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“The Impact of Weight on the Performance of a Race Horse”

By Larry Wellman (Dec 19, 1999)

Probably one of the most asked questions in horse racing is: What effect does a jockey’s weight have on a horse’s performance? If a horse picks up five pounds from his last race, what effect in lengths, time, Beyer numbers, or any other rating system might we expect? Is a pound extra more critical at sprints or routes? These are just a few of the questions handicappers ask themselves daily trying to decipher the information in their past performance whether the Daily Racing Form, Equibase Program, or any of the many online past performance services provided on the internet. Over the next few pages I will give you some incite in how weight carried effects the performance of a horse during a race and how to decide if you need to consider weight as a handicapping factor. Racing secretaries try to set race conditions using weight as a means to even out a race. Handicap races for older horses can have a significant range of weights sometimes over 20 pounds between the top weight horse and other members of the race. Horses like Forgo, Kelso, and Dr. Fager carried weights in the 130 plus range throughout their careers. Did they have weight limits?

I will show two different analyses for the impact of weight carried by a horse. The first will be using Newton’s second law where force equals mass times acceleration otherwise know as F=ma. The second analysis will used actual exercise physiology results. Each analysis is based on dead weight at the center of gravity of the horse. Later I will discuss live weight and location of weight. Dead weight can be considered the same as weight due to extra body fat.

The force function of the above equation can be modeled analytically so that the function can be integrated to get velocity and then displacement. I won’t go into the equation in detail, however it has an exponential term (constant * exp (-t/tau)) with a time constant (tau). The following table shows the results for analytically solution. The table has the fractional times of a race for two different horses. Total weight of horse and riders is 1190 and 1210 pounds with a horse weight of 1080 pounds. Times shown in the table are in seconds. The race that is simulated has a gate run-up of 65 feet. There are five additional rows shown in the table. The second row (delta, seconds) is the time difference between the two horses. The next row is the seconds per pound of weight. The sec/5 lbs. row is how many seconds for five extra pounds. The lbs/fifth sec row is for how many pounds required to increase running time one fifth of a second (.20 seconds). The last row is from another part of the analysis that calculated the number of pounds required to cost a horse one length (10 feet).

Table 1:

Weight, lbs.	2f	4f	6f	8f	10f
Horse 1 - 1190	24.02	47.69	71.56	95.65	121.06
Horse 2 - 1210	24.41	48.48	72.77	97.27	123.13
delta, sec	(0.391)	(0.794)	(1.205)	(1.622)	(2.068)
sec/lb	0.020	0.040	0.060	0.081	0.103
sec/5lbs.	0.098	0.198	0.301	0.406	0.517
lbs/fifth sec	10.24	5.04	3.32	2.47	1.93
lbs/10ft	9.3	4.58	3.05	2.3	1.8

If you want to convert to Beyer Speed Figure (BSF) we can use the following from “Beyer on Speed” page 20. At 6f one length = 2.4 points or 1-fifth sec =2.8 points and at 8f one length = 1.8 points. Beyer also says on page 106-107 that a kilogram (2.2 pounds) equals one point in his system at all distances or a pound is worth 0.45 points. From the above analysis we have 3.05 pounds equals one length at 6f. This makes each pound worth 0.80 points and about the same at 8f. It appears that Beyer number is a little low. As a rough rule of thumb we could use one half to one point for each pound of weight shift at all distances or one to two pounds for each point. Beyer developed his rules from actual horse race results where a horse fitness level changes. The analytical analysis is based on a fixed fitness level. Sheet players use a weight shift of five pounds as one point.

The second analysis is based on data collected by researchers from the exercise physiology and experimental biology fields. Some of this information will be based on actual thoroughbred (TB) horses, some on other mammals, and humans. I will give references for the academia type of handicappers if they would like to do their own research. In some cases I might offer some opinions on how to use this information. I will try to keep the technical terms to a minimum or give a related term.

C.R. Taylor (1970) showed that the energy cost of running in all animals from a mouse to an elephant is directly related to the running speed. He also showed that larger animals have a lower energy cost on a per pound basis. Energy is determined based on the amount of oxygen the animal uses while running. Laboratory test results show that the oxygen consumption for a dog and a horse are linear (straight line) as running speed increases with the smaller animal having a steeper increase (slope) in oxygen consumption, Figure 1. Eaton (1988) showed that from laboratory test that a horse has an outstanding capability to consume oxygen almost twice what the top human runners. Oxygen consumption is measured by milliliters of oxygen per kilogram of body weight per minute with a horse having a maximum oxygen consumption (Vo2max) range from 104 to 170 ml/kg/min on the average. You can think of the oxygen term as horsepower per body weight or a measure of class. Higher oxygen consumption the more horsepower generated the better the horse (higher class). This concept will be the basis for showing how weight carried effects the performance. Taylor (1980) showed that when rats, dogs, horses, and humans carry an extra load that the oxygen consumption increased in direct proportion to the added load. For example if the load was 10 percent of body mass, the oxygen consumption was increased by 10 percent, and so on, as shown in Figure 2. Thus a larger horse would carry less percentage then a smaller horse when carrying the same weight.

Potard (1998) measured the oxygen consumption during laboratory test on TB showing a maximum oxygen consumption peaking at 136 ml/kg/min at around 12-13 meters per second (m/sec) or 36 to 40 feet per second (ft/sec) while running with the equivalent of 10 percent of body weight as a draft load. These test results were measured on horizontal motorized treadmills and usually without the wind component being simulated. In addition, a treadmill does not simulate an actual track. Lejeune (1998) measured humans running in sand and shown that the cost of running increases between 1.2-1.6 times that of running on a treadmill. In this discussion I will use a value of 1.2 for the additional cost of running in sand to simulate the track.

The term Vo2max represents the highest level of oxygen consumption the individual horse can obtain. The energy supplied to run at higher speed comes from energy stored in the muscles. At high speeds above Vo2max the horse is running with what is known as an oxygen debt. Oxygen debt is measured by the amount of lactic acid that is accumulated in the muscles and/or blood. Each TB has their own limits of lactic acid based on training and genetics. In the following example I will use a baseline Vo2max value of 136 ml/kg/min to show how weight effects performance. Table 2 shows the velocity versus oxygen consumption for a TB horse. Column one is the horse speed (feet per second, ft/sec) on the treadmill, column two through five are the oxygen consumption values (ml/kg/min) on the treadmill, column three for running on sand (simulate the race track), column four is for a jockey weight of 130 pounds and the last column for a jockey of 110 pounds. The table shows that at a constant speed the oxygen required increases as the load condition increases running in sand or carrying weight.

Table 2:

Velocity Oxygen Running Jockey Jockey

Consumption in Sand Weight Weight

Vel, ft/sec	O2, ml/kg/min	1.2	130	110
5	15.5	18.6	20.84	20.49
10	26	31.2	34.96	34.38
15	36.5	43.8	49.07	48.26
20	47	56.4	63.19	62.14
25	57.5	69	77.31	76.03
30	68	81.6	91.42	89.91
35	78.5	94.2	105.54	103.79
40	89	106.8	119.66	117.68
45	99.5	119.4	133.77	131.56
50	110	132	147.89	145.44
55	120.5	144.6	162.01	159.33
60	131	157.2	176.12	173.21
65	141.5	169.8	190.24	187.09
70	152	182.4	204.36	200.98
75	162.5	195	218.47	214.86
80	173	207.6	232.59	228.74
85	183.5	220.2	246.71	242.63

In our example we said our horse had a maximum oxygen consumption of 136 ml/kg/min, which occurs at a velocity just over 50 ft/sec on the racetrack without a rider. Add a rider and the speed at maximum oxygen consumption occurs at around 45 ft/sec at 130 pounds and about 47 ft/sec for 110 pounds. Table 2 shows the oxygen values for speeds up to 85 ft/sec even though maximum speed for a horse is only in the 70 plus ft/sec range without a rider.

We will use the above table to calculate the velocity for each rider at a constant oxygen consumption level giving a velocity difference. Using the velocity difference and time we can then calculate the distance difference for the two riders. Eaton (1995) showed that a horse running at a velocity corresponding to 105 percent of Vo2max can run at that speed for about 165 seconds, at 115 percent for only 98 seconds and finally at 125 percent for only 57 seconds. Using these time values and the information from Table 2 we get the results shown in Table 3. Table 3 shows that at each value of oxygen consumption level and at each time value the amount of weight which would cost a horse 10 feet (one standard horse length) and one fifth of a second of time.

Table 3:

Time> sec.	57		98		165
O2, ml/kg*min	lb/10ft	lb/.20 sec	lb/10ft	lb/.20 sec	lb/10ft	lb/.20 sec
15.5	38.03	2.40	22.12	1.40	13.14	0.83
26	22.67	3.13	13.19	1.82	7.83	1.08
36.5	16.15	3.44	9.39	2.00	5.58	1.19
47	12.54	3.61	7.29	2.10	4.33	1.25
57.5	10.25	3.72	5.96	2.17	3.54	1.29
68	8.67	3.80	5.04	2.21	2.99	1.31
78.5	7.51	3.85	4.37	2.24	2.59	1.33
89	6.62	3.90	3.85	2.27	2.29	1.35
99.5	5.92	3.93	3.45	2.28	2.05	1.36
110	5.36	3.96	3.12	2.30	1.85	1.37
120.5	4.89	3.98	2.85	2.31	1.69	1.37
131	4.50	4.00	2.62	2.32	1.55	1.38
141.5	4.17	4.01	2.42	2.33	1.44	1.39
152	3.88	4.03	2.26	2.34	1.34	1.39
162.5	3.63	4.04	2.11	2.35	1.25	1.39
173	3.41	4.05	1.98	2.35	1.18	1.40
183.5	3.21	4.06	1.87	2.36	1.11	1.40

Based on Table 2 we see that racing speeds occur at oxygen consumption levels in the 145-160 ml/kg/min. Lets take a closer look at the values for oxygen consumption of 152 ml/kg/min a middle value. We’ll make an assumption that each one of the time values corresponds to a race distance so we can compare it to Table 1. We’ll say 57 seconds is 5f, 98 seconds is 8f, and 165 seconds is 12f race distances.

From Table 3:

Time> sec.	57		98		165
O2, ml/kg*min	lb/10ft	lb/.20 sec	lb/10ft	lb/.20 sec	lb/10ft	lb/.20 sec

152

3.88

4.03

2.26

2.34

1.34

1.39

Our results show that at 57 seconds that 3.88 pounds is required for each horse length. Table 1 showed that at 4f, 4.58 pounds was required and at 6f, 3.05 pounds. Splitting the difference to get 5f puts us at 3.81 pounds. At 98 seconds we had 2.26 pounds per length and 2.3 pounds for 8f. So from two different methods one analytical and one based on exercise physiology test of the oxygen consumption of a TB, basically give us the same answer. This analysis was for dead weight located at the center of gravity of the horse

Now a jockey position is at a location above the center of gravity on the horse. The natural question to ask is there any positive or negative effect because of the jockey as live weight? Unfortunately I could not locate any research data of oxygen consumption with riders versus dead weight.

The effect of weight at other locations other then the center of gravity can have a significant impact on oxygen consumption levels. Myers (1985) found that the cost of adding a given mass to the limbs is significantly greater than adding it to the center of mass and that this effect becomes more pronounced as the limb loads are moved distally (towards the foot). Miller (1987) showed a 0.8% increase in oxygen consumption for ankle weights of 100 grams on human runners. A 600-gram weight at the center of gravity would result in the same increase of oxygen consumption. Relative to horse racing, any increase in extra weight carried along the leg and at the hoof could impact on a horse’s performance. For example on a muddy or sloppy day a large horse (large hoof) could end up carrying extra dirt in it’s hoof relative to a smaller horse (smaller hoof). Also the come from behind type of runner could have additional weight along the legs from mud being thrown back from horse’s in front. This extra weight could be enough to cost a win.

In summary I’ve shown that weight carried impacts a horses performance and this impact changes as the distance of the race changes. The impact of weight along the legs and at the hoof has more impact then weight at the center of gravity. Relative to handicapping we can adjust Beyer Speed Figures (BSF) for a weight shift using from one to two pounds being worth one point in BSF. We can also see that horses could have a limit on the amount to their weight carrying ability. Weight carrying ability is based on their size and their maximum oxygen consumption level.

Reference:

Taylor, CR, Schmidt-Nielsen, K, and Raab, JL (1970) Scaling of energetic cost of running to body size in mammals, Amer J Physiol 219:1104-7

Evans DL, Rose RJ, Determination and repeatability of maximum oxygen uptake and other cardio respiratory measurements in the exercising horse. Equine Vet J 1988 Mar, 20(2):94-8

Taylor, CR, Heglund, NC, McMahon, TA, and Looney, TR (1980) Energetic cost of generating muscular force during running. J Exp Biol 86:9-18

Potard , U. Silke Birlenbach, Leith, DE, and Fedde, MR, Force, speed, and oxygen consumption in thoroughbred and draft horses., J Appl Physiol 1998 84(6):2052-9

Lejeune, TM, Willems, PA, Heglund, NC, (1998) Mechanics and energetics of human locomtion on sand. J Exp Biol 201:13; 2071-80

Eaton MD, Evans DL, Hodgson DR, Rose RJ, Maximal accumulated oxygen deficit in thoroughbred horses. J Appl Physiol 1995 Apr, 78(4):1564-8

Myers MJ, Steudel K, Effect of limb mass and its distribution on the energetic cost of running. J Exp Biol 1985 May;116:363-73

Miller JF, Stamford BA, Intensity and energy cost of weighted walking vs. running for men and women. J Appl Physiol 1987 Apr;62(4):1497-501