Respiratory Physiology & Neurobiology 195 (2014) 61–62
Contents lists available at ScienceDirect
Respiratory Physiology & Neurobiology journal homepage: www.elsevier.com/locate/resphysiol
Reply to Letter to the Editor Sex-related differences in muscle deoxygenation during ramp incremental exercise: Response to Peltonen et al. In their Letter to the Editor regarding our paper “Sex-related differences in muscle deoxygenation during ramp incremental exercise” Peltonen and co-workers state that “conclusions, on sex differences are affected by the methods used to follow overall exercise responses and specifically the way to analyze [HHb]”. Peltonen et al. attempt to discredit our analysis of the [HHb] response (normalized to the individual amplitude of the total exercise response); however, the statement that sex differences are affected by the method with which the [HHb] signal is analyzed could easily be applied to the overall conclusions that these authors made in their own study. Peltonen et al. argue that normalizing the [HHb] from 0 to 100% of its amplitude response creates differences in the interpretation of the data compared to analyzing the response simply using the absolute M units. We could not agree more with this concept! Indeed, a recent paper from our group specifically deals with the problems associated with using absolute M units and trying to make inferences about fractional O2 extraction or arterialvenous O2 differences (a-vO2diff ) (Murias et al., 2012). Indeed, we have noted that subjects exercising at the same absolute moderate intensity, with similar VO2 amplitude and likely similar blood flow, have distinctly different amplitudes in the [HHb] signal ranging from ∼5 M to ∼20 M (Murias et al., 2014). As Peltonen et al. pointed out in their paper, subcutaneous fat tissue is an important factor (although not the only one) affecting the total amplitude of the [HHb] signal. Only in relatively lean women have we been able to obtain a reasonable NIRS signal of exercise related deoxygenation and even then, since women typically have a thicker subcutaneous fat tissue over the quadriceps muscle (e.g. Jutte et al., 2012; Eston et al., 2005), this is a particularly important problem when comparing sex differences as the total amplitude of the signal is usually larger in men. In fact, Peltonen et al. reported a mean total amplitude of ∼15.0 M and 3.7 M in men versus women, respectively. What does this mean in terms of relative O2 extraction? Were men capable of extracting four times more O2 than women? Likely not! As with other signals (e.g. heart rate), O2 extraction (or a-vO2diff ) is driven by the relative exercise intensity, and thus it would be expected that at maximal exercise, O2 extraction in the area of NIRS inspection, is near maximal. Nevertheless, related to the issue of subcutaneous fat, Peltonen et al. argue that, despite the fact that NIRS recordings have been shown to be affected by adipose tissue thickness, their results “represent real sex-specific responses in the monitored tissues” as: “. . .the anthropometric values indicate that the women subjects were relatively lean (body fat percentage on average 21.5%); the amplitude and level for NIRS changes from rest to unloaded cycling was similar in men and women for all tissues and all variables; http://dx.doi.org/10.1016/j.resp.2014.01.010 1569-9048/© 2014 Elsevier B.V. All rights reserved.
NIRS parameters reached similar values after 5 min recovery in both groups”. We truly have a difficult time understanding how any of these observations make their data “real sex-specific responses”. Even if we accepted that body fat percentage for their group of women was relatively low (21.5%), this value was significantly greater than the value observed and reported in the men (14.1%). Additionally, it is unlikely that both groups had similar subcutaneous adipose tissue thickness in the area of NIRS interrogation. Such a sex-specific similarity has not even been reported among the leanest of high-level athletes, for example, Legaz Arrese et al. (2005) reported a mean thigh skinfold of ∼7 mm and ∼16 mm from 56 and 130 top-class male and female runners, respectively. As for NIRS data being similar for unloaded exercise and returning to the baseline level after recovery, again, it is difficult to understand how this proves that the data are “real” and why the total signal amplitude in response to exercise would be fourfold greater in the men than in the women. Thus, it is difficult to understand what the absolute units of [HHb] are telling us (with respect to actual O2 extraction and/or a-vO2diff ), and we consider normalization of the data a critical step for interpretation. In fact, Peltonen et al. say in their letter to the editor that when they normalized the data using the same approach as we did, women tended to show greater reliance on O2 extraction. Surprisingly, given the large amplitude differences to which these data would have been normalized, this difference was not significant. That said, it might simply be that, in their group, men and women were not different! The topic of sex-related differences in hemodynamic responses to exercise remains open for further study. Our data showed a greater O2 extraction, consistent with a poorer microvascular O2 delivery to the exercising tissue in women. In fact, based on the work from Dr. Proctor’s laboratory and measurements of bulk blood flow (Parker et al., 2007) we originally expected the opposite response to the one we observed. Now we also appreciate that differences occur between bulk blood flow (femoral artery) and microvascular delivery of O2 (which then affect O2 extraction). In conclusion, we believe that our approach for normalizing the [HHb] data is correct and we would not discount the sex-related differences we proposed based on the analysis and interpretation of the Peltonen et al. study.
References Eston, R.G., Rowlands, A.G., Charlesworth, S., Davies, J., Hoppitt, T., 2005. Prediction of DXA-determined whole body fat from skinfolds: importance of including skinfolds from the thigh and calf in young, healthy men and women. Eur. J. Clin. Nutr. 59, 695–702. Jutte, L.S., Hawkins, J., Miller, K.C., Long, B.C., Knight, K.L., 2012. Skinfold thickness at 8 common cryotherapy sites in various athletic populations. J. Athl. Train. 47, 170–177. Legaz Arrese, A., Gonzalez Badillo, J.J., Serrano Ostarez, E., 2005. Differences in skinfold thicknesses and fat distribution among top-class runners. J. Sports Med. Phys. Fitness 45, 512–517.
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Reply to Letter to the Editor / Respiratory Physiology & Neurobiology 195 (2014) 61–62
Murias, J.M., Spencer, M.D., Pogliaghi, S., Paterson, D.H., 2012. Noninvasive estimation of microvascular O2 provision during exercise on-transients in healthy young males. Am. J. Physiol. Regul. Integr. Comp. Physiol. 303, R815–R823. Murias, J.M., Spencer, M.D., Paterson, D.H., 2014. The critical role of O2 provision in the dynamic adjustment of oxidative phosphorylation. Exerc. Sport Sci. Rev. 42, 4–11. Parker, B.A., Smithmyer, S.L., Pelberg, J.A., Mishkin, A.D., Herr, M.D., Proctor, D.N., 2007. Sex differences in leg vasodilation during graded knee extensor exercise in young adults. J. Appl. Physiol. 103, 1583–1591. Peltonen, J.E., Hagglund, H., Koskela-Koivisto, T., Koponen, A.S., Aho, J.M., Rissanen, A.P., Shoemaker, J.K., Tiitinen, A., Tikkanen, H.O., 2013. Alveolar gas exchange, oxygen delivery and tissue deoxygenation in men and women during incremental exercise. Respir. Physiol. Neurobiol. 188, 102–112.
Juan M. Murias Faculty of Kinesiology, University of Calgary, Calgary, AB, Canada
Daniel A. Keir a,b Matthew D. Spencer a,b Donald H. Paterson a,b,∗ a Canadian Centre for Activity and Aging, London, ON, Canada b School of Kinesiology, The University of Western Ontario, London, ON, Canada ∗ Corresponding author at: School of Kinesiology, University of Western Ontario, London, ON, Canada N6A 3K7. Tel.: +1 519 661 1606; fax: +1 519 661 2008. E-mail address: [email protected] (D.H. Paterson)
Contents lists available at ScienceDirect
Respiratory Physiology & Neurobiology journal homepage: www.elsevier.com/locate/resphysiol
Reply to Letter to the Editor Sex-related differences in muscle deoxygenation during ramp incremental exercise: Response to Peltonen et al. In their Letter to the Editor regarding our paper “Sex-related differences in muscle deoxygenation during ramp incremental exercise” Peltonen and co-workers state that “conclusions, on sex differences are affected by the methods used to follow overall exercise responses and specifically the way to analyze [HHb]”. Peltonen et al. attempt to discredit our analysis of the [HHb] response (normalized to the individual amplitude of the total exercise response); however, the statement that sex differences are affected by the method with which the [HHb] signal is analyzed could easily be applied to the overall conclusions that these authors made in their own study. Peltonen et al. argue that normalizing the [HHb] from 0 to 100% of its amplitude response creates differences in the interpretation of the data compared to analyzing the response simply using the absolute M units. We could not agree more with this concept! Indeed, a recent paper from our group specifically deals with the problems associated with using absolute M units and trying to make inferences about fractional O2 extraction or arterialvenous O2 differences (a-vO2diff ) (Murias et al., 2012). Indeed, we have noted that subjects exercising at the same absolute moderate intensity, with similar VO2 amplitude and likely similar blood flow, have distinctly different amplitudes in the [HHb] signal ranging from ∼5 M to ∼20 M (Murias et al., 2014). As Peltonen et al. pointed out in their paper, subcutaneous fat tissue is an important factor (although not the only one) affecting the total amplitude of the [HHb] signal. Only in relatively lean women have we been able to obtain a reasonable NIRS signal of exercise related deoxygenation and even then, since women typically have a thicker subcutaneous fat tissue over the quadriceps muscle (e.g. Jutte et al., 2012; Eston et al., 2005), this is a particularly important problem when comparing sex differences as the total amplitude of the signal is usually larger in men. In fact, Peltonen et al. reported a mean total amplitude of ∼15.0 M and 3.7 M in men versus women, respectively. What does this mean in terms of relative O2 extraction? Were men capable of extracting four times more O2 than women? Likely not! As with other signals (e.g. heart rate), O2 extraction (or a-vO2diff ) is driven by the relative exercise intensity, and thus it would be expected that at maximal exercise, O2 extraction in the area of NIRS inspection, is near maximal. Nevertheless, related to the issue of subcutaneous fat, Peltonen et al. argue that, despite the fact that NIRS recordings have been shown to be affected by adipose tissue thickness, their results “represent real sex-specific responses in the monitored tissues” as: “. . .the anthropometric values indicate that the women subjects were relatively lean (body fat percentage on average 21.5%); the amplitude and level for NIRS changes from rest to unloaded cycling was similar in men and women for all tissues and all variables; http://dx.doi.org/10.1016/j.resp.2014.01.010 1569-9048/© 2014 Elsevier B.V. All rights reserved.
NIRS parameters reached similar values after 5 min recovery in both groups”. We truly have a difficult time understanding how any of these observations make their data “real sex-specific responses”. Even if we accepted that body fat percentage for their group of women was relatively low (21.5%), this value was significantly greater than the value observed and reported in the men (14.1%). Additionally, it is unlikely that both groups had similar subcutaneous adipose tissue thickness in the area of NIRS interrogation. Such a sex-specific similarity has not even been reported among the leanest of high-level athletes, for example, Legaz Arrese et al. (2005) reported a mean thigh skinfold of ∼7 mm and ∼16 mm from 56 and 130 top-class male and female runners, respectively. As for NIRS data being similar for unloaded exercise and returning to the baseline level after recovery, again, it is difficult to understand how this proves that the data are “real” and why the total signal amplitude in response to exercise would be fourfold greater in the men than in the women. Thus, it is difficult to understand what the absolute units of [HHb] are telling us (with respect to actual O2 extraction and/or a-vO2diff ), and we consider normalization of the data a critical step for interpretation. In fact, Peltonen et al. say in their letter to the editor that when they normalized the data using the same approach as we did, women tended to show greater reliance on O2 extraction. Surprisingly, given the large amplitude differences to which these data would have been normalized, this difference was not significant. That said, it might simply be that, in their group, men and women were not different! The topic of sex-related differences in hemodynamic responses to exercise remains open for further study. Our data showed a greater O2 extraction, consistent with a poorer microvascular O2 delivery to the exercising tissue in women. In fact, based on the work from Dr. Proctor’s laboratory and measurements of bulk blood flow (Parker et al., 2007) we originally expected the opposite response to the one we observed. Now we also appreciate that differences occur between bulk blood flow (femoral artery) and microvascular delivery of O2 (which then affect O2 extraction). In conclusion, we believe that our approach for normalizing the [HHb] data is correct and we would not discount the sex-related differences we proposed based on the analysis and interpretation of the Peltonen et al. study.
References Eston, R.G., Rowlands, A.G., Charlesworth, S., Davies, J., Hoppitt, T., 2005. Prediction of DXA-determined whole body fat from skinfolds: importance of including skinfolds from the thigh and calf in young, healthy men and women. Eur. J. Clin. Nutr. 59, 695–702. Jutte, L.S., Hawkins, J., Miller, K.C., Long, B.C., Knight, K.L., 2012. Skinfold thickness at 8 common cryotherapy sites in various athletic populations. J. Athl. Train. 47, 170–177. Legaz Arrese, A., Gonzalez Badillo, J.J., Serrano Ostarez, E., 2005. Differences in skinfold thicknesses and fat distribution among top-class runners. J. Sports Med. Phys. Fitness 45, 512–517.
62
Reply to Letter to the Editor / Respiratory Physiology & Neurobiology 195 (2014) 61–62
Murias, J.M., Spencer, M.D., Pogliaghi, S., Paterson, D.H., 2012. Noninvasive estimation of microvascular O2 provision during exercise on-transients in healthy young males. Am. J. Physiol. Regul. Integr. Comp. Physiol. 303, R815–R823. Murias, J.M., Spencer, M.D., Paterson, D.H., 2014. The critical role of O2 provision in the dynamic adjustment of oxidative phosphorylation. Exerc. Sport Sci. Rev. 42, 4–11. Parker, B.A., Smithmyer, S.L., Pelberg, J.A., Mishkin, A.D., Herr, M.D., Proctor, D.N., 2007. Sex differences in leg vasodilation during graded knee extensor exercise in young adults. J. Appl. Physiol. 103, 1583–1591. Peltonen, J.E., Hagglund, H., Koskela-Koivisto, T., Koponen, A.S., Aho, J.M., Rissanen, A.P., Shoemaker, J.K., Tiitinen, A., Tikkanen, H.O., 2013. Alveolar gas exchange, oxygen delivery and tissue deoxygenation in men and women during incremental exercise. Respir. Physiol. Neurobiol. 188, 102–112.
Juan M. Murias Faculty of Kinesiology, University of Calgary, Calgary, AB, Canada
Daniel A. Keir a,b Matthew D. Spencer a,b Donald H. Paterson a,b,∗ a Canadian Centre for Activity and Aging, London, ON, Canada b School of Kinesiology, The University of Western Ontario, London, ON, Canada ∗ Corresponding author at: School of Kinesiology, University of Western Ontario, London, ON, Canada N6A 3K7. Tel.: +1 519 661 1606; fax: +1 519 661 2008. E-mail address: [email protected] (D.H. Paterson)
