The Comparative Effectiveness Of Isotonic And Isokinetic Strength Trainings On Quadriceps Peak Torque

Introduction

The lack of force generation by the muscles is an impairment contributing to an inability to perform everyday or sports activities. The musculature around the knee, especially the quadriceps, is important in the prevention of injuries, as well as in the enhancement of the knee function. Isotonic and isokinetic exercises are commonly used methods of developing quadriceps muscle strength in both sports medicine and rehabilitation [7, 11, 16, 23].

During isotonic contractions, the neuromuscular system has to overcome an initial resistance (constant throughout the movement) to move the lever arm [20]. On the other hand, isokinetic contractions provide muscle training throughout the range of motion of the joint at a pre-set, constant speed of contractions with a constantly accommodating resistance [25].

There is an ample volume of literature regarding the comparison of both isotonic as well as isokinetic strength training programs on quadriceps muscle strength development. However, controversy still exists regarding the use of these exercises for strength development. Several researchers have indicated isotonic exercises to be more efficacious than isokinetic exercises for muscle training during rehabilitation [17, 21]. In contrast, Cordova et al. [9] and Wojtys et al. [24] reported isokinetic strength training to be at the upper edge over isotonic training.

Indeed, there is a scarcity of evidence regarding the comparison of the effect of open chain and specific training regimen types of isotonic and isokinetic exercise programs on the peak torque of the quadriceps muscle. The relationship between joint torque and angular velocity is often used to quantify dynamic strength, similar to the force-velocity relationship in isolated muscle [12]. The ability to perform a dynamic task often depends on both torque production and the speed of contraction, the product of which is power [23]. Therefore performance during dynamic muscle contractions can be examined by measuring torque across a range of velocities.

Henceforth, the present study was undertaken to evaluate the comparative effectiveness of isotonic and isokinetic strength training programs on quadriceps peak torque in order to find the most appropriate method of developing muscle strength amongst healthy young males. Significant neurophysiological adaptations to resistance training occur in 4 weeks [15]. Henceforth, it was hypothesized that both strength training methods will enhance the quadriceps peak torque after 4 weeks. As previous research has documented both training methods to be superior to each other, we further hypothesized that either of the two exercises may be better than the other.

Materials and methods

Samples

Sixty healthy young males of 19-25 years of age were randomly taken from Maharishi Markendeshwar University, Mullana. Out of the sixty, only fifty-one subjects met the inclusion criteria of having normal Body Mass Index (BMI) value i.e. 18.5 - 24.9 Kg/m2. Out of these fifty-one subjects, six were removed as they matched the exclusion criteria of either having history or evidence of lower extremity musculoskeletal or neurological problems, cardio-vascular, respiratory or major systemic disorders, insufficient strength to complete base line testing or participation in any type of vigorous or recreational activity, aerobic or weight training program during 6 months prior to the study. The remaining forty-five male subjects were familiarized with the experimental protocol and informed about the possible risks and benefits involved in the study both verbally and in writing before giving written consent. The study was approved by Institutional Medical Ethics Committee.

Instrumentation

Easytech Genu 3 Isokinetic dynamometer, manufactured by Biomed-Inc., New Delhi was used for both assessment of Peak torque (Newton-meter) and isokinetic strength training. Quadriceps strengthening table, manufactured by Biomed-Inc, New Delhi was used for isotonic strength training. A static lower limb bicycle ergometer was used for warm-up prior to each assessment and training session. A weighing machine and an anthropometric rod were used to measure body weight and height, respectively.

Testing Protocol

Forty-five subjects were randomly assigned into 3 groups of 15 subjects each (n=15): Group A - Isotonic group, Group B - Isokinetic group, Group C – No exercise Control group. All the subjects of the three groups were pre-tested and post-tested with a gap of four weeks using isokinetic dynamometer to evaluate peak torque of the quadriceps. The subjects were asked to perform 10 repetitions at 2200/sec followed by 10 repetitions at 1000/sec with a rest interval of 1 minute between the sets. The peak torque of the quadriceps at both velocities was recorded. A general body warm-up of 5-minute easy stationary cycling followed by a 5-minute rest interval was included before each testing.

Training Protocol

Group A (Isotonic group):
Each subject was asked to do general body warm-up of 5-minute easy stationary cycling followed by a 5-minute rest interval before each exercise session. 1RM was estimated using Brzyncki [6] equation:

• 1RM = Weight lifted during n RM / (1.0278 - 0.0278(n))

10 RM was determined through 75% of the 1 RM [15]. Each exercise session consisted of three sets of 10 repetitions and 1-minute rest between the sets. Each exercise session was performed three times a week for four weeks.

Isotonic protocol
• 10 repetitions with 50% 10 RM
• 10 repetitions with 75% 10 RM
• 10 repetitions with 100% 10 RM

Group B (Isokinetic group):
Subjects were asked to do general body warm-up of 5-minute easy stationary cycling followed by a 5-minute rest interval before exercise.

Isokinetic protocol

Group C (Control group)
The subjects in the control group were tested twice with the gap of 4 weeks.

Statistical Analysis

All data are presented as the mean ± standard deviation (SD). The data were analyzed for statistical significance by using the statistical package for social sciences (SPSS 16.0) software. Two way ANOVA followed by post hoc multiple Scheffe analysis was done.

Results

The Mean ± Standard Deviation (SD) values for Group A, Group B, Group C at Pre and Post testing at 2200/sec and 1000/sec are shown in Table 1, Table 2, and Table 3, respectively. The comparison of peak torque values at 2200/sec (F=3.17, p>0.05) and 1000/sec (F=1.56, p>0.05) at baseline with one way ANOVA showed statistically non significant differences in all the groups. One way ANOVA revealed statistically significant changes only at 2200/sec (F= 4.47, p<0.05) (Table 4). Post hoc multiple Scheffe analysis showed peak torque of isotonic group to be higher than in the isokinetic group. The control group showed least peak torque values.

Table 1. Intra group comparison for peak torque at 2200/sec and 1000/sec for Group A (Isotonic group).

Table 2. Intra group comparison for peak torque at 2200/sec and 1000/sec for Group B (Isokinetic group).

Table 3. Intra group comparison for peak torque at 2200/sec and 1000/sec for Group C (Isokinetic group).

Table 4. Post test ANOVA between and within groups.

Figure 1. Intra group comparison of peak torque (Nm) at 220 deg/sec for group A, B and C.

Figure 2. Intra group comparison of peak torque (Nm) at 110 deg/sec for group A, B and C.

Discussion

The findings from the present study suggest that both Isotonic (at 2200/sec and 1000/sec) and Isokinetic (at 2200/sec) strength training programs improved the peak torque of the quadriceps after 4 weeks. On the other hand, non-experimental control group did not yield any significant changes in the intra-group comparison.

As previously stated, there have been decades of compiling a substantial database relevant to the effects of both types of strength trainings on various muscle groups throughout the body [1, 4, 5, 18, 19]. Although both isotonic and isokinetic contractions present different biomechanical characteristics, they both load the neuromuscular system [22]. An increase in the number and frequency of firing motor units has been observed after isotonic and isokinetic strength training [21]. Remaund et al. [20] suggested that specific changes on the torque-angle and torque-velocity relationship may be induced with both isotonic and isokinetic strength training.

Furthermore, the isotonic group revealed the highest peak torque value on post-training at 2200/sec, followed by the isokinetic and control groups respectively. Besides Kovaleski et al. [17] and Schmitz and Westwood [21], similar results have also been reported by Smith and Melton [22]. There may be many underlying factors responsible for such findings. Greater motor unit activation per unit of work performed with isotonic exercise as compared to isokinetic exercises may be one of the reasons for better improvement with isotonic strength training as observed by Schmitz and Westwood [21]. Kovaleski et al. [17] also found greater strength improvement with isotonic training than with isokinetic training on isometric tests at 100, 300, 500, 700 and 900 of knee flexion. They reported similar findings for isotonic and isokinetic power.

According to Connelly and Vandervoot [8], isotonic exercises are the closest form of exercise to normal movement, that is, they are close to functional movement. So, it would not be surprising to find that these exercises increase muscle strength at double the speed of isometric and isokinetic exercises in the untrained population.

Another important explanation for lower peak torque values following isokinetic strength training may be that it provides constantly accommodating resistance rather than maximal resistance during the exercise because the resistance provided by the isokinetic dynamometer matches the force exerted on the attachment arm by the user through the range of motion of an exercise [10]. Hostler [13] has reported that optimal strength development can occur only when a muscle contracts against a degree of resistance high enough to reach a maximal or near maximal effort, which is enough to cause fatigue to the trained muscle.

Moreover, isokinetic exercise involves three phases of movement; acceleration, constant velocity, and deceleration. The acceleration phase, the rate of velocity development, represents the beginning part of the motion and is performed without resistance. The constant velocity phase follows the acceleration phase of movement and corresponds to the match between mechanically imposed velocity and the subject’s movement. The third phase of motion, the deceleration phase, represents slowing down of the device prior to contacting the end stop [3]. By definition, only the constant velocity portion of the range of motion represents load range [4]. Thus, maximal loading of the neuromuscular system may occur only during the second phase of the isokinetic exercise.

Another important factor is co-activation of the hamstring muscle during isokinetic knee extension which may have led to weaker work:repetition ratio with isokinetic strength training. Co-activation of the hamstring muscle during isokinetic knee extension exercises has been reported by many researchers in the past [2, 14, 20].

Conclusions and practical application

The findings of this study can be summarized as follows: both isotonic and isokinetic exercises are effective in increasing the quadriceps muscle strength. However, isotonic exercises are more effective than the isokinetic exercises in increasing the peak torque at high velocity i.e. 2200/sec. Thus, considering the above findings, isotonic strength training should be used in order to enhance strength indices and possibly in competition results in sprint and power sports requiring more quadriceps strength. Future studies need to focus on other factors such as different velocities, repetitions, duration of training, as well as examining the interaction of fatigue and velocity spectrum training on larger sample sizes. Since it is an easily understandable and applicable method, we recommend health professionals to add open chain isotonic training in rehabilitation program following injury.