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Why study isometric training at all?
Most people in the strength and conditioning industry today train dynamically and largely avoid deliberate isometric training. However, the research suggests that it can be a very effective training method and, if done across a range of angles, it does not necessarily have to limit strength gains to specific ranges of motion. In this article, Chris Beardsley (@SandCResearch) takes a look at a study that presents an interesting breakthrough.
The study: Strength training: Isometric training at a range of joint angles versus dynamic training, by Folland, Hawker, Leach, Little and Jones, Journal of Sports Sciences (2005)
What is the background?
Large increases in strength through isometric training
Despite its lack of popularity in the mainstream fitness industry, a number of researchers have observed that very large and rapid increases in strength occur as a result of isometric training. For example:
- Lindh (1979) noted an increase of 30% in 5 weeks
- Young (1985) noted an increase of 40% in 8 weeks
- Thepaut- Mathieu (1988) reported an increase of 25 – 54% in 5 weeks
- Weir (1995) found that strength increased by 27% in 6 weeks
Compared to concentric-only, eccentric-only training or even traditional (i.e. stretch-shortening cycle) training, this kind of result is pretty good. Let’s take a look at a direct comparison. The chart below shows a study by Jones (1987), in which some of the other dynamic options were compared with isometric training:
The length-specific adaptations of isometric training
Traditionally, it has been said that the major limitation of isometric training is that it produces very muscle length-specific adaptations. This is more commonly expressed as “angle-specific” strength, referring to the angle of the joint at which the strength is expressed. Essentially, researchers have usually found that training at a specific joint angle (which is what happens when you train isometrically) leads to large strength gains at that angle but not at all other angles. In the chart below, taken from Bandy (1993), we can see how training at different angles leads to angle-specific strength gains. You can see that when training at 30-degrees, for example, the greatest increases are in the 15-45 degree range. While when training at 90 degrees, the greatest increases are in the 75-90 range.
Researchers have therefore generally concluded that with dynamic training (i.e. eccentric-only, concentric-only or stretch-shortening cycle), the strength increases throughout the whole range of motion (ROM) are smaller but more evenly distributed.
Training isometrically at a range of angles
Since isometric training has been found to generate great increases in strength that are limited to specific joint angles, it is possible that training isometrically at a range of angles might be more effective over the whole ROM at a joint than dynamic training. Today’s study looks at exactly that question. The researchers noted that because individuals respond so differently to strength training, they would need a huge sample to be certain that differences between the isometric and dynamic programs were caused by the programs and not by the individual responses to training. Therefore, they decided to use the same individuals for both training programs, using different sides of the body for each. They noted that this could be confounded by the crossover effect, which is where training one limb has an effect on the contralateral limb.
What did the researchers do?
The researchers recruited 33 recreationally active male subjects who had not undertaken any leg strength training in the last 6 months. They supervised 9 weeks of unilateral leg training, three times per week, for the quadriceps muscle group. The subjects performed isometric leg extensions with one leg and dynamic leg extensions with the other. For the isometric training, the subjects performed 4 sets of 10 reps of 2 seconds duration. Each set was performed at a different knee angle: 50, 70, 90 and 110 degrees. The subjects took a 2-second rest between contractions and a 2-minute rest between each set. The contractions were monitored with a strain gauge and the subjects aimed for 75% of their maximum force output. For the dynamic training, the subjects performed 4 sets of 10 reps on a variable resistance leg extension machine, taking a short pause between lifts and 2 minutes rest between sets. The variable resistance machine ensured that the dynamic training was isokinetic (constant torque). The researchers tested the strength of each leg of the subjects pre- and post-training. The researchers measured isokinetic strength at three speeds (45, 150 and 300 degrees/s) and isometric strength at four angles of knee flexion (50, 70, 90 and 110 degrees).
The researchers found that both the isometric and the dynamic strength training significantly increased the isometric strength of both legs at a range of angles. However, the gains in isometric strength were greater in the isometrically–trained leg than in the dynamically-trained leg at all angles measured. Additionally, the relative improvements in isometric strength were not equal at all joint angles for both isometrically-trained and dynamically-trained legs. However, there was a strong pattern observed for both types of training. The chart below shows how both types of training appeared to lead to similar strength gains at the different angles tested:
The researchers found that both types of training significantly increased the isokinetic strength at a range of speeds. The improvements in isokinetic strength were similar with both training programs.
What did the researchers conclude?
The researchers drew four conclusions, as follows:
- Both programs led to significant improvements in both isometric and isokinetic strength.
- Both programs led to similar gains in isokinetic strength.
- Isometric training at four joint angles led to significantly greater gains in isometric strength at those joint angles than training dynamically.
- Isometric training at four joint angles did not result in the highly angle-specific adaptations that have been reported for isometric training at just one joint angle.
However, I think the most interesting observation that can be made here is that the gains in isometric strength at specific joint angles are not equal at each joint angle (see the final chart above). Given the same volume of training, the gains in strength are larger at some joint angles than others. Isokinetic training leads to a similar pattern of gains at these same joint angles, suggesting that the phenomenon behind this uneven pattern of gains is independent of the isometric or dynamic mode of lifting.
What were the limitations?
As always with studies, there are always more questions at the end than at the beginning. While the researchers discovered that training isometrically across a range of joint angles produces similar results in full ROM isokinetic strength, there are still questions outstanding, because of the way in which the study was constructed, as follows:
- The researchers compared isometric training with isokinetic (accommodating resistance) training. Therefore, we do not know how isometric training stacks up against the various types of traditional isoinertial training. This is quite important, as most sports-specific training uses isoinertial resistance.
- The researchers selected four joint angles at 20-degree increments. Therefore, we do not know whether three angles at 30-degree increments or five angles at 15-degree increments (for example) would produce better or worse results.
- The researchers only tested the isometric strength of the subjects at the same joint angles that they trained at. Therefore, this study cannot help address the question regarding the transferability of isometric training at one joint angle to other joint angles, since all angles tested were trained at.
- Both programs led to different strength gains at different joint angles (see the chart above), even though the isometric program trained each of these joint angles with equal volume and the isokinetic training required equal force at all joint angles. This is a fascinating result but the study does not help us understand why it occurs.
What are the practical implications?
Isometric training at several equally spaced joint angles can produce similar full-ROM isokinetic strength gains as full-ROM isokinetic training. This could be useful for training with minimal equipment or for designing bodyweight training programs for building strength through full ranges of motion using only static positions.
Both isometric and isokinetic training do not build strength equally at all joint angles. Therefore, if your sport or task requires strength at a specific joint angle, it might be necessary to look at targeted methods of increasing the strength at that specific angle, either by partials, eccentrics, isometrics at long or short lengths, or dynamic exercises that stress a particular ROM.