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Archived Comments for: Whole body oxygen uptake and evoked knee torque in response to low frequency electrical stimulation of the quadriceps muscles: V • O 2 frequency response to NMES

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  1. Commentary on Conor M Minogue, Brian M Caulfield and Madeleine M Lowery: ¿Whole Body Oxygen Uptake and Evoked Knee Torque in Response to Low Frequency Electrical Stimulation of the Quadriceps Muscles¿.

    Joseph Mizrahi, Technion, Israel Institute of Technology

    21 January 2015

    This paper suggests a method for invoking whole-body exercise resulting from isometric activation of the knee extensor muscles by means of low-frequency electrical stimulation. The interest in eliciting cardio-pulmonary function, as is required in subjects with exercise limitations, is rather contrary to conventional approach in functional electrical stimulation of muscles whereby more effective performance is associated with lower energy consumption (1). Thus, `optimal¿ stimulation (expressed by means of stimulation frequency) is associated here with high oxygen uptake of the subject.

    This interesting paper stimulates open issues including the following:

    1. This is a multi-scale problem: from isolated muscle preparation (by referring to Hill¿s equation), through muscle organ to the whole body system. A clear distinction between these scales is somewhat missing and a proposed transition would have been desirable. For instance, the authors made an assumption that the observed pulmonary oxygen uptake was all taken by the stimulated muscle. While this assumption may be practically justifiable, some evidence would have been required.

    2. The model equations deal with energy balance. This is understandable because of the very nature of the study: focus on energy expenditure in whole-body physical exercise. However, these equations relate to small-scale, isolated muscle preparation and do not treat whole-body, global, behavior. While bioenergetics at the small scale is adequately expressed by means of intracellular parameters, such as phosphorus metabolites (e.g. inorganic phosphate, phosphocreatine, intracellular pH) and calcium ions (2), at the system level more global indirect (and time-delayed) parameters such as general acidosis are used.

    3. Another point about the muscle equations is that for the muscle organ, a distinction between non-specific parameters (assumed to be the same for all muscles, such as nominal stress-strain curves of tendons) and specific parameters (which are subject-dependent, such as rest length, cross sectional area and muscle mass) would have been desirable (3).

    4. Intermittent stimulation: Published work on metabolic parameters recorded by magnetic resonance spectroscopy have revealed a variation in the phosphorus metabolites in parallel to the force output during the fatiguing process of muscle activated by electrical stimulation and a recovery course of these metabolites to their rest values when stimulation is ceased. These records have also formed the basis of fatigue-recovery models in intermittent stimulation, allowing better predicting the muscle output in this mode of stimulation (4). It could have been interesting to include this information.

    5. There is an inferred assumption that there is no unintended activation of the antagonists by the applied stimulation on the quadriceps and that the measured torque and is attributed to the quadriceps only. However, such activation has been reported present and significant under fatigued conditions and could affect the torque partition between the antagonists (5). Experimental evidence to support the assumption of no antagonist activation is desirable here, especially in view of the relatively high stimulus intensity (average 88 mA) and stimulus duration (600 microsec).

    Extension of the protocol beyond isometric activation could have been interesting and beneficial: apart from increasing the scope of application, it would eliminate the question of possible high restraining force which, in low frequency twitch-like activation, would give rise to undesired impact forces at the restraints and at the knee joint.

    Key References:

    1. Energy cost and physiological reactions to effort during activation of paraplegics by functional electrical stimulation. E. Isakov, J. Mizrahi, D. Graupe, E. Becker and T. Najenson Scandinavian Journal of Rehabilitation Medicine 1985, Suppl. 12:102-107.
    2. In Vivo P-31 NMR studies of paraplegic's muscles activated by functional electrical stimulation. M. Levy, T. Kushnir, J. Mizrahi and Y. Itzchak, 1993, Magnetic Resonance in Medicine 29:53-58.
    3. Muscle and Tendon: Properties, Models, Scaling and Applications to Biomechanics and Motor Control. F.E. Zajac 1988, CRC Critical Reviews in Biomedical Engineering, CRC Press, Boca Raton, FL.
    4. A model of fatigue and recovery in paraplegic's quadriceps muscle when subjected to intermittent stimulation. Y. Giat, J. Mizrahi and M. Levy 1996, ASME Journal of Biomechanical Engineering, 118:357-366.
    5. Transcutaneous FES of paralyzed quadriceps: Is knee torque affected by unintended activation of the hamstrings? O. Levin, J. Mizrahi, and E. Isakov J. 2000, Electromyography and Kinesiology, 10:47-58.

    Competing interests

    I declare that I have no competing interests