Fatigue Resistance: An Intriguing Difference in Gender

Muscle fatigue is a
multifaceted phenomenon resulting from a combination of impairments throughout
the human neuromuscular system (Hicks, Kent-Braun & Ditor 2001; Russ et al.
2008). The definition of muscle fatigue has been modified throughout the years
as research has brought forth more understanding of the components contributing
to fatigue.

Traditionally, muscle fatigue has been defined as the muscle’s
inability to maintain an expected force (Barry & Enoka 2007). In the past
decade, Barry and Enoka have written that it corresponds to an exercise-induced
decline in the capability of the muscle to generate force or power, regardless
of whether or not the task can be continued. According to this description,
muscle fatigue slowly starts after the onset of sustained activity, even though
an individual can continue performing a muscular task.

Numerous studies have shown that women have a greater resistance
to fatigue than men; therefore, women are able to sustain continuous and
intermittent muscle contractions at low to moderate intensities longer than men
(Clark et al. 2003; Fulco et al. 2001; Hunter & Enoka 2001; Hunter et al.
2004; Russ et al. 2008; Russ & Kent-Braun 2003; Thompson et al. 2007; Wust
et al. 2008; Yoon et al. 2007). This trend has been observed in a variety of
muscles using assorted training protocols; however, the physiological
mechanisms for the differences between males and females are not completely
understood. Differences in muscle mass, exercise intensity, utilization of
foodstuffs during metabolism (reactions that produce ATP) and neuromuscular
activation have all been suggested as contributing factors for the fatigue
differences between the sexes (Russ et al. 2008; Hicks, Kent-Braun & Ditor
2001; Russ & Kent-Braun 2003; Wust et al. 2008; Thompson et al. 2007; Yoon
et al. 2007).

Muscle Mass and Exercise Intensity

Generally, men can generate a higher
absolute muscle force when performing the same relative (percent of maximal
voluntary contraction) workload as women during a muscle contraction (Hicks,
Kent-Braun & Ditor 2001). Several researchers have suggested that this
higher absolute muscle force during the same relative workload causes
intramuscular pressures, compressing the blood vessels that feed the muscles
and slightly inhibiting oxygen supply to the working muscles (Russ &
Kent-Braun 2003; Thompson et al. 2007; Yoon et al. 2007). To delay fatigue in
working muscles, oxygen is necessary during sustained contractions to allow for
the continuation of oxidative
phosphorylation (aerobic metabolism). The constriction of
blood vessels also delays the removal of metabolic byproducts (carbon dioxide,
hydrogen ions, lactate) from the muscle, thus contributing to fatigability.

Researchers have shown that women are capable of longer endurance
times than men when performing low- to moderate-intensity isometric
contractions in several muscle groups, including the adductor pollicis, elbow
flexors, extrinsic finger flexors and knee extensors (Hunter & Enoka 2001).
As the intensity of the contraction increases above moderate levels, the
difference in gender fatigability is less observable. Thus the intensity of
the exercise is a major discerning factor in the fatigue resistance differences
seen between females and males.

Other researchers have also shown differences in the time to
fatigue between men and women when they are matched for
maximal voluntary contraction of the target muscle site. When testing the elbow
flexor muscles, Hunter et al. (2004) showed that women had a greater time to
task failure during intermittent submaximal muscle contractions than their
strength-matched male counterparts. These findings are consistent with Fulco et
al. (1999) for intermittent contractions of the adductor pollicis muscle
performed by strength-matched males and females. These authors showed that
women had a longer time to task failure at 50% of maximal voluntary contraction
of the adductor pollicis muscle. Note that the differences in time to fatigue
between men and women are most apparent in submaximal (not maximal)
contractions.

Mean Arterial Blood Pressure and Blood Flow

Some investigations have studied the
blood flow to the working muscles and the mean arterial pressure (MAP).
MAP is the average blood pressure during a cardiac cycle and is estimated by
the following equation: diastolic pressure + &#8531 (systolic pressure – diastolic pressure). Yoon
et al. (2007) found that women had lower MAP than men during submaximal contractions of
the forearm muscles but not at 80% of voluntary maximal
contraction. Other researchers have also found that females had lower MAP than
men, even when subjects were matched for absolute strength of the target
muscles (Hunter & Enoka 2001; Thompson et al. 2007; Hunter et al. 2004).

There are several possible explanations for the differences in
MAP responses between males and females:

1. less muscle mass involvement in females than
   males (at same relative workload)
2. lower absolute muscle contraction differences,
   resulting in less blood flow constriction, in females
3. gender differences in motor unit activation of
   the nervous system
4. gender differences in utilization of substrates
   (foodstuffs such as carbohydrates and fats)
5. lower production of metabolite byproducts
   (i.e., hydrogen ions, carbon dioxide and lactate) in females (Hunter &
   Enoka 2001; Hicks, Kent-Braun & Ditor 2001; Hunter et al. 2004)

Some studies have tried to control for blood flow constriction to
the working muscles by placing the muscles in ischemic conditions
(where blood vessels are constricted). Russ and Kent-Braun (2003) studied men
and women performing intermittent submaximal muscle contractions of the
dorsiflexor muscles of the ankle during both free-flow and ischemic conditions.
They found that women had less fatigue than men during the free-flow condition,
but the difference was eliminated during the ischemic condition. Similarly,
Wust et al. (2008) tested the quadriceps muscles for fatigue differences
between males and females under normal-blood-flow and ischemic-blood-flow
conditions. To achieve ischemia, a pneumatic (compressed-air) thigh cuff was
placed around the upper thigh and inflated to 240 millimeters of mercury to
impede blood supply to the leg before and during the fatigue tests. For both
the normal-blood-flow condition and the ischemic condition, women showed less
fatigability than men during quadriceps contraction.

Neuromuscular Activation

Some investigators have looked at
electromyography (EMG) during muscle contraction to assess the patterns of
muscle contraction and recruitment (Clark et al. 2003; Hunter & Enoka 2001;
Hunter et al. 2004; Thompson et al. 2007; Yoon et al. 2007). During sustained
muscle contraction, the neuromuscular system strives to maintain force
production by recruiting additional nonfatigued motor units, recruiting larger
motor units and increasing the firing rate of activated motor units (Thompson
et al. 2007; Yoon et al. 2007). An EMG is able to detect muscle contraction and
recruitment by the electrical impulses sent through the body for muscle
contraction. Differences in the activation of recruitment patterns within a
target muscle and its agonists affect the fatigue rate of that muscle group.

Some investigators have observed gender differences in the way
muscles are used and in their recruitment patterns, whereas other researchers
have found no differences between the sexes. During higher-intensity exercise
(≥ 80% maximal voluntary contraction), there is usually no difference between the sexes in either muscle
activation or the recruitment of muscles (Yoon et al. 2007). More gender
comparison studies are needed to elucidate this area of the
neuromuscular-activation and fatigue debate.

Practical Considerations

Some fascinating physiological and
metabolic fatigability characteristics exist in the submaximal muscle-force-producing
capacity of women, which are illustrated in Figure 1. It should be noted that
the fatigue differences between men and women are more apparent with fit
females, as “low-fit” individuals (males and females) tend to fatigue rather
rapidly owing to the untrained state of their musculature. However, as many
personal trainers and fitness professionals have observed in their professional
experience, these physiological phenomena help to explain scientifically why
moderately to highly trained females are very capable of doing advanced
multiple-set and multiple-exercise program designs as well as completing
frequent resistance training workouts on a weekly basis.

SIDEBAR: Figure 1. Contributing Mechanisms of Fatigue Resistance Characteristics in Females

SIDEBAR: Substrate Utilization and Estrogen

Differences in metabolism exist between males and females.
Several studies have shown that males have greater glycolytic (carbohydrate) capacity and
rely more on glycolytic pathways, whereas females rely more on oxidative
phosphorylation (fat and carbohydrate) during sustained cardiovascular exercise
(Braun & Horton 2001; Tarnopolsky et al. 1990). During continuous aerobic
exercise, women have been found to have a lower
respiratory exchange ratio (a laboratory measurement used during
aerobic exercise to determine foodstuffs being used for fuel), indicating that
women rely more on fat for fuel during this submaximal exercise (Braun &
Horton 2001; Tarnopolsky et al. 1990). Muscle biopsy research shows that the
common glycolytic enzymes (phosphofructokinase, pyruvate kinase and lactate
dehydrogenase) that break down carbohydrate are less active in women, possibly
decreasing their potential for glycolytic-pathway energy production
(Tarnopolsky 2008). Thus, it is proposed that females will make greater use of
the longer-lasting fat metabolism pathway during cardiovascular exercise.

Estrogen has been shown to
influence the utilization of different fuels (i.e.,fats, proteins and
carbohydrates), especially during long endurance exercise (Tarnopolsky 2008).
Females typically rely less on carbohydrate and muscle glycogen stores and more
on fat oxidation during endurance exercise, even with carbohydrate-loading
diets. This finding has led researchers to believe that estrogen has
glycogen-sparing characteristics (Braun & Horton 2001; Tarnopolsky 2008;
Tarnopolsky et al. 1990).

Brenda Critchfield, ATC, LAT, CSCS, is completing her master’s
degree in exercise science at the University of New Mexico in Albu­querque
(UNMA). She is a teaching assistant in the UNM athletic training education
program, in charge of women’s volleyball, swimming and diving.

Len Kravitz, PhD, is the program coordinator of exercise
science and a researcher at UNMA, where he recently won the Outstanding Teacher
of the Year Award. In 2006 he was honored as the Can-Fit-Pro International
Presenter of the Year and as the ACE Fitness Educator of the Year.

References

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