Abstract: Resistance exercise has a positive effect on many tissues, including heart, bone, skeletal muscle, and nervous tissue. Eccentric muscle actions offer a unique and a potentially beneficial form of exercise for maintaining and improving health. During resistance exercise, the effects of gravity, and mechanical properties of the sarcomere and connective tissue in skeletal muscle allow a greater muscle load during an eccentric (lengthening) muscle contraction than a concentric (shortening) muscle contraction. Consequently, older patients, patients with muscle or limb movement limitations or injuries, as well as cancer patients may be able to benefit from isolated eccentric muscle actions. There are specific physiological responses to eccentric muscle contractions. This review will describe the effects of different eccentric muscle contraction protocols on endocrine responses that could have positive effects on different tissues and recommend direction for future research.
Alex’s Notes: Resistance training is usually performed with concentric (CON) and eccentric (ECC) motions during the same set and with the same load. However, thanks to gravity and connective tissue properties, we are significantly stronger during the ECC portion of a lift than during the CON portion. Depending on the exercise, this discrepancy can be as great as 20-60% in men and upwards of 161% in women. At the same time, however, there is less recruitment of motor units due to the economics of ECC motions, which results in less metabolic stress. Thus, ECC focused training can be of benefit in older persons or individuals with limited limb movement. This short review by Robert Kraemer (look him up, his work is some of the best) discusses what is known regarding different endocrine responses and adaptations to concentric and eccentric exercise, with a focus on anabolic and glucoregulatory hormones.
The three anabolic hormones that Kraemer focused on were testosterone, growth hormone (GH), and insulin-like growth factor-1 (IGF-1). Several studies have compared the anabolic hormone response to CON and ECC exercise, but many were faulted by unequal loading. For instance, one study recruited ten young men to complete four sets of 12 repetitions of either CON or ECC only contractions and concluded that while both protocols increased testosterone and GH, the CON elicited greater GH increases. However, the loading for both protocols was 80% of the CON 10RM, which suggests that the greater GH responses were likely due to greater relative workload for the CON trial. To overcome these discrepancies, follow-up work utilized a similar exercise layout with loads that corresponded to the 1 rep-max of either CON or ECC and found that both produced similar testosterone and GH responses.
Overall, Kraemer concludes that both CON and ECC motions will produce similar testosterone, GH, and IGF-1 responses when loaded for a specific CON 1RM or ECC 1RM. However, since loads are usually loaded for the CON 1RM, the ECC motions will elicit a significantly lower GH response than CON. There is also some evidence that increasing time under tension for ECC motions will lead to greater GH output.
This category encompasses insulin, c-peptide, and amylin. I’m sure we are all familiar with insulin, and C-peptide is a product of proinsulin cleavage that is mainly used as a marker for insulin release.Amylin is co-secreted from pancreatic β-cells with insulin in response to glucose and regulates blood glucose by reducing gastric emptying and inhibiting hepatic glycogenolysis.
There is a belief that excessive muscle damage causes insulin resistance. However, studies using oral glucose tolerance tests (OGTT) to determine glucose disposal after up or downhill running (CON vs ECC) found no difference between the protocols. Other studies done with weight lifting found a similar lack of difference between CON and ECC dominated contractions. Kraemer concludes that,
“1) muscle damage produced by excessive ECC muscle contractions does not affect insulin and glucose responses to an OGTT indicating glucose tolerance, and 2) that ECC muscle actions of downhill running produce muscle damage that elicits reduced glucose uptake during a euglycemic–hyperinsulinemic clamp.”
Cortisol & Ghrelin
Cortisol is a glucocorticoid that plays a role in fatty acid mobilization, catabolic effects on muscle (needed for recovery), as well as immunosuppressive and anti-inflammatory actions. Ghrelin is an orexigenic hormone produced primarily in the stomach that transmits “hunger.” One study found that cycling (primarily CON) and downhill running (primarily ECC) did not produce significant changes in either hormone. However, work by Kraemer found cortisol responses to be lower during ECC exercise and was associated with perceived exertion and pain scale ratings. Kraemer concludes that,
“Some evidence suggests that total ghrelin is reduced with CON muscle actions which are related to increased GH concentrations, whereas ECC muscle actions do not elicit a ghrelin response. As for cortisol, if CON and ECC muscle actions are loaded relative to a CON 1-RM, data suggest that cortisol levels will increase or decline less over time with CON muscle actions compared to ECC muscle actions.”