Strength and conditioning coache’s and trainer’s focus will change with respect to the degree of athlete preparation, injury prevention, and injury rehabilitation needed. The requirements and situational needs of athletes typically require a blending of certain muscle actions: isometric, concentric, eccentric, or a coupling of an eccentric-concentric muscle action termed a stretch-shortening cycle (SSC). Successful trainers understand these muscle actions and how to program them into a resistance training program. Most human motion involves this eccentric-concentric coupling or SSC muscle action. The eccentric phase (the lower portion of a movement) of this coupling provides the focus of this article. We will take a look at relevant and recent research that explains the applications and benefits of eccentric training.
Eccentric loading for tendon injury rehabilitation
Achilles tendinosis is a degenerative process due to overuse where there are no inflammatory cells, but changes of the collagen fiber structure result in an inability of the tendon to adapt to changes in loading patterns [1, 2].
Eccentric strength training has gained popularity in the rehabilitation of Achilles tendinosis. The eccentric training theory promoted the importance of structural adaptation of the symptomatic tendon, so it could deal with the increased repetitive load and prevent injury [3]. Rehab programs involving eccentric training using loads greater than bodyweight have provided positive results in treating Achilles tendinosis, with decreased pain and a higher percentage of patients returning to preinjury levels of physical activity [4-6].
A minimum of 12 weeks is recommended for eccentric strengthening programs, given the research into the time course of tendon remodeling and regeneration. Three sets of 15 reps, with moderately increasing the load over time appears the most common loading progression [7].
Optimizing the length of muscle for injury prevention
Believe it or not, muscle has an optimum length for producing peak tension. As muscle lengthens beyond its optimal length, tension levels decrease. This descending portion of length/tension is thought to be the region of vulnerability in which muscle strain injuries occur. Many researchers also believe that athletes who produce peak tension at shorter than normal muscle lengths are more likely to suffer acute muscle strains [1, 8, 9].
It’s suggested that muscle strain injuries could be reduced if the optimum length is shifted to a longer length. The only form of training consistently shown to increase the optimum length of tension development is eccentric contractions [1, 10, 11]. The magnitude of the change in length depends on three variables: 1) load of eccentric exercise, 2) volume of eccentric exercise and 3) length of the muscle during eccentric muscle actions. Research shows the greatest shifts used protocols with either high volume or high load at long muscle lengths [7].
Supramaximal loading for strength and hypertrophy
Humans are able to recruit fewer motor units during an eccentric muscle action that a concentric contraction at a given or absolute load. This demonstrates that the neural efficiency of eccentrics is greater. Therefore, it has been suggested to maximize neural activation and subsequent strength adaptation during eccentric muscle actions; greater loads are required [12, 13]. Therefore, the eccentric portion of a traditional resistance exercise is under-loaded even when the concentric portion is at maximum. Some research suggests that participants may be as much as 20-60% stronger eccentrically than concentrically [14].
Most of the research shows similar improvements in strength with supramaximal eccentric and standard resistance training utilizing both concentric and eccentric muscle actions. The issue with some of the current research is that despite using supramaximal eccentric training, eccentric strength is rarely assessed. When taking into account the well-documented principle of training specificity, eccentric strength has been shown to be enhanced by eccentric specific training [15].
It’s important to note that supramaximal eccentric training has no advantage over standard resistance training with regard to hypertrophy of muscle. There is some speculation that the mode of contraction (concentric vs. eccentric) may influence the degree of muscle hypertrophy. We know that we can handle higher forces during eccentric compared to concentric contractions; consequently, the concentric strength will determine the absolute loading conditions during conventional resistance exercise that utilizes both types of muscle actions. For example, the amount of weight used to lower the bar on a biceps curl (an eccentric action of our biceps muscle) is greater than what we can handle to raise the bar on a biceps curl (a concentric action of our biceps muscle). Therefore, the amount of weight we can use to raise the bar on a biceps curl determines the load used during resistance exercise utilizing both concentric and eccentric contractions. Although there is speculation of one type of muscle action being preferred over the other; there is no research that shows eccentric only contractions enhance muscle hypertrophy with a prolonged training program.
Improving sports performance via SSC optimization
When a muscle needs to overcome resistance, or contract concentrically, its ability to do so may be determined by whether that concentric contraction was preceded by an eccentric muscle action. Concentric force production in isolation is relatively low compared with concentric contractions that are coupled with an initial eccentric muscle action [16]. This pairing is termed the stretch-shortening cycle (SSC) as defined previously in this article. Eccentric training is known to enhance this SSC and cause a more powerful concentric contraction. An example of this is the rebound in basketball where the athlete performs a small initial eccentric contraction by squatting down a little before jumping up to rebound the ball.
Recent research using nutrient timing with whey protein combined with eccentric and concentric contractions on tendon and muscle size.
Recent research investigated whether whey protein combined with prolonged eccentric only resistance training demonstrated a superior means in promoting skeletal muscle and tendon size compared to a concentric only and placebo intervention. This study investigated the effect of 12 weeks of either maximal eccentric or concentric resistance training combined with either a high-leucine whey protein hydrolysate + carbohydrate supplement or isoenergetic (same # of calories) carbohydrate only placebo, on quadriceps muscle and patellar tendon size [17].
Here is what they found:
- Conducting either eccentric only or concentric only training promoted tendon as well as muscle hypertrophy.
- The training-induced hypertrophy of both tendon and muscle were further enhanced with a high-leucine whey protein hydrolysate supplement.
- A novel finding is that patellar tendon size can be increased with resistance training combined with whey protein hydrolysate supplementation. There was no further enhancement in muscle or tendon size from the placebo (carbohydrate only) condition.
- These effects were observed to be independent of contraction (eccentric vs. concentric) modality. It’s suggested that there is no benefit in utilizing eccentric only contractions when trying to enhance muscle size. This means that you wouldn’t get bigger biceps by just lowering the bar (eccentric contraction) and not raising it (concentric contraction) while doing biceps curls during your training cycle.
Summary
Observations from this research suggest that resistance training enhanced tendon and muscle size following 12 weeks of resistance training. Muscle and tendon size was further enhanced by high-leucine whey protein hydrolysate supplementation compared to isoenergetic (same calories) placebo (carbohydrate only) supplementation. There was no advantage of performing a certain contraction mode (concentric vs. eccentric) when trying to enhance muscle and tendon size. It’s best to utilize both modes (concentric and eccentric) during a structured resistance training program.
Implications of this research
It’s clear that muscle size can be enhanced no matter if you perform eccentric only or concentric only actions.
In support of previous research; nutrient timing with high-leucine whey protein hydrolysate enhances muscle hypertrophy.
It seems that timed protein ingestion as utilized in the metabolic window is a strong determinant for inducing muscle hypertrophy.
The synergistic effect of combining high-leucine whey protein hydrolysate and resistance exercise on tendon hypertrophy could have important clinical implications since enhancing tendon size and strength may lower the mechanical stress on the tendon during exercise and potentially assist in tendon rehabilitation following injury.
References:
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