Introduction
Rowing movement is systemic exercise in which many muscle groups such as the upper limbs, trunk, and lower limbs are mobilized, being a complex exercise in which approximately 70% of the full-body muscles are used [1]. Rowing ergometers have been developed to improve cardiorespiratory endurance fitness as a system of aerobic exercise in addition to running, walking, swimming, and bicycle exercises. They are widely utilized for training by persons at fitness gyms [2] to special training by rowers. Training for boat races depends on the climate or environment. When on-water training is impossible, track and field training [3,4], player assessment [5-7], selection of rowers, and boat positioning [8,9] are performed using rowing ergometers. Furthermore, rowing ergometers are used to establish beginners’ forms.
In most cases, fixed-type rowing ergometers (FIX) that pull a handle against air/inertial resistance under stretcher fixation and moving only the seat are used. McNeely [10] compared performance on a FIX with 2,000 m on-water performance, and reported that the former did not accurately reflect the latter. [9,11-13] It was reported that the timing of “catch” and force sense of rowers on a FIX differ from those during actual on-water rowing motions, and that spatiotemporal variables, such as the stroke length and stroke frequency, differ. To overcome these problems, a slide-type rowing ergometer (SLD) in which both the seat and stretcher move was developed [14]. The main background of SLD development is the rower's of force exertion to the stretcher resemble those during on-water motions. This background, however, have not fully proved by the research data. We newly examined to force exertion to the stretcher by analyzing handle/stretcher force dynamics for each stroke phase and compare these from SLD to FIX type situations.
In recent years, the use of SLD has become increasingly important in the training of rowing athletes. This is because the force applied during rowing on an SLD closely resembles the sensation of rowing on water. Additionally, with FIX, the greater load during pulling tends to increase the risk of injury, particularly in the upper limbs and back. As shown in <Figure 1>, the SLD has a structure to slide to the opposite side of the handle’s pulling direction in accordance with the legs’ kicking motions, with the seat, loading, and stretcher parts placed on a rail with a roller. On an SLD, only the machine’s mass (approximately 27kg) serves as the load, making acceleration after the catch easier compared to a FIX, where the athlete’s body weight is also a load, resulting a higher stroke rate. As a consequence, the SLD allows for a significantly shorter drive time and a higher stroke rate compared to the FIX, thus replicating the motion on water more effectively. On the other hand, FIX requires the athlete to accelerate their body weight, which tends to result in a lower stroke rate compared to both SLD and rowing on water. This difference in stroke rate can greatly impact rowing performance. For example, the stroke rate of a medalist in the single sculls event at the 2000 Sydney Olympics was 35.2 SPM [15]. While this differs from studies comparing stroke rates on FIX and SLD ergometers (SLD: 31.5 SPM vs. FIX: 28.7 SPM) [5], the SLD ergometer closely mirrors the stroke rate on water, suggesting it can better simulate on water conditions. Therefore, during a 2000m race simulation (which typically takes around 6 to 7 minutes), it is expected that the SLD rowing ergometer will consistently maintain a higher stroke rate throughout the race. However, there is no sufficient scientific evidence on differences between SLD and FIX. Few studies have been conducted on the characteristics of the slide ergometer from the viewpoints of handle or stretcher forces [5,16].
Futhermore, the results are not consistent among studies comparing rowing movement between SLD and FIX ergometers. As the number of cases is small, the characteristics of mechanical output from the slide ergometer remain to be clarified.
The purpose of this study was to compare the mechanical outputs including stretcher and handle forces and lower limb articular kinematics during rowing movement between FIX and SLD ergometers during 2,000-m boat race simulation trials.
Material and Methods
Subjects
The subjects were eight junior rowers (two males and six females, age: 16.5 ± 0.5 years, height: 164.5 ± 8.9 cm, weight: 60.1 ± 7.0 kg) who belong to high school in Yokohama City, Japan. They have rowing experience for over least one year and are training almost daily so as to get well-skilled rowing techniques. To the subjects, the purpose, contents, and safety of this study were explained in advance, and written consent was obtained before measurement. Written informed consent was obtained from each subject and all procedures were performed and conducted according to the guidelines of the Declaration of Helsinki and approved by both the Institutional Review Board of Nippon Sport Science university (Approval No.: 019-H061). Each subject performed 2000 m time simulation trials at lab both on the FIX and SLD type ergometers on a day. The order of FIX and SLD trials was designed as swapping the people randomized to FIX (1 male + 3 females) vs SLD (another 1 male + left 3 females) (where 6 females were randomized) and then swapping the people randomized to male and female, i.e. FIX->SLD and SLD->FIX. The interval between two trials for each subject set more than three hours. Temperature and humidity were controlled as same situations between two trials and nutrition and water intake was not controlled up to two-hours to the end of the trial.
Experimental equipment
This study was performed using an air-resistance-type rowing ergometer (Model C Indoor Rower by Concept 2). As shown in <Figure 1>, under the FIX condition (upper row), an ergometer was installed on the floor of a laboratory. Under the SLD condition (lower row), an ergometer was attached on to two slides (Concept 2). A rotary encoder (RS232-C, Vine Inc., Japan) was attached to the axis of rotation at the flywheel of the rowing ergometer, and a tension load cell (small LUR-A-2KNSA1, Kyowa Electronic Instruments Co., Ltd. Tokyo) was incorporated in the junction between the handle and tow chain. To measure the seat velocity, a rotary encoder was attached to the lower row of the seat. To measure the force acting on the footplate (stretcher), strain gauge-type force plates (KLU-100KA-SH11, Vine Inc., Japan) were attached to the stretcher.
For imaging of the rowing movement, the video were recorded at 200 fps using a high-speed camera (DITECT Co., Ltd.). Synchronous recording was performed using a trigger box (PH1461A, Bcam Sync Generator, DKH), and the data were imported into a computer via an A/D converter (Power Lab/16sp 1kHz, AD Instruments).
Experiment protocol
The subjects were instructed to sufficiently warm up on FIX and SLD ergometers before measurement. The 8 subjects simulated a 2,000-m boat race on FIX and SLD ergometers. They were instructed to do rowing movement at the same pace as kept during an actual race. During the test race, each subject was told the moving distance, and every 500-m lap time was recorded. A sufficient rest time was established between a test race on the FIX and that on the SLD ergometer so that there might be no influence of fatigue.
Calculation of mechanical outputs from the ergometers
Concerning one stroke of rowing movement in this experiment, a motion from a catch until the next catch was defined as one stroke. The stroke rate was established as the reciprocal of the stroke time, and the stroke rate per minute (stroke/min) was calculated <Figure 2>.
The handle and stretcher forces (N) during the drive phase were respectively obtained from a load cell attached to the handle and stretcher. Rate of force developments in each handle and stretcher force were determined by force developments from catch to maximal divided by corresponding time. The handle power (W) during the drive phase was calculated by multiplying the handle velocity obtained from the rotary encoder by the handle force.
Videography and motion analysis
Four strokes at a distance of approximately 500 m during a 2,000-m time trial were selected for the analysis. For rowing motion analysis, digitizing of body segment land markers was performed by Dempster’s link segment model using a motion analysis system (Frame-DIAS V, DKH, Japan), and the position coordinates of the subject’s marking points were calculated using the two-dimensional direct linear transformation (DLT) methods. The body position data obtained on video analysis were smoothed with a zero-phase Butterworth low pass digital filter after determining an optimal cutoff frequency (4Hz) by residual analysis. As shown in <Figure 2>, hip joint angle was defined as an angle involving the acromion, greater trochanter and lateral femoral tuberosity, the knee joint angle as an angle involving lateral femoral tuberosity and lateral malleolus, and the ankle joint angle as an angle involving the lateral femoral tuberosity, lateral malleolus and distal lateral side of the fifth metatarsal. Furthermore, all extension directions of joint angles were defined as plus (+).
Statistical analysis
All values were expressed as the mean±standard deviation. The spatio-temporal variables, mechanical output variables, and lower limb kinematics variables were compared between fixed and slide ergometers using the paired t-test. A p value of 0.05 level was regarded as significant. All statistical analyses were performed using SPSS software (IBM SPSS Statistics Version 27.0).
Results
The comparison of spatiotemporal variables between FIX and SLD ergometers were shown in <Table 1>. There was no significant difference in the 2,000-m time between the SLD (8:28.5±37.1) and FIX (8:30.22±33.5) ergometers. The drive time on the SLD (0.94±0.08 seconds) was significantly shorter than on the FIX (1.00±0.08 seconds) (p<0.01). The recovery time on the SLD (0.95±0.14 seconds) was significantly shorter than on the FIX (1.04±0.11 seconds) (p<0.05). The stroke rate on the SLD (32.07±3.94 strokes/min) was significantly higher than on the FIX (29.34±2.09 strokes/min) (p<0.05).
The comparison of mechanical outputs between FIX and SLD ergometers were shown in <Table 2>. Typical changes in the handle force, handle power, and stretcher force were shown in <Figure 3>. The handle peak force on the SLD (0.96±0.16 N/BW) was significantly smaller than on the FIX (1.02±0.22 N/BW) (p<0.05). The handle RFD on the slide ergometer (2.56±0.62 N/BW/s) was significantly greater than on the FIX (1.17±0.24 N/BW/s) (p<0.01). The handle rate of power development on the SLD (3.98±1.34 W/BW/s) was significantly greater than on the FIX (2.27±0.19 W/BW/s) (p<0.01). The stretcher peak force on the SLD (1.46±0.15 N/BW) was significantly greater than on the FIX (1.26±0.23 N/BW) (p<0.01). The stretcher RFD on the SLD (0.43±0.10 N/BW/s) was significantly greater than on the FIX (0.32±0.07 N/BW/s) (p<0.01).
The hip, knee, and ankle angles at “catch” and “finish” positions, ROM, and maximum angular velocity/acceleration during the stroke phase on Fix and SLD ergometers were shown in <Table 3>. There were no significant differences in the hip, knee, or ankle angles at “catch” or “finish” positions as well as ROM and maximum anglar velocities between the FIX and SLD ergometers. On the other hand, there was a significant difference in the maximum angular acceleration of the knee between the SLD (908.1±246.3 deg/s2) and FIX (740.7±147.2 deg/s2) (p<0.05). Maximum angular acceleration of the hip also had a tendency to be greater on the SLD.
Discussion
This study compared mechanical outputs and lower limb articular kinematics during rowing movement between FIX and SLD ergometers and clarify the characteristics of rowing movement on the slide ergometer.
There was no significant differences in 2000-m time between FIX and SLD ergometer conditions. However, in slide condition, higher stroke rate, higher stretcher peak force as well as higher RFD in both handle/stretcher forces were observed. We discuss those characteristics of SLD ergometer rowing from view points of spatio-temporal parameters and mechanical variables along with rower limb kinematics.
Spatio-temporal and mechanical variables
There was no significant difference in the 2,000-m time between the FIX and SLD ergometers. Therefore, there may be no difference in rowing performance between the two conditions. However, the drive time and recovery time on the SLD were significantly shorter than on the FIX, resulting higher stroke rate.
Several studies reported that the stroke rate increased on the SLD during an incremental rowing test using FIX and SLD ergometers [5,16-18]. Holsgaard-Larsen and Jensen compared spatiotemporal variables between FIX and SLD ergometers with maintaining the intensity of exercise as 70% of the peak power. They reported that the stroke rate on the SLD was higher than on the FIX, and that this was related to a short recovery time. Furthermore, another study found an increase in the stroke rate on the RowPerfect ergometer of which the stretcher moves in the anteroposterior direction [3]. This supports previous studies describing a high stroke rate during rowing movement on the SLD ergometer. Concerning the reason why there were such a high stroke rate and a decrease in the recovery time on the SLD, moving only the mass of the ergometer, but not the rower’s body mass, may relatively accelerate “catch” motions, contributing to a high stroke rate and a decrease in the recovery time.
The handle peak force during rowing on the SLD was significantly smaller than on the FIX ergometer. Vinther et al. [19] investigated 22 subjects belonging to the national team of Denmark, and reported that the handle peak force on the SLD was significantly smaller than on the FIX ergometer. Furthermore, some studies showed similar results on a dynamic ergometer of which the stretcher moves forward [3,20]. A previous study examined the handle peak force in beginners and male/female rowers using FIX and SLD Concept 2 ergometers, as adopted in this study, and reported that the handle peak force on the fixed ergometer was higher in both males and females [5]. Holsgaard-Larsen and Jensen [16] also reported similar results by comparing spatiotemporal variables between FIX and SLD ergometers. According to a study comparing handle forces under FIX/SLD ergometer and on-water conditions, the handle force pattern on the SLD resembled that under the on-water condition [21]. This suggests that the slide ergometer does not markedly depend on upper limb force development. Thus, a decrease in load per stroke may reduce the risk of injury, such as stress fractures of the ribs.
When focusing on lower limb force development, the stretcher peak force on the SLD was significantly greater than on the FIX ergometer. Colloud et al. [3] used a dynamic ergometer of which the stretcher moves forward during drive, and reported that the stretcher force on the SLD was higher than on the FIX ergometer. On the other hand, Millar et al. [21] used two slide instruments, as adopted in this study, and found that there was no difference in the stretcher force between SLD and FIX ergometers. As the reasons why these results differed from those of previous studies, Colloud et al. [3] used a different type of ergometer(RowPerfect), and only four the number of subjects in the study by Millar et al. [21].
Lower limb kinematics
Although there were no significant differences in the knee and hip extension angles, the maximum angular accelerations of the knee and hip tended to be greater and were significantly greater on the SLD ergometer. This suggests that the SLD facilitates more ballistic knee and hip extension in comparison with the FIX ergometer.
Lamb [22] compared on-water rowing with rowing movement using the FIX, and reported that rowing movement was similar between the two conditions. Furthermore, Elliott et al. [8] showed that the use of a dynamic ergometer of which the stretcher moves forward provided on-water rowing-like motions. These results support that differences in ergometer mechanism do not markedly influence rowing movement.
This also reflects an ergometer type-related difference in the pattern of muscle utilization. Under the SLD condition, the handle force, power, and stretcher force development rapidly rose in the former half of a stroke, <Figure 3>. A factor for this may be associated with changes in the maximum angular acceleration of the knee and hip under the SLD condition. This reflects that rapid force exertion by hip and knee extensor muscles is required from a “catch” motion until the former half of the stroke phase under the SLD condition in comparison with the FIX condition.
Based on these results, the SLD may provide training resembling on-water rowing dynamics. In particular, force development immediately after a “catch” motion is prompt, and lower limb muscle activation is emphasized; therefore, the slide ergometer may contribute to an improvement in on-water performance.
As the reason, under the SLD condition, the force sense at the time of catch, which is based on rowers’ subjective impressions, resembles on-water rowing movement. Furthermore, the SLD primarily depends on biarticular leg muscles (Shaharudin et al. [23]), and the extension force of the legs is emphasized. Thus, it also may reduce low back pain in rowing beginners, providing a better training tool. These results suggest that the SLD provides training resembling on-water rowing movement or force exertion patterns. In particular, major and biarticular muscles that coordinate with simultaneous extension of the knee and hip are activated, and this may contribute to an improvement in on-water performance. It may be necessary to adequately select an ergometer in accordance with the purpose of training. In the future, the long-term effects of training in consideration with the characteristics of each ergometer should be emphasized. In this study, we analyzed the FIX and SLD ergometers focusing on the 500m spots during the race. However, in the future, it will be necessary to investigate the drive time and recovery time associated with the stroke rate in each spot of a 2000m race and compare the changes related to fatigue.
Conclusions
We compared mechanical outputs and lower limb articular kinematics during rowing movement between fixed and slide ergometers. As a result, the following conclusions were obtained:
(1) The stroke rate on the slide ergometer was higher, and the drive time was shorter.
(2) Rate of force developments in both handle and stretcher on the slide ergometer were higher, suggesting its superiority in ballistic force exertion. On the other hand, the fixed ergometer was advantageous with respect to the handle force output.
(3) Maximum angular accelerations of the knee and hip were higher at the beginning of the drive phase on the slide ergometer.
Based on these results, as an application for training, the slide ergometer is characteristics as to acquire the timing of exhibiting a great force at the moment of catch, and may contribute to an improvement in on-water rowing performance. On the other hand, the fixed ergometer may be effective in strengthening upper limb muscle strength. Selection of two type ergometer depends on individualization and specialization of the rowing training undertaken.
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