By Avery Hinks When performing exercises like a wall-sit, a plank, or holding a box, your muscles are active and producing force, but not creating movement. In the above exercises, muscles are performing “isometric” contractions, contracting without moving the joints they’re connected to. These exercises are different from exercises that involve movement, such as squats, push-ups, or most exercises you’d see at a gym. In those cases, muscles are instead performing “concentric” and “eccentric” contractions. You could argue that exercises involving concentric and eccentric contractions have better applicability to sports and everyday life, and you’d be correct! When you kick a ball, you are performing concentric contractions of your quadriceps. When you walk up the stairs, your legs are performing concentric contractions. When you lift an object off the floor, you’re performing concentric contractions of your arms and legs. When you set the object back down, those same muscles are performing eccentric contractions. However, what if someone has a limited range of motion, such as while recovering from an injury? In that case, it would be harder to perform exercises involving concentric and eccentric contractions. Here, strength training with isometric exercise is an important alternative. Additionally, some sports do in fact require isometric strength. For example, when using grip strength in mountain climbing, resisting an opponent in wrestling, or stabilizing the body during skiing. But the benefits of isometric strength training may extend beyond performance in isometric actions. Our lab has shown that isometric training can improve performance during movement as well! Our isometric strength training interventionIn 2019, we conducted an isometric training study with a wide variety of measurements that we published in three separate papers. Seven men and six women participated in 8 weeks of isometric strength training 3 days per week. The muscle group they performed strength training on was the “dorsiflexors,” which are responsible for pulling the top of your foot closer to your shin. The “tibialis anterior” is the primary dorsiflexor muscle, so focusing on the dorsiflexors allowed us to target mainly that one muscle during training. Our setup for this training program is pictured below. Additionally, we wanted to investigate whether the muscle length used during isometric training can make a difference. To do this, each participant trained one of their legs with the tibialis anterior at a long muscle length, and the other leg with the tibialis anterior at a short muscle length. Benefits for muscle structure and isometric strengthMuscle thickness and fascicle pennation angle (PA on the graph below) measured by ultrasound can provide an indication of changes in muscle mass. Following training at both muscle lengths, the thickness and pennation angle of the tibialis anterior increased, as shown below. Muscle thickness and pennation angle also often relate to a muscle’s isometric strength (for more information, see our previous knowledge translation). Correspondingly, maximum isometric strength of the dorsiflexors (“MVC Torque” on the graph below) also increased by about 10% following training at both muscle lengths. These improvements in strength are indicated by the red line on the graph below, which represents the average of all participants. The above measurements of isometric strength were obtained at only one joint angle. This distinction is important because, in real life, the body needs to generate isometric strength at more than one joint angle. When we looked into strength adaptations at other joint angles, we observed different results between training at a short muscle length and training at a long muscle length! First, a quick lesson on what we call the “force-length relationship.” For a muscle to generate force, proteins called actin and myosin must bind together. This phenomenon occurs in microscopic units called sarcomeres, which there are thousands of within a whole muscle. When a muscle is in a stretched or shortened position, these sarcomeres stretch and shorten as well. In stretched or shortened positions, it is harder for actin and myosin to bind together and produce force. With this in mind, force initially increases with increasing muscle length, and we call this the “ascending limb.” There is then a range of muscle lengths in which strength is optimal, called the “plateau region.” Lastly, force decreases as muscle length continues to increase, and we call that the “descending limb.” The width of the plateau region is especially important because it tells us the range of joint angles at which strength is close to optimal. We investigated how the shape of the force-length relationship changed following isometric training at a short compared to a long muscle length. While training at a short muscle length improved strength at some joint angles, the width of the plateau region did not change. Following training at a long muscle length, however, the width of the plateau region widened considerably (indicated by the red box on the graph below). In other words, isometric training at a long muscle length can bring strength close to optimal across a wider range of joint angles. More importantly, it can improve strength at joint angles other than the angle used in training! Widening of the force-length relationship is often associated with in an increase in muscle fascicle length (FL on the graph below). We only observed an increase in fascicle length following isometric training at a long muscle length, so with that in mind, the widening of the plateau region is not surprising. This elongation of muscle fascicles likely occurred because the muscle needed to adapt to acting at a long muscle length during training. Does isometric training improve muscle performance during movement? Muscle strength is not only measured in isometric contractions. As mentioned earlier, concentric contractions may be more applicable to everyday movements. We saw that isometric training can also improve strength during concentric contractions (called “dynamic torque” in the graph below). This result did not differ between training at a short and long muscle length. We can also investigate a muscle’s performance during movement by assessing maximum power. Power equals the force produced by the muscle during movement multiplied by the velocity of the movement. Therefore, maximum power incorporates both the strength and the speed of a muscle. This cross between strength and speed is important for explosive movements, such as jumping, sprinting, or powerlifting. We assessed maximum muscle power in four different conditions of movement:
Conditions using a heavier weight and a larger range of motion are intended to be more difficult. Following isometric training at a short muscle length, maximum power only increased in the two small range of motion conditions. Following training at a long muscle length, maximum power increased in the small range of motion conditions even more, and also increased in the large range of motion, 10% weight condition. These results are shown in the graph below, with the red lines representing the average of all participants. We can also assess a muscle’s performance during concentric contractions by looking at the “rate of velocity development.” Rate of velocity development is basically just acceleration (like what you do with your car when merging onto a highway), and describes how fast a muscle can increase its speed. This measurement may seem trivial, but it has applications in everyday life. Most prominently, if you suddenly fall, your muscles must accelerate to catch you. With both isometric training groups combined, we observed increases in rate of velocity development (RVD in the graph below) during all four concentric contraction conditions. Altogether, even though isometric training involves only static contractions, it can improve performance in dynamic contractions—especially when training is performed at a long muscle length! Conclusion Isometric strength training has sport-specific applications and may be useful during recovery from an injury that restricts range of motion. Some people may also prefer the static nature of isometric training compared to conventional resistance training (see the photo below from isophit.com). And that’s okay! Our study determined that isometric training is a viable option for eliciting positive adaptations in muscle structure and improving performance in both static and dynamic actions. When isometric training is performed at a long muscle length, these adaptations seem to be even more pronounced! References Akagi R, Hinks A, Power GA. Differential changes in muscle architecture and neuromuscular fatigability induced by isometric resistance training at short and long muscle-tendon unit lengths. J Appl Physiol (1985). 2020 Jul 1;129(1):173-184. doi: 10.1152/japplphysiol.00280.2020. Epub 2020 Jun 18. PMID: 32552430; PMCID: PMC7469237.
Davidson B, Hinks A, Dalton BH, Akagi R, Power GA. Power attenuation from restricting range of motion is minimized in subjects with fast RTD and following isometric training. J Appl Physiol (1985). 2022 Feb 1;132(2):497-510. doi: 10.1152/japplphysiol.00688.2021. Epub 2022 Jan 13. PMID: 35023762. Hinks A, Davidson B, Akagi R, Power GA. Influence of isometric training at short and long muscle-tendon unit lengths on the history dependence of force. Scand J Med Sci Sports. 2021 Feb;31(2):325-338. doi: 10.1111/sms.13842. Epub 2020 Oct 19. PMID: 33038040.
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