, Walter; Syme, Douglas A.
    Background: The force-length relationship describes the amount of force a muscle can produce as a function of its length, typically during maximal isometric contractions. When activation is submaximal, it has been shown that the length at which force production is highest (the optimum length) is shifted towards longer lengths. The usual explanation for this shift is a length-dependent increase in calcium sensitivity due to an increased proximity of the myofilaments with increases in length, which has been dubbed the “length dependence of activation” (LDA). Some recent research has suggested that shortening-induced force depression, (a phenomenon which describes the relative reduction in muscle force when a muscle is actively shortening to a given length compared to contracting isometrically at that same length) has the potential to alter a muscle’s optimum length in an activation-dependent way, thereby affecting any activation dependent shifts in optimum length. Purpose: The purpose of this study was to determine whether shortening-induced force depression would impact the relative shift in optimum length that occurs with submaximal activation. Methods: Rat medial gastrocnemius muscle was isolated and stimulated at 200 Hz triplet pulses and single pulse stimulation over a range of lengths, both with and without additional in-series compliance provided by a small piece of silicon tubing in series with the muscle tendon unit. Peak force and optimum length were calculated by curve-fitting of the force-length data, and these data were compared between low- and high-compliance conditions as well as with maximal and submaximal activation. Results: Additional in-series compliance led to a significant decrease in force at both maximal and submaximal activation, but did not impact the optimum length for either condition. Therefore, in our model, shortening-induced force depression did not impact the length dependence of activation. Conclusion: The additional in-series compliance allowed greater muscle shortening upon activation, which was accompanied by a reduction in force, however the post-shortening length at which the muscle produced the most force was unchanged. The best explanation for the length dependence of activation in skeletal muscle remains a length dependent increase in calcium sensitivity.
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    Conditions for cardiac muscle myosin regulatory light chain phosphorylation
    (2000) Willis, Janine Corinth; MacIntosh, Brian R.
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    Developing procedures and software for correcting artifacts in motion data
    (2019-04-25) Chen, Alexander Wen; MacIntosh, Brian R.; Edwards, William Brent; Syme, Douglas A.; Bertram, John Edward Arthur
    Kinesiology relies upon accurate, reliable recordings of movements. Two common techniques for obtaining such recordings are sonomicrometry and optical marker-based motion capture. Although commonly used, sonomicrometry and motion capture are often limited by the presence of artifacts that require correction. Unfortunately, the standard approach to artifact correction in kinesiology is a manual, time-intensive process that requires expertise and painstaking effort to verify. Therefore, to improve the efficiency and consistency of motion data analysis, we have developed procedures and software tools for the correction of artifacts in sonomicrometry and motion capture. Our work on sonomicrometry provides a framework for artifact correction that is more transparent, easier to use, and requires fewer and more limited manipulations from the user compared to existing approaches. Our work on motion capture provides a biomechanically-principled approach to cleaning motion capture data using a dynamic Bayesian network.
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    Exercise and Dietary Interventions in a Rat Model of Cardiac Adaptation
    (2020-12-09) Boldt, Kevin; Herzog, Walter; Syme, Douglas A.; MacIntosh, Brian R.; Reimer, Raylene A.
    Cardiovascular disease is one of the leading causes of death and disability, and is responsible for approximately one third of all deaths. Conversely, effective functioning of the heart is critical for performance in many sports and recreational activities. Therefore, developing effective strategies for preventing cardiovascular disease and promoting enhanced function of the heart is paramount. The overarching purpose of this thesis was to evaluate how cardiac muscle develops and ways to improve its function through dietary and exercise intervention. The work of this thesis was conducted in three phases. In the first phase, we compared both the structural and mechanical adaptations of the heart in response to aerobic and resistance exercise training. We concluded that exercise affects the structure and function of the heart, and its cellular components in a manner that is specific to the exercise protocol, and that there may be additional benefit of combining aerobic and resistance exercise training. In the second phase, we evaluated the effects of systematic protein supplementation in addition to aerobic exercise on the structural and mechanical properties of the heart. We concluded from these findings that a whey supplemented high-protein diet did not provide additional benefit for cardiac adaptation following aerobic exercise. In the final phase, we compared contractile properties of the heart muscle over the first year of life in rats. We concluded from these findings that many of the previously observed changes in systolic function associated with aging, occur between 12 and 21-33 months of age, while early signs of increased diastolic stiffness manifest sooner. Overall, though more work is required, this thesis presents evidence that structural and mechanical properties of the heart are adaptable and can be modified by either positive (exercise and diet) or negative (obesity and aging) factors.
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    Force-velocity properties and fatigue in skeletal muscle
    (2007) Devrome, Andrea Noelle; MacIntosh, Brian R.
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    Identifying exercise intensity "thresholds": Implications for metabolic responses, performance, and exercise intensity prescription.
    (2019-08-28) Iannetta, Danilo; Murias, Juan M.; Millet, Guillaume Y.; MacIntosh, Brian R.; Paterson, Donald Hugh; Bomhof, Marc R.; Vanhatalo, Anni
    The exercise intensity spectrum, from rest to maximal oxygen uptake (V̇O2max), can be partitioned into three domains of intensity: moderate, heavy, and severe. These domains are demarcated by the lactate threshold (LT) (moderate-to-heavy) and critical power (CP) or maximal lactate steady-state (MLSS) (heavy-to-severe), with the respiratory compensation point (RCP) of the ramp-incremental exercise also being proposed as a marker of the heavy-to-severe boundary of exercise intensity. Although the physiological concepts underpinning these thresholds are well established, methodological issues associated with their determination may lead to inaccuracies and contrasting interpretations regarding their equivalence. The general purpose of this thesis was to find solutions to some of the issues associated with the determination of these “thresholds” and demonstrate why their accurate determination is fundamental in exercise physiology. Using a variety of exercise protocols it was demonstrated that: i) current methods to compute the mean response time (MRT) of V̇O2 during ramp-exercise are inaccurate – the novel method proposed was valid and more reproducible than these methods; ii) exercising slightly above MLSS, although characterized in this study by a stable V̇O2 response, disproportionally impaired maximal exercise capacity; iii) if the V̇O2 dynamics during ramp-incremental exercise are carefully considered, the work rates at RCP and CP/MLSS are not different – refuting the idea that the RCP is not a valid surrogate of the heavy-to-severe boundary of exercise intensity; iv) current methods to prescribe exercise intensity based on fixed-percentage of maximum values (e.g., V̇O2max) do not provide an accurate procedure by which to control exercise intensity. Collectively, these findings provide solutions/explanations to some of the issues related to the correct identification of these exercise thresholds and suggest that their correct identification is of extreme importance when interpreting their physiological implications and to guarantee an accurate exercise intensity prescription.
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    Length dependence of fatigue and of repetitive contractions
    (2005) MacNaughton, Meredith Brooke; MacIntosh, Brian R.
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    Length-dependence of twitch potentiation in skeletal muscle
    (1998) Rassier, Dilson Jose Etcheverry; MacIntosh, Brian R.
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    Motor unit recruitment patterns during cycling
    (2006) Rozitis, Antra Ilze; MacIntosh, Brian R.
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    Muscle length adaptations to high-velocity training in young adults with Cerebral Palsy
    (2019-07-08) Gallinger, Tessa Leigh; MacIntosh, Brian R.; Condliffe, Elizabeth G.; Fletcher, Jared R.; Syme, Douglas A.
    Fascicle length is an important determinant of a muscle’s shortening velocity and excursion. Changes in fascicle length have been shown in typically developing adults following a period of high-velocity training (HVT); however, studies confirming this adaptation in Cerebral Palsy (CP) muscle are lacking. Improvements in fascicle length may allow these individuals to improve their muscular power output and therefore enhance functional movements including those used in sport performance. The purpose of this study was to evaluate high-velocity training as an intervention strategy to increase medial gastrocnemius (MG) muscle fascicle length and improve peak power outputs in CP. 12 young adults with CP (GMFCS I or II, 15-30 years) were randomly allocated to no training (CP-NT)(n=8), or training (CP-T) (n=8) for 10 weeks. 10 typically developing (TD) recreationally trained young adults (17-26 years) were also recruited for the study. The training consisted of 1-hour sessions bi-weekly for 10 weeks (20 sessions), with exercises involving sprints, plyometrics and agility tasks. Triceps surae muscle force-power-velocity and force-length relationships were quantified pre- and post-training using dynamometry and ultrasound. Triceps surae muscle moment arm was significantly different between TD (35.4 ± 3.6) and CP (29.1  5.5mm, p=0.002). The CP-T group had a significant increase in resting fascicle length (+4.1 mm, p<0.002) at a common ankle angle of 105°, but no significant change in power output and peak velocity. High velocity training was able to increase fascicle length, but additional training may be needed to enhance power output.
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    Neuromuscular fatigue of prolonged exercises in normoxia and hypoxia revisited: methodological developments and fatigue etiology
    (2018-07-11) Bartolomeu de Mira, José Miguel; Millet, Guillaume Y.; Rupp, Thomas; Messonnier, Laurent André; Vergès, Samuel; Goodall, Stuart R.; Samozino, Pierre; Deley, Gaëlle; MacIntosh, Brian R.
    The understanding of knee extensor function under fatigue due to whole-body exercise was until recently limited due to short-term recovery. Indeed, subjects needed to be moved from a treadmill/cycle ergometer to an isometric chair, which may underestimate fatigue. This thesis addressed this issue thanks to an innovative ergometer where fatigue can be induced and measured on the same bike, allowing to revisit fatigue etiology in hypoxia and after endurance training. Also, to assess central fatigue, VATMS is traditionally assessed by delivering transcranial magnetic stimulation during three voluntary contractions, each interspersed with 5-10 s of rest (TRADI). In study 1, we assessed whether TRADI methodology resulted in central fatigue underestimation when compared to a continuous approach, with no rest in between voluntary contractions. Post-fatigue, VATMS was heavily underestimated with TRADI. Thus, we propose the continuous method to assess central fatigue. In addition of the lag time explained above, fatigue in hypoxia is poorly understood because of (i) the lack of control in arterial saturation and (ii) different exercise durations in hypoxia vs. normoxia. In study 2, fatigue was assessed on the new cycle ergometer during cycling and right at exhaustion (EXH) in normoxia, moderate and severe hypoxia at relative and absolute workloads. The relative sessions performed in hypoxia were shorter than in normoxia and yet presented lower peripheral fatigue at EXH. This suggests a brain-hypoxic effect that may not only happen in severe hypoxia as previously suggested but also in moderate hypoxia. In study 3, the innovative ergometer was used to revisit the effects of cycling training on neuromuscular fatigue measured during and immediately after cycling exercise. After training, peripheral fatigue was either similar or attenuated at EXH compared to pre-training. The lower or similar fatigue after endurance training suggests that, unlike previously suggested, fatigue may not be upregulated after training.
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    Nonlocalized postactivation performance enhancement (PAPE) effects in trained athletes: a pilot study
    (Applied Physiology Nutrition and Metabolism, 2017-10) Cuenca-Fernández, Francisco; Smith, Ian Curtis; Jordan, Matthew J.; MacIntosh, Brian R.; López-Contreras, Gracia; Arellano, Raúl; Herzog, Walter
    Fifteen trained athletes were assessed for postactivation performance enhancement (PAPE) of squat jumps (SJs) and power push-ups (PPUs) following upper body activation, lower body activation, upper and lower body activation, and rest. SJ improved similarly across all 4 conditions. PPU could not be assessed. Since the test protocol of SJ and PPU involved upper and lower body activation and caused PAPE in SJ, future work is required to determine if a nonlocalized PAPE effect exists.
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    Skeletal muscle alterations in young and mature rats after injection of monocrotaline
    (2009) Mazursky, Konstantin; MacIntosh, Brian R.
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    Skeletal muscle fatigue and increased endurance time with breathing heliox in healthy humans
    (2010) Sadeghian Shahi, Mohammad Reza; MacIntosh, Brian R.
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    A Stochastic Simulation of Skeletal Muscle Calcium Transients in a Structurally Realistic Sarcomere Model using MCell
    (PLOS Computational Biology, 2019-01-30) Holash, Robert John; MacIntosh, Brian R.
    Skeletal muscle contraction is initiated when an action potential triggers the release of Ca2+ into the sarcomere in a process referred to as excitation-contraction coupling. The speed and scale of this process makes direct observation very challenging and invasive. To determine how the concentration of Ca2+changes within the myofibril during a single activation, several simulation models have been developed. These models follow a common pattern; divide the half sarcomere into a series of compartments, then use ordinary differential equations to solve reactions occurring within and between the compartments. To further develop this type of simulation, we have created a realistic structural model of a skeletal muscle myofibrillar half-sarcomere using MCell software that incorporates the myofilament lattice structure. Using this simulation model, we were successful in reproducing the averaged calcium transient during a single activation consistent with both the experimental and previous simulation results. In addition, our simulation demonstrated that the inclusion of the myofilament lattice within our model produced an asymmetric distribution of Ca2+, with more Ca2+ accumulating near the Z-disk and less Ca2+ reaching the m-line. This asymmetric distribution of Ca2+ is apparent when we examine how the Ca2+are bound to the troponin-c proteins along the actin filaments. Our simulation model also allowed us to produce advanced visualizations of this process, including two simulation animations, allowing us to view Ca2+ release, diffusion, binding and uptake within the myofibrillar half-sarcomere.
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