Eccentric Training – An Introduction by Matt Jordan, PhD:
If we were discussing a personality trait, ‘eccentric’ would characterize an odd or strange individual. Some might characterize Stu as being eccentric. If we were discussing an object, ‘eccentric’ would refer to its non-centered position. However, when it comes to muscle, the term ‘eccentric’ refers to an action whereby a muscle is lengthened under tension.
Not only are eccentric muscle actions often used by strength coaches as a potent training stimulus to strengthen skeletal muscles but they have also posed unique challenges to the scientific theories underpinning muscle contraction.
In his seminal paper in 1957, Andrew Huxley, the father of the cross bridge model of muscle contraction, noted the discrepancies between his mathematical models of muscle contraction and the empirical evidence obtained during lengthening contractions. The slope of the force-velocity relationship just beyond maximum isometric force was much steeper for lengthening contractions compared to shortening contractions and the energy liberated during lengthening contractions was less than his mathematical models predicted.
Additionally, when an active muscle is forcibly stretched to a new length, the new tension that is generated exceeds the value obtained at that length if no pre-stretch occurred. This phenomenon termed ‘residual force enhancement’ is another fundamental muscle property related to lengthening or eccentric contractions that is not well explained by existing scientific theories on skeletal muscle contraction.
In his book “Reflections on Muscle” published in 1980, Huxley discusses eccentric actions frequently. Huxley remarks: “I imagine that special features have been evolved which allow this elongation (of muscles) to take place without damaging the muscle … I suspect that many of the unexplained phenomena, such as those I have just described here (associated with muscle lengthening) are related to these special features, and have little relation to the processes that take place during shortening”.
It seems the nature of eccentric muscle actions are mysterious and might contain hidden secrets. Is this why experienced strength coaches find so much potential with eccentric training methods? Because if one thing is clear, eccentric training methods are of critical importance for preparing the elite athlete.
In these next two blog-posts, Dr Angus Ross delves into the practical applications and considerations for eccentric training. Having read these two posts, I can tell you that what you are about to read will be an excellently written and well-referenced series on why and how eccentric training methods should be used.
|photo courtesy justynwarner.com|
Part I: Eccentric Strength Training – Dr Angus Ross
This is the first of my two guest blogs on Eccentric (ECC) training, with Part I covering some background in regards to some of the concepts of ECC training for speed and power sports, and Part II covering some practical application and programming options.
As some brief background, my work is predominantly in New Zealand Olympic power sports i.e. a relatively small pond with limited depth, where more often than not our top athletes are relatively unchallenged for their Olympic berths. The benefit of this, however, is that we are lucky in being able to plan in the long- to medium-term for performance at pinnacle events. With that in mind, it may give some perspective to my use of eccentric training, and where I see it fitting in during a training block. Nevertheless please consider my proposed approach may not suit dissimilar competition schedules – though I do hope some of the principles are of value.
Eccentric Strength Training:
As has been mentioned in previous posts in this series, ‘eccentric-emphasized’ strength training involves yielding to – or lowering of – typically supra-maximal loads (relative to the concentric phase). There are a variety of methods for inducing ECC overload. This is a growing area in both sport-related and commercial fitness with an increasing number of ECC-based devices coming on the market; nonetheless there are also many options that can be done on standard equipment with a little imagination.
Why use ECC training?
There are lots of pros and cons to be considered as ECC training has the potential to strongly influence other training units; it needs to be used judiciously to create a positive impact on performance. The text to follow is largely based around the direct performance implications with respect to ECC training and the potential to develop a contractile system that is both stronger and faster. Perhaps the Holy Grail is using ECC training to move the force/velocity curve upwards and to the right with particular gains at high speed (see Figure 1). We may not quite be there yet in terms of complete understanding – but options are discussed in the text to follow. There remains much to be learnt on many fronts with ECC training, and it should be noted that ECC work is also heavily prescribed in tendon, rehabilitation and therapeutic work, so it has training implications well beyond the scope of this post.
POTENTIAL BENEFICIAL MUSCULO-TENDINOUS ADAPTATIONS
Faster muscle contractile properties:
Training muscle to contract harder and faster has obvious performance benefits for many sports. Evidence suggests that there are at least two potential mechanisms by which ECC training may be able to up-regulate the contractile speed of muscle:
Fibre type shifts: In contrast to most isotonic resistance training studies (which typically show a decrease in the percentage of the fast IIx myosin or IIb muscle – Harridge, 2007), ECC training may offer the opportunity to shift myosin in the opposite direction – i.e. towards fast twitch (FT). The evidence is by no means conclusive but there is just enough of it to make it an interesting option!
Eight-weeks of high speed (180 degrees per second) ECC elbow flexor work significantly increased the percentage of FT & particularly IIb muscle fibres (Paddon-Jones et al., 2001), and a single bout of ECC exercise caused the proliferation of muscle satellite cells towards FT fibre type (IIx) following (Cermak et al., 2013) and or greater anabolic signalling of FT muscle than in slow twitch muscle (Tannerstedt et al., 2009)
Increasing relative cross sectional area of FT muscle with ECC resistance training (as compared to concentric only training) (Vikne et al., 2006; Hortobagyi et al., 1996)
Number of sarcomeres in series: Eccentric training also has potential for increasing contractile velocity by adding sarcomeres in series (Lynn & Morgan 1994 ; Friden et al., 1984) and increasing muscle fascicle length (Timmins et al., 2015; Franchi et al., 2014; Seynnes et al., 2007; Butterfield et al., 2005). Evidence suggests that more sarcomeres in series has an additive effect on contraction velocity – perhaps having a greater effect on contractile speed than fibre type (Sacks & Roy, 1982). Not surprisingly, muscle fascicle length is positively correlated with sprint running performance in some papers (Kumagai et al., 2000); although given the multi-factorial nature of sprinting, it doesn’t discriminate between good sprinters (10.70s 100m runners) and elites (~10.30s) (Karamanidis et al., 2011). Notably, not all studies concur with regard to greater changes in fascicle length and sarcomeres in series with ECC training. Furthermore, and somewhat counterintuitively, even when significantly greater gains in fascicle length have been demonstrated post-ECC work, immediate performance gains especially in concentric (CON) tasks are not always forthcoming – more on this later.
Additional Muscle/Tendon Architecture changes:
Muscle Adaptation: The greater training loads of eccentric exercise promote greater hypertrophy (and in the most part strength – depending on how it is being measured) adaptations for an equal period of training than that observed from conventional weight training (Higbie et al., 1996; Farthing et al., 2004; Roig et al., 2009). Similarly, greater increases in collagen expression within muscle also occur with ECC training as compared to CON training (Heinemeier et al., 2007).
Tendon – changes in cross sectional area (CSA)? Some evidence suggests that the distal part of the musculo-tendinous unit is where the greatest stress and adaptation occurs with ECC training (e.g. Franchi et al., 2004; Seger et al., 1998), and certainly changes do occur with tendon stiffness (see section below). Perhaps due to the relatively avascular nature of tendons, however, there is less evidence on direct changes in tendon CSA with short term ECC training interventions.
Fascia – Anecdotally at least, it appears that fascia also responds and develops in response to tension (Schliep & Muller 2013), and hence the high stresses experienced in ECC exercise means it is likely to be a potent stressor for remodeling fascia. The increase of collagen expression in response to ECC training seen in muscle work (see muscle adaptation point above) appears to corroborate this assumption.
|Figure 2: Torque exerted by the knee-extensor muscles and EMG of Vastus Lateralis (from Tesch, et. al, 1990)|
Greater muscle/tendon stiffness: It is beyond the scope of this blog-post to go into the topic of stiffness in any great detail. In brief however, stiffness is the resistance to change in length of a tissue or joint (or series of joints – like a leg) and there are active (affected by muscle activation) and passive elements (the intrinsic stiffness qualities of the tissue) that affect the overall stiffness outcome. Noting that it is possible to be simultaneously both very stiff and very elastic – think of a heavy duty spring that returns nearly all of the energy when it is compressed. It is interesting to note that certain components of stiffness are related positively to performance outcomes in stretch shortening cycle (SSC) sports, with great jumpers and sprinters typically displaying great stiffness qualities. Greater stiffness potentially increases the rate of force application – a particularly critical factor in high speed sports with limited time to maximize force application. An obvious example of this would be sprinting, given both the brevity of ground contact time in sprint running and the importance of massive forces to increase speed (Weyand et al., 2000 & 2010). Gains in leg-spring stiffness have also been positively associated with increased running economy in endurance runners (Albracht & Arampatzis, 2013), and improved peak power in sprint cyclists (Watsford et al., 2010). As with most things however, more stiffness ad infinitum is not always better – i.e. you need to be able to actually compress a spring if you want to get any rebound out it, so it’s good to measure and know how ECC training affects the stiffness (both active and passive) of the leg.
So what happens to stiffness with ECC training?
ECC training appears to increase leg spring stiffness significantly more than concentric only work (Elmer et al., 2011; Lindstedt et al., 2001). It is likely that the ECC training effects both active (i.e. neurally driven – e.g. co-contraction, decreased inhibition, etc.) as well as passive elements of stiffness. Where in the ‘series elastic component’ (muscle + tendon) the passive adaptations occur it is not entirely clear – though the likelihood is that both muscle and tendon are modified with ECC loading. Some evidence suggests that high load ECC work particularly may increase the stiffness of the tendon (Malliaras et al., 2013; Foure et al., 2013) and as alluded to in the previous section, potentially there may also be increased collagen expression within the muscle. In practical terms, ECC training can be used to directly target increases in stiffness – potentially a genuine performance enhancer – particularly for a highly compliant or ‘floppy’ athlete that may have great elastic qualities but sub-optimal speed and slow ground contact times.
|photo courtesy justynwarner.com|
POSITIVE APPLIED PERFORMANCE ADAPTATIONS
Performance adaptations as result of an ECC training intervention are clearly what most of us as coaches or practitioners are interested in. Obviously the performance outcomes are dictated by both the soft tissue adaptions discussed above as well as neural adaptations. Evidence suggests that ECC contractions differ from CON with both greater cortical activation (Fang et al., 2001) and lesser muscle activation despite greater force (see Figure 2). With ECC training, an athlete learns to cope with severe ECC stress and thus have an improved ability to activate agonists (via adaptation in muscle spindle feedback), decreased antagonist co-contraction as well as potential decreased agonist inhibition from Golgi tendon organs. So the neural alterations in conjunction with the musculo-tendinous adaptations can give rise to our primary objectives of performance gains. In addition to strength adaptations already mentioned these can include:
Stretch Shortening Cycle (SSC) performance: Eccentric strength is positively related to power output in SSC performance (Miyaguchi et al 2008). Similarly, gains from both purely eccentric loading (Elmer et al., 2011; Linstedt et al., 2001) and eccentric-accentuated jump training (Sheppard et al., 2008 – see Figure 3) have shown superior vertical jump adaptations in comparison to concentric only or traditional jump training. Eccentric-accentuated strength training also appears to facilitate gains in maximum velocity sprint running (Askling et al., 2003) as well as acceleration (30m time) and drop jump performance (Liu et al., 2013). At this stage, it is unclear whether the speed of the ECC intervention is critical for the SSC adaptations. It is, however, worth noting the Elmer research (eccentric cycling) and some of the passive leg press training from the Taiwan group (Liu et al., 2013) (see www.youtube.com/watch?v=9sjkWTNP4n4) would be described as high speed ECC dominant training.
Concentric (CON) Performance/Power: It could be reasonably expected that the potential gains in muscle contractile speed (detailed above) would be mirrored in performance in concentric performance. However gains in ‘concentric only’ power e.g. cycling, swimming, kayak, & perhaps rowing and-or isometric force from eccentric training are not always immediately forthcoming from a period of eccentric loading (Gross et al., 2010; Elmer et al., 2011; Leong et al., 2014). Indeed the data from Leong and associates (2014) demonstrated that there may be a delay in CON performance adaptation with ECC training. Notably, CON power production was greater 8 weeks post cessation of an ECC training block than it was when tested 1 week after completing the ECC training.
With that in mind, potential negatives of eccentric loading (see below) need to be factored in before adopting an eccentric training protocol in CON sports.
|Figure 3: accentuated eccentric jump squat|
POTENTIAL CONTRAINDICATIONS OF ECCENTRIC TRAINING
Muscle Damage/Soreness: As alluded to above, part of the mechanism of adaptation to ECC training may be related to greater muscle damage that occurs – with evidence suggesting that FT fibres in particular get hammered from ECC stress (Friden et al., 1984; Linnamo et al., 2000). Similarly, we see greater delayed on-set of muscle soreness (DOMS) with ECC loading than that observed from conventional strength training (at least for initial sessions). If muscle tissue is stressed too severely, performance may be substantially impaired for several weeks or longer (Mackey et al., 2004; Sayers & Clarkson, 2001). This is potentially reflected in the delayed CON performance adaptations discussed above.
Fascial damage/pain response: Evidence suggests that much of the DOMS response to ECC work may be related directly to damage to, or inflammation of, the fascial network (Lau et al., 2015; Gibson et al., 2009). My take on both the fascial pain response and remodeling that likely occurs is that acutely there will be changes in afferent feedback during exercise and it may mean it is difficult to maintain an optimal technical model in your sport during the initial phases of a heavy ECC training phase. Acknowledging this, and working with and around it, becomes a critical consideration in a training plan.
|photo courtesy justynwarner.com|
Summary & Recommendations: Damage and inflammation occurs with ECC training at both a muscle (especially FT muscle) and fascial level, which may negatively affect performance in the short term especially in high tension and/or speed ballistic tasks. With that in mind, my recommendation is that ECC training should be commenced using conservative load assignments, using limited sets and reps on training initial sessions to minimize DOMS – noting there is a strong protective effect against DOMS with one or two sessions of graduated ECC loading. In my experience, such an approach minimizes the disruption to other training units which can be adversely affected, and total training stress can be progressed relatively rapidly over days and weeks.
Mode-Speed Specificity: There is some suggestion in the ECC training literature that adaptations from ECC training are more mode- and speed-specific than equivalent concentric training (Roig et al., 2009). Given that the objective for most of us is to transfer strength gains to dynamic sport performance it is worth noting that higher speed ECC loading (180o s-1 ) may give better adaptation in strength, hypertrophy and fibre-type than that seen in slower (~30o s-1) training (Farthing et al., 2004; Paddon-Jones et al., 2000).
My impression is that once some basic preparatory training has been done, adaptations to faster ECC work is likely to be a superior modality for athletes. It is apparent however, that much of the ECC strength training literature is somewhat lacking in ecological validity – i.e. what is done in research does not necessarily reflect general training practice. The vast majority of ECC studies (unlike in typical athlete training) do not combine the ECC loading with additional concentric training or applied SSC exercise, perhaps partially explaining the mode-specificity findings with ECC work. In one of the few studies that has combined ECC training with other modalities and applied testing, Cook and associates (2013) demonstrated additive performance gains in sprint running when ECC work was combined with over-speed running. Hence, results in applied tasks perhaps could-would be further enhanced with a concurrent training approach.
My recommendation would certainly not be to do exclusively ECC work in training (in the designated ECC phase), but rather to do it as the dominant modality in conjunction with some dynamic sport-specific training.
ECC force modulation and impact on performance: A further observation of ECC exercise is that athletes with the ability to accurately modulate eccentric force production may have a genuine advantage in certain sports. Vogt and associates (2014) noted a strong relationship between accuracy of force modulation on an eccentric recumbent bike and competitive slalom ski race performance. Similarly, some anecdotal evidence with multi-event athletes (decathlon & heptathlon) suggests that some of the very top performers are able to dampen or stiffen the leg spring to suit the demands of the event at hand to a greater degree than those athletes below medal level. Perhaps demonstrating superior control of ECC force modulation to optimize impulse or RFD depending on the constraints of the task. I would expect that elite performers in many team sports may also exhibit the same sorts of characteristics with the unpredictable nature of the contractile demands imposed on the athletes. To my mind at least, it makes sense that perhaps training the ECC qualities may both improve performance directly with regard to muscle strength and power qualities, and some of the rhythmic ECC training modalities (e.g. eccentric cycling, passive squatting) may also offer a motor learning benefit.
Ultimately, perhaps empowering athletes with the physical and neural tools to drive either the force-application or force-damping in a superior fashion to optimize performance in the specific task at hand.
Rest and Restoration vs Detraining: With the extreme muscle tensions produced with overloaded ECC training, clearly there are both potential negative and positive responses to such loading – and perhaps the optimal time of recovery post an ECC training phase will depend on the demands of the desired performance outcome, as well as the magnitude of the prior training stress. There is, however, some evidence to suggest that ECC training strength adaptations are relatively well preserved over up to 8 weeks, and appear more resistant to detraining than those from CON training (Colliander & Tesch, 1992). Notably however changes in fascicle length derived from ECC training appear less well maintained – with data suggesting that these may return to baseline after 28 days of no ECC stimulus (Timmins et al., 2015). Arguably, this muddies the water somewhat – as a coach you must weigh up the potential positive effects of rest on the improved ability to activate a muscle without inhibition, as well as possible shift towards type IIx myosin versus the potential negatives of a reduction of fascicle length and potential atrophy.
|Figure 4: performance impact of eccentric training - overview|
As summarized in Figure 4 above ECC training – and arguably fast ECC training in particular – appears to offer superior adaptations relative to traditional strength training for developing the anatomical tools for high speed contractions and robust delivery of force, particularly in SSC-dominant sports. However, muscle damage and resulting potential power impairment in the short- to medium-term needs to be factored in when planning the implementation of an ECC training intervention to maximize the transfer of power to applied performance. Needs of both the individual athlete and event performance must be considered in the taper to a major event.
I really want to thank Angus for his thoughts; there are few folks in the world who are as comfortable in academia as they are in the weight room, or on the track. Angus is definitely one of them, and I’m really grateful he agreed to write this section. Stay tuned for the next post, where Angus delves into the practical application of eccentric training; as well, I will share my own thoughts, and how they have developed over time.