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Putting isometric contractions on hold for strength gains
The immediate benefits of a resistance training program include improved strength, larger muscle volume, stronger bones, and tendons that withstand the burdens of added load. For athletes, strength training also helps improve performance and prevent injuries, especially when tailored to the needs of their sport. The challenge is finding the time to adhere to a resistance training program while also completing sport-specific drills, cardiovascular training, and competitions. Often times, if an athlete must make compromises, the time in the weight gym is the first thing to go.
Researchers from Taiwan wondered if there might be a more efficient way to grow muscle and gain strength(1). They proposed that incorporating an isometric hold between resistance training sets might speed muscle adaptations, thus growing muscle, strength, and endurance, compared to a traditional weight training program. The study enrolled 35 resistance-trained males, ages 18-35, in the study. After taking baseline measurements of muscle thickness ( of the upper arm and upper leg using ultrasound), muscle strength, and muscle endurance (both measures conducted using the incline bench press and leg press exercises), the investigators randomly assigned participants to either a traditional resistance training program (TRAD) or one that incorporated isometric holds (ISO).
All subjects performed the same weight training program of three sets of eight to 12 repetitions of six exercises, three times per week for eight weeks (see table 1). Load was adjusted for each individual to maintain momentary concentric failure at the end of each set. All participants were allotted two minutes of rest between sets. However, immediately after finishing each set, the ISO group executed a no-load isometric hold of either the elbow flexors, elbow extensors, or the quadriceps muscles for 30 seconds. They then rested for the remainder of their allotted time.
Table 1: Exercises performed by all participants
- Flat barbell
- Bench press
- Barbell military press
- Wide grip lat pulldown
- Seated cable row
- Barbell back squat
- Machine leg press
The upper extremity isometric holds consisted of positioning the arms close to the body and then squeezing the elbows in either as much flexion or extension as possible and holding that position for 30 seconds. The lower extremity hold was performed in sitting with knees extended as much as possible. All participants were instructed to refrain from any other type of exercise during the study. The investigators also tried to control for diet, asking each subject to keep a daily food diary. They also provided all participants with the same protein shake supplement on weight training days.
Of the 35 men that started the study, 27 completed the program – 13 in the TRAD group and 14 in the ISO group. Adherence to the training schedule was good with an average of 92% of all workouts completed. The changes in muscle thickness between the two groups were similar for the upper arm. However, the ISO group showed modest gains in the size of the upper leg muscles when compared to the TRAD group. The change in upper extremity strength was similar between groups while the change in strength for the lower extremity seemed to favor the TRAD group. However, the statistics seemed skewed due to the progress of one individual. When his results were left out, the change scores became more comparable. The ISO group demonstrated small gains in muscular endurance compared to the TRAD group when measuring the number of repetitions each subject could perform bench pressing 50% of their 1RM.
While theoretically isometric holds should add to the muscular adaptations of a traditional strength training program, the results of this study fail to support this approach. Of note, the isometric holds were performed with the muscles in the shortest position possible. According to a study conducted at the University of Michigan, optimal isometric force production occurs with the muscle in a somewhat elongated position(2). For instance, the best position for triceps isometric force production is between 125º to 130º elbow flexion, up to 90º shoulder flexion, and 90º supination, depending on the targeted portion of the muscle(1). The best position for isometric force production in elbow flexion is around 125º with 90º supination. Knee extension isometric force is best produced with at least 28º degrees of knee flexion.
Conceptually an isometric contraction with greater force may improve muscle adaptations and growth compared to those executed in the shortened position. However, they don’t seem at this time to provide a short cut to obtaining the benefits of a consistent and well-executed strength training program. The ability of added isometric contractions to hasten muscle gains when injured deserves more exploration. If adding to a rehabilitation strengthening program consider positioning the muscle in the most advantageous position.
- Front Physiol. 2020 Jan;10:1571
- J Sport Rehabil. 2016 Nov 1; 25(4): jsr.2015-0118