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J Athl Train. 2001 Oct-Dec; 36(4): 384–387.
PMCID: PMC155433

Knee Extensor Electromyographic Activity-to-Work Ratio is Greater With Isotonic Than Isokinetic Contractions



To determine whether isotonic or isokinetic contractions produced greater electromyographic (EMG) activity per unit of work during isotonic and isokinetic knee-extension exercise.

Design and Setting:

Subjects performed three 3-second maximal voluntary isometric contractions of the dominant knee extensors for EMG normalization. Exercise testing performed on the Biodex System 3 Dynamometer involved 10 isokinetic contractions at 180°·s−1 and 10 isotonic contractions with the resistance set at 50% of the previously recorded maximal voluntary isometric contraction.


Recreationally active college students (10 men and 11 women).


Surface EMG signals were collected from the vastus medialis and lateralis muscles and then integrated (IEMG) over the concentric phase of each repetition for both exercises. The IEMG was divided by the total work performed during the concentric phase for each exercise (IEMG/W).


We analyzed the IEMG/W data using a 1-between (sex), 2-within (exercise and muscle) repeated-measures analysis of variance. There was a significant main effect for exercise, with the isotonic IEMG/W value being significantly greater than the isokinetic IEMG/W value. Additionally, the IEMG/W relationship did not appear to be affected by sex or individual muscle tested.


Per unit of work performed, the isotonic contractions resulted in greater motor unit recruitment or an increased rate of firing, or both. This finding may have implications for the early phase of rehabilitation, when goals include complete motor unit recruitment of injured or atrophied muscles.

Keywords: rehabilitation, IEMG, dynamometry

Two common types of active resistance exercise are reported in the sports medicine literature: isotonic (constant load) and isokinetic (constant velocity or accommodating). Force-production capability of muscle can be increased by either method of resistance, provided the normal stimulation patterns of the muscle are exceeded.1 With the advent of the isokinetic method of muscle resistance came the natural comparisons with the more common methods of isotonic resistance training. With either method, training regimens of the past and the present are most often based on the number of sets and repetitions completed.2

Work is performed when the muscle shortens and a limb is moved through space. Isokinetics are an accommodating type of exercise that theoretically allows the muscle to perform more work over the same range of motion compared with isotonic exercise.3,4 This allows the exercising muscle to generate its maximal mechanical output at all angles throughout the range of motion, which allows for changes in the musculoskeletal lever system. With isotonic resistance, the muscle must simply overcome inertia to move the limb through space.5 It has been speculated that isotonic resistance may not be entirely efficient for loading the muscle throughout the physiologic range of motion.3 Efficiency in this context occurs when the type of resistance matches the maximal muscle-joint complex torque output throughout the entire range of motion.3

The effectiveness of isotonic versus isokinetic contractions in the development and assessment of muscle strength has long been investigated.3,6 The superiority of both isotonics7 and isokinetics8 has been demonstrated. Kovaleski et al7 showed that an equal number of sets and repetitions for isotonic and isokinetic training groups resulted in greater strength gains for the isotonic group. In contrast, Wojtys et al8 reported that isokinetics were superior to isotonics in training groups that completed an equal number of sets and repetitions during a 6-week training protocol. However, Wojtys et al8 did not account for specificity of contraction type and testing.

To increase the force-generating capacity of muscle, one must increase normal activation levels.1 Activation levels can be increased by increasing the number of stimulated muscle fibers or increasing the rate at which the muscle fibers are recruited, or both. Surface electromyography (EMG) is a tool that may be useful in the indexing of such phenomena because it can measure the voltage associated with recruitment of motor units. Integrated EMG (IEMG) has been described as a measurement for estimating the number of motor units firing and the firing frequency of motor units.1

As described previously, isotonics and isokinetics are 2 dynamic methods used to improve muscle strength. Because the goal of both training methods is to increase the number of motor units being recruited or the frequency at which they are recruited (during maximal contractions), surface EMG could be used to measure motor unit activation during each type of contraction. Previous researchers have investigated differences in activation levels of maximal isotonic and maximal isokinetic contractions. No differences were noted between the IEMG of the 2 contraction types of the biceps brachii.5

The previously described information provides insight into the differences between isokinetic and isotonic contractions. To correctly determine which type of resistance may be best suited to recruit more motor units or increase the frequency at which motor units fire, we can investigate the relative activation level of the motor units per unit of work performed. If muscle activation level is normalized to the amount of work done, the contraction type that provides for optimal recruitment of the muscle could be determined. This may be of importance in the early phase of rehabilitation, when completeness of central drive rather than intensity (force level) is important.9 Therefore, our purpose was to determine whether isotonic or isokinetic contractions produced greater EMG activity per unit of work during knee-extension exercise.



Twenty-one volunteers (10 men, 11 women; age, 20.3 ± 1.6 years; height, 175.9 ± 10.5 cm; mass, 74.4 ± 15.6 kg) were recruited from the general college population. All subjects gave informed consent as approved by the university institutional review board before participating in the study. The board also approved the study.


All muscle testing was performed on the Biodex System 3 Dynamometer (Biodex Medical Systems, Shirley, NY). Torque, velocity, and position analog data were collected from the Biodex during all muscle performance measures and digitized for storage and later analysis. Simultaneously, the surface EMG signal from the vastus lateralis and vastus medialis muscles was collected with the Therapeutics Unlimited Model 544 System (Therapeutics Unlimited Inc, Iowa City, IA) (input resistance, 15 MΩ at 100 Hz; common mode-rejection ratio, 87 dB at 60 Hz; sampling frequency bandwidth, 20 to 4000 Hz) using preamplified electrodes (Therapeutics Unlimited; preamplification gain, 35; interelectrode distance, 22 mm; electrode diameter, 8 mm) and subsequently recorded at a frequency of 1000 Hz on a Pentium-based microcomputer. The EMG activity from both muscles was collected to ensure that we obtained the valid signal from the muscles, because EMG and force contribution should be equal from the respective muscles during knee extension.10,11 Therefore, any EMG differences between the vastus lateralis and vastus medialis would alert us to problems with the EMG data collection. All data extraction was performed with custom LabVIEW Software programming (National Instruments Corp, Austin, TX).

Setup Procedures

To prepare each subject for EMG surface-electrode placement, the skin was shaved at each electrode location, followed by abrasion and alcohol cleansing to help reduce skin impedance. Preamplified electrodes were placed midway between the muscle belly and the distal tendinous insertion of the vastus lateralis and medialis muscles and were left in place until the completion of the experiment. A reference surface electrode was placed over the contralateral lateral malleolus. After a 5-minute warm-up on a cycle ergometer, the subject was seated in the chair of the dynamometer. All testing was performed on the dominant knee. Knee dominance was determined by asking the subject which leg would be used to kick a ball. The anatomical axis of the knee was aligned with the axis of the dynamometer, and the distal aspect of the arm of the dynamometer was placed 4 cm proximal to the medial malleolus. The dynamometer seat back was placed at 100°. The ankle was fastened to the dynamometer arm, and the chest, thigh, and waist were fastened to the dynamometer seat with hook-and-loop tape stabilization straps to minimize extraneous movements.

Experimental Protocol

Subjects underwent isotonic and isokinetic exercise protocols on the same day, separated by 5 minutes. Testing order was stratified by sex, then randomly assigned. Total work was calculated from the concentric knee extension data for each exercise.

Exercise Session

Each subject performed 3 submaximal voluntary isometric contractions for isometric familiarization purposes. This was followed by three 3-second maximal voluntary isometric contractions (MVICs) of the quadriceps muscle. Isometric testing was performed with the knee in 90° of flexion.12

For all exercise testing, the subject performed concentric knee extension from 90° to 0° of knee extension.13 For isokinetic testing, the dynamometer's resistance was set at 180°·s−1. This velocity was based on clinical practice and a previous investigation using 180°·s−1 to compare isokinetic with isotonic knee extension.14 The subject performed 5 to 10 isokinetic concentric warm-up repetitions at submaximal and maximal effort for familiarization. For data inclusion, the subject then performed 10 maximal concentric knee extensions. The subject was instructed to kick out “as fast and hard as possible” and then allow the dynamometer to passively return the limb to the starting position (90°) before beginning the next contraction.

For isotonic testing, the resistance of the dynamometer was set at 50% of the peak torque previously recorded during the MVICs. We used this 50% value because it was the highest resistance at which subjects (those not included in the study) could completely extend their knees to 0° in pilot testing. The subjects performed 5 to 10 warm-up repetitions at this resistance. For all isotonic contractions, the subjects were instructed to complete the 90° motion in approximately 1 second. This method is based on anecdotal information to mimic exercise commonly performed in the athletic training rehabilitation setting. We instructed the subjects to allow the dynamometer to passively return the leg to the starting position. For data inclusion, the subjects then performed 10 continuous concentric knee extensions.

Mechanical Data Extraction

Peak torque was obtained from 1 of the 3 isometric contractions with the highest torque produced during that isometric contraction. Work (joules) was defined as the product of the average torque and displacement in units of radians. Total work was calculated from the isokinetic (Wisok) and isotonic (Wisot) exercise.

EMG Data Extraction

The EMG data were forward and backward low-pass filtered at 500 Hz using a second-order Butterworth filter. Data were then full-wave rectified and normalized to the mean amplitude of a 1-second interval that encompassed the MVIC. For each exercise, the EMG data from the concentric portion of the knee extension were integrated over the 10 repetitions and subsequently normalized to the work performed isokinetically (IEMG/Wisok) and isotonically (IEMG/Wisot).

Statistical Analysis

We performed a 1-between (sex = male or female), 2-within (exercise = isokinetic or isotonic, muscle = vastus medialis or vastus lateralis) repeated-measures analysis of variance on the dependent variable of IEMG/W calculated following each exercise. The α level for all statistical tests was set at P < .05.


We found a significant main effect for type of exercise, with IEMG/Wisot being greater than IEMG/Wisok (isotonic, 1156.2 ± 347.3 mV·s·J−1; isokinetic, 629.1 ± 334.5 mV·s·J−1; F1,19 = 33.6; P < .001). Main effects for sex (F1,19 = 0.21, P = .391) and muscle (F1,19 = 0.054, P = .819) were not significant. There were no significant interactions for exercise by muscle (F1,19 = 0.015, P = .904), exercise by sex (F1,19 = 0.048, P = .829), or sex by exercise by muscle (F1,19 = 0.713, P = .409).


Our current data are evidence that, per unit of work performed, motor unit activation is greater during isotonic exercise than during isokinetic exercise. Our novel method of normalizing IEMG to total work makes comparisons with previous studies difficult. Previous researchers have attempted to determine which contraction type (isotonic or isokinetic) can best recruit motor units as measured through surface EMG activity.5,13 In both studies, maximal isotonic and isokinetic contractions of the knee extensors were used, and no significant difference in IEMG activity was reported. These investigators attempted to determine the more effective way to strengthen muscle, but no consideration was given to the amount of work performed during the contractions.

A limitation to our study is that we compared the EMG amplitude from a maximal contraction (isokinetic) with the EMG amplitude from a submaximal contraction (isotonic). We studied these contraction types to better understand differences in motor unit recruitment between isokinetic and isotonic resistance. These findings support the use of isotonic resistance training in the early stage of rehabilitation when central drive to the muscle (motor unit recruitment) may be more important than absolute muscle force production.9

The relationship between EMG and muscle force production in the quadriceps muscle must be considered because we are operating under the assumption that as EMG increases, force also increases linearly.15 Previous research has supported the concept that IEMG does not have a fixed relationship to quadriceps force production at specific points in the range of motion.16 However, we were interested in the EMG across the entire range of motion. As joint angles and muscle lengths change, they may change the EMG-force relationship, making the relationship nonlinear.17 By including the entire range of motion and calculating the IEMG for the entire contraction, we believe that the concerns about EMG and force being nonlinearly related throughout the range of motion are allayed.

Muscle force production is a combination of central factors (such as stimuli to the higher motor centers and motor neuron excitability18) and peripheral factors (such as muscle pH19 and phosphocreatine depletion20). Thus, the ability of muscle to perform work should also be investigated at the peripheral level, because central command may not completely explain the resultant force of muscle contraction.21 We were able to locate one investigation in which isometric contractions were reported to perform more work using fewer energy resources at the peripheral level than concentric isokinetic contractions, using measures of metabolic strain and adenosine triphosphate turnover.22 However, no comparison was made between isotonic and isokinetic contractions.

In our study, isotonic contractions resulted in greater motor unit activation per unit of work performed as measured by our IEMG/W ratio. During the early stages of rehabilitation, it may be more important to maximize the neural drive (ie, increase IEMG) than to increase absolute force levels.9 Increased forces may result in detrimental forces being placed on the recovering injury, such as increased quadriceps force increasing shear forces across the tibiofemoral joint.23

Although we found no differences between men and women, we thought it important to examine any differences between the sexes that would differentiate the use of specific contraction types. In the clinical setting, previous researchers have demonstrated that specific training regimens may decrease incidence of injury around the knee.24 Various training regimens may include isotonics or isokinetics or both; thus, we need to determine differences that would facilitate the rehabilitative process. Our results show that optimal motor unit activation during isotonic and isokinetic resistance exercise appears to be independent of sex. Therefore, the type of resistance may not be important during resistance training for men and women.


The development of strength is a complex phenomenon, regardless of the type of exercise performed. Based on our results, we recommend that clinicians incorporate isotonic exercise early in the rehabilitation process, when motor unit recruitment is of primary importance. With greater motor unit activity per unit of work performed by the muscle-joint system, the potential benefits of less stress placed on the musculoskeletal lesion and more complete or more rapid (or both) motor unit recruitment could serve to enhance the early phase of rehabilitation.


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