Hormonal Responses to Resistance Exercise Variables
Luis M. Alvidrez and Len Kravitz, Ph.D.

Introduction:
Exercise places a major challenge on the body due to the increased energy needs and physiological demands put upon the body's nervous, muscular, cardiovascular, metabolic and respiratory systems. As a body goes through these responses and subsequent adaptations to exercise, there are a number of complex hormonal interactions occurring simultaneously. For example, hormones can increase blood pressure, stimulate protein synthesis, and increase the body's metabolic rate. With resistance training variables, several hormonal responses and consequent adaptations which have been observed will be reviewed in this article.

Hormones '101': The Basics
Hormones, secreted by endocrine (hormone secreting) glands in the body, are substances that regulate the function of body cells, tissues, organs or systems. Hormones are released from a number of 'traditional' glands, such as the pituitary, testes, ovaries, pancreas, thyroid and the adrenal cortex. More recently, science has also documented hormone secretion from 'non-traditional' sites such as the heart, kidney, liver, and adipose tissue. In reference to gender, the major differences in male and female endocrinology (the study of the hormone secreting glands) come down to the differing reproductive structures (testes versus ovaries). Males produce high levels of testosterone whereas females have higher levels of estrogen and progesterone, and lower levels of testosterone when compared to males.

Hormone Function in Resistance Exercise: Energy Production
With resistance exercise there is an immediate increase in epinephrine and norepinephrine (Kraemer and Ratamess, 2005). These hormones increase blood glucose and are important for increasing force production, muscle contraction rate, and energy production (i.e., the synthesis of ATP-the energy currency of cells). These hormones actually begin to rise prior to the resistance training workout (Kraemer and Ratamess, 2005). This is an anticipatory response of the body preparing for the challenging exercise to follow. Interestingly, the elevated blood glucose levels do not typically lead to an increase in insulin unless protein/carbohydrate supplementation precedes the workout (Kraemer and Ratamess, 2005). The increased uptake of blood glucose by the skeletal muscle is occurring due to the increase in function of the cell's glucose transporters, which increase glucose uptake and thus glucose metabolism in the muscle cell. Thus, regular resistance exercise training has been shown to increase 'insulin sensitivity', meaning the body can intake and utilize glucose more effectively (Pollock et al., 2000).
Practical Application: Energy Production
During resistance exercise there is a cascade of events that leads to an increase in several hormones that very specifically help deliver needed glucose for energy production to the working muscle cells.

Hormone Function in Resistance Exercise: Volume of Training
In resistance exercise, total volume is easily calculated by the number of reps x sets x weight that is performed in either a single session of resistance exercise or during a long-term resistance training program. Marx et al. (2001) examined the long term training (6-month training regime) adaptations associated with a low-volume (circuit) resistance training program versus a periodized high-volume resistance program in college-aged women. The study showed that the periodized higher volume resistance program had higher testosterone, insulin-like growth factor-1, and decreased levels of cortisol after the 24 weeks of training when compared to the circuit program. Greater increases in muscular strength, power, and speed were also seen in the high-volume group.
Smilios et al. (2003) examined the acute effects of the number of sets on testosterone, cortisol, and growth hormone responses after maximum strength (5 reps at 88% of 1RM, 3-min rest) and muscular hypertrophy (10 reps at 75% of 1RM, 3-min rest) protocols with 2, 4 and 6 sets of each exercise in 11 physically active (2-8 years resistance training experience) young men. Subjects also did a strength endurance (15 reps at 60% of 1RM, 1-min rest) protocol with 2 and 4 sets. In the muscular strength protocol, the number of sets did not affect the hormonal profile. In the muscular hypertrophy and strength endurance protocol, there was an increase in cortisol and growth hormone levels in four sets of exercise versus two sets. In this study, there was no significant increase in testosterone in any of the testing conditions. Contrariwise, Kramer and Ratamess (2005) summarize that protocols high in volume do tend to produce acute hormonal elevations in testosterone, as well as cortisol, and growth hormone.
Practical Application: Volume of Training
Acute and chronic research shows that higher-volume resistance programs tend to elicit the greatest hormonal responses.

Hormone Function in Resistance Exercise: Training to Failure versus Non-Failure
In a unique recent study, Izquierdo et al. (2006) examined hormonal responses in an 11-week resistance training to failure (one group) vs. non-failure (second group) followed by an identical (both groups) 5-week peaking period of maximal strength and power protocol. Subjects were 42 physically active males randomly assigned to the two groups. The results showed that 11 weeks of training to failure and not-to-failure resulted in similar gains in 1RM strength, muscle power output of the arm and leg extensor muscles, and maximal number of repetitions in the squat. However, after the identical 5-week peaking period of maximal strength and power training, the non-failure group showed greater increases in strength, power, resting testosterone levels, and reduced coritsol levels when compared to the failure group. The failure group did have a greater increase in muscular endurance in bench press repetitions and a decrease in insulin-like growth factor 1 (IGF-1), a muscle building hormone.
Practical Application: Training to Failure versus Non-Failure
Taking each set to failure may not be as an important factor as once felt when trying to increase muscular strength, power, and hormonal response for clients. By taking each set to failure, a trainer may actually make clients more susceptible to overtraining and decreased hormonal and muscle power adaptations.

Hormone Function in Resistance Exercise: Rest Period
In a recent study, Ahtiainen et al. (2005) examined a shorter rest period (2 minutes) in comparison to a longer rest period (5 minutes) in a 6-month long strength training protocol (2 heavy resistance training loading sessions per week for the lower body) with 13 recreationally trained men. Workout volume (reps x sets x weight) was equated for the two groups. It was shown that there were no differences between strength, mass, or hormonal profile (testosterone, cortisol, and growth hormone) in either the short versus the long rest periods in this 24-week study.
Practical Application: Rest Period
From a time perspective, trainers are always working to create the most time-efficient workouts for their busy clients. Previous research (Kraemer et al. 1990) suggested that a shorter rest period (one minute versus three minutes) elicited slightly higher acute hormonal responses; however, this newer 6-month study suggests no significant hormonal difference between the 2-minute versus 5-minute rest period on strength, mass, and hormone elevations.

Hormone Function in Resistance Exercise: Concentric versus Eccentric Training
During conventional resistance exercise, there is a sequential concentric and eccentric muscle action. In training adaptations and hormonal responses, concentric muscle actions produced a greater amount of growth hormone when compared to an eccentric muscle action (Durand et al., 2003). Durand and colleagues compared both the concentric and eccentric muscle actions with the same absolute load. However, when compared using a relative load, both concentric and eccentric muscle actions produced similar growth hormone and testosterone responses (Kraemer et al., 2006).

Practical Application: Concentric versus Eccentric Training
From the hormonal response perspective, trainers are encouraged to vary resistance training schemes to incorporate and emphasize concentric and eccentric training protocols.

Hormone Function in Resistance Exercise: Forced versus Maximum Repetitions
Forced repetitions are a popular method for adding intensity to any resistance exercise program. To perform a forced repetition requires the person to perform repetitions after the person has gone to failure. This type of training requires the assistance of a trainer (or workout partner). Maximum repetitions is synonymous with training to failure. Ahtiainen et al. (2004) investigated the hormonal responses of forced repetitions versus maximal repetitions in experienced strength athletes compared to nonathletes. Although the hormonal (testosterone, growth hormone, cortisol) increased similarly with both training loads (in both groups), the testosterone increases in the experiences weightlifters were significantly greater as compared to the maximal repetitions protocol.
Practical Application: Forces versus Maximum Repetitions
Training clients to take sets to and beyond failure should be used in moderation. As previously stated, always taking each set to failure can have negative effects on strength, power, and hormone responses (Izquierdo et al., 2006). Then again, it appears that the more trained the client, the greater the muscle developing hormonal response when forced repetitions are periodically incorporated.

Hormone Function in Resistance Exercise: Final Considerations
Resistance exercise has been shown to dramatically affect acute hormonal responses in the body after training. These responses play a huge role not only in immediate tissue remodeling and growth, but as well as to long term strength, power, and hypertrophy gains. Resistance exercise protocols that stress large muscle mass (multi-joint exercises), are high in volume, and moderate to high intensity, tend to produce the greatest hormonal elevations for optimal muscular fitness benefits (Kraemer et al., 2005).

These hormones actually begin to rise prior to the resistance training workout (Kraemer and Ratamess, 2005). This is an anticipatory response of the body preparing for the challenging exercise to follow

Wardie wrote:I think it's safe to assume that the best strength gains come from not training to failure, I would of liked something relating to muscular size though.

Good post but it would have been good if the study factored in training frequency

Wardie wrote:The thing is, the study was based on women, who are a little different to men when it comes to training from what I remember.

Fatigue Resistance: An Intriguing Difference in Gender
Brenda Critchfield, M.S. and Len Kravitz, Ph.D.

Introduction
Muscle fatigue is a multifaceted phenomenon resulting from a combination of impairments throughout the human neuromuscular system (Hicks et al. 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 is defined as the muscle's inability to maintain an expected force (Barry & Enoka 2007). In the past decade, Barry and Enoka expand that the definition of muscle fatigue in research 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 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 to produce ATP), and neuromuscular activation have all been suggested as contributing factors for the fatigue differences between the sexes (Russ et al. 2008; Hicks et al. 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) work load as women during a muscle contraction (Hicks et al. 2001). It has been suggested by several researchers that this higher absolute muscle force during the same relative work load causes intramuscular pressures compressing the blood vessels feeding the muscles; therefore, oxygen supply is slightly inhibited 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 will also delay the removal of metabolic byproducts (carbon dioxide, hydrogen ions, lactate) in the muscle, thus contributing to fatiguability. Researchers have shown that women are capable of longer endurance times compared to men when performing low to moderate intensity isometric contractions in several muscles 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 intensities the difference in gender fatiguability is less observable. Thus the intensity of the exercise is a major discerning factor to the fatigue resistance differences seen between females and males.
In addition, other researchers have 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% maximal voluntary contraction of the adductor pollicis muscle (Fulco et al., 1999). Note that the time to fatigue differences 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 blood pressure (MAP). MAP is the average blood pressure during a cardiac cycle and is estimated by the following equation: diastolic pressure + 1/3(systolic pressure - diastolic pressure. Yoon et al. 2007 looked at the MAP during submaximal contractions of the forearm muscles and found women had lower arterial pressure than men during intermittent submaximal contractions, but not at 80% of voluntary maximal contraction. Other researchers have also found females had a lower MAP than men, even when the subjects were matched for absolute strength of the target muscles (Hunter & Enoka 2001; Thompson et al. 2007; Hunter et al., 2004). Possible explanations for the differences in the MAP responses between males and females are, 1) less muscle mass involvement in females (at same relative work load), 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, 3) different utilization of substrates (foodstuffs such as carbohydrates and fats), and 4) lower metabolite byproduct production (i.e., hydrogen ions, carbon dioxide, and lactate) between men and women (Hunter & Enoka 2001; Hicks et al. 2001; Hunter et al. 2004).

Some studies have tried to control for the blood flow constriction to the working muscles by placing the muscles in ischemic (constriction of blood vessels) conditions. 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 blood conditions 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 (Russ and Kent-Braun, 2003). 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 mmHg 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 fatiguability than men during quadriceps contraction.

Substrate Utilization and Estrogen
Differences between males and females exist in metabolism. Several studies show that males have greater glycolytic (carbohydrate) capacity and rely more on glycolytic pathways, while females rely more on oxidative phosphorylation (fat and carbohydrate) during sustained cardiovascular exercise (Braun & Horton 2001; Tarnopolsky et al. 1990). Women have been shown to have a lower respiratory exchange ratio (a laboratory measurement during aerobic exercise to determine foodstuffs being used for fuel) during continuous aerobic exercise, 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 women have lower activities of the common glycolytic enzymes (phosphofructokinase, pyruvate kinase, and lactate dehydrogenase) that break down carbohydrate, possibly decreasing their potential of glycolytic pathway production of energy (Tarnopolsky 2008). Thus, it is proposed that females will have greater utilization 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, carbohydrates), especially during long endurance exercise (Tarnopolsky 2008). It has been shown that 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).

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 increasing the recruitment of additional non-fatigued motor units, recruiting larger motor units, and increasing the firing rate of activated motor units (Thompson et al. 2007; Yoon et al. 2007). Electromyography 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 will affect the fatigue rate of that muscle group. Some investigators have found a difference in the way muscles were utilized, and that the recruitment patterns are different in men than in women; whereas others have found no difference between the sexes. However, during higher intensity exercise (&Mac179; 80% maximal voluntary contraction) there is usually no difference between the sexes in the muscle activation or the recruitment of muscles (Yoon et al. 2007). This area of neuromuscular activation and fatigue factors needs to be elucidated with more gender comparison studies.

Practical Observations to the Personal Trainer and Fitness Professional
Some fascinating physiological and metabolic fatiguability characteristics exist in the submaximal muscle force producing capacity of women. 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 due to the untrained state of their musculature. However, as many personal trainers and fitness professionals have observed in their professional experience, these physiological phenomenon help to scientifically explain why moderately to highly trained females are very capable of doing more advanced multiple-set and multiple-exercise program designs, as well as completing more frequent resistance training workouts during the week.