Muscles are responsible for creating forces that allow movement and stability of the body, the process that enables them to do this is muscle contractions. Muscle contractions occur when muscle fibers shorten, causing tension and force production. Muscle contractions are the result of activation of muscle fibers by nerve impulses.
There are 3 types of muscle contractions: isokinetic, isotonic and isometric.
Each type of contraction involves different mechanisms and produces different effects on the muscles and surrounding tissues, and understanding these differences is essential for effective training and rehabilitation.
Isokinetic muscle contractions are a type of contractions where the rate of muscle contraction remains constant throughout the entire range of motion. This differs from traditional isotonic contractions where the speed of movement can vary depending on the resistance. In an isokinetic contraction, the resistance is controlled by a machine that adapts to the force produced by the muscle. This means that the resistance remains the same throughout the range of motion, and the muscle is forced to contract at a constant rate. Isokinetic machines are commonly used in rehabilitation and sports performance training. The primary benefit of isokinetic muscle contractions is that they allow maximal muscle activation throughout the entire range of motion, which can lead to increased strength and improved muscle endurance. In addition, because the machine controls the resistance, there is less risk of injury from sudden increases in resistance that can occur with traditional isotonic contractions.
Isotonic muscle contractions occur when the tension in the muscle fibers remains constant throughout the contraction, but the length of the muscle changes. There are two types of isotonic contractions: concentric and eccentric.
Concentric contractions occur when muscle fibers shorten as they contract. This type of contraction generates force and is used to create movement, such as lifting a weight during a biceps curl. Concentric contractions are also known as “positive” contractions because they involve the shortening of muscle fibers against resistance. During a concentric contraction, muscle fibers generate force by bringing the beginning and end of the muscle closer together. Muscle fibers also become shorter and thicker, and the tension within the muscle increases. This increase in tension allows the muscle to lift heavier loads, move faster or perform more repetitions.
Eccentric contractions occur when muscle fibers lengthen as they contract. This type of contraction creates force, but it is used to slow down or control movement, not to create it. An example of an eccentric contraction is when you lower the weight during a biceps curl. Eccentric contractions are also known as “negative” contractions because they involve lengthening of the muscle fibers against resistance. During an eccentric contraction, muscle fibers generate force by resisting an external load. This resistance causes the muscle fibers to lengthen while still generating force. The tension within the muscle also increases, but not to the same extent as during a concentric contraction. Eccentric contractions are important for building muscle strength and improving muscle function.
Isometric contractions occur when muscle fibers create tension but do not change length. In other words, the muscle contracts, but there is no visible movement. Isometric muscle contractions are important for maintaining posture and joint stability and are commonly used in rehabilitation and strength training. During an isometric contraction, the muscle fibers create tension, but the opposing forces are equal, resulting in no movement. Isometric contractions can occur at any point in the range of motion and can be performed at different intensities. For example, holding a static plank is an example of isometric contraction of the abdominal muscles, while pushing against a wall or holding weights in a static position is an example of isometric contraction of the muscles of the upper body. Isometric contractions have several benefits, including improving joint stability, increasing strength, and reducing the risk of injury. Isometric contractions can also be performed at any time, in any place, without the need for equipment. However, isometric contractions also have some limitations. They only provide an increase in strength at the angle at which the contraction is performed, and do not provide cardiovascular benefits.
We will try to bring the latest discoveries closer and look at the results together.
Previous studies have reported increases in strength using high (80% 1-RM) versus light load (30% 1-RM) performance of various exercises in previously untrained women, performed to failure, strength gains and hypertrophy were observed in recreationally active men following different resistance training protocols performed to failure at 20%, 40%, 60% or 80%1-RM.
However, the authors observed greater hypertrophic effects after 80% to 20% 1-RM protocol. Other authors have found similar hypertrophic but greater strength gains induced by high versus low load in resistance-trained men. More recently, some systematic reviews with meta-analysis have compared the effects between high and low workload on different parameters.
The first meta-analysis summarized the effects of high (>> 60%1-RM) versus low load (≤ 60%1-RM) training on muscle strength and hypertrophy, performed to failure by healthy subjects for at least six weeks.
The authors have showed that strength gains in dynamic 1-RM were greater when heavy loads were used (effect size: 0.58, 95% CI 0.28 to 0.89), although gains in isometric strength were similar (effect size: 0.16, 95% CI – 0.10 to 0.41) and hypertrophic effects were obtained (effect size: 0.03, 95% CI – 0.08 to 0.14).
Another meta-analysis showed that the hypertrophic effects of high and low loads were similar in type-I (effect size: 0.28, 95% CI − 0.27 to 0.82) and type-II fibers (effect size: 0.30 , 95% CI − 0.05 to 0.66), although it has previously been suggested that the lack of difference may be mediated by other factors such as training to failure, since in protocols that include training to failure, high loads act more effectively to stimulate increases in the sizes of all type of fiber.
Taken together, both high and low loads can be effective for resistance training, provided other factors (eg, training to failure) are taken into account.
However, heavy loads appear to be more suitable for maximizing power gains. Therefore, load is the primary factor to be considered and should be included in the calculation of total exercise volume.
The authors found a graded dose-response relationship regarding hypertrophic adaptations.
Interestingly, when looking for the minimum total number of repetitions required to increase strength in resistance-trained men, a total of 6–12 repetitions per exercise performed twice per week appears to be a sufficient stimulus.
Nevertheless, the hypertrophic stimulus resulted in an overall dose-response relationship regardless of sex, age, and body region.
To summarize, the total number of repetitions must be stated, primarily because the dose-response principle may apply to hypertrophic muscles, but not to strength adaptations. By the way, the total number of repetitions should be considered part of the volume of the exercise and should not be considered the volume itself.
Recently, two different meta-analyses compared the effects of repetitions to failure on strength and hypertrophy. When total volume was not matched (although it was not declared how), one meta-analysis reported greater overall increases in strength (effect size: 0.34, 95% CI 0.00 to 0.67) and power (effect size: 0, 61, 95% CI 0.08 to 1.15) when training was not performed to failure and a greater hypertrophic response when training was performed to failure (effect size: 0.82, 95% CI 0.09 to 1.56).
Notably, the authors reported a small but significant advantage in hypertrophic adaptations in resistance-trained subjects when training to failure (effect size: 0.15, 95% CI 0.03 to 0.26).
Finally, performing repetitions to failure appears as the primary stimulus when using light loads, as discussed above.