Smooth movement: the benefits of isokinetic training

Andrew Hamilton looks at recent evidence for the benefits of using isokinetic strength training in rehabilitation.

The goals of clinicians and trainers involved in the care and rehab of athletes are simple: firstly, to try and prevent the occurrence of injury, and secondly, to help rehabilitate those athletes that succumb to injury as rapidly as fully as possible. In many cases of injury rehab, the use of strength and conditioning is a crucial tool in the box.

Speed, resistance and distance

Muscular overload is necessary for any strength and conditioning protocol (all of which require the application of external resistance), and there are many ways to achieve this. The types of applied resistance include:

• Isotonic – muscles work against constant weight/loading
• Isometric – muscles work at a constant (fixed) position regardless of load
• Isokinetic – muscles work at a constant velocity regardless of load

Most strength and conditioning settings employ isotonic loading thanks to the availability of free and machine weights, which are conveniently adjusted and relatively inexpensive. However, in theory, at least, isokinetic training devices offer some distinct advantages. In particular, isokinetic training applies maximum tension and loading to a muscle at all points through its range of motion; this is in contrast to conventional resistance (isotonic training). In Isotonic training, the loading occurs within a particular range of motion, meaning that at other points in the movement, the loading is less than optimal.

Perhaps more significantly in an athletic context, isokinetic loading allows maximal force development at various limb speeds, and at high velocities. In isotonic loading, limb movement rarely exceeds 60 degrees/second; however, in most functional sporting movements, limb movement exceeds 90 degrees/second, with some exceeding 200 degrees/second⁽¹⁾. Another potential advantage of isokinetic loading is the ability to measure and monitor the relationship between joint angle and force produced (see figure 1). This information can provide clinicians with considerable insight into how effective/resilient a joint is under load, particularly during the functional demands of a particular sport.

Figure 1a: Changes in the angle-torque relationship for two leg extension training protocols⁽²⁾

NB: HF = high fatigue training; LF = low-fatigue training. Isokinetic leg extension performed at a constant speed; joint angle measured in rads - from 2.09 rad (120°) to 0.26 rad (15°)

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Isokinetic strength training in rehab

There’s no doubt that isokinetic training can deliver potential advantages, particularly for high-velocity limb movements. But what about its role in injury rehab? Is isokinetic training an effective protocol for injury rehabilitation, and does it provide additional benefits above and beyond those derived from conventional resistance training? Very few studies have directly compared the use of isokinetic vs. isotonic training protocols for injury rehab, but there’s certainly some evidence for the benefits of isokinetic training.

In a 2019 study, Chinese researchers investigated the effect of isokinetic training on knee function and graft remodeling after anterior cruciate ligament (ACL) reconstruction⁽³⁾. In this study, 40 patients who had undergone arthroscopic ACL reconstruction using hamstring tendon grafts were randomly divided into two groups:

• Isokinetic post-op training group
• Control group (who trained using conventional resistance methods)

Importantly, there was no significant difference in gender, age, body mass index, side of injury, the interval between injury and operation, and preoperative International Knee Documentation Committee (IKDC) score between the two groups.

Both groups of patients underwent staged rehabilitation treatment; however, the isokinetic group replaced the traditional intervention with the corresponding isokinetic strength training in the three to six months after the operation. The study used traditional rehabilitation intervention for the control group. The peak torque (PT) of knee extension and flexion and hamstring quadriceps ratio (H/Q) were measured at three, six, and 12-months post-op, and MRI examination was also performed at these times to evaluate graft remodeling. A further arthroscopy was performed at 24-months post-op to assess the shape, tension, and degree of vascularization of grafts. The results were as follows:

• At the 24-month arthroscopy, the IKDC score was 90.45 in isokinetic group and 89.32 in control group – both significantly improved compared with preoperative scores but not significantly different between the two groups. However, histological scores in the isokinetic group were superior to those in the control group.
• At 6 and 12 months, the peak torque figures of knee extension and flexion in isokinetic group were higher than those in control group.
• The H/Q ratio scores at 6 and 12 months were higher in isokinetic group than in control group.
• There was no significant difference in MRI scores between the two groups at 3 and 6 months, but the MRI scores at 12 months were significantly higher in isokinetic group.

In their summary, the researchers concluded that

“using an isokinetic training system to develop a suitable post-surgical isokinetic rehabilitation training program was helpful in early muscle strength recovery, early graft remodelling, and even long-term histological results after ACL reconstruction.”

In another study on the use of isokinetic training for rehab following ACL reconstruction (also published 2019), Brazilian researchers compared the effects of conventional (constant load) eccentric training and isokinetic eccentric training on quadriceps muscle mass, strength and functional performance in recreational athletes following anterior cruciate ligament (ACL) reconstruction⁽⁴⁾. To do this, 30 recreational male athletes (average age 25 years old) undergoing ACL reconstruction received a standard rehabilitation program. Before rehab commenced, the athletes were randomized to either a conventional rehab group (who performed conventional isotonic training on an extensor chair) or an isokinetic group (who performed isokinetic training using an isokinetic dynamometer- see figure 2).

Figure 2: Using an isokinetic dynamometer for hamstring/quadriceps training and testing

Click on the image above for an informative overview of knee extension/flexion strength testing/training on an isokinetic dynamometer.

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Both groups performed two sessions per week of rehab training for six weeks. Assessments of quadriceps muscle mass (using MRI), strength (through isokinetic dynamometry), and self-awareness functionality (via a questionnaire) were performed before and after the training programs. Single leg hop test performance was assessed only at post-training evaluation. The results showed that the isokinetic-trained subjects experienced greater improvements than the conventionally trained subjects for all muscle mass outcomes (an average of 17-23% gains in the isokinetic group vs. 5-9% in the conventional group). The same was true for peak torque improvements. The isokinetic versus conventional gains were +34% versus +20% for isometric peak torque and 85% compared to +23% for eccentric peak torque. Interestingly. however, there was no between-group difference for concentric peak torque or single-leg hop test scores.

Partial meniscectomy and quadriceps strength

Yet another study suggests that isokinetic eccentric training may be more effective than constant-load eccentric training in rehab – this time for quadriceps rehabilitation following a partial meniscectomy⁽⁵⁾. In this study, 32 recreational male athletes (average age 27 years old) who had undergone a partial meniscectomy performed a six-week quadriceps strength training program. Before the rehab training, however, the athletes were randomized into two groups: conventional-load training group or an isokinetic training group. Before and after the rehab program, all the athletes were assessed for quadriceps muscle mass, strength, and function.

As expected, both rehab groups experienced enhanced muscle mass, strength, and functionality outcomes over the six-week intervention period. However, the isokinetic group experienced more significant gains in muscle mass, muscle strength, and the Lysholm knee-score scale. The researchers unsurprisingly concluded that, following a partial meniscectomy, isokinetic eccentric training is more effective than conventional eccentric training to restore quadriceps muscle mass, strength, and functional capacity.

Utilizing isokinetic training

Controlled studies comparing isokinetic and isotonic strength training use in athlete injury rehab are few. This makes it difficult to draw definitive conclusions about its efficacy. However, given the current evidence, clinicians should feel confident that isokinetic training is a good alternative to isotonic/isometric training, if not a superior option.

The relative paucity of research means that evidence-based specific guidelines for clinicians wishing to introduce isokinetic training during rehab are limited. An excellent review by Ellenbecker and Davies, found here, outlines one approach. They recommend an eight-stage method beginning with sub-maximal isometric exercises and short-arc isotonic exercises, before introducing isokinetic training at stage 5 (see figure 3)⁽⁶⁾.

Figure 3: Stages of Ellenbecker and Davies' resistive exercise progression continuum

Isokinetic training is introduced into rehab at stage 5, following appropriate isometric and short-arc isotonic training. Note that stage 8 involves full range of motion (ROM) isokinetic training, providing a further level of training adaptation above and beyond full range isotonic training.

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Short-arc exercises are often started using submaximal isokinetics, due to the accommodating resistance inherent in isokinetic exercise, making them safe for the patient's healing tissue. Velocities in short-arc isokinetics should range from 60 to 180 degrees/second. Velocities slower than 60 degrees/second increase joint compressive forces and often create inhibition responses, thus prompt athletes to keep the velocity above this range. Meanwhile, velocities greater than 180 degrees/second for short-arc isokinetic exercises create too broad a range of free-limb acceleration, resulting in insufficient loading of the muscles.

Isokinetic exercise contains three major components: acceleration, deceleration, and effective ‘load range’. The load range is the actual portion of the range of motion in which the preset angular velocity is met by the patient and a true isokinetic load is imparted to the patient. A patient will experience a larger load range at slower contractile velocities, with a statistically shorter load range at faster contractile velocities⁽⁷⁾. When setting the range of motion for an isokinetic exercise, the acceleration and deceleration components must be factored in, which means a greater range than desired for training will be required.

Summary

Although the use of isokinetic strength training in injury rehab is supported by the general literature, there is a paucity of data regarding its efficacy when used in athletic populations and compared to conventional (isotonic) rehab training. The limited data that does exist, however, points toward isokinetic training as a superior alternative to more conventional means. When incorporated into a rehab strength training program, use a staged approach starting with low-range isotonic and isometric exercises followed by isokinetic training.

References

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