Data are presented as means ± SD unless otherwise noted. Results of data collected in this study (sumo wrestlers and untrained controls) are shown at the top of Table 1, and are compared with previous results collected from combat, aerobic, and power sport participants. Sumo wrestlers, compared to untrained controls, were taller (1.77 ± 0. 03 vs. 1.72 ± 0.03 m; p < 0.05), and had larger (p < 0.05) body mass (117.0 ± 4.9 vs. 56.1 ± 9.8 kg), percent fat (24.0 ± 1.4 vs. 13.3 ± 4.5), fat-free mass (88.9 ± 4.2 vs. 48.4 ± 6.8 kg), predicted SMM (48.2 ± 2.9 vs. 20.6 ± 4.7 kg) and VO2max (3.6 ± 1.3 vs. 2.5 ± 0.7 L·min-1). Heart rate at maximum was not different between sumo wrestlers and controls (189 ± 8 vs. 192 ± 7 bpm, respectively; p > 0.05). Relative VO2max to body mass, however, was significantly lower in sumo wrestlers compared to untrained controls (31.1 ± 1.3 vs. 44.6 ± 9.1 ml·kg-1·min-1, respectively; p < 0.05).
The predicted SMM and VO2max/SMM results are in reasonable agreement with previous results using MRI to quantify SMM (Sanada et al., 2005). Mean VO2max/SMM was significantly different (p < 0.05) among aerobic athletes (164.8 ± 18.3 ml·kgSMM-1·min-1), combat athletes (which was not different from untrained controls; 131.4 ± 9.3 and 128.6 ± 13.6 ml·kgSMM-1·min-1, respectively), power athletes (96.5 ± 5.3 ml·kgSMM-1·min-1), and sumo wrestlers (71.4 ± 5.3 ml·kgSMM-1·min- 1).
Figure 1 was created by using previous data (weighted mean results) from combat, aerobic, and power sport participants (from Table 1), and data collected from sumo wrestlers and untrained controls in the current study, and shows the relationship between SMM and absolute VO2max for these participants. There appears to be an overall positive relationship between SMM and absolute VO2max until ~ 45 kg of predicted SMM; the relationship appears to weaken after this point.
Figure 2 is a plot of the relationship between percent fat and VO2max/SMM from sumo and untrained controls, and previously collected data (weighted mean results) from combat, aerobic, and power athletes. There is a strong negative correlation between these variables (r = - 0.75; p < 0.05).
DISCUSSION
To our knowledge, this is the first time that VO2max results have been reported for sumo wrestlers. However, in order to make better comparisons of VO2max among participants of varying fat and body mass, we divided VO2max by the amount of predicted skeletal muscle mass (SMM) - SMM being the functional tissue for oxygen uptake during strenuous physical work. The calculation of VO2max /SMM should provide an indication of the "aerobic muscle quality" of the athlete's skeletal muscle. We use the term "aerobic muscle quality" to refer to the amount of oxygen consumed per skeletal muscle mass. The higher this value, the higher the aerobic muscle quality. We found that sumo wrestlers have a low "aerobic muscle quality" (low VO2max /SMM), compared to untrained controls and other combat sports, and aerobic and power sports (Table 1). Why might the aerobic muscle quality, i.e. VO2max /SMM, be low in sumo wrestlers?
Maximum oxygen uptake is equal to the product of cardiac output (itself a function of heart rate and stroke volume) and the arterial-venous oxygen differences in the blood. Clearly one or more of these factors must be affected in sumo wrestlers to account for the low VO2max /SMM. It is possible that the massive skeletal muscle hypertrophy in sumo wrestlers is not matched by increases in heart size and/or stroke volume; that is, cardiac output may be low per unit of SMM for sumo wrestlers, resulting in abnormally low VO2max /SMM values. However, a recent study of professional Japanese sumo wrestlers suggests that left ventricular dilation is indeed very large in these athletes (median left ventricular [LV] end-diastolic volume ~ 60 mm), although it should be noted that LV function is normal, and LV dimensions were significantly correlated with body size, which would lead one to assume that sumo wrestlers have normal LV sizes and stroke volumes for their amount of skeletal muscle mass (Kinoshita, et al., 2003). Additionally, we noted normal (within ± 5 % of predicted) maximal heart rates (see Results) in sumo wrestlers at VO2max. This suggests cardiac output of these athletes is not compromised.
The low VO2max /SMM results for sumo wrestlers should therefore be the cause of alterations in arterial-venous oxygen content differences, indicating a "peripheral" limitation. It is possible that the massive skeletal muscle hypertrophy in sumo wrestlers could decrease the capillary density, or mitochondrial density, of skeletal muscle, resulting in less oxygen delivered or utilized per volume of myofibrillar tissue and thus a lower VO2max /SMM. Another possibility could be that the high fat, high calorie diet and/or high fat storage mass in sumo wrestlers results in higher than normal intramuscular stores of lipid (obesity has been shown to increase intramuscular triglyceride stores; Gray et al., 2003; Goodpaster et al., 2000). This could result in a "diluting" of myofibrillar content and a decrease in VO2max /SMM, although the amounts of triglycerides stored would have to be substantial.
Perhaps the low VO2max /SMM in sumo wrestlers is related to the content of aerobic enzymes in their skeletal muscle. The training and competition style of sumo is very brief and anaerobic - most competitions are over in < 1 minute. Thus, the aerobic energetic system probably receives little stress during sumo training and competition. On the other hand, grappling style sports (i.e. wrestling, Judo) probably require a combination of aerobic and repeated anaerobic types of energy (Amtmann and Cotton, 2005). This is because they typically last for > 2 minutes, with times of very high intensity interspersed throughout the competition. Likewise, striking combat sports (i.e. karate, boxing, Kendo) also require a combination of aerobic and repeated anaerobic types of energy. Therefore increases in skeletal muscle aerobic enzymes (and thus increases in VO2max /SMM) should occur in other combat sports and aerobic sports, but not in sumo wrestlers. Perplexing, however, is the fact that sumo wrestlers appear to have a lower VO2max /SMM than other power athletes, and untrained controls (Table 1). More research examining the intracellular content and structure of sumo wrestler's skeletal muscle is needed.
Sumo wrestlers have a higher fat-free mass (FFM), fat mass, and predicted skeletal muscle mass (SMM) compared to other combative sports (Table 1). Indeed, sumo wrestlers have some of the largest absolute FFM (and thus, SMM) measured in humans (Hattori, et a., 1999, Kanehisa, et al., 1997, 1998, Kondo, et al., 1994, Yamauchi, et al., 2004). Why might these attributes be helpful in the sport of sumo wrestling? These attributes make the wrestler difficult to push off his feet or move. The large fat mass of sumo wrestlers plays a part in lowering the center of gravity of the body, providing stability to the wrestler. The large muscle mass allows the sumo wrestlers to create enormous absolute force and power to move their opponent, and also provides stability.
The large SMM and fat mass (FM) in sumo wrestlers may also play a role in the shape of results in Figure 1: the relationship between SMM and absolute VO2max is generally linear until about 45 kg of SMM; then the relationship begins to disappear (right hand side of Figure 1) for the athletes with large SMM, i.e. power athletes and sumo wrestlers. This may indicate that athletes with a very large SMM incur a "penalty" in the form of an aerobic plateau. It is well known that energy cost per distance increases with increases in body mass in humans (McArdle et al., 2001). The results of Figure 2 support this concept: VO2max /SMM is negatively related to percent body fat, indicating that excess fat is detrimental to work because it increases the carried load, which increases oxygen costs. Although the data analyzed are limited in scope, the ceiling of increasing SMM in human athletes without incurring a "penalty" in the guise of plateaued or decreasing aerobic performance seems to be ~ 45 kg of SMM based on the data analyzed (Figure 1).
CONCLUSIONS
In conclusion, sumo wrestlers have a low VO2max /skeletal muscle mass compared to untrained controls and combat, aerobic, and power sport athletes, which is currently unexplainable, but may be related to the lack of aerobic training particular to sumo wrestling practice/competition, and/or the large fat mass in these athletes. The enormous fat and skeletal muscle mass in sumo wrestlers is unique in the combat sports, and in all of sport. Further investigation into sumo wrestler aerobic skeletal muscle function is warranted.
KEY POINTS
Sumo wrestlers have a high absolute VO2max compared to untrained controls.
However, sumo wrestlers have a low VO2max /kg of skeletal muscle mass compared to other combat sports, other strength/power sports, and untrained controls.
The reason for this is unknown, but is probably related to alterations in sumo skeletal muscle compared to other sports.
Based on the present and previous data, there appears to be a "ceiling" at which increases in skeletal muscle mass do not result in additional increases in absolute VO2max.
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Fitness and Nutrition 
venous oxygen differences in the blood. Clearly one or more of these factors must be affected in sumo wrestlers to account for the low VO2max /SMM. It is possible that the massive skeletal muscle hypertrophy in sumo wrestlers is not matched by increases in heart size and/or stroke volume; that is, cardiac output may be low per unit of SMM for sumo wrestlers, resulting in abnormally low VO2max /SMM values. However, a recent study of professional Japanese sumo wrestlers suggests that left ventricular dilation is indeed very large in these athletes (median left ventricular [LV] end-diastolic volume ~ 60 mm), although it should be noted that LV function is normal, and LV dimensions were significantly correlated with body size, which would lead one to assume that sumo wrestlers have normal LV sizes and stroke volumes for their amount of skeletal muscle mass (Kinoshita, et al., 2003). Additionally, we noted normal (within ± 5 % of predicted) maximal heart rates (see Results) in sumo wrestlers at VO2max. This suggests cardiac output of these athletes is not compromised.
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