There are three main types of muscular contraction associated with strength and conditioning training. These types of contractions are concentric, eccentric and isometric. Each type of contraction has a place in training, and there are positive and negative aspects for each. While eccentric contractions provide the highest amount of tension during execution, isometric contractions come in second place, with concentric contractions trailing at third place (Bompa & Buzzichelli, 2015).
There are numerous training programs that are designed around each of the three types of contractions. Numerous research studies have analyzed many aspects of each type of contraction. Much debate has arisen over which type of training and contractions are the best. The data shows that each contraction type has strengths and limitations. A well rounded program should maximize the benefits of each, while avoiding the limitations. Isometric contractions are usually used the least, and are often misunderstood. The research shows that isometric contractions can be used to great benefit when applied properly (Bompa & Buzzichelli, 2015).
The concept behind isometric training revolves around two main methods. The first method is achieving an isometric contraction, by trying to lift a heavy weight that is beyond the muscle’s capability. The second method focuses on trying to move an inanimate or unmovable object. Both techniques result in a static contraction, where the length of the muscle does not change. Isometric training has been around for quite some time. One of the popular strongmen of older times was Alexander Zass. He was a prisoner of war during World War I. During his captivity, he worked on his strength by performing isometric contractions, against the steel bars and chains of his cell. He later went on to sell his isometric training program through mail order courses (Read, 2015).
Some of the first recorded research studies that outlined the benefits of isometric training, occurred in the 1950’s and 1960’s. During this time period, isometric training gained in popularity, and numerous training programs were created. Programs were designed for athletes, fighters, bodybuilders, and strongmen. Programs were even developed for the average, non-athletic citizen (Raizis, 2017).
While isometric training attained its peak in popularity during the 1960’s, it soon faded from the spotlight, and was replaced by many other fitness fads and trends. Some notable fitness icons that promoted isometric training included the great martial arts star, Bruce Lee, and fitness guru, Jack Lalanne. Bruce Lee was well known for a unique isometric exercise, where he attempted to move a steel bar, which was permanently attached to a squat rack. While he obviously never moved the steel bar, he did become so strong, that he put a curved bend in it (Read, 2015).
Isometric training has little functional use, as it is stationary in nature. But it does provide considerable gains in strength. It is also very effective for trunk, core, and abdominal stabilization, and strength. Positive results in rehabilitation therapy have also been shown with isometric training. Because the nature of the contraction is stationary, people who are recovering from skeletal, and bone related injuries can benefit from isometric training (Raizis, 2017).
Some of the other benefits of isometric training include the minimal time, equipment, and space required to perform it. Isometric training is also capable of considerable motor unit recruitment and activation. Many believe that isometric training is one of the more superior methods of motor unit recruitment. The earlier research studies showed that a single session of isometric training per day, at seventy five percent of maximal output, over ten weeks, raised strength levels by up to five percent, per week. Other research concluded that isometric training caused isometric strength gains to continue, even after the training protocol had concluded. Some of the studies outlined that isometric contractions of only six seconds could cause increases in strength, equal to a much larger number of dynamic isotonic contractions. The studies also suggested that in certain circumstances, ten minutes of isometric training could be the equivalent of sixty minutes of regular resistance training (Barry, 2015).
No special equipment is need for isometric training. During the 1960’s, when isometric training was very popular, many companies developed training devices specifically for isometric contractions. This never took off and isometric specific equipment quickly disappeared. Today, isometric training can utilize existing equipment, or body weight. Standard squat racks with pins and safety bars, can be utilized in a number of ways, with common items such as barbells. There are dozens of ways to perform isometric training with body weight alone. An old exercise that was popular once upon a time, simply had people put their hands together and apply force, for a period of time. Large spaces are not required for isometric training. It can be done in a standing, seated, prone, or supine position (Barry, 2015).
Isometric training does have its limitations. When isometric training is performed as the main training method, muscular elasticity, coordination and speed can be compromised. Critics of isometric training often say that this method of training only produces strength gains at specific joint angles, and is therefore limited. Other research has shown that this is not entirely accurate. Isometric training has been shown to produce strength increases for up to fifteen degrees, on each side of the joint angle that was trained (Kubo, Ishigaki, & Ikebukuro, 2017).
With isometric training, most people do not experience the common post workout fatigue, and soreness that accompanies regular resistance training. However, isometric training is said to produce a very deceptive, central nervous system fatigue, which can negatively impact performance. For this reason, supporters of isometric training recommend that training sessions be limited to about ten minutes. Adequate recovery time is just as important with isometric training, as it is with other methods (Kubo, Ishigaki, & Ikebukuro, 2017).
There are certain health risks associated with isometric training. Isometric training is not recommended for people with heart, blood pressure, or circulation problems. During isometric contractions, blood flow to the muscle is temporarily halted, which increases blood pressure. This could be a serious problem for certain people. Isometric training also dramatically increases intrathoracic pressure as contractions are conducted while breathing is momentarily suspended. This increase in intrathoracic pressure could cause medical concerns for people with certain conditions. Medical clearance is recommended for those with blood pressure related conditions, before beginning any isometric training. Some current research does indicate that isometric training, performed under certain conditions, could potentially help to lower blood pressure (Millar, McGowan, Cornelissen, Araujo, & Swaine, 2014).
The recommended method of incorporating isometric training, into a modern strength and conditioning program, uses the functional isometric contraction. This method is used in conjunction with weight training. Significant strength gains can be achieved by using functional isometric contractions. These would be utilized throughout various joint angles, or sticking points, in Olympic weightlifting, powerlifting and other resistance exercises. Combining isotonic and isometric training together, in a balanced strength program, can provide optimal results for the athlete (Millar, McGowan, Cornelissen, Araujo, & Swaine, 2014).
During my twenty years in the Army, misinformation regarding strength training was abundant. The military is transfixed on aerobic training. For many years, the military has promoted a singular concept in which all movement revolves around aerobic fitness. Science never seemed to enter the picture, until recently. Misinformation in the military is generally handed down from generations of leaders telling subordinates, of how it was done in the past. There was absolutely no knowledge of metabolic energy pathways, or of the need for anaerobic training to develop strength and power. After completing hundreds of long distance runs and countless high repetitions of exercises, the realization that performance transference existed, resonated with me to this day. Five mile runs did not provide any cardiorespiratory benefits, when the actual tasks involved picking up heavy objects, jumping over obstacles, and sprinting to a position, that provided cover and concealment (Bompa & Buzzichelli, 2015).
Despite the fact that long distance cardio training did not enhance performance, for tasks requiring anaerobic energy pathways, the guidance was to simply run more. During the times that strength training could be conducted in a gym, the emphasis was on light resistance and high repetitions. The well indoctrinated, aerobic based leaders would tell myths that heavy weight training would cause excessive mass, slow a soldier down, and get them killed. There was no knowledge of the benefits of strength and power development. The reality was that the soldiers needed strength and power development, for many of the same reasons that athletes do. Soldiers must be able to sprint, from one position to another, while under direct fire. There is a strength requirement when soldiers must physically pick up wounded comrades, and move them to safety. Negotiating obstacles, and maneuvering through difficult terrain, can require the use of all metabolic energy pathways. But yet the focus remained steadfast. Run more and do more repetitions to maximize endurance. Picking up a wounded soldier, with body armor and gear on, is anaerobic. No amount of five mile runs will assist with that task. Because of the excessive aerobic training, overtraining symptoms became common. Sarcopenia, osteopenia, and the skinny fat syndrome were present in soldiers, across the entire Army. The lack of power and strength training not only hindered performance, but caused many health issues as well (Mientka, 2013).
When power and strength training was added into the programming, along with aerobic training, performance increased dramatically. Physical fitness test scores were raised, and actual job related tasks were performed at much higher levels. By incorporating the different metabolic energy pathways into the fitness program, the limitations of the previous programming were overcome. The aerobic based leaders continued to dismiss these science based results. The modern military is finally accepting the science, and new programs addressing power and strength are being developed. Sprinting, plyometrics, and heavy resistance training are invaluable assets to any military program. Hopefully, the days of being told to just run more, are coming to an end (Stevens, 2017).
Bompa, T.O., & Buzzichelli, C.A. (2015). Periodization training for sports (3rd ed.). Champaign, IL: Human Kinetics.
Anaerobic and aerobic training both result in specific, acute, and chronic adaptations. Acute adaptations to both anaerobic and aerobic training include responses, and changes, that occur during and shortly after the training. Chronic adaptations to both anaerobic and aerobic training include changes in the body that occur after numerous training sessions. These changes are longer lasting and continue on after the training sessions. While both anaerobic and aerobic training produce acute and chronic adaptations, the changes between the two types of training are very different in nature. The differences between the adaptations are largely influenced by the metabolic energy pathways used by each type of training (Coburn & Malek, 2012).
The results of anaerobic training concentrate around short duration, high power events, such as sprinting, plyometric jumps, and resistance exercises. The metabolic energy pathways used by anaerobic training include the phosphagen system and glycolysis. Acute adaptations of anaerobic training include muscular, neurological, and endocrine changes. Neurological responses include increases in motor unit recruitment and EMG amplitude. Muscular changes involve increases in hydrogen ion concentrations, ammonia levels, and inorganic phosphate concentrations. There is usually a slight decrease or no change to ATP concentrations. CP and glycogen concentrations decrease. Endocrine changes include increases in the concentrations of epinephrine, cortisol, testosterone, and growth hormone (Pike, 2015).
Chronic adaptations to anaerobic training include changes to muscle fiber characteristics, muscle enzymes, muscle substrates, and muscle performance. Structural, body composition, and neurological changes also occur. The changes to muscle fibers include increases in type I & II CSA fibers, and type IIa fibers. There are decreases in type IIx fibers and usually no change to type I fibers. Increases in muscle size, and the number of muscle fibers are also common. Structural changes include increases in bone density, bone mass, and connective tissue strength. Muscle enzyme changes include increases in the absolute levels of glycolytic enzymes, and phosphagen system enzymes. The concentrations of glycolytic enzymes, and phosphagen system enzymes may also increase. Muscle substrate changes include increases in the absolute levels of ATP, and CP. The concentrations of ATP, and CP may also increase. There are usually decreases in ATP, CP, and lactate during exercise. Muscle performance changes include increases in muscle strength, power, and endurance. Body composition changes include a decrease in body fat percentage, with an increase in fat free mass. The metabolic rate usually increases as well. Neurological changes include a decrease in cocontraction, with an increase in motor unit firing rate. There is usually an increase in EMG amplitude during maximal voluntary contraction, and motor unit recruitment. Additional changes in the endocrine, immune, and cardiorespiratory systems promote an increase in force, velocity, and power capabilities (Fitzherbet, 2012).
Aerobic endurance training causes the body to respond to stressors, through a variety of alterations and changes, in numerous physiological processes and systems. The aerobic system is suited for activities that are normally longer in duration, and less intense, than those that require the anaerobic system. Distance running, mud runs, marathons, and triathlons are some examples of activities requiring the aerobic system. Intensity, frequency and duration are key variables that impact the response of the aerobic system. Acute adaptations of aerobic training include cardiovascular, respiratory, metabolic, and endocrine changes. Cardiovascular changes include increases in heart rate, stroke volume, cardiac output, blood flow to coronary vasculature, skeletal muscle blood flow, mean arterial pressure, hematocrit, and systolic blood pressure. Other adaptations also include decreases in splanchnic blood flow, plasma volume, and total peripheral resistance. Respiratory changes include increases in pulmonary minute ventilation, breathing rate, tidal volume, and the respiratory exchange ratio. Metabolic changes include increases in oxygen consumption, blood lactate, and arteriovenous oxygen difference. There is a decrease in blood PH levels. Endocrine changes include increases in glucagon, catecholamines, and growth hormone. There is a decrease in insulin. Cortisol levels decrease during low to moderate intensity exercise, and increase during moderate to high intensity exercise (Murphy, 2015).
Chronic adaptations to aerobic training include changes to the respiratory system, blood, heart, muscle, bones, metabolism, body composition, and performance. Changes to the respiratory system include increases in respiratory muscle aerobic enzymes, and ventilatory muscle endurance. Changes to the blood include increases in blood, plasma, and red blood cell volumes. Heart changes include increases in coronary arteriole densities, and diameters, left ventricular muscle thickness, and end-diastolic chamber diameter. Sometimes there is also an increase in myocardial capillary density. Metabolism changes include an increase in lactate threshold. Muscle changes include increases in capillary density, myoglobin, mitochondria density, oxidative enzymes, triglyceride stores, and glycogen stores. Sometimes there is an increase in type I fiber cross-sectional areas. Usually, there are no changes to the muscle cross-sectional areas, type IIa, and type IIx fiber cross-sectional areas. Normally, there are no changes or an increase in bone mineral density. Body composition changes include decreases in body fat percentage, fat mass, and overall body mass. There are usually no changes to fat free mass. Performance changes include an increase in cardiorespiratory endurance. There are usually no changes in muscular strength, anaerobic power, vertical jump, and sprint speed (Lewis, 2013).
In order to isolate and enhance certain adaptations to anaerobic and aerobic training, specific exercise programs can be designed. Lactate threshold training is an example of a specific style of training that benefits aerobic performance. Conducting specific training to increase the lactate threshold causes further adaptations to the athlete’s metabolism. Raising an athlete’s lactate threshold allow the body to move further, at a higher intensity level, for longer periods of time. The increased ability of the athlete to maximize lactate clearance increases performance dramatically. A variety of exercise protocols exist where intensity and distance are manipulated during each training session. Over a period of time, these specific adaptions increase the lactate threshold and clearance capability. This ultimately improves aerobic performance (Messonnier, Emhoff, Fattor, Horning, Carlson, & Brooks, 2013).
To isolate and enhance certain, specific adaptations to anaerobic training, specialized plyometric workouts can be designed. This training maximizes the benefits of the stretch shortening cycle. By using different single and double leg plyometric exercises, in a variety of repetition and set schemes, with varying intensity levels, numerous physiological adaptations can be enhanced. This style of specific training has been shown to improve overall power, strength, stored elastic energy, stretch reﬂex response, motor unit recruitment, muscle fiber size, rates of force production, inhibition of antagonist muscles, activation, and cocontraction of synergistic muscles, and increased Type I and Type II muscle ﬁber area. By conducting plyometric training numerous, specific, anaerobic adaptations are isolated and enhanced. This improves overall anaerobic performance as well (Luebbers, Potteiger, Hulver, Hyfault, Carper, & Lockwood, 2003).
Coburn, J.W., & Malek, M.H. (2012). NSCA’s essentials of personal training (2nd ed.). Champaign, IL: Human Kinetics.
The human body utilizes three metabolic energy systems to replenish adenosine triphosphate (ATP). ATP provides energy for all movements, exercises, and muscular activity. These energy systems include the phosphagen system, the glycolysis system, and the oxidative system. The phosphagen system, and the glycolysis system are anaerobic, and do not require oxygen. The glycolysis system is subdivided into two sub systems, which are fast glycolysis and slow glycolysis. The oxidative system is aerobic and requires oxygen. The three macronutrients, proteins, fats, and carbohydrates, are the main food sources for energy. Carbohydrates are the only macronutrient that can be utilized for energy, without oxygen. All physical movements and exercises require the use of one or more of the three energy systems. The type and duration of the movement, or exercise determines which energy system will be utilized (Coburn & Malek, 2012).
For short duration, high intensity, explosive, power movements, the phosphagen system is utilized. The phosphagen system is used for about the first ten seconds of activity. Examples of activities that rely on the phosphagen system include short sprints, plyometric actions such as box jumps, and powerful actions such as an Olympic lift. Even if the physical activity lasts longer than ten seconds, the phosphagen system acts first. ATP and creatine phosphate are the primary elements used by the phosphagen system. The enzymes myosin adenosine triphosphatase (ATPase) and creatine kinase play a role in the breakdown, and regeneration of ATP. The ATP is broken down to release its energy. Myosin ATPase facilitates the breakdown of the ATP into adenosine diphosphate (ADP) and phosphate. This catabolic action releases the energy from the ATP so that it can be utilized by the body. Once this occurs, the ATP must be regenerated. The adenosine diphosphate (ADP) levels rise, which activates the creatine kinase. The creatine kinase breaks down the creatine phosphate, which provides a phosphate group to combine with the ADP. This anabolic sequence produces ATP. This cycle repeats rapidly, and provides a high rate of energy, for a very short duration (Karp, 2009).
As physical exertion moves beyond the first ten seconds, the glycolysis system activates. Glycolysis breaks down carbohydrates from blood glucose, and stored glycogen to form ATP. The glycolysis system provides energy for high intensity activity, which lasts up to two minutes. For intense activities lasting for about thirty seconds, fast glycolysis is used. With fast glycolysis, pyruvate is converted to lactate, to produce ATP. The lactate can be then sent to the liver to be converted back into glucose through the Cori cycle. As the activity goes beyond thirty seconds, slow glycolysis begins to take over. With slow glycolysis, the pyruvate is sent to the mitochondria, through the Krebs cycle, to produce more ATP. This occurs from approximately thirty seconds to two minutes of intense exercise. The intensity of the activity or exercise will decline as the sequence moves through the phosphagen system, to fast glycolysis, and on to slow glycolysis. Examples of activities that could utilize the glycolysis system include a four hundred yard run, a high repetition set of a barbell exercise, or a full court press in basketball (Kelso, 2017).
At approximately the two minute mark and beyond, the oxidative system takes over from the slow glycolysis system. This is where the activity switches from anaerobic to aerobic, and oxygen is required. The oxidative system utilizes both carbohydrates and fats to fuel ATP production. Protein is not usually metabolized as fuel, unless the activity lasts for over ninety minutes. With the oxidative system, ATP production can occur through the Krebs cycle, electron transport chain, and beta oxidation. In addition to being used to produce ATP for longer, slower activities, the oxidative system also works while at rest. While at rest, fats are predominantly used as fuel. Once activity begins, it switches to carbohydrates. Once glucose levels start to become depleted, the system reverts back to fats. As the glycolysis system fades and the oxidative system takes over, the pyruvate is sent to the mitochondria, and converted into acetylCoA. The acetylCoA is then sent through the Krebs cycle for more ATP production. As glucose levels decline, fats are metabolized through the electron transport chain, and beta oxidation process, to be converted to acetylCoA. The acetylCoA then enters the Krebs cycle to produce more ATP. If the activity is very long in duration, protein can assist in energy production through gluconeogenesis and the Krebs cycle. The protein is broken down into its amino acids, which are then either converted to glucose or acetylCoA. Protein breakdown is usually minimal, as glucose and fats are normally present in sufficient quantities. for most activities. Examples of activities utilizing the oxidative system include rest, medium to long distance runs, triathlons, and manual labor during a work day (Pegg, 2013).
Coburn, J.W., & Malek, M.H. (2012). NSCA’s essentials of personal training (2nd ed.). Champaign, IL: Human Kinetics.
Muscular contraction begins with the nervous system sending a stimulus to the muscle fibers. This stimulus occurs at the neuromuscular junction. There is one neuromuscular junction for each muscle fiber. The neuromuscular junction contains structures that include the axon terminal of the neuron, the motor endplate, and the synaptic cleft, or neuromuscular cleft. From this point, the actions of the sliding filament theory begin. The actin and myosin filaments slide past each other, causing the muscle to shorten or lengthen. The filaments do not change in length during this action. Excitatory neurotransmitter acetylcholine (ACh), is released at the neuromuscular junction, after an action potential passes along the length of a neuron. ACh is released into the synaptic cleft between the axon terminal of the neuron and the muscle fiber. This occurs in direct response to the action potential (Coburn & Malek, 2012).
From that point, the Ach binds with ACh receptors, on the motor endplate of the muscle fiber. This occurs after the Ach moves across the synaptic cleft. This causes another action potential to be created. This new action potential moves along the sarcolemma of the muscle fiber. T-tubules are utilized by this action potential to travel to the interior of the muscle fiber. This movement causes the release of stored calcium from the sarcoplasmic reticulum. The calcium is released into the sarcoplasm. Once in the sarcoplasm, the calcium moves to the troponin molecules. The calcium then binds with the troponin molecules. These the troponin molecules are located along the length of the actin filaments. The shape of the troponin changes after the calcium binds to it. Tropomyosin is attached to the troponin. This change in shape causes the tropomyosin to expose the binding sites on actin, to the myosin head (Krans, 2010).
The exposed binding sites on the actin are able to attach to the myosin, forming a cross bridge. This myosin head of attachment pulls the actin filament toward the center of the sarcomere. The effort of the myosin pulling on the actin depletes the energy of the myosin. The depleted myosin must then detach from the cross bridge and reenergize itself. This requires a new adenosine triphosphate (ATP) molecule to be bound to the myosin. Once the ATP molecule is bound to the myosin, it can then detach and energize itself. The energizing of the myosin comes from the enzyme myosin adenosine triphosphatase (ATPase). The ATPase splits the ATP molecule. This energizes the myosin and allows for the cross bridge sequence to occur again. This sequence can continue as long as the muscle fiber is being stimulated to contract, by its motor neuron. The myosin pulling the actin toward the center of the sarcomere shortens the muscle. This process is the muscle contracting. The success or failure of this sequence is determined the external forces pulling against the cross bridge (Szent-Györgyi, 2004).
There are three basic types of contractions that can occur when the sliding filament theory is activated. Muscle contractions are usually pulling against an external force such as a barbell. When the myosin is pulling the actin through the cross bridge, the resistance determines the outcome. The contraction generates more force than the resistance, causing a concentric contraction. The external force is greater than the contraction force, causing a lengthening of the muscle. This is an eccentric contraction. The contraction force is equal to the resistance force, which stalls movement. This is an isometric contraction. The muscle will always attempt to shorten as the myosin pulls on the actin. The resistance level will cause one of the three types of contractions to occur. ATP is the primary fuel source for this sequence (Lefkowith, 2014).
Coburn, J.W., & Malek, M.H. (2012). NSCA’s essentials of personal training (2nd ed.). Champaign, IL: Human Kinetics.