Monitoring the improvement of muscle performance using isokinetic dynamometry: A comparative analysis based on the full range of motion vs. the valid isokinetic sector

Abstract

BACKGROUND:

Isokinetic dynamometers measure moment and calculate work and power values, which are generally used to interpret muscle performance.

OBJECTIVE:

This study aimed to determine the differences of peak moment (PM), work, and power between FRoM and calculated valid isokinetic sector (VIS) data to define the possibility of the misinterpretation of isokinetic dynamometer data.

METHODS:

Fourteen professional male soccer players who had ACL reconstruction were recruited to the muscle strengthening program which was conducted four days a week for six weeks with increasing training intensity each week. Isokinetic muscle peak moment, work, and power of knee extensors and flexors were measured at angular velocities of 60, 120, 180 and 240 ${}^{\circ }$ /s.

RESULTS:

The results of the standard full-RoM (FRoM) report were compared with the calculated VIS data. Analyses of the FRoM data showed that PM, work, and power values for both extensor and flexor muscle groups improved significantly. However, a comparison of FRoM and VIS data showed significant differences for the work and power values at 120, 180 and 240 ${}^{\circ }$ /s.

CONCLUSION:

The evaluation of FRoM isokinetic data may cause improper interpretations in muscle performance, particularly for contractions with higher angular velocities.

1. Introduction

Isokinetic dynamometers measure moment and calculate work and power values, which are generally used to interpret muscle performance. These devices are also used for effective muscle strengthening rehabilitation applications. An important feature of isokinetic dynamometer is to train isolated muscle groups at predetermined angular velocities. During training moment-sensed, real time resistance adjustments against the angular movement gives an opportunity to prevent excessive loading of the contracting muscle group. The unique specification of this device makes it possible to train with minimized excessive loading-induced complications. During rehabilitation, the angular velocities and the number of repeated contractions are adjusted to change the training intensity. It is possible to define various training models to improve muscle strength with an isokinetic device [1, 2, 3, 4] e.g. in rehabilitation programs following ACL (Anterior crucial ligament) reconstruction surgery [5, 6, 7].

Contractions performed with isokinetic dynamometers involve three phases of movement; acceleration, constant velocity, and deceleration [5]. The constant velocity portion of the range of motion (RoM) represents the valid (also known as the ‘true’) isokinetic sector (VIS) and by definition, corresponds to the real isokinetic portion of contraction. In other words, the values measured during acceleration and deceleration phases are not isokinetic and they may not be considered for this purpose [5, 8].

An evaluation of the dynamometric data based on FRoM or limited to the VIS may show significant differences [8]. The purpose of this study was to determine the differences of PM, work, and power between FRoM and VIS data throughout a rehabilitation period and to try to define the possibility of misinterpreting isokinetic dynamometer data.

2. Methods

2.1 Subjects

Fourteen professional male soccer players (aged 19–38 with a height of 176.4 $\pm$ 5 cm (mean $\pm$ SD) and body weight of 73.6 $\pm$ 7 kg) who had ACL reconstruction volunteered to participate in this study. The patients completed the postoperative fourth month before the initiation of isokinetic training. After a conclusive consultation with the Department of Physical Medicine and Rehabilitation, the patients were informed about the entire rehabilitation period and the associated risks and benefits. An informed consent from each patient was obtained before their participation in the rehabilitation period. During the rehabilitation period, none of the patients had any complications that interrupted the training program. The study was approved by the local Research Ethics Committee and performed according to the Helsinki declaration.

2.2 Muscle strength measurement

Concentric PM of knee extensors and flexors was measured with a Cybex Norm dynamometer (Computerized Sports Medicine Inc., USA). Patients were seated on the dynamometer’s chair with the backrest angle at 90 ${}^{\circ }$ . Stabilization straps were placed over the pelvis and chest to fix the torso. The axis of knee rotation was aligned with the axis of the dynamometer’s armature rotation. The ankle cuff was attached approximately 3 cm above the dorsal surface of the foot. Gravity correction was performed before contraction bouts. The patients warmed-up for approximately 10 min on the treadmill before each measurement.

Table 1

Muscle performance testing protocol (angular velocity $\times$ number of repetitions). Patients rested for 1 minute between contraction bouts

Muscle performance testing protocol
(240 ${}^{\circ }$ /s $\times$ 7) – 60 seconds rest (Data not used)
(240 ${}^{\circ }$ /s $\times$ 7) – 60 seconds rest
(180 ${}^{\circ }$ /s $\times$ 7) – 60 seconds rest
(120 ${}^{\circ }$ /s $\times$ 7) – 60 seconds rest
(60 ${}^{\circ }$ /s $\times$ 7) – 60 seconds rest

The first set of the test protocol involving seven contractions of 240 ${}^{\circ }$ /s was performed to adapt the patients to the device and the obtained data were not analyzed. For all testing sessions, patients performed the same maximal contraction protocol from high (240 ${}^{\circ }$ /s) to slow (60 ${}^{\circ }$ /s) angular velocities with seven repetitions in each set (Table 1). The range of motion adjusted from full extension (0 degree) to 90 degrees of flexion. The patients were instructed to contract maximally over the complete range of motion for every contraction and rested for 1 minute between each set. During the testing session, the patients were given verbal encouragement to ensure that maximum effort was exerted. Following the rehabilitation period, initial muscle performance measurement protocol was repeated to evaluate the effect of regular isokinetic strength training. The final evaluation was performed at least 24 hours later than the last training session and patients were advised to stop training during this time period.

Table 2

The composition of training program throughout rehabilitation period of six weeks. [(angular velocity $\times$ number of repetitions) $\times$ number of sets)]

Week 1	Week 2	Week 3	Week 4	Week 5	Week 6
(240 $\times$ 10) $\times$ 2	(210 $\times$ 10) $\times$ 2	(180 $\times$ 10) $\times$ 2	(180 $\times$ 10) $\times$ 2	(240 $\times$ 10) $\times$ 1	(240 $\times$ 10) $\times$ 1
(210 $\times$ 10) $\times$ 2	(180 $\times$ 10) $\times$ 2	(150 $\times$ 10) $\times$ 2	(150 $\times$ 10) $\times$ 2	(180 $\times$ 10) $\times$ 1	(180 $\times$ 10) $\times$ 1
(180 $\times$ 10) $\times$ 2	(150 $\times$ 10) $\times$ 2	(120 $\times$ 7) $\times$ 2	(120 $\times$ 7) $\times$ 2	(120 $\times$ 7) $\times$ 1	(120 $\times$ 7) $\times$ 1
(210 $\times$ 10) $\times$ 2	(180 $\times$ 10) $\times$ 2	(150 $\times$ 10) $\times$ 2	(150 $\times$ 10) $\times$ 2	(60 $\times$ 7) $\times$ 3	(60 $\times$ 7) $\times$ 3
(240 $\times$ 10) $\times$ 2	(210 $\times$ 10) $\times$ 2	(180 $\times$ 10) $\times$ 2	(180 $\times$ 10) $\times$ 2	(120 $\times$ 7) $\times$ 1	(120 $\times$ 7) $\times$ 1
				(180 $\times$ 10) $\times$ 1	(180 $\times$ 10) $\times$ 1

2.3 Rehabilitation program

Patients trained for four days a week (1 ${}^{\text{st}}$ , 2 ${}^{\text{nd}}$ , 4 ${}^{\text{th}}$ , and 5 ${}^{\text{th}}$ days of the week) for six weeks, with training intensity increasing each week, as presented in Table 2.

During the rehabilitation program, angular velocity values progressively decreased in order to increase the intensity of the training. At the beginning, the lowest angular velocity was adjusted to 180 ${}^{\circ }$ /s and reduced to 150 ${}^{\circ }$ /s in the second week. The angular velocity was decreased to 120 ${}^{\circ }$ /s in the third and fourth weeks, and subsequently the patients were trained with 60 ${}^{\circ }$ /s at the fifth and sixth weeks. The patients were allowed to rest for 60 seconds between training sets. Subjects performed ten repetitions per set for the angular velocities ranging between 150–240 ${}^{\circ }$ /s. However, because of the physically challenging effect of the contractions induced by 120 ${}^{\circ }$ /s and 60 ${}^{\circ }$ /s, the number of repetitions was reduced to seven for each set. Patients were performed the contractions reciprocally and consecutively.

2.4 VIS data calculation

FRoM and VIS data of knee extensor and flexor muscle groups for PM, work, and power values were compared by evaluating pre- and post-training test results. The detailed procedure for calculating the VIS data was given elsewhere [8].

Briefly, the acceleration and deceleration phases of contraction were determined by taking the first derivative of velocity with respect to the range of motion. The data points near zero (fluctuations equal to 0 $\pm$ 0.2) in the first derivative curve were accepted as isokinetic (VIS) and used to calculate VIS’s PM, work, and power. The values obtained by the analysis of the data were normalized with respect to body weight.

2.5 Statistical analysis

Data were presented as mean $\pm$ standard deviation. Data were tested for normality of distribution by the Shapiro-Wilk tests. The initial and final test data for PM, work and power were compared by using the paired sampled-t test. The Wilcoxon signed ranks test was used to evaluate the statistical difference between FRoM and VIS data. Statistical significance was set at $p<$ 0.05 with the confidence interval as 95%. All of the statistical evaluations were conducted using SPSS Statistics version 22.

Figure 1.

Valid isokinetic sector (VIS) ratio for A) extensor and B) flexor muscle groups.

Table 3

Pre- and post-training FRoM test findings for A) PM, B) Work, C) Power ( ${}^{*}p<$ 0.001, ${}^{\&}p<$ 0.01, ${}^{\mathrm{\#}}p<$ 0.05)

Angular velocity ${}^{\circ }$ /s	Extension (FRoM)		Flexion (FRoM)
	Pre-training	Post-training	Pre-training	Post-training
A) PM (Nm/kg body weight)
60	2.22 $\pm$ 0.45	2.78 $\pm$ 0.39 ${}^{*}$	1.45 $\pm$ 0.41	1.72 $\pm$ 0.43 ${}^{\mathrm{\#}}$
120	1.92 $\pm$ 0.47	2.33 $\pm$ 0.37 ${}^{*}$	1.25 $\pm$ 0.35	1.47 $\pm$ 0.36 ${}^{\&}$
180	1.64 $\pm$ 0.35	2.02 $\pm$ 0.33 ${}^{*}$	1.11 $\pm$ 0.35	1.37 $\pm$ 0.30 ${}^{*}$
240	1.40 $\pm$ 0.26	1.75 $\pm$ 0.29 ${}^{*}$	0.97 $\pm$ 0.30	1.23 $\pm$ 0.29 ${}^{\&}$
B) Work (Nm/kg body weight)
60	2.51 $\pm$ 0.52	3.12 $\pm$ 0.45 ${}^{*}$	1.72 $\pm$ 0.43	2.00 $\pm$ 0.48
120	2.15 $\pm$ 0.63	2.78 $\pm$ 0.51 ${}^{*}$	1.60 $\pm$ 0.36	1.80 $\pm$ 0.42
180	1.94 $\pm$ 0.42	2.33 $\pm$ 0.38 ${}^{*}$	1.36 $\pm$ 0.36	1.65 $\pm$ 0.37 ${}^{\&}$
240	1.63 $\pm$ 0.36	2.01 $\pm$ 0.36 ${}^{*}$	1.17 $\pm$ 0.37	1.46 $\pm$ 0.37 ${}^{\mathrm{\#}}$
C) Power (W/ kg body weight)
60	1.61 $\pm$ 0.33	1.98 $\pm$ 0.28 ${}^{*}$	1.08 $\pm$ 0.33	1.30 $\pm$ 0.31 ${}^{\&}$
120	2.66 $\pm$ 0.59	3.27 $\pm$ 0.50 ${}^{*}$	1.83 $\pm$ 0.53	2.17 $\pm$ 0.51 ${}^{\&}$
180	3.26 $\pm$ 0.71	3.96 $\pm$ 0.61 ${}^{*}$	2.22 $\pm$ 0.73	2.82 $\pm$ 0.61 ${}^{*}$
240	3.15 $\pm$ 1.08	4.29 $\pm$ 0.70 ${}^{*}$	2.41 $\pm$ 0.86	3.15 $\pm$ 0.82 ${}^{\&}$

Table 4

Pre- and post-training VIS findings for A) PM, B) Work, C) Power ( ${}^{*}p<$ 0.001, ${}^{\&}p<$ 0.01, ${}^{\mathrm{\#}}p<$ 0.05)

Angular velocity ${}^{\circ }$ /s	Extension (VIS)		Flexion (VIS)
	Pre-training	Post-training	Pre-training	Post-training
A) PM (Nm/kg body weight)
60	2.17 $\pm$ 0.40	2.70 $\pm$ 0.38 ${}^{*}$	1.58 $\pm$ 0.42	1.84 $\pm$ 0.40 ${}^{\mathrm{\#}}$
120	1.87 $\pm$ 0.40	2.22 $\pm$ 0.34 ${}^{*}$	1.37 $\pm$ 0.34	1.61 $\pm$ 0.35 ${}^{\&}$
180	1.52 $\pm$ 0.35	1.89 $\pm$ 0.30 ${}^{*}$	1.22 $\pm$ 0.35	1.45 $\pm$ 0.28 ${}^{\&}$
240	0.86 $\pm$ 0.50	1.16 $\pm$ 0.57 ${}^{\mathrm{\#}}$	0.84 $\pm$ 0.42	1.21 $\pm$ 0.29 ${}^{\&}$
B) Work (Nm/kg body weight)
60	2.17 $\pm$ 0.43	2.71 $\pm$ 0.41 ${}^{*}$	1.65 $\pm$ 0.49	1.98 $\pm$ 0.45 ${}^{\&}$
120	1.56 $\pm$ 0.38	1.88 $\pm$ 0.30 ${}^{\&}$	1.21 $\pm$ 0.33	1.41 $\pm$ 0.33 ${}^{\mathrm{\#}}$
180	0.93 $\pm$ 0.29	1.13 $\pm$ 0.29 ${}^{\mathrm{\#}}$	0.76 $\pm$ 0.25	0.88 $\pm$ 0.20 ${}^{\mathrm{\#}}$
240	0.20 $\pm$ 0.14	0.20 $\pm$ 0.20	0.19 $\pm$ 0.12	0.47 $\pm$ 0.37 ${}^{\mathrm{\#}}$
C) Power (W/kg)
60	1.67 $\pm$ 0.34	2.04 $\pm$ 0.30 ${}^{*}$	1.27 $\pm$ 0.37	1.50 $\pm$ 0.33 ${}^{\mathrm{\#}}$
120	3.23 $\pm$ 0.86	3.67 $\pm$ 0.60 ${}^{\mathrm{\#}}$	2.43 $\pm$ 0.66	2.81 $\pm$ 0.63 ${}^{\&}$
180	4.02 $\pm$ 1.10	4.84 $\pm$ 0.82 ${}^{\&}$	3.28 $\pm$ 0.99	3.95 $\pm$ 0.84 ${}^{\&}$
240	3.11 $\pm$ 1.87	4.21 $\pm$ 2.60	3.21 $\pm$ 1.67	4.54 $\pm$ 1.18 ${}^{\&}$

Figure 2.

Comparison of pre- and post-training FRoM- and VIS-derived PM, work and power data of the extensor muscle groups. The first data group in each angular velocity represent pre-training and the second represents post-training test results. $\bullet$ – FRoM Data, $\circ$ – VIS Data. A) PM, B) Work, C) Power (* $p<$ 0.001, & $p<$ 0.01, # $p<$ 0.05).

Figure 3.

Comparison of pre- and post-training FRToM- and VIS-derived PM, work and power data of the flexor muscle groups. The first data group in each angular velocity represent pre-training and the second represents post-training test results. $\bullet$ – FRoM Data, $\circ$ – VIS Data. A) Peak torque, B) Work, C) Power (* $p<$ 0.001, & $p<$ 0.01, # $p<$ 0.05).

3. Results

1.

The percentage of VIS range out of the FRoM range of motion, did not change significantly after training at all angular velocities (Fig. 1A and B) and displayed similarity for both muscle groups. The isokinetic portion was approximately 80 ${}^{\circ }$ at 60 ${}^{\circ }$ /s and decreased below to 20 ${}^{\circ }$ at the highest tested contraction velocity value, 240 ${}^{\circ }$ /s.

2.

The post-training extension and flexion PM improved significantly for both FRoM and VIS at all angular velocity (Tables 3 and 4A). However, no improvement took place in FRoM-based flexor work at 60 and 120 ${}^{\circ }$ /s (Table 3B) or in VIS-based extensor work and power at 240 ${}^{\circ }$ /s (Table 4C). All other parameters for both FRoM and VIS data were significant (Tables 3B and C, 4B and C).

3.

Extensors performance: VIS vs. FRoM. The analysis of pre- and post-training data showed that the difference between FRoM and VIS data was significant at 240 ${}^{\circ }$ /s for PM (Fig. 2A). On the other hand, the VIS-based work values were significantly lower than FRoM’s at all velocities (Fig. 2B). VIS-based power was significantly higher than FRoM’s at 180 ${}^{\circ }$ /s for pre- and post-training while both differed at pre-training (120 ${}^{\circ }$ /s) as shown in Fig. 2C.

4.

Flexors performance: VIS vs. FRoM. The difference between FRoM and VIS for PM values was not significant at all angular velocities (Fig. 3A). VIS-based work values at pre- and post-training (180 ${}^{\circ }$ /s and 240 ${}^{\circ }$ /s) was significantly lower than the FRoM’s while the difference between FRoM and VIS work data for post-training at 120 ${}^{\circ }$ /s was significant (Fig. 3B). Moreover, pre- and post-training VIS power values were significantly higher than those derived from FRoM at 120, 180 and 240 ${}^{\circ }$ /s for post-training (Fig. 3C).

5.

Correlation of percent improvement: FRoM and VIS. The correlation scores for knee extension became low and non-significant at 180 ${}^{\circ }$ /s and nearly zero at 240 ${}^{\circ }$ /s (Table 5). In sharp contrast the opposite correlational values for the flexors were robust and significant at all angular velocities for PM, work and power except the power value of 180 ${}^{\circ }$ /s (Table 6).

4. Discussion

The major finding of this study is that the evaluation of isokinetic dynamometer FRoM data without considering VIS may cause erroneous results and misinterpretation of training progress. Even though similar PM values were obtained from FRoM and VIS data, the determination of significant differences between work and power underlines the importance of VIS for proper data analysis, mainly at high angular velocities.

Table 5

Correlation of percent improvement between FRoM vs. calculated valid isokinetic sector (VIS) data for knee extension

Angular	PM		Work		Power
velocity ( ${}^{\circ }$ /s)	$r$	$P$	$r$	$P$	$R$	$P$
60	0.692	0.006	0.885	0.000	0.917	0.000
120	0.719	0.006	0.302	0.316	0.648	0.017
180	0.366	0.199	0.615	0.019	0.585	0.028
240	0.05	0.878	0.105	0.774	0.084	0.807

Table 6

Correlation of percent improvement between FRoM vs. calculated valid isokinetic sector (VIS) data for knee flexion

Angular	PM		Work		Power
velocity ( ${}^{\circ }$ /s)	$r$	$P$	$r$	$P$	$R$	$P$
60	0.985	0.000	0.975	0.000	0.984	0.000
120	0.974	0.000	0.868	0.000	0.885	0.000
180	0.955	0.000	0.20	0.048	0.053	0.856
240	0.739	0.003	0.542	0.045	0.878	0.000

The essential purpose of strength training is primarily emphasized as increasing muscle PM, work, and power. During isokinetic rehabilitation programs, various angular velocity contractions are performed to increase muscle strength. The implementation of low angular velocity contractions in training programs aims to improve functional capacity and increase PM [6]. In addition to angular velocity, the number of repetitons and sets per training, resting periods between sets, and training sessions per week are the other main variables participating in creating an effective training program [1, 3, 9, 10, 11]. Angular velocities of 60, 120, and 180 ${}^{\circ }$ /s are commonly used for knee isokinetic rehabilitation. As the knee joint could not reach at angular velocities above 270 ${}^{\circ }$ /s, the maximal angular velocity for training, in this study, was adjusted to 240 ${}^{\circ }$ /s [8]. Intermediate velocity values of 150 ${}^{\circ }$ /s and 210 ${}^{\circ }$ /s were included in the training program to achieve a gradual decrease to a final velocity of 60 ${}^{\circ }$ /s. The resting duration between sets was adjusted to 60-s to allow the patients to recover before the next set [12].

Changes in PM, work and power are commonly used to determine the progress in a rehabilitation program [6, 9]. In the present study, the evaluation of the FRoM data showed a significant improvement in these variables which indicates that the performed muscle strength training protocol was effective. However, as presented in Figs 2 and 3, a statistically significant difference between FRoM and VIS data revealed the possibility of a misinterpretation of muscle strength improvement, especially at high angular velocities.

Angular velocity and VIS percentage must be considered before making critical decisions about the correct timing to terminate the rehabilitation program [13, 14]. In using isokinetic dynamometry to measure knee muscles strength in patients, it was seen that at low angular velocities both the extensor and flexor muscles performed such that the ratio of the VIS to the FRoM remained roughly around 80%. However, with increasing angular velocity, this ratio decreases significantly, reaching below 20% at 240 ${}^{\circ }$ /s. In low velocities, the VIS accounts for a significant portion of the entire range of motion and therefore, as expected, the PM, work and power measurements are in accordance with the FRoM data (Tables 5 and 6). However, with increasing angular velocities, the measurements of the aforementioned variables displayed significant differences from the FRoM data gathered. This difference may be especially apparent in measurements of work which is directly calculated using the area underneath the moment vs. range of motion. The results of this study suggest that considering VIS data may be extremely important to interpret the efficacy of isokinetic training programs. The faulty evaluation of muscle performance may result in premature interruption of the rehabilitation program and cause unexpected secondary injuries during training and/or competition [14].

5. Conclusion

The decrease in the measured values of work at high velocities may lead to the misinterpretation of the status of the patient leading forming misinformed judgments and rehabilitation plans. Even though the percentage of performance improvements of the two groups of athletes that were treated using the FRoM and processed data are close to one another, this consistency is not sufficient in validating judgments made relying on erroneous data. It is particularly important to emphasize that determining PM, work and power data based on a valid isokinetic sector is essential for decision making regarding the termination of a training program.