Chapter 5—Fitness for Firefighting

This chapter shows how muscular and aerobic fitness are related to performance in firefighting. We explain each component of fitness and describe how training improves work performance and reduces the risk of injury.

Aerobic Fitness

Studies of firefighters and other field workers confirm the link between aerobic fitness and performance. Fit workers accomplish more with less fatigue, and they perform better in hot environments. They cope with and recover faster from long shifts and reduced rest, and they miss fewer days of work because of illness and injury. In short, aerobic fitness is an important factor in prolonged arduous work. Table 5.1 shows the energy costs for common firefighting tasks.

Table 5.1—Energy costs of common wildland firefighting tasks. The energy costs are estimates for someone weighing 150 pounds. Add or subtract 10 percent for each 15 pounds above or below 150 pounds.

Wildland Firefighting Tasks	Energy Cost
	cal/min	mL/kg • min
Using a hand tool (for instance, digging or chopping with a Pulaski, combi tool, McLeod, or brush hook)	7.5	22.5
Lifting and carrying light loads (examples are clearing loose brush or trees, deploying or repositioning hose, throwing dirt with a shovel, firing operations, or structure protection)	6.8	20.0
Chain sawing (felling, bucking, limbing)	6.2	18.0
Packing heavy loads (pumps, hose packs, 5-gallon water bags)	7.5 (flat) 10.0 (hill)	22.5 29.4
Hiking with light loads (field pack and tools)	6.5	19.0
Performing under adverse conditions (including long work shifts; rough, steep terrain; heat, cold, altitude, smoke; insufficient food, inadequate fluid replacement, lack of sleep)	6.5-10+	19-30
Emergency responses (fast pull-out to safety zone, rescue, or evacuation assistance to others)	10.0+	29.4+
Chopping wood	7.5	21.4
Tree felling (ax)	8.5	25.0
Stacking wood	5.8	17.0
Shoveling	6.8	20.0

The body metabolizes fat and carbohydrate when producing energy during long periods of hard work. Aerobic fitness is defined as the maximal capacity to take in, transport, and use oxygen.

Aerobic fitness indicates the maximal capacity of the respiratory system (taking in oxygen), the circulatory system (transporting oxygen), and the muscles (using oxygen). Although the maximal oxygen intake (VO₂ max) is usually measured with a laboratory treadmill test, it can be estimated with simple field tests (see chapters 6 and 8).

When the body can't meet the energy demands of strenuous work with aerobic metabolism, it begins to use the limited supplies of anaerobic energy. Continued reliance on anaerobic energy rapidly leads to fatigue.

The two ways to measure aerobic fitness are aerobic capacity (liters per minute) and aerobic power (milliliters per kilogram-minute). Aerobic capacity is related to performance in cycling and rowing, while aerobic power is a better measure of performance when workers are carrying their own body weight, such as when they are hiking and climbing hills. But when workers are carrying a heavy load (70 pounds), performance is more related to aerobic capacity (VO₂), which defines the size of the engine. Now let's consider two additional dimensions of aerobic fitness: the first and second lactate thresholds.

Lactate Thresholds—Lactic acid is both an energy carrier and a metabolic byproduct of intense effort. Its accumulation is a sign that you are using energy faster than it can be produced aerobically. Too much lactic acid interferes with the muscles' capabilities. Lactic acid and the high levels of carbon dioxide produced during vigorous effort are associated with labored breathing, fatigue, and discomfort.

Blood lactate is a byproduct of anaerobic glycolysis. In a progressive treadmill test, going from a walk to a jog to a run-lactate rises slowly at first, then more rapidly (figure 5.1). The transition from slow oxidative muscle fibers to fast glycolytic fibers is associated with the increase in lactic acid.

Figure 5.1—Lactate thresholds. Reprinted, with permission, from B.J. Sharkey
and S.E. Gaskill, 2007, Fitness & Health, 6th ed. (Champaign, IL: Human Kinetics), 73.

The first lactate threshold (LT1) occurs at about 50 percent of your VO₂ max. The first threshold defines a level of exertion that can be sustained for several hours to all day, depending on its level. As you go from a jog to a run, you involve additional fast-twitch strength muscle fibers and produce more lactate.

The second lactate threshold (LT2) defines the upper reaches of aerobic metabolism. It is highly correlated to performance in competitive events (such as a 10-kilometer run) lasting from 30 minutes to 3 hours (Sharkey and Gaskill 2006).

Training can raise VO₂ max and both lactate thresholds. Eventually the VO₂ max will plateau, but the lactate thresholds will continue to improve (figure 5.2). As training improves the muscle's oxidative capacity, more work can be done without an increase in lactic acid. The first lactate threshold defines the ability to perform prolonged arduous work. Individuals with an improved LT1 can sustain 50 percent or more of their VO₂ max for 8 hours or longer. If a worker has a VO₂ max of 50 milliliters/kilogram-minute, and his LT1 is 50 percent of his max, his VO₂ at LT1 is 25 milliliters/kilogram-minute.

Figure 5.2—Improvements in VO₂ max and lactate thresholds.

Studies in the workplace indicate that LT1 defines work output in some physically demanding occupations. In a study by Gaskill and others (2002), wildland firefighters were divided into two groups: those with higher and those with lower levels of LT1. The firefighters were fitted with electronic motion sensors to determine work activity and sent out to work for 9 days on an actual fire. Those with a high LT1 had an average of 43.7 milliliters/kilogramminute VO₂, while those with a lower LT1 had an average of 34.6 milliliters/ kilogram-minute VO₂. Those with the high LT1 performed more work throughout the day. They sustained a higher work rate without feeling more fatigued (figure 5.3).

Figure 5.3—This graph shows the work (kilocalories per day) done by a hotshot crew over a 9-day period.
The crew was divided into fit and less fit groups. The fit group did more work per day than the less fit group.

Sustainable Fitness—Steve Gaskill calls the VO₂ at LT1 "sustainable fitness." It defines the workload a firefighter can sustain throughout the working day. You can train sustainable fitness (LT1) by gradually increasing the duration of time spent at or somewhat above LT1. Of course, the training should involve the actual work or a close approximation.

In the field, sustainable fitness is measured with sustained work. Wildland firefighters must pass the Pack Test, a 3-mile (4.8-kilometer) hike with a 45-pound (20.5-kilogram) pack, completed within 45 minutes. The energy cost of the test is the same as the work of wildland firefighting, so firefighters are asked to demonstrate the ability to sustain the workload for the time it takes to complete the 3-mile hike. Those who complete the test in less than 45 minutes demonstrate the ability to sustain a higher work rate. Completing the test in 45 minutes predicts a VO₂ max of 45 milliliters/kilogram-minute.

Efficiency—Another factor that has a significant impact on work capacity is a worker's efficiency or economy of motion. Efficient workers use less energy to accomplish a given task, allowing them to work at a lower percentage of their maximum capacity. Efficiency conserves energy and prolongs performance. With appropriate instruction and practice, workers can learn to use tools and accomplish tasks with a minimum of wasted motion. Efficiency can help compensate somewhat for differences in VO₂ max or the lactate thresholds. The ideal worker has a high VO₂ max and lactate thresholds combined with skill and economy of motion.

Muscular Fitness

Firefighters with more strength and muscular endurance are better able to carry the loads and use wildland firefighting tools than those who are not as fit. Muscular fitness protects against lower back and other overuse injuries and the accidents and hazards found in dangerous environments like the fireline. Muscular fitness also contributes to everyday life, allowing workers to perform their daily tasks with vigor and a greater margin of safety.

Muscular fitness is an essential part of the total health and fitness program. It:

• Maintains the muscle mass needed to burn fat
• Prevents or reduces the risk of lower back and other overuse injuries
• Maintains performance, mobility, and independence well into the retirement years
• Maintains bone density and reduces the risk of osteoporosis

The primary components of muscular fitness are strength, muscular endurance, power, and flexibility.

All these components contribute to work capacity. We will focus on explaining how strength, endurance, power, and flexibility aid work performance while lowering the risk of overuse and other job-related injuries.

Muscular strength is the ability to lift heavy objects; muscular endurance is the ability to lift smaller objects repeatedly, and power is the ability to do work rapidly, for instance, when swinging an ax.

Muscular Strength

Strength, the maximal weight that can be lifted by a specific muscle group, is highly related to the cross-sectional area of the muscle. While strength is influenced by heredity, it improves with training. Although strength declines slowly with age, it declines more slowly when muscles are used regularly. Strength training yields results, regardless of age.

The average woman has about half the arm and shoulder strength and three fourths the leg strength of the average man. Part of this difference can be attributed to the difference in body weight between the average man and the average woman. When strength is expressed per unit of body weight, strength differences between the genders are reduced. Workers who need additional strength can safely engage in strength training to improve their ability to carry out field tasks. Women can reduce the "strength gap" by engaging in a systematic weight training program.

Strength is the primary factor limiting work capacity when heavy lifting is involved, when using heavy tools, or when heavy loads must be transported.

Strength, muscular endurance, and aerobic fitness combine to set limits on work capacity for repeated lifting, when working with handtools, or when moderate loads are involved. Figure 5.4 illustrates how strength and aerobic fitness interact. The curves on the figure show combinations of work rate, workload, and aerobic fitness that can be sustained for prolonged work shifts.

Figure 5.4—Relationship of strength and aerobic
fitness. Reprinted, with permission, from B.J.
Sharkey and S.E. Gaskill, 2007, Fitness & Health,
6th ed. (Champaign, IL: Human Kinetics), 321.

For stronger individuals, a given workload constitutes a lower percentage of their maximum strength, allowing improved performance. The ideal combination involves above-average strength and aerobic fitness. For example, a worker with a VO₂ max of 55 can lift a loaded shovel using just 20 percent of maximal strength and will be able to sustain a work rate of more than 10 contractions (shovel loads) per minute. A worker with a VO₂ max of 45, for whom a loaded shovel constitutes 50 percent of maximum strength, will be able to sustain less than five contractions per minute. Field studies of wildland firefighters verify these predictions.

Some workers may have high levels of aerobic fitness but relatively low muscular strength. They may compensate by lifting a lighter load more often. Similarly, strong workers compensate for low aerobic fitness by lifting heavier loads more slowly. Skillful workers use less energy to accomplish a task. The best production rates are accomplished by workers who possess above-average strength and muscular endurance, along with aerobic fitness, skill, efficiency, and experience.

Muscular Endurance

Endurance is an essential component of work capacity. It defines the ability to keep working and is measured by the muscle's ability to lift a load repetitively. For many forestry field tasks, repetitive work with handtools is the name of the game. Training improves muscular endurance by improving the aerobic energy capabilities of the muscles used. Training enhances the action of aerobic enzymes and the capillary supply of specific muscle fibers. Training to improve work capacity must be specific to the task and muscles used (table 5.5).

Table 5.5—The strength-endurance continuum. Reprinted, with permission, from B.J. Sharkey and S.E. Gaskill, 2007, Fitness & Health, 6th ed. (Champaign, IL: Human Kinetics), 153.

	Strength	Short-term (anaerobic) endurance	Intermediate endurance	Long-term endurance
For	Maximum force	Brief (2-3 min) persistence with heavy load	Persistence with intermediate load	Persistence with lighter load
Prescription	4-8 RM* (slow) 3 sets rest > 3 min	12-15 RM 3 sets rest 1-3 min	30-50 RM 2 sets rest < 1 min	Over 100 RM 1 set
Improves	Contractile protein ATP** and CP*** Connective tissue	Some strength and anaerobic metabolism (glycolysis)	Some endurance and anaerobic metabolism Slight improvement in strength (for untrained individuals)	Aerobic enzymes Mitochondria Oxygen and fat utilization
Doesn't improve	Oxygen intake Endurance	Oxygen intake		Strength
*RM = Repetitions maximum **ATP = Adenosine triphosphate ***CP = Creatine phosphate

Most work tasks require more endurance than strength. Once an individual has the strength needed to accomplish a task, training should focus on endurance. Endurance is developed with training that emphasizes repetitions. You'll find training advice in chapter 7.

Flexibility

Flexibility is the range of motion our limbs can move through. Skin, connective tissue, and conditions within joints restrict this range. Injuries can occur when a limb is forced beyond its normal range. Improved flexibility can reduce the potential for injury. While excessive flexibility isn't necessary, a certain amount helps workers get over, under, and around obstacles, lessening the risk of injury in most forms of vigorous activity.

Range of motion increases when joints and muscles are warm, so do some general physical activity before stretching. A few minutes of dynamic stretching (see appendix H) before work or sport improves flexibility and performance, reduces soreness, and could lower the risk of injury. Stretching won't prevent muscle soreness, but it does help reduce soreness. Use gentle stretching movements. Avoid vigorous bobbing, which tightens muscles as it invokes a reflex contraction in the muscle you are trying to stretch. Contracting the muscles briefly before a stretch allows more complete relaxation and a better stretch.

Stretching is important to maintain the range of motion during training. Lack of flexibility in the back and hamstring muscles contributes to lower back problems. Stretching may reduce the risk of repetitive trauma injuries. Attention to flexibility must be a lifelong pursuit if you are to maintain range of motion and avoid problems in your lower back.

Specificity of Exercise—Exercise and the effects of a particular kind of training are specific to the muscles and metabolic pathways used in the training. Firefighting and many field tasks require prolonged work with the arms. It is essential to train the muscles that will be used on the job. Untrained arm muscles may fatigue, even though a worker has a high VO₂ max based on a leg test and leg training. A high VO₂ max ensures the cardiovascular and respiratory capacity for work, but does not ensure specific training of the arm and trunk muscles used to perform prolonged arduous work with handtools. Athletes recognize the importance of specificity and follow general off season training with sport-specific training as the season approaches. The same principles apply to firefighters and forestry field workers who should engage in job-specific tasks (shoveling, wood cutting) to prepare for work.

Muscle Fiber Types

Muscles have two primary fiber types, the slow-twitch fibers needed for endurance and the fast-twitch fibers needed for strength and power. Endurance athletes have more slow-twitch fibers, while strength or power athletes have more fast-twitch fibers. Firefighting requires both types of fibers. Slow twitch fibers use energy and oxygen efficiently for sustained energy output. These fibers contract more slowly than fast-twitch fibers, producing less power. Wildland firefighting sometimes requires both strength and endurance (see table 5.6).

Table 5.6—Characteristics of slow-twitch and fast-twitch muscle fibers. Reprinted, with permission, from B.J. Sharkey and S.E. Gaskill, 2007, Fitness & Health, 6th ed. (Champaign, IL: Human Kinetics), 46.

Characteristics	Slow oxidative (SO)	Fast oxidative glycolytic (FOG)	Fast glycolytic (FG)
Average fiber percentage	50%	35%	15%
Speed of contraction	Slow	Fast	Fast
Force of contraction	Low	High	High
Size	Smaller	Large	Large
Fatigability	Fatigue resistant	Less resistant	Easily fatigued
Aerobic capacity	High	Medium	Low
Capillary density	High	High	Low
Anaerobic capacity	Low	Medium	High

The characteristics of these two fiber types are quite different. The optimal muscle for endurance can never be maximally strong or powerful. Likewise, the muscle fibers that produce the most force are not optimal for endurance. Training will tend to move muscle characteristics toward one type or the other. The fibers are quite different in their characteristics and how they respond to training. This difference in the fibers' characteristics requires specific training so wildland firefighters can develop adequate strength and the capacity for long-duration effort. In addition, firefighters need adequate strength and power for times when it is needed, such as when dragging a charged hose or when a smokejumper packs out 110 pounds of gear.

Muscle Proteins—The proteins actin and myosin allow muscles to contract. These proteins are more pronounced in the strength (fast-twitch) muscle fibers. Enzyme proteins in the mitochondria are needed for endurance. These enzyme proteins, which enhance the oxidation of carbohydrate and fat, are more pronounced in the slow-twitch muscle fibers. Strength training increases the contractile proteins in both types of fibers. Endurance training improves enzyme protein and the fibers' ability to produce energy from the oxidation of fat and carbohydrate. Since adaptations only take place in the fibers actually used in training, specific training is required for the tasks you will perform in the field.