Abstract
This study investigated whether low-load isotonic training would elicit a greater improvement in muscle strength at the same fascicle length, rather than the same joint angle, adopted during training. Sixteen healthy men (24.1 ± 2.5 years) were randomly divided into an intervention group and a control group. Before (Pre) and after (Post) training, one repetition maximum (1RM), maximum isometric and isokinetic strength were measured. Maximum isometric strength were measured every 15 ° through the range of ankle joint position from 15° dorsiflexion to 30° plantarflexion. Maximum isokinetic strength was measured at 20°/s from 15° dorsiflexion to 30° plantarflexion. Fascicle length of medial gastrocnemius (MG) was measured using B-mode ultrasound. Isotonic resistance training with 20%1RM of low-intensity on ankle plantarflexion at 5°/s from 15° to 30°plantarflexion, was performed 3 days a week for 4 weeks. After intervention, maximum isometric strength increased significantly only at 15° dorsiflexion in the intervention group. Maximum isokinetic strength increased significantly only at 8° ~ 12° dorsiflexion the intervention group. The fascicle length at isotonic training condition was similar to the fascicle length at 15° -0° dorsiflexion during maximum voluntary contraction, which was not the range of ankle angle used in training (15° ~ 30°plantarflexion). Our findings suggest that low-load isotonic training at a shortened fascicle length may be effective for improving muscle strength at a lengthened fascicle length because of specificity of the fascicle length than the joint angle.
Introduction
It was reported that muscle strength specifically improved at training angle. In isometric training, Kitai and Sale (1989) reported that training of ankle plantarflexion using maximum voluntary isometric contraction at an ankle angle of 0° (i.e., 90° angle between the tibia and sole of foot) produced improvement in maximum isometric strength only around 0° of ankle position (between −5° and 5°). These results demonstrate the ‘so called’ joint angle specificity on training effects (Kitai and Sale 1989). Moreover, Graves et al. (1989) reported that isotonic training of knee extension with high load at an angle range from 0° to 60° or from 60° to 120° improved maximum isometric strength depended on training angle, respectively (Graves et al. 1989). Furthermore, Barak et al. (2004) reported that concentric isokinetic training (30°/s) of knee extension with maximal contraction at an angle range from 30° to 60°improved maximum isometric strength only at an angle of 45°knee flexion (Barak et al. 2004). In this way, muscle strength was specifically improved at training angle used by not only isometric training but also isotonic and isokinetic training. On the other hand, to be contrary to the joint angle specificity in isometric training described above, some reports have demonstrated that improvement in maximum muscle strength can be obtained over a wide range of joint angles around the training angle at isometric training (Rasch and Pierson 1964; Rasch et al. 1961; Weir et al. 1994; Weir et al. 1995) or eccentric isokinetic training (Barak et al. 2004). Thus, at present, because a consensus view about joint angle specificity has not been achieved, further research is required more investigation.
In general, high load more than 60% of maximum voluntary contraction have been needed to gain strength improvement in resistance training (Ratamess et al. 2009). Therefore, all previous studies of investigating the joint angle specificity of training effect used high load more than 70% maximum voluntary isometric contraction or 60% one repetition maximum. However, our recent study showed that isometric training with low-intensity of 30% maximum voluntary contraction at shortened fascicle length was effective for improving muscle strength at different angle from training angle, that is, muscle strength at lengthened (more dorsiflexion) position which would be equal with fascicle length during training. Our findings suggest that the effects of low-intensity training depend on not joint angle specificity but the fascicle length. The fascicle length is affected by both joint angle and contraction level. In high-load training, because exerted forces during training would be equal with the measuring maximum isometric strength, the fascicle length during training would be also equal at the same joint angle with the measuring maximum strength. This fact suggests that it is uncertain which the training effects depend on the joint angle or the fascicle length in high-load training. However, our previous study of low-load isometric training showed that muscle strength improved at a different angle from a training angle, and that improvement of muscle strength was obtained at the same fascicle length rather than at the same joint angle, used during training (Tanaka et al. 2016). Therefore, in isotonic training, it also could be thought that improvement of muscle strength is affected by fascicle length which is defined as load and joint angle. However, influence of the fascicle length during low-load isotonic training on improvement of muscle strength is unclear.
Therefore, the purpose of present study was to examine whether improvement of muscle strength depends on only the joint angle or the fascicle length with low-load isotonic plantarflexion training. We hypothesized that low-load isotonic training would have a greater improvement of muscle strength at the same fascicle length, rather than at the same joint angle used during training.
Subjects
Sixteen healthy men (age 24.1 ± 2.5 years), who were non-athletes and had not been involved in any regular stretching or resistance training, participated in this study. Subjects with a history of neuromuscular disease or musculoskeletal injury involving the lower limbs were excluded. The subjects were randomly assigned to the intervention group (n = 8) or to the control group (n = 8) using a computerized random number function in Microsoft Excel. All subjects were fully informed of the procedures and purpose of the study, which conformed to the Declaration of Helsinki. Written informed consent was obtained from all subjects. This study was approved by the ethics committee of Kyoto University Graduate School and the Faculty of Medicine (R-0216).
Study protocol
The experimental design of this study was a randomized controlled trial. A flowchart of the experimental protocol is shown in Figure 1. Prior to obtaining pre-training measurements, all subjects were familiarized with the maximum voluntary contraction procedure for plantarflexion. In the initial week of the experiment, subjects attended 3 familiarization sessions, practicing maximum voluntary contraction of isometric, isokinetic and isotonic. On maximum voluntary isometric contraction, subjects practiced at the 4 ankle joint positions used for testing, set at 15° intervals over the range from 15° dorsiflexion to 30° plantarflexion. On maximum voluntary isokinetic contraction, subjects practiced at 20°/s over the range from 15° dorsiflexion to 30° plantarflexion. On maximum voluntary isotonic contraction (1RM; One Repetition Maximum), subjects practiced at three ranges of an ankle joint angle (from 15° dorsiflexion to 0° , from 0°to 15° plantarflexion, and from 15° plantarflexion to 30° plantarflexion). An ankle joint angle of 0° (neutral position) was defined as a 90° angle between the fibula and fifth metatarsal bone. Following the familiarization sessions, muscle strength during maximum voluntary contraction of the ankle plantarflexors and the fascicle length of the medial gastrocnemius (MG) were measured (Pre). The same measurements were performed after 4 weeks of intervention (Post), resulting in a total experimental period of 5 weeks. All measurements and analysis were implemented only at Kyoto University.
Training protocol
The intervention group performed isotonic resistance training of ankle plantarflexion at an intensity of 20%1RM, 3 days per week, for 4 weeks, using a Biodex dynamometer (Biodex System 4, Biodex Medical Systems Inc., Shirley, New York, USA.). The isotonic resistance training protocol consisted of 3 sets of 20 repetitions of plantarflexion contraction of only concentric phase, performed at 15°~30° plantarflexion, with a 1-s rest between each contraction, and a 2-min rest between sets. The subjects performed for 3-s over the range of ankle angle at 15° ~30° at the speed of 5°/s. The examiner supervised all training session and checked whether the subjects could exert at the proper speed. Subjects in the control group did not receive any intervention
Measurement procedure of muscle strength
Muscle strength was determined by measuring the maximum isometric strength, maximum isokinetic strength and 1RM. Muscle strength was measured using a Biodex dynamometer with 1000Hz sampling rate. For measurement, the ankle joint of the dominant leg was securely attached by velcro strap to the footplate of the dynamometer. Soft cloth was inserted between the velcro strap and instep to prevent unwanted movement of the ankle joint. The trunk and distal thigh were securely fixed by the dynamometer belts to keep the hip joint position of 80° flexion and the knee joint in full extension. The subjects grasped horizontal bars attached to the dynamometer. Before each test, subjects underwent a warm up of 10~20 submaximal isometric contractions. After more 1 min of rest, subjects were asked to generate maximum voluntary isometric and isokinetic contraction in randomized order. The measurement of 1RM would make large fatigue because it would need a lot of maximal contraction. Therefore, 1RM was measured at last. The whole measurement time was about 60 minute (-min).
Maximum isometric strength was measured at 4 ankle positions, set at 15° intervals over the range from 15° dorsiflexion to 30° plantarflexion, with the order of maximum voluntary isometric contraction randomized across participants. Maximum voluntary isometric contraction was exerted for 5 seconds (-s) at each of the 4 ankle joint angles, with more than 2-min of rest provided between each maximum voluntary isometric contraction. Maximum isometric strength was measured twice at each ankle joint angles, and the greater value of two measures was used for analysis.
Maximum isokinetic strength at 20°/s was measured by every 1°over the range from 15° dorsiflexion to 30° plantarflexion. Maximum isokinetic strength was measured twice with more than 2-min of rest between trials. The greater value of two measures was used for analysis.
1RM was measured at three ranges of an ankle joint angle in randomized order (from 15° dorsiflexion to 0°, from 0° to 15° plantarflexion, and from 15° plantarflexion to 30° plantarflexion). The load was started from the 1RM value at familiarization session, and increased by 5~10 Nm until the subject was unable to plantarflex to the required fully range of motion. When the subject couldn’t plantarflex at full range of motion, the subject challenged the load which decreased by 5Nm. The last acceptable plantarflexion with the highest possible load was determined as 1RM. A rest period was allotted between each attempt to ensure recovery.
Measurements of fascicle length and muscle thickness
The fascicle length of the MG was measured at the proximal 30% of the lower leg length(Akagi and Takahashi 2014), using B-mode ultrasound imaging (LOGIQ e, General Electric, Duluth, GA, USA) with an 8-MHz linear array probe (6 cm). The ultrasound settings used by the measurements were set at 58–70 dB gain. Depth and Dynamic focus of the equipment settings were controlled to achieve a clear image of the muscle thickness and the fascicle length of the MG. The fascicle length was measured at isotonic training condition (at 15° ~30° plantarflexion with 20%1RM) and during muscle strength measurements. In measurement of the fascicle length at maximum isometric strength, the images were preserved when exerted force displayed on the dynamometer monitor reached a plateau. The fascicle length during isokinetic strength measurement and isotonic training condition were measured using the function of moving imaging, and static imaging every 0.1-s was recorded.
The fascicle length was estimated from these images based on the methods which evaluated the distance along a straight line, between extension lines from the aponeurosis and the origin of the fascicle (Figure 2). Ando et al. (Ando et al. 2014) demonstrated that this method is useful technique for estimating the fascicle length of quadriceps muscle. This method has been used to determine the fascicle length of the quadriceps muscle in a number of previous studies (Blazevich et al. 2007; Csapo et al. 2011; Ema et al. 2013). Moreover, the reliability of this method for the measurements of the MG has been shown by our previous study (Tanaka et al. 2016). The intraclass correlation coefficients (ICC 1.1) value for the fascicle length of the MG was valid in both inter session (ICC > 0.9) and inter day (ICC > 0.75), because ICC value higher than 0.75 is considered valid (Lee et al. 1989). The fascicle length was measured for each condition using image processing software (ImageJ, version 1.48, National Institutes of Health, Bethesda, MD, USA).
To examine morphological changes, the fascicle length and the muscle thickness of MG, soleus (SOL), and lateral gastrocnemius (LG) at rest with ankle position of 0° were measured at Pre and Post. The fascicle length and the muscle thickness of MG, LG and SOL were measured at the proximal 30% of the lower leg length. The muscle thickness was measured to assess whether the improvement in muscle strength is due to morphologic changes such as muscle hypertrophy. The muscle thickness was measured by measuring the line drawn perpendicular from the surface to the deep aponeurosis at transverse plane. To accurately measure the muscle thickness without including non-contractile tissue, the measurement between the inside edges of the aponeurosis was used. Previous studies have shown the reliability of the ultrasound technique for measuring muscle thickness of triceps surae (Maganaris et al. 1998; Narici et al. 1996).
Statistical analysis
Statistical analysis was performed using SPSS (version 22.0, SPSS Japan Inc., Tokyo, Japan). Normality of the data was evaluated using a Shapiro-Wilk test. Group differences for characteristics and maximum muscle strength at baseline were assessed using an unpaired t-test.
Split-plot ANOVA, using two factors (group × time), was used to analyze the effects on maximum muscle strength. Post-hoc analysis was used a paired t-test to determine the differences between the value at Pre and Post in both groups. Two-way repeated measures analysis of variance (ANOVA) using two factors (ankle joint angle × time) was used to determine the differences in isokinetic strength in both groups. Post-hoc analysis was used a paired t-test determine the differences between the value at Pre and Post in both groups.
Paired t-test was used to determine the differences in the fascicle length between at isotonic training condition (at 15° ~30° plantarflexion with 20%1RM) and during muscle strength measurements at each angle in Post in intervention group. The fascicle length at training condition was calculated as the mean value at 15° ~30° plantarflexion of Pre and Post.
Paired t-test was used to determine the differences in the fascicle length and muscle thickness between Pre and Post. Differences were considered to be statistically significant at an alpha level of 0.05
Results
Characteristics of the subjects
No subjects dropped out, and all subjects in the intervention group completed the training sessions. Therefore, all data for all subjects in the intervention and control groups were entered in the analysis.
The characteristics of the subjects are shown in Table 1. There were no significant differences in age, height, and body mass between the intervention and the control groups. There were no significant differences in any variable of maximum muscle strength at baseline between intervention and control groups.
Effects of isotonic training intervention on maximum muscle strength
Maximum muscle strength at Pre and Post were shown in Table 2.
Split-plot ANOVA, using two factors (group × time), showed no significant interaction but significant main effect only at the ankle positions of 15° dorsiflexion. At 15° dorsiflexion, Post value of maximum isometric strength was significant higher than Pre in the intervention group, but there was no significant change in the control group. However, there were no significant differences at 0°, 15° and 30°plantarflexion in both intervention group and control group.
As for maximum isokinetic strength, there were no significant changes between Pre and Post in both groups.
Split-plot ANOVA, using two factors (group × time), showed no significant interaction and main effect of 1RM at any range of ankle angle.
Changes in isokinetic strength over the range of ankle angle between 15° dorsiflexion and 30° plantarflexion were shown in figure 3. Two-way repeated measures ANOVA (ankle joint angle × time) showed significant interaction in intervention group. Post-hoc analysis showed that there were significant increases in isokinetic strength at 12° ~ 8° dorsiflexion. However, there were no significant differences at any range of ankle angle in control group.
Differences in the fascicle length between at isotonic training condition and during muscle strength measurements
In intervention group, the mean value of the fascicle length at the training condition (i.e., 20%1RM from 15° to 30° plantarflexion) was 3.54 ± 0.64cm.
The fascicle length during maximum isometric strength measurement in intervention group at 15° dorsiflexion, 0°, 15°, and 30° plantarflexion were 3.71 ± 1.03cm, 3.18 ± 0.82 cm, 2.72 ± 0.67 cm, and 2.42 ± 0.50 cm, respectively. There were significant differences in the fascicle length between training condition and maximum isometric contraction at 15°and 30° plantarflexion. On the other hand, there were no significant difference between training condition and maximum isometric contraction at 15° dorsiflexion and 0°.
The fascicle length during maximum isokinetic strength measurement at range of ankle angle from 15° dorsiflexion to 30° plantarflexion, was showed in Table 3. There were no significant differences in the fascicle length between at the training condition and isokinetic strength measurement from 5° to 1° dorsiflexion.
Effects of intervention on muscle thickness and fascicle length
The results of change in muscle thickness and fascicle length were shown in Table 4. No significant differences in the muscle thickness and the fascicle length between Pre- and Post-measurement in both groups.
Discussion
To our knowledge, the present study is the first report which investigated the effects of low-load isotonic training at shortened fascicle length on muscle strength including various contraction modes and joint angles in detail. The results of this study showed that the isotonic training with 20% 1RM at ankle angle from 15° to 30° plantarflexion resulted in significant improvement in maximum isometric strength at 15° dorsiflexion and isokinetic strength at 8° -12° dorsiflexion.
Our results showed that there was a significant improvement of maximum isometric strength at 15° dorsiflexion only in intervention group. Although intervention group performed isotonic training at ankle angle from 15° to 30° plantarflexion, maximum isometric strength improved only at 15° dorsiflexion. That is, there was no significant change in isometric strength at training angle. This result is inconsistent with the joint angle specificity of training effect. We hypothesized that the effects of strength training depends on the fascicle length rather than the joint angle during training. In this study, there was no significant difference in fascicle length during between low-load training at 15° – 30° plantarflexion (3.54 ± 0.64 cm) and maximum isometric strength measurement at 15° dorsiflexion (3.71 ± 1.03 cm). This result implies that the fascicle length during training was equal with that of maximum voluntary isometric contraction regardless of the different ankle angle. Our findings suggest that low-load isotonic training may be effective for improving maximum isometric strength not at the same joint angle but the same fascicle length during training, and that the effects of isotonic training with shortened fascicle length could produce at more lengthened fascicle position.
Our results showed that isokinetic strength improved significantly at range of 12° to 8° dorsiflexion, although there was no significant increase in maximum isokinetic strength. These results suggests that training at 15° – 30° plantarflexion could not be produced the effect of improvement in maximum isokinetic strength at the wide range from 15° dorsiflexion to 30° plantarflexion. Similar to the result of maximum isometric strength, the result of isokinetic strength was also inconsistent with the joint angle specificity of training effect. As for the fascicle length during isokinetic contraction, fascicle length at 5° to 0° dorsiflexion of maximum isokinetic contraction was almost same as fascicle length during training. Unfortunately, the result of our study was inconsistent with our hypothesis that improvement of muscle strength depends on the fascicle length during training, because isokinetic strength improved at range of 12° to 8°dorsiflexion. The discrepancy may be due to lack of perfect synchronization between a dynamometer and ultrasound imaging. In this study, initial movement of dynamometer and initial muscle contraction on ultrasound imaging were synchronized manually. If muscle contraction occurred before movement of dynamometer, the data of the fascicle length in ultrasound imaging and the joint angle wouldn't be matched. Because of the inadequate synchronization, isokinetic strength might not improve at the same fascicle. Further investigation of the fascicle length by proper synchronization between a dynamometer and ultrasound imaging would be needed and interesting.
There were no significant changes in 1RM at all ranges of angle from 15° dorsiflexion to 0°, from 0° to 15° plantarflexion, and from 15° to 30° plantarflexion. 1RM reflects the weakest strength value each at ranges of angle. Generally, strength of plantarflexion decreases with shortened fascicle length at angle of more plantarflexion according to force-length relationship. Therefore, it may be possible that 1RM of ankle angle from 15° dorsiflexion to 0°, from 0° to 15° plantarflexion, and from 15° to 30° plantarflexion were influenced by maximum muscle strength at 0°, 15°, and 30° plantarflexion, respectively. In actual fact, the present study showed that there were no significant improvements in maximum isometric strength at 0°, 15°, and 30° plantarflexion, although there was significant improvement in maximum isometric strength at 15° dorsiflexion. Therefore, this training at shortened fascicle length might not produce improvement in 1RM from 15° dorsiflexion to 0°, from 0° to 15° plantarflexion, and from 15° to 30° plantarflexion.
The limitation of this study is that we did not investigate the effect of neural adaptation. Improvement in muscle strength after resistance training depends on neural adaptations over the initial period of training (i.e., <4 weeks), follows by morphological muscle adaptations after 6–8 weeks, which mainly contributes to the strength gains (Kraemer et al. 1996; Moritani and Devries 1979) . Our results showed no changes in morphological measurements such as the fascicle length or the muscle thickness after training. Therefore, the improvement in maximum muscle strength after a 4-week intervention in this study may be influenced by neural adaptations, such as the increases in muscle activity of agonist muscles and decreases in antagonist coactivation (Garfinkel and Cafarelli 1992). Further investigation is necessary to clarify the interaction between neural adaptation mechanisms and the effects of specificity in the fascicle length during training.
Conclusions
The results of the present study showed that low-load isotonic training with shortened fascicle length improved the isometric and isokinetic strength at different angle from training condition. The results suggest that the effects of improvement in muscle strength could be depended on the fascicle length rather than the joint angle during isotonic training. Thus, low-load training with shortened fascicle length could improve muscle strength at more muscle lengthened position than training position. It is possible that low-load training at shortened muscle length may be suitable and safer for the elderly who cannot perform high-load training or for patients who have restricted range of joint motion.