In the first part of this article series, “Muscle Contractions, Part 1: Different Types,” the various types of muscle contractions and their features were explored. These contractions were classified by their length change and muscle tension characteristics and the various nomenclatures associated with these characteristics were examined. In this second part of the series, the different contractions are compared and their strengths and weakness addressed. Finally, means of applying this knowledge into training applications are discussed.
Concentric (Miometric) vs Eccentric (Pliometric) Contractions
Apart from the differences in muscle length changes, there are several other notable differences between concentric (miometric) and eccentric (pliometric) contractions. These are:
- Eccentric (pliometric) contractions are approximately 45 to 50 percent more forceful than concentric contractions thereby providing the ability to move greater loads (Marieb & Hoehn, 2007; Jones & Rutherford, 1987). This additional strength also makes eccentric contractions the prime contractions for decelerating speed and absorbing load. Catching a ball is an example of decelerating an external load while bending your knees when landing from a jump, a movement of "weight acceptance" is an example of decelerating an internal load.
- During eccentric (pliometric) contractions fewer muscle fibers are recruited when compared to concentric contractions (Plowman & Smith, 2007; Jenkins, 2005; Fleck & Kraemer, 2004). With fewer fibers recruited some cross-bridges do not cycle, less oxygen is therefore needed and the overall energy cost of performing the work is lower. With this in mind, it is estimated that the energy cost of performing concentric (miometric) contractions is 3 to 9 times higher than performing the same task eccentrically (pliometrically) (Plowman & Smith, 2007).
- As the muscle fiber filaments are being pulled apart while under contractile tension, eccentric (pliometric) activity is acknowledged as damaging muscle to a greater extent than concentric activity (Plowman & Smith, 2007). As such the finding that eccentric activity is known for developing Delayed Onset Muscle Soreness or DOMS (McArdle et al., 2006; Plowman & Smith, 2007; Fleck & Kraemer, 2004; Escamilla & Wickham, 2003) is understandable. This damage has been found to last (and raise resting metabolic rates) for up to 48 hours (Dolezal et al., 2000).
Eccentric training involves utilizing only the eccentric (pliometric) phase of an exercise. The weight is traditionally lowered by the lifter and lifted by either another person or mechanically. An alternate method when training alone has the lifter eccentrically "lower" the load with one limb while concentrically ‘raising’ the load with both limbs.
With the ability to move more load eccentrically (pliometrically), eccentric training is considered by some to hold some benefit over concentric training or traditional dynamic training (a concentric and eccentric contraction per repetition) for strength and muscle development. Whether this actually occurs however is debatable, with research providing mixed results. Some research findings are shown in Table 1 below.
|Table 1: Overview of sample eccentric training research
|Jones & Rutherford, 1987
||Unilateral isometric training, other leg control or Unilateral concentric-only, other leg eccentric-only
- Isometric training of one leg resulted in a significant increase in force on trained leg.
- Forces generated during eccentric training were 45% higher than during concentric training.
- Similar changes in strength and muscle cross-sectional area were found between eccentric and concentric only training.
- Both eccentric and concentric only training increased cross-sectional area.
|Higbie et al., 1996
||Isokinetic concentric-only and isokinetic eccentric-only contractions
- Eccentric isokinetic training is more effective than concentric isokinetic training for developing strength in eccentric isokinetic movements.
- Concentric isokinetic training is more effective than Eccentric isokinetic training for developing strength in concentric isokinetic muscle actions.
- Gains in strength are highly dependent on the muscle action used for training and testing.
|Hortobagyi et al.,1996
||Concentric-only and eccentric-only loads with equal resistance
- Eccentric training improved eccentric and isometric strength more than concentric training improved concentric and isometric strength.
|Marler et al., 1999
||Both groups performed concentric-only contractions at 70% 1-RM. The eccentric group performed an additional set at between 10% to 100% above 1-RM eccentrically - only
- Eccentric and concentric training produce similar strength gains.
|Folland et al., 2001
||One arm dynamic training with opposite arm performing the same dynamic training, but after completing a single set (60 reps over 10 mins) of maximal eccentric work.
- There was no significant difference in strength between the two arms.
- Eccentric training may compromise strength gains for several weeks.
|Hortobagyi et al., 2001
||Both groups performed the same dynamic exercise with one group applying a 50% overload to the eccentric phase of the dynamic movement
- Eccentric overloading may increase strength rapidly for a short period to a greater extent than traditional dynamic exercise.
|Symons et al., 2005
||Three groups, concentric-only, isometric-only and eccentric-only performed maximal voluntary contractions for their specific contraction type.
- The eccentric isokinetic training group did not demonstrate gains in voluntary strength superior to those of the isometric and concentric groups
- Concentric contractions in particular had a significant impact on concentric work and concentric power.
More recently, a detailed meta-analysis of eccentric and concentric training research, conducted by Roig et al., (2009), found that when eccentric training was performed at a higher intensity (compared to concentric training):
- Total strength and eccentric strength increased more significantly.
- Strength gains after eccentric training appeared more specific in terms of velocity and mode of contraction compared to concentric contractions.
- Eccentric training was considered to be more effective in promoting increases in muscle mass (measured as muscle girth).
- Eccentric training showed a trend towards increased muscle cross sectional area (measured with magnetic resonance imaging or computerized tomography).
These findings should be viewed with caution however as the papers included in the meta-analysis employed concentric-only and eccentric-only conditions and did not compare either of these directionally isolated movements against a dynamic contraction employing both eccentric and concentric movements.
Apart from the difference in strength and hypertrophy gains, there is the question of need and relevance, especially in light of the increased muscle soreness post training.
Eccentric training may be of little value to an Olympic lifter or swimmer as they employ predominantly concentric contractions. As Shield and Young (1995, p.183) state...“Olympic weight lifters….employ many lifts which require little or no eccentric activity…(they) simply drop the weight to the ground prior to the next lift.”
Conversely, strength lifters (those who compete in the 1 RM Bench Press and Squat) may find that stronger eccentric contractions assist in the initial lowering of the weight. Thereby allowing for better form and providing a greater reserve for the concentric phase. "Lifters who can lift heavier weights lower the resistance more slowly." (Fleck & Kraemer 2004, p.45 citing Madsen and McLaughlin 1984 and McLaughlin et al., 1977).
While excessive eccentric loads may be a mechanism of injury (LaStayo et al., 2003), eccentric training is commonly used in rehabilitation with the premise being to gradually strengthen muscles eccentrically, thereby allowing them to absorb more energy before failing (LaStayo et al., 2003). Often prescribed in tendinopathy treatment (Taunton et al., 2003; Brukner & Khan, 2002; Reid, 1992), eccentric training also has a very high level of evidence for improving functional outcomes in anterior knee pain treatment (Australian Acute Musculoskeletal Pain Guidelines Group, 2004). While eccentric training may be of benefit for treating specific injuries, care must be taken when eccentrically training other muscles that injured patients may need for mobility. Consider the difficulty a patient may exhibit trying to negotiate the day on crutches when suffering DOMS in the upper limbs.
Isometric training involves holding an isometric contraction at a given angle or group of angles against an opposing resistance. When compared to dynamic training, research has found that isometric training improves only isometric strength where as dynamic contractions improve both dynamic strength and isometric strength (Tricoli et al., 2001). Furthermore, gains in isometric strength have been found to exist only at the given joint angle trained (Bera et al., 2007; McArdle et al., 2006) and possibly up to around 20 degrees either side of the joint angle (Howley & Franks, 2007). This is not to suggest however that isometric training has no place in a conditioning program. Apart from improving sport specific requirements (i.e. events which require the ability to maintain isometric contractions, like gymnastics, shooting, archery etc) isometric training may still hold some value in improving dynamic strength. With the greatest weight that can be lifted during a dynamic contraction limited to the weakest or "sticking" point in the movement range (Newton, 2007; Zatsiorsky & Kramer, 2006), isometric training may hold value in increasing strength at that specific joint angle (Fleck & Kramer, 2004).
Additional considerations, that should be given to performing isometric contractions for conditioning purposes, include:
- Isometric contractions utilize less energy that dynamic contractions (Cramer, 2008).
- Maximal isometric contractions produce more strength gains than submaximal isometric contractions (McArdle et al., 2006).
- Isometric training is known to dramatically increase blood pressure (Bera et al., 2007).
Movement Through Range
It is important to appreciate that the force developed by these contractions varies with the velocity of the contractions (research the Force-Velocity Curve for more information). Furthermore, it is also important to realize that just because on contraction type can generate move force than another it does not mean that it recruits more neural drive or motor units. In fact the reverse is more likely. Consider for example that eccentric contractions are stronger and produce more force than concentric contractions, yet have been found to use fewer motor units and recruit fewer muscle fibers (Plowman & Smith, 2007; Jenkins, 2005; Fleck & Kraemer, 2004).
Isotonic vs. Isokinetic Contractions
Machines, by providing variable resistance and accommodating resistance, are regarded as the only means of providing an isokinetic or near isokinetic contraction. They are therefore often sided with isokinetic contractions in comparisons (see Note at end of this section). The same applies to "free" weight or dynamic constant external resistance (DCER) exercises and the conceptual "isotonic" contraction.
Strength Gains and Hypertrophy
During isokinetic contractions maximal effort is applied throughout the range of motion as opposed to isotonic / isoinertial contractions which exert maximal effort only at the ‘sticking point’ or weakest angle (Kraemer et al., 2007; Wilson, 1993 as cited by Keogh et al., 1996, p.7). As such, isokinetic training may allow greater strength gains due to higher levels of stimulation created by a longer maximal effort (Keogh et al., 1996). In light of this Zatsiorsky and Kraemer (2006, p.122 ) state that “...studies have repeatedly failed to demonstrate that accommodating resistance exercises...... hold an advantage over free weight exercises for increasing muscular strength and inducing muscle hypertrophy.”
There has however been research suggesting that isotonic training is superior to isokinetic training for strength and power development (Kean et al., 2005; Kovaleski et al., 1994), with Kovaleski et al., (1994) claiming that isotonic training is more able to meet sports training movement needs and hence is the preferred medium for sports training.
With these findings in mind, several experts still agree that there is a lack of substantial evidence indicating the superiority of either isotonic (“free weight” / DCER) training or isokinetic (variable / accommodating resistance training) in increasing strength or changing body shape (Zatsiorsky & Kraemer, 2006; Fleck & Kraemer, 2004; Shield, 1995).
When determining which of the contraction types to use in a training program, the following factors should also be considered:
- By using machine weights to perform isokinetic contractions movement lines are fixed and velocity remains constant, making it a useful tool in early rehabilitation (Bera et al., 2007; Howley & Franks, 2007; Zatsiorsky & Kramer, 2006).
- While isotonic based exercises, using DCER, can employ movement across six degrees of freedom (permissible movement directions), isokinetic machines are typically limited to one (Zatsiorsky & Kramer, 2006).
- While some isokinetic machines are capable of providing for compound exercises (e.g. Squat) they cannot provide for complex actions (e.g. wood chop) and are of little value for power athletes employing these lifts (Shield, 1995).
- The angular velocity of the isokinetic movement is typically low (Zatsiorsky & Kraemer, 2006). This is understandable as the primary function of the isokinetic machine is to maintain a constant velocity. This would mean that the specificity of acceleration and acceleration – deceleration patterns of movement, as experienced in free weights and most natural athletic movements, is lost (Zatsiorsky & Kramer, 2006; Shield, 1995).
- Studies have suggested that isokinetic training generally improves strength gains at specific to the training velocity (Howley & Franks, 2007).
Note: While isokinetic contractions are closely associated with machine weights due to the need for machines to control the movement velocity, not all of the machines used in weight training are isokinetic. Likewise, not all machines that limit velocity allow for the performance of isokinetic contractions (Harman, 2008).
In summary, eccentric contractions have been found to be stronger, use less energy and create increased muscle soreness when compared to concentric contractions, while research is still inconclusive whether eccentric or concentric contractions outweigh one another for strength and size gains. Eccentric only training can be used when concentric repetitions are not possible to allow movement practice while concentric only training can limit muscle soreness.
Isometric contraction training may only increases strength in a limited range and increase blood pressure but these contractions can be used to increase strength in exercise weak points and improve efficiency of dynamic movement by limiting unwanted movement.
Isokinetic contractions are closely associated with machine weights due to the need for machines to control the movement velocity and where commonly only associated with concentric contractions due to limitations with these specific machines. Isotonic contractions are used to describe moving free weight but are now appreciated to not provide achieve a state of constant tension even though the load is constant. With this in mind, some machines, like those with accommodating resistance attempt to enable this form of contraction.
With this in mind, isotonic contraction training has not been found to have any advantage over Isokinetic contraction training for strength and size gains while the later has notable restrictions in movement performance.
Taking the muscle contraction background established in Part 1 of the article series, "Muscle Contractions, Part 1: Different Types," and the comparisons between these contractions discussed in this article, the final installment of the series, “Muscle Contractions, Part 3: How to Use Them – Practical” will provide practical training guidelines and applications.
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