The fitness industry may arguably be one of the fastest growing occupations in the United States. A recent news release from IDEA has noted a significant growth rate in the industry. Interestingly, the origin of personal training is undocumented and anecdotal at best.
From the mid 1980s to the present, the wealth of technology designed to ease the worker’s job and the decreased mandatory physical education in schools has begun to take its toll on the public. In 1985 the International Obesity Task Force (IOTF) deemed the prevalence of obesity an epidemic. Today approximately one third (33.4 percent) of adults are estimated to be obese. Meanwhile, daily activity levels continue to decline. People are working longer hours, moving less and no longer spending as much of their free time engaged in physical activity.
This new technological environment produces more inactive and non-functional people. According to numbers from the Bureau of Labor Statistics (1983 vs. 1998), it is apparent that more people today are spending time in office related jobs and more hours at work (three percent increase). This lends itself to more sitting and less activity at work. Furthermore, most jobs today, as opposed to 15 or 20 years ago, are augmented with much more automation. This also leads to a decrease in daily functional activity, in turn leading to dysfunction and increased incidents of injury when physical activity is resumed.
Evidence of Dysfunction and Increased Injury
Research supports the concept that decreased activity may lead to dysfunction and ultimately injury. Some of the major topics reviewed include LBP, knee injuries, chronic diseases in the adult population and musculoskeletal injuries.
Low Back Pain (LBP)
In 1997, a study was conducted that looked at the epidemiology of low back pain (LBP) in low and middle-income countries around the world.
It was hypothesized that lower income rural countries would exhibit higher rates of low back pain due to harder physical labor versus higher income countries, such as the United States, where urbanized office work predominated. Contrary to the hypothesis, however, it was noted that a higher incidence of low back pain was found in countries with higher income levels (urban, industrialized, office work) versus rural lower income countries (hard physical labor). Further, it was demonstrated that low back pain was significantly more predominant among workers in an enclosed workspace (i.e., office) than in the rural lower income countries. Thus it was concluded that low back pain may not be a result of hard physical labor and that low back pain is rising with the progression of urbanization and industrialization.
Low back pain is also one the major forms of musculoskeletal degeneration seen in the adult population as it effects nearly 80 percent of all adults.
Other forms of degeneration seen in the elderly include arthritis and osteoporosis. It has been suggested that these degenerative conditions may result from a loss in muscle mass and strength due to disuse and not necessarily the result of age.
The incidence of knee injuries is also a concern. It has been shown that an estimated 80 to 100,000 anterior cruciate ligament (ACL) injuries occur annually in the United States in the general population and approximately 70 percent of these are non-contact injuries. The prime ages for an ACL injury occur between 15 and 25 years of age. Further, it has been shown that ACL injuries have a strong correlation to acquiring arthritis in the affected knee. It is also suggested that enhancing the lack of neuromuscular stabilization (body control) may alleviate this high incidence of non-contact injuries. This comes as no surprise when you take into account the decreased activity of teenagers as evident by the incidence of obesity, the lack and mandatory physical education in schools and the abundance of automation and technology (i.e., video games) seen in the American home.
Chronic Diseases in the Adult Population
Physical activity has been demonstrated to reduce the risk of chronic disease related to lifestyle such as increased triglyceride and cholesterol levels, obesity, glucose tolerance and high blood pressure, coronary heart disease and strokes. More importantly, however, is that some research indicates that discontinuing or significantly decreasing physical activity can actually lead to an increased risk of lifestyle disease.
In 1988, more than 12 percent of the American population suffered a musculoskeletal impairment. More than half of these (51.7 percent) were spinal or back related (shoulders and lower extremity comprised 11 percent). These injuries resulted in over 60 million days spent in bed and the numbers continue to rise into the new millennium.
Furthermore, it’s been noted that 43 percent of work related injuries are sprains and strains with over 60 percent involving the trunk. These work-related injuries comprised more than 39 million days of restricted activity, or approximately nine days per back episode. The total monetary value of these musculoskeletal injuries was estimated at approximately 120 billion dollars.
At least 80 percent of all Americans will suffer from low back pain at some time in their life. Other researchers have shown that men who spend over half their workday sitting in a car have a 300 percent increased chance of disc herniation. It has also been noted that unnatural posture due to improper sitting will result in increased neck, mid and lower back, shoulder and leg pain.
Musculoskeletal disorders can greatly effect our quality of life, and thus it has become much more important in today’s society to focus on our health and well being. Many individuals do realize the need for exercise. In fact, the number of people becoming involved in recreation and leisure activities has increased over the last two decades. Ironically, however, these numbers parallel an increase in injuries to the musculoskeletal system.
Other factors associated with etiology of dysfunction and injuries have been investigated. Factors such as physical fitness, over training, skeletal abnormalities, technique and warm up have been suggested. Also the importance of ensuring that all components of the body are properly prepared for the stress placed on them regardless of their perceived contribution. Furthermore, training programs and apparatus used to condition the musculoskeletal system often neglect essential parts of the body such as the core (hips, upper and lower back and neck). This results in a weakened kinetic chain.
Simply, the extent to which we condition our musculoskeletal system will directly influence our risk of injury. The less conditioned our musculoskeletal system is the higher the risk of injury. Therefore, as our daily lives include less physical activity, the less prepared we will be to partake in recreational and leisure activities such as resistance training, weekend sports or just simply playing on the playground.
Current Training Programs
Research has been conducted on the effectiveness of training programs on sedentary adults. It has been shown that the intensity required by a sedentary person who is trying to improve his cardiorespiratory fitness level might put him in a state of excessive overload. It has also been shown that in the initial six weeks of training, there was a 50 to 90 percent injury rate. This occurred in training programs specifically designed to minimize risk of injury. Needless to say, they concluded that the musculoskeletal system is very easily over trained when it is deconditioned.
It is important to note that “deconditioned” does not simply mean the person is out of breath upon climbing a flight of stairs or that he is overweight. Deconditioned refers to a state in which a person has muscle imbalances, decreased flexibility and/or a lack of core and joint stability. All of these conditions can greatly affect the ability of the human body to produce proper movement and can eventually lead to injury.
Many of these injuries occur in the transverse plane. This is evident in the increased incidence in ACL injuries, especially among females. Statistically, this is not surprising when it is noted that most training programs do not emphasize multi-planar movements (movement in different degrees of the sagittal, frontal and transverse planes) through the full muscle action spectrum (concentric, eccentric and isometric) in a proprioceptively enriched environment . A proprioceptively enriched environment is an environment that challenges the internal balance and stabilization mechanisms of the body. Examples of this would include performing a dumbbell (DB) chest press on a stability ball or simply performing a single leg squat.
Ironically, it is the fitness educators who are at fault for the industry’s inability to meet the needs of today’s client. At worst, we should have recognized the trend toward non functional living and taken measures to stay one step ahead. At best, we should have recognized the decrease in functionality of our clients and began to address this need immediately. We tend to exclusively emphasize the cosmetic appearance of our appendages at the expense of providing a structurally sound kinetic chain.
Thus we find ourselves in a new state of training rooted in structural disarray where the client of today has been physically molded by furniture, gravity and inactivity. When combined with the increased number of office jobs and the continual decrease in everyday activity, we have the postural deficiency seen in people today. Simply, today’s client is not adept to function initially at a level that was seen 15 to 20 years ago. Therefore, the current training programs seen in today’s literature cannot be the same as the programs of the past.
We need to address this new clientele by restructuring our thought process. Our new mind-set should cater toward creating programs that address the functional capacity of the client as part of a proper individualized program. This will best be achieved by introducing an integrated approach to designing training programs. In other words, the training program must be designed with consideration toward the person, the environment and the tasks to be performed. It was for this reason the Optimum Performance Training (OPT) model was created.
Optimum Performance Training - An Integrated Training Model
Integrated training is a concept that applies all forms of training, such as integrated flexibility training, integrated cardiorespiratory training, neuromuscular stabilization (balance), core stabilization and reactive neuromuscular training (power) and integrated strength training in a progressive system. The model conceptualizes a training program for a society with more structural imbalance and susceptibility to injury than ever before. It is a process of programming that systematically progresses any client to any goal. Depicted in Figure 1, the Optimum Performance Training model is built upon a foundation of scientific training principles.
Figure 1.The Optimum Performance Training Model
The Optimum Performance Training programming scheme is based on the scientific rationale of human movement science. Each stage has a designated purpose that provides the client with a systematic approach for optimum progression toward their individual goals as well as addressing their specific needs. Now more than ever it is imperative that the trainer fully understand all of the proper scientific components of programming as well as the right order those components must be addressed in order to allow their client to achieve success.
As mentioned above, the Optimum Performance Training model is divided into three different stages. These include the Stabilization, Strength and Power. Each stage is built upon and dependent on the prior stage. Within each stage there exists specific phases of training that progress a client through the stage. It is imperative that fitness professionals understand the scientific rationale behind each stage in order to properly utilize the model.
The Stabilization Stage consists of two phases of training, Corrective Exercise Training (CET) and Integrated Stabilization Training (IST). The major components that make up corrective exercise training (CET) and integrated stabilization training (IST) are corrective flexibility, core stabilization training, neuromuscular stabilization training (balance), some forms of reactive neuromuscular training (power) and integrated strength training. The progression for this stage of training is proprioceptively based. This means that difficulty is increased by introducing more challenge to the balance and stabilization systems of the body versus simply increasing the load.
Remember, progression for the Stabilization Stage is proprioception, meaning that intensity is increased by challenging the balance and stabilization systems of the body versus simply increasing the load. This provides more sensory input to the nervous system creating better movement responses, and it burns more calories than traditional forms of training.
Figure 2. Components of the Stabilization Stage
The main focus of the Stabilization Stage is to increase stabilization strength and develop optimal neuromuscular efficiency. Stabilization strength is the ability of the stabilizing muscles to provide dynamic joint stabilization and postural equilibrium during functional activities. Neuromuscular efficiency is the ability of the neuromuscular system to allow agonists, antagonists, stabilizers and neutralizers to work synergistically to produce force (concentric), reduce force (eccentric) and dynamically stabilize (isometric) the entire kinetic chain in all three planes of motion.
It has already been pointed out that the typical client entering the fitness world today is not as adept to withstand the traditional forms of programming due to their decreased functional capacity. Therefore, any alterations in the alignment of the kinetic chain will effect the quality of movement and perpetuate faulty movement patterns. These faulty movement patterns will in turn alter the activation sequence or firing order of the muscles involved, disturbing specific functional movement patterns and decreasing neuromuscular efficiency.
If a client enters the fitness environment with active pain, following a surgery or is under the care of a medical practitioner, they may need to start in Phase 1, Corrective Exercise Training. First and foremost, the fitness professional should refer the client to a medical practitioner and consult with the current practitioner prior to initiating a workout program. Following proper advice from the consulting medical practitioner, the fitness professional will initially focus on corrective exercise strategies that are aimed at correcting muscular imbalances, increasing flexibility and extensibility as well as creating joint and postural stabilization. This is achieved by utilizing a proprioceptively enriched environment, proper flexibility techniques followed by balance and core stabilization exercises to re-educate the neuromuscular system. Proper Correct ive Exercise Training prepares and educates the kinetic chain for Integrated Stabilization Training (Phase 2).
Stabilization and neuromuscular efficiency can only be obtained by having the appropriate combination of alignment and stability strength to maintain that alignment. Phase 2 of the Optimum Performance Training model, Integrated Stabilization Training, provides the needed stimuli to acquire stabilization and neuromuscular efficiency through the use of proprioceptively enriched exercises and progressions. This is where most typical clients will begin. The goal is to increase their joint and postural stabilization. This must be done prior to advancing into the Strength and Power stages. Research has shown that inefficient stabilization can lead to altered force production in muscles, increased stress at the joints, tissue overload and eventually injury.
In the Stabilization Stage, the client begins in the most unstable environment that they can safely manage. This forces their intrinsic stabilization mechanisms to produce joint and postural stability, allowing them to enhance their stabilization strength and neuromuscular efficiency. For example, instead of starting a client off on a seated chest press machine where the machine dictates the path of motion as well as joint and postural stability, the client may start by performing a dumbbell chest press lying supine on a stability ball or a standing cable chest press. In these examples, the client must not only control their joint stability but their postural stability as well.
Stabilization training not only addresses needed structural deficiencies, it also provides superior stimuli for altering body composition. By performing exercises in a proprioceptively enriched environment (unstable), the body is forced to recruit more muscles to stabilize itself. In doing so, more calories are expended.
The Strength Stage of training follows the successful completion of the Stabilization Stage. In this stage of the Optimum Performance Training model, higher levels of balance and core stabilization are warranted and Stabilization Equivalent Training (SET) replaces integrated stabilization training (IST). The emphasis in the Strength Stage is on dynamic joint stabilization, stabilization endurance and improving intra-muscular and inter-muscular coordination. Dynamic joint stabilization is the ability of the stabilizing muscles of a joint to produce optimum stabilization during functional, multi-planar movements. Stabilization endurance is the ability of the stabilization mechanisms of the kinetic chain to sustain proper levels of stabilization to allow for prolonged neuromuscular efficiency. I ntra-muscular coordination is the ability of the neuromuscular system to allow optimal levels of motor unit recruitment and synchronization within a muscle while inter-muscular coordination is the ability of the neuromuscular system to allow all muscles to work together with proper activation and timing between them.
These concepts are achieved by performing two exercises in a “super-set” manner (back-to-back without rest) with similar joint dynamics. One exercise is more traditional and performed in a more stable environment while the other is an integrated exercise performed in a less stable more proprioceptively enriched environment. The principle behind this method is to work the prime movers predominantly with the first exercise to elicit motor unit recruitment. By immediately following with an exercise that challenges the stabilizers, neuromuscular and postural stabilization endurance and dynamic joint stabilization are produced. An example of this would be performing a DB bench press followed by push-ups on a stability ball.
Again, it must be emphasized that completing the Stabilization Stage of training is crucial for success in the Strength Stage. The Stabilization Stage prepares the kinetic chain to engage in more demanding exercise by correcting muscle imbalances, increasing flexibility and extensibility of the muscles as well as increasing the joint and postural stabilization mechanisms.
Other components of the Strength Stage are Muscular Development Training (MDT) and Maximal Strength Training (MST). These two components of training can be utilized as more special forms of training and a progression within the Strength Stage. In essence, training in this stage turns toward that which more closely begins to mimic the client’s goal. Figure 3 depicts the components of the Strength Stage.
Figure 3. Components of the Strength Stage Power
The Power Stage of the Optimum Performance Training model should only be entered upon successful completion of the two previous stages. This stage of training emphasizes the development of speed and power. This is achieved through two major phases of training termed Elastic Equivalent Training (EET) and Maximal Power Training (MPT).
The premise behind Elastic Equivalent Training is the execution of a strength exercise followed by a plyometric/power exercise of similar joint dynamics. This is to elicit maximal motor unit recruitment of fast-twitch muscle fibers and improve the rate of force production. Utilization of Maximal Power Training is a progression to produce maximal acceleration, motor unit recruitment and rate of force production.
The speed and power with which people can produce muscular actions determine successful performance in many sports as well as everyday activities. This stage of training is necessary to enhance the speed spectrum that the body is allowed to operate within. The speed with which muscles are able to exert force is dictated by the neuromuscular system. Therefore, the body will only be able to move within a set range of speed that is determined by the nervous system. Thus this stage of the Optimum Performance Training model uses high levels of balance, core stabilization and reactive training.
Figure 4. Components of the Power Stage
The uniqueness of the OPT model is that it packages scientific principles with practical application to achieve consistent results with all clients.
The next eight articles will describe and detail the necessary components of an integrated training program. They include:
- Integrated Assessments
- Integrated Flexibility Training
- Core Stabilization Training
- Neuromuscular Stabilization Training
- Reactive Neuromuscular Training
- Integrated Strength Training
- Integrated Speed and Agility Training
- Program Design
Each of these articles will provide the trainer with information regarding how each component specifically fits into the OPT model and how to realistically apply the information given.
The typical client entering the fitness world today seeking the assistance of a trainer has a lower initial functional capacity than seen in previous decades. This is due to technology and automation advances that are aimed at easing the working environment and provide entertainment (video games) as well as the decrease in mandatory physical activity such as PE in schools. The evidence lies in the increase in muscle imbalances, the decrease in flexibility and extensibility of the muscular system, the decreased stabilization strength and endurance as well as the decreased proprioceptive abilities (balance) of today’s typical client.
Fitness professionals must develop a new thought process concerning exercise programming for today’s clientele. Programming for today’s clientele must address factors such as flexibility, core, balance, power and strength. Most importantly, however, is utilizing a proper systematic progression that is based in human movement science and has been applied in the real world for a variety of populations.
- Anonymous. Bureau of Labor Statistics 1999. http://www.stats.bls.gov/news.release/osh2.nws.htm
- Anonymous. International Obesity Task Force 1999. www.iotf.org
- Anonymous. Trends in personal training. IDEA International Personal Trainer Summit 1999; Mar 5-7, Baltimore, MD.
- Borsa PA, Lephart SM, Kocher MS, Lephart SP: Functional assessment and rehabilitation of shoulder proprioception for glenohumeral instability. J Sports Rehab 1994; 3:84-104.
- Bullock-Saxton, J: Muscles and Joint: Inter-relationships with pain and movement dysfunction. Course Manual, November 1997.
- Caspersen CJ, Pereira MA, Curran KM. Changes in physical activity patterns in the United States, by sex and cross-sectional age. Med Sci Sports Exerc 2000; 32(9):1601-1609.
- Chaffin DB, Andersson GJ, Martin BJ. Occupational Biomechanics. New York: Wiley-Interscience; 1999.
- Chaitow L: Soft Tissue Manipulation. Rochester, Healing Arts Press, 1991.
- Clark M. Integrated training for the new millennium. 2000.
- Dominiguez RH: Total Body Training. Moving Force Systems. East Dundee, IL, 1982.
- Edgerton VR, Wolf S, Roy RR: Theoretical basis for patterning EMG amplitudes to assess muscle dysfunction. Med Sci Sports Exerc 28(6)744-751, 1996.
- Evans W, Rosenberg I. Biomarkers. New York: Simon and Schuster 1992.
- Gambetta V. The Gambetta method. Sarasota, FL: Gambetta Sports Training System, Inc. 1998.
- Gillquist J, Messner K. Anterior cruciate ligament reconstruction and the long term incidence of gonarthrosis. Sports Med 1999; 27:143-56.
- Griffin LY, Agel J, Albohm MJ et al. Noncontact anterior cruciate ligament injuries: risk factors and prevention strategies. J Am Acad Orthop Surg 2000; May-June 8(3):141-50.
- Guyer B, Ellers B. Childhood injuries in the United States: Mortality, morbidity, and cost. American Journal of Diseases in Children 1990: 144: 649-52.
- Hammer WI. Chapter 12. Muscle imbalance and postfacilitation stretch. In Hammer WI editor. 2nd edition. Functional soft tissue examination and treatment by manual methods. Gaithsburg, MD: Aspen Publishers, Inc.; 1999.
- Heus R, Wertheim AH, Havenith G. Human energy expenditure when walking on a moving platform. Eur J Appl Physiol Occup Physiol 1998;100(2):133-48.
- Hodges PW, Richardson CA: Inefficient muscular stabilization of the lumbar spine associated with low back pain. Spine 1996; 21(22):2640-2650.
- Hodges PW, Richardson CA: Neuromotor dysfunction of the trunk musculature in low back pain patients. In: Proceedings of the International Congress of the World Confederation of Physical Therapists, Washington, DC, 1995.
- Hurley BF, Hagberg JM. Optimizing health in older persons: Aerobic or strength training? In Holsey JO, editor. Exerc Sports Sci Rev Volume 26. Baltimore: Williams & Wilkins; 1998. p. 61-89.
- Huston LJ, Greenfiled ML, Wojtys EM. Anterior cruciate ligament injuries in the female athlete. Potential risk factors. Clin Orthop 2000; 372:50-63.
- Janda V, Vavrova M: Sensory motor stimulation video. Body control systems. Brisbane, Australia, 1990.
- Janda V: Muscle function testing. London, Butterworth, 1983.
- Janda V: Muscle weakness and inhibition in back pain syndromes. In Grieve GP; Modern Manual Therapy of the Vertebral Column. New York, Churchill Livingstone, 1986.
- Janda V: Muscles, central nervous system regulation, and back problems. In Korr IM (ed): Neurobiologic Mechanisms in Manipulative Therapy. New York, Plennum Press, 1978.
- Janda V: Physical Therapy of the cervical and thoracic spine. In Grant R (ed). New York, Churchill Livingstone, 1988.
- Jesse J: Hidden causes of injury, prevention, and correction for running athletes. The Athletic Press. Pasadena, CA, 1977.
- Jones BH, Cowan DN, Knapik J. Exercise, training, and injuries. Sports Medicine 1994; 18(3): 202-14.
- Kelsey JL, Hardy RJ. Driving motor vehicles as a risk factor for acute herniated lumbar intervertebral discs. American Journal of Epidemiology 1975; 102: 63-73.
- Kelsey JL. An epidemiological study of acute herniated lumbar discs. Rheumatoogy and Rehabilitation 1975; 14:144-45.
- Kelsey JL. An epidemiological study of the relationship between occupations and acute herniated lumbar discs. International Journal of Epidemiology 1975; 4: 197-204.
- Kiely DK, Wolf PA, Cupples LA, Beiser AS, Kannel WB. Physical activity and stroke risk: the Framingham Study. Am J Epidemiol 1994; 140:608-20.
- Larsson L, Grimby G, Karlsson J. Muscle strength and speed of movement in relation to age and muscle morphology. J Appl Physiol 1979; 46; 451-6.
- Lee IM, Hennekens CH, Berger K, Buring JE, Manson JE. Exercise and risk of stroke in male physicians. Stroke 1999; 30:1-6.
- Leon AS, Connett J. Physical activity and 10.5 year mortality in the Multiple Risk Factor Intervention Trial (MRFIT). Int J Epidemiol 1991; 20:690-7.
- Lewit K: Muscular and articular factors in movement restriction. Manual Medicine 1:83-85, 1985.
- Liebenson CL: Rehabilitation of the spine. Baltimore, Williams and Wilkins, 1996.
- O’Sullivan PE, Twomey L, Allison G, Sinclair J, Miller K, Knox J: Altered patterns of abdominal muscle activation in patients with chronic low back pain. Australian Journal of Physiotherapy 1997; 43(2):91-98.
- Ogita F, Stam RP, Tazawa HO, Toussaint HM, Hollander AP. Oxygen uptake in one-legged and two-legged exercise. Med Sci Sports Exerc 2000;32(10):1737-42.
- Paffenbarger MD, Kampert JB, LE IM, Hyde RT, Leung RW, Wing Al. Changes in physical activity and other lifeway patterns influencing longevity. Med Sci Sports Exerc 1994; 26(7):857-65.
- Praemer A, Furner S, Rice DP. Musculoskeletal conditions in the United States. Rosemont, IL: Academy of Orthopedic Surgeons; 1992.
- Prate RR, Pratt M, Blair SN, et al. Physical activity and public health. A reccomendation from the Centers of Disease Control and Prevention and the American College of Sports Medicine. JAMA 1995; 273(5):402-7.
- Sahrmann S: Diagnosis and treatment of muscle imbalances and musculoskeletal pain syndrome. Course manual. St. Louis, MO, 1997.
- Sherman SE, Agostino RBD, Silbershatz H, Kannel WB. Comparison of past versus recent physical activity in the prevention of premature death and coronary artery disease. Am Heart J 1999; 138:900-7.
- Steindler A. Kinesiology of the human body. Springfield, IL: Charles C. Thomas; 1964.
- Stephens T. Secular trends in adult physical activity: Boom or bust? Research Quarterly for Exercise and Sport 1987; 58(2): 94-105.
- Towner EL, Jarvis SN, Walsh SM, Aynsley-Green A. Measuring exposure to injury risk in schoolchildren aged 11-14. British Medical Journal 1994; 308(6926): 449-52.
- Voight M, Draovitch P: Plyometrics. In: Albert M (ed), Eccentric Muscle Training in Sports and Orthopedics, pp. 45-73. New York: Churchill Livingstone, 1991.
- Volinn E. The epidemiology of low back pain in the rest of the world. A review of surveys in low- and middle-income countries. Spine 1997 Aug; 22(15): 1747-54.
- Wannamethee SG, Sharper AG, Whincup PH, Walker M. Role of risk factors for major coronary heart disease events with increasing length of follow-up. Heart 1999; 81:374-9.
- Watkins J. Structure and Function of the musculoskeletal system. Champaign, IL: Human Kinetics; 1999.
- Wescott WL, Baechle TR. Strength training for seniors. Champaign, IL: Human Kinetics; 1999.
- Whiting WC, Zernicke RF. Biomechanics of musculoskeletal injury. Champaign, IL: Human Kinetics; 1998.
- Williford HN, Olson MS, Gauger S, Duey WJ, Blessing DL. Cardiovascular and metabolic costs of forward, backward, and lateral motion. Med Sci Sports Exerc 1998;30(9)1419-23.
- Zohar J. Preventative conditioning for maximum safety and performance. Sports Coach 1973; 42(9): 65, 113-15.