Gym rats understand that strength training and endurance training will affect their bodies in different ways, but they may not understand the actual science behind this. A new study from University of Jyväskylä in Finland explains this difference. While all types of training activate the gene PGC-1-alpha, the type of proteins produced by this gene during different forms of exercises are not the same, so your body responds by creating different effects: more or less muscle.

Strength training (also called resistance training) targets the muscles, causing them to contract against an external resistance (such as weights) in order to increase strength, tone, mass, and mobility, explains the LiveStrong website. By comparison, endurance training targets either the muscular or cardiovascular systems, in the first case helping muscles sustain their activity over a long period of time, and in the second case helping you to sustain your activity over a long period of time — even when your heart is racing. Commonly, strength training is working out with weights, endurance training is aerobic exercise, including jogging.

All good, but most of us understand how exercise affects our bodies in not just one but any number of ways. So how do you isolate the particular effects of a single exercise?

For the new study, the University of Jyväskylä researchers concentrated on isoforms.

What are isoforms?

Proteins are the workhorses of your body. Not only do they flip the “on/off” switch for everyday physiological processes, they also regulate your body’s internal activities, speeding things up or slowing things down as necessary. Your body has many proteins, with the recipe for making them written on sections of your genes (DNA). Although one gene may “code” for different proteins, sometimes more than one protein can come from the same gene by way of which stretch of the DNA is "read."

Isoforms, then, are proteins produced when only a section of a gene’s code is read.

Past research has shown the PGC-1-alpha gene can produce more than one protein or isoform, and whichever is produced depends on the exercise.

For the current study, then, the researchers searched for isoforms in the thigh muscles of 19 healthy men divided into two groups, one performing high-intensity resistance exercises, the other moderate-intensity endurance exercises. The researchers obtained thigh muscle biopsies before and after exercise and studied them.

The researchers found that both types of exercises produced three isoforms (specifically, PGC-1-alpha exon 1b, PGC-1-alpha exon 1b, and truncated PGC-1-alpha). However, only endurance exercise produced one other isoform (PGC-1-alpha exon 1a).

The researchers believe their results improve the understanding of how one exercise can influence protein cascades in our bodies while supporting the idea that the isoforms of the PGC-1a gene play an important role in muscle adaptation.

Source: Silvennoinen M, Ahtiainen JP, Hulmi JJ, et al. PGC-1 isoforms and their target genes are expressed differently in human skeletal muscle following resistance and endurance exercise. Physiological Reports. 2015.