Regardless of the activity, the muscles of the skeletal system must contract for movement to take place or to prevent movements from taking place. With this in mind, understanding the type of contraction being performed and its relationship to movement and other contractions is important. Unfortunately, when it comes to the types of contractions that a muscle performs even the experts’ interpretations vary slightly. Nomenclatures associated with contractions are therefore wide and varied.
In Part 1 of this article, "Muscle Contractions, Part 1: Different Types," the different types of muscle contractions will be identified and classed, and the basic fundamentals of each discussed. In Part 2, the different types of muscle contractions will be compared to one another, while Part 3 will provide practical application guidelines.
In regards to a muscle’s length there are three widely recognized types of contractions:
- Isometric contractions,
- Concentric contractions, and
- Eccentric contractions.
An isometric (iso – same, metric – length) contraction occurs where tension is developed with no change in muscle length or joint angle (see Figure 1). As there is no change in joint angle this contraction is also known as a static contraction (Kraemer & Vingren, 2007; McArdle et al., 2006; Adams, 2005). An everyday example would be holding a cup of coffee in your hands or holding a mobile phone to your ear.
Figure 1. An isometric contraction.
A concentric contraction occurs when a muscle shortens under tension and there is a decrease in joint angle (see Figure 2). This contraction is also known as a "miometric" (mio – shorten, metric – length (Zatsiorsky & Kramer, 2006; McArdle et al., 2006; Faulkner, 2003; Faulkner et al., 1997)) contraction or "positive" work / contractions (Egger et al., 1998). As this action involves limb movement, a concentric or miometric contraction is considered part of a dynamic contraction. An everyday example would be lifting an object off the table or standing up from a chair.
Figure 2. A concentric contraction.
An eccentric contraction occurs when a muscle lengthens under tension and there is an increase in joint angle (see Figure 3). This contraction is also known as a "pliometric" (plio – lengthen / more, metric – length (Zatsiorsky & Kramer, 2006; McArdle et al., 2006; Faulkner, 2003; Faulkner et al., 1997)) contraction or "negative" work / contractions (LaStayo et al., 2003; Bompa et al., 2003; Egger et al.,1998). Since, like concentric (miometric) contractions, eccentric or pliometric contractions involve limb movement, they too are considered part of a dynamic contraction. An everyday example would be walking down stairs or sitting down into a chair.
It is important to make the distinction between the terms pliometric contraction (being a lengthening contraction) and plyometric action (being a term coined by athletic coach Fred Wilt to replace the term “jump training” (Chu, 1992)). While plyometric actions (“jump” training) employ eccentric / pliometric contractions (muscle lengthening), they also employ concentric contractions and are movement speed dependent.
Where concentric (miometric) contractions generally accelerate body segments (Tan, 2006), eccentric (pliometric) contractions play an important role in acting to absorb energy. This energy is then either dissipated and the movement decelerated, or is transferred to a concentric (miometric) contraction (Tan, 2006; LaStayo et al., 2003; Lindstedt et al., 2001), increasing contraction strength and/or acceleration potential. An example of absorbing the energy and decelerating a movement would be dropping into a squat when jumping off a step while an example of energy transfer and creating acceleration potential would be a rapid mini-squat before jumping up. (See concepts like the "stretch-shortening cycle" or the "myotatic reflex" for more information.)
Figure 3. An eccentric contraction.
With the basis for the terms concentric being as "having the same center" and of eccentric as "not having the same center," these terms, while consistent with remodeling of the heart muscle, do not accurately reflect the changes in muscle length. Consequently, Faulkner (2003, p.458) states that, "The adjectives 'concentric' and 'eccentric' are misleading and inappropriate and should not be used to describe the contractions of skeletal muscles." As such, the terms pliometric or lengthening contraction and miometric or shortening contraction may be more appropriate. Unfortunately, the terms concentric and eccentric are those commonly used in the fitness and academic fields. In order to therefore discuss these contractions in a method that is readily understood, the terms "concentric" and "eccentric" are still used.
Contractions by tension
The second muscle contraction classification relates to the type of tension employed during a contraction, rather than the changes in muscle length. This category can be subdivided into:
- Isotonic contractions, and
- Isokinetic contractions.
The translation of the term isotonic (iso – same, tonic – tension) implies that isotonic contractions are those that maintain constant tension over the full range of motion. Most texts, however, agree that this is not the case (Plowman & Smith, 2007; Zatsiorsky & Kramer, 2006; McArdle et al., 2006; Fleck & Kraemer, 2004; Nordin & Frankel, 2001; Egger et al., 1998). Put succinctly by Zatsiorsky and Kraemer (2006, p.110), "If external resistance (weight lifted) is constant, the tension exerted by a muscle varies during shortening because of factors such as the change in muscle moment arm."
Unfortunately, exercises that utilize "free weights," these being those where external resistances or loads do not vary (Fleck & Kramer, 2004; Bompa,1993), were thought to generate contractions that provide a constant tension to the contracting muscle. As such "free weights" were associated with isotonic contractions, as the constant load was mistakenly thought to provide a constant tension to the muscle.
With this in mind the term "isoinertial" (iso - same, inertial - resistance) may be more accurate than the term "isotonic" to describe the contractions performed lifting a constant loads, like plate weights and dumbbells (Adams, 2005; Nordin & Frankel, 2001; Colduck, 1997).
Isokinetic (iso - same, kinetic - motion) contractions maintain a constant limb motion or velocity throughout the range of motion employed (McArdle et al., 2006; Adams, 2005; Nordin & Frankel, 2001; Egger, et al., 1998; Keogh, Wilson & Weatherby, 1996). By controlling the velocity of execution, isokinetic contractions allow the muscle to apply maximal force throughout the full range of motion (Egger et al., 1998; Keogh, et al., 1996). Due to the need for the resistance to vary as the angle-of-pull (Jenkins, 2005) changes, it is difficult to perform this type of contraction without specialized equipment (Egger et al., 1998). Variable or accommodating resistance equipment, by changing the amount of load moved (through the use of cams, pneumatics and electronics for example), can match the force applied by the user and therefore provide a constant resistance through exercise range. With this in mind, an important consideration is made by Shield (1995, p.32) in regards to machines that apply a variable resistance:
It is also apparent that the design of any variable resistance device must be based on the "average" strength curves developed from studies of particular populations. No single curve generated in this fashion can accommodate for all individuals because those with different anthropometric characteristics (limb length and trunk girths) will exhibit deviations from the average curve.
There is also still a belief that isokinetic contractions can only be performed concentrically (miometrically). Baechle & Groves (1998, p.xxv), for example, state that "isokinetic equipment involves only concentric activity." You will find that many references indirectly support this statement by mentioning eccentric and concentric contractions when considering isotonic contractions but not when considering those isokinetic in nature. This is not, however, the case and both concentric (miometric) and eccentric (pliometric) contractions can be performed on some isokinetic devices (Fleck & Kraemer, 2004; Keogh et al., 1996). While Passive Isokinetic Systems, like Hydra, Cybex and Universal, as examples, are only able to provide concentric activities, Active Isokinetic Systems, like Biodex, Kin-Com and Keiser, as examples, can however be used to perform concentric and eccentric movements (Tan, 2006).
In summary, muscles perform various types of contractions. These contractions can be classified by their length change characteristics and their muscle tension characteristics. Furthermore, a given characteristic (e.g, decrease in joint angle) may be represented by several different nomenclatures (e.g. "concentric," "miometric" or "positive"). With this in mind, practitioners must be conversant with not only the different types and characteristics of muscle contractions, but also the different names used to describe them.
Taking the muscle contraction background established in Part 1 of this article series, the second part of this series "Muscle Contractions, Part 2: How to Use Them – Theory" will cross compare the contractions and discuss how to apply this knowledge while the third instalment "Muscle Contractions, Part 3: How to Use Them – Practical" will provide practical training guidelines and applications.
- Adams, S.K. (2005) Hand Grip Torque Strength. International encyclopedia of ergonomics and human factors, Vol 1, 2nd Edition. Ed. W. Karwowski. Taylor and Francis.
- Baechle, T.R., & Groves, B.R. (1998). Weight Training. Steps to Success, 2nd Edition . Champaign,IL : Human Kinetics.
- Bompa,T.O., Di Pasquale, M.G. & Cornacchia, L.J., (2003). Serious Strength Training, 2nd Edition. Champaign,IL : Human Kinetics.
- Colduck,G. (1997). Physical training instructors’ course : Lecture summaries for the Subject 4 SGT (Rehabilitation). Bne,QLD. 2 Field Hospital.
- Egger, G., Champion, N. & Boulton, A., (1998). Fitness Leaders Handbook, 4th Edition. Kenhurst, Sydney: Kangaroo Press.
- Faulkner, J. A., Brooks, S. V., Dennis, R. G., Kuzon, W. M., Devor, S. T., Lynch, G. S. & Macpherson, P. C. D., (1997). Terminology in Muscle Mechanics: the Case for Miometric, Isometric, and Pliometric Contractions, Medicine & Science in Sports & Exercise, 29 (5s): p. 26.
- Faulkner, J.A. (2003). Terminology for contractions of muscles during shortening, while isometric, and during lengthening. Journal of Applied Physiology (95) : pp. 455–459.
- Fleck, S.J., & Kraemer, W.J. (2004). Designing resistance training programs, 3rd Edition . Champaign,IL : Human Kinetics.
- Jenkins, S.P.R. (2005). Sports Science Handbook: The Essential Guide to Kinesiology. Sport and Exercise Science: Vol 2: I-Z . Multi-Science Publishing.
- Keogh, J., Wilson, G. & Weatherby, R., (1996): Alternative weight training techniques (part 1). Strength and Conditioning Coach 4 (4): pp. 7–13.
- Kraemer, W.J. & Vingren, J.L. (2007) Muscle Anatomy 101 in Strength Training, ed. L.E. Brown, National Strength & Conditioning Association, Human Kinetics, pp. 3-28.
- LaStayo, P.C., Woolf, J.M., Lewek, M.D., Snyder-Mackler, L., Trude-Reich., et al., (2003). Eccentric Muscle Contractions: Their Contribution to Injury, Prevention, Rehabilitation, and Sport, Journal of Orthopedic Sports Physical Therapy. Vol 33 (10) , pp.557-571.
- Lindstedt, S.L., LaStayo, P.C. & Reich, T.E. (2001). When Active Muscles Lenghten: Properties and Consequences of Eccentric Contractions, News of Physiology Science, Vol 16 , pp. 256-261.
- Mc Ardle, W.D., Katch, F.I., & Katch, V.I. (2006). Essentials of Exercise Physiology, 3rd Edition. Lippincott Williams & Wilkins.
- Nordin, M. & Frankel, V.H. (2001). Basic Biomechanics of the Musculoskeletal System, 3rd Edition. Wolters Kluwer Health.
- Plowman, S. & Smith, D.L. (2007). Exercise Physiology. Health, Fitness, and Performance, 2nd Edition. Lippincott Williams & Wilkins.
- Shield, T., (1995). Resistance training equipment. Fitlink, 2nd quarter. Coorparoo, Bne: Fitlink. p. 30-33.
- Tan, J.C. (2006). Practical manual of physical medicine and rehabilitation. Elsevier Health Sciences.
- Zatsiorsky, V.M. & Kramer, W.J. (2006). Science and Practice of Strength Training, 2nd Edition. Champaign, IL: Human Kinetics.