Edited by: Jared Porter, Southern Illinois University, USA
Reviewed by: Will Wu, California State University, USA; Jason B. Winchester, East Tennessee State University, USA
*Correspondence: Mathias Reiser, Institute of Sport Science, Justus Liebig University Giessen, Kugelberg 62, 35394 Giessen, Germany. e-mail:
This article was submitted to Frontiers in Movement Science and Sport Psychology, a specialty of Frontiers in Psychology.
This is an open-access article subject to a non-exclusive license between the authors and Frontiers Media SA, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and other Frontiers conditions are complied with.
The purpose of this training study was to determine the magnitude of strength gains following a high-intensity resistance training (i.e., improvement of neuromuscular coordination) that can be achieved by imagery of the respective muscle contraction imagined maximal isometric contraction (IMC training). Prior to the experimental intervention, subjects completed a 4-week standardized strength training program. 3 groups with different combinations of real maximum voluntary contraction (MVC) and mental (IMC) strength training (M75, M50, M25; numbers indicate percentages of mental trials) were compared to a MVC-only training group (M0) and a control condition without strength training (CO). Training sessions (altogether 12) consisted of four sets of two maximal 5-s isometric contractions with 10 s rest between sets of either MVC or IMC training. Task-specific effects of IMC training were tested in four strength exercises commonly used in practical settings (bench pressing, leg pressing, triceps extension, and calf raising). Maximum isometric voluntary contraction force (MVC) was measured before and after the experimental training intervention and again 1 week after cessation of the program. IMC groups (M25, M50, M75) showed slightly smaller increases in MVC (3.0% to 4.2%) than M0 (5.1%), but significantly stronger improvements than CO (−0.2%). Compared to further strength gains in M0 after 1 week (9.4% altogether), IMC groups showed no “delayed” improvement, but the attained training effects remained stable. It is concluded that high-intensity strength training sessions can be partly replaced by IMC training sessions without any considerable reduction of strength gains.
Traditionally, mental training has been applied to tasks with mainly coordinative affordances. It is based on vivid mental images containing a perceptual experience in the absence of overt behavior and therefore with no sensory input (Annett,
In the Yue and Cole study, two groups of participants completed a 4-week training of isometric force production with the little finger by either physical maximum voluntary contraction (MVC) or mental training imagined maximal isometric contraction (IMC). Both groups showed a comparable strength gain (MVC: 29.8%, IMC: 22%) that differed significantly from a control group with no training (3.7%). Moreover, strength gains were also found for the contralateral side not included in the training regime. This transfer to the contralateral part of the body highlights the importance of central neural processes in strength production, because peripheral processes (e.g., on the muscular level) are unlikely to substantially contribute to strength gains on the contralateral side (Carroll et al.,
Using the same task as Yue and Cole (
Small muscle groups | Large muscle groups | ||||||
---|---|---|---|---|---|---|---|
Yue and Cole ( |
Smith et al. ( |
Ranganathan et al. ( |
Herbert and Gandevia ( |
Zijdewind et al. ( |
Ranganathan et al. ( |
Reiser ( |
|
Duration of one IMC (s) | 15 | 5 | 5 | 10 | 10 | 5 | 5 |
IMCs per unit | 15 | 20 | 50 | 6 | 50 | 50 | 8 |
IMC duration per unit (s) | 225 | 100 | 250 | 60 | 500 | 250 | 40 |
Units per week | 5 | 2 | 5 | 3 | 5 | 5 | 4 |
IMC duration per week (s) | 1125 | 200 | 1250 | 180 | 2500 | 1250 | 160 |
Training weeks | 4 | 4 | 12 | 8 | 7 | 12 | 4 |
Total IMC duration (min) | 75 | 13.3 | 250 | 24 | 292 | 250 | 10.7 |
IMC effect (%) | 22.0 | 23.3 | 35 | 6.8 ns | 21.8 | 13.5 | 5.7 |
Some clinical studies also provide evidence for IMC training effects on neuromuscular activation. For example, Newsom et al. (
A large number of brain mapping studies have shown that the same neural areas are activated during either physical or mental simulation of motor actions. This holds not only for cortical areas such as the premotor cortex (PMC), the supplementary motor area (SMA), and the primary motor cortex (M1), but also for subcortical areas such as the basal ganglia and the cerebellum (Lotze et al.,
Nonetheless, it is still unclear to what extent IMC effects shown in laboratory experiments can be transferred across specific field conditions. One particular practical application might be to reduce the demands on the musculoskeletal system associated with “heavy load training” sessions. In order to improve maximal voluntary neural activation, maximal force training sessions are typically applied with very high loads (>90% 1RM). Hence, IMC training could be a suitable way of contributing to a reduction of stress on passive structures (e.g., avoiding overtraining) while simultaneously preserving maximal voluntary neural activation.
Because it is not clear how far strength training sessions can be replaced by IMC training without a substantial reduction of strength gains, the objective of the present study was to determine what would be a useful proportion of physical and mental training. The level of effectiveness of mental training within physical training was tested by using different ratios of physical to mental training (3:1, 2:2, 1:3). These conditions were then compared to exclusive physical training without any mental rehearsal. In general, we predict similar strength gains for the IMC groups compared to the exclusively physical training group. To address applied problems in sports settings, subjects completed IMC interventions after a 4-week standardized training program.
All subjects completed the same 4-week resistance training with submaximal loads prior to the actual intervention followed by a supervised 4-week training program consisting of maximal isometric contractions (MVC) and IMC respectively (see Figure
Maximum isometric voluntary contraction force (MVC) was measured before and after the intervention and again 1 week after cessation of the program. Groups were matched in terms of pretest performance.
A total of 43 healthy sport students (20 female) from the Institute of Sports Science of the Justus Liebig University Giessen, Germany (mean age = 22.7 years, SD = 2.3 years) participated in this study. The subjects had experience with strength training (mean 2 h/week) and were familiar with the exercises, but none of them had recently undertaken an isometric high-resistance training. Testing with the movement imagery questionnaire (MIQ; Hall and Martin,
Four training exercises were performed that differed in terms of extremity (upper vs. lower) and complexity (single-joint vs. multijoint). Exercises were the bench press (upper, multi), leg press (lower, multi), triceps extension (upper, single), and calf raise (lower, single). To compare IMC and physical training, the respective strength gains are given as percentages, because subjects trained different exercises (resulting in different absolute strength gains). This makes it possible to summarize the four strength exercises in the statistical analysis. Training and test exercises were identical in the treatment groups but not in the control condition.
The measurement of maximum isometric contractions force was carried out on standard strength training devices. For test contractions, subjects performed two 5-s maximal efforts separated by a 90-s resting interval. Instruction focused on gradually increasing force to a peak after approximately 2 s (Figures
Bench pressing was performed on a multipress. Using steel chains and carbines, the barbell was attached to the frame of the bench press in order to produce isometric conditions. The chain length was adjusted individually so that subjects could train with an arm and elbow flexion of approximately 90°. Working positions could be reproduced precisely by means of markings on the barbell and a device to adjust the horizontal distance of the subject relative to the barbell.
Leg pressing was tested on a sled 45° leg press. Again, the sled could be fixed to produce isometric conditions. Chains were adjusted so that subjects achieved a 100° angle at their knees. Triceps extension force was measured unilaterally. Subjects were seated on a preacher bench with their upper arm supported. They grasped a handle attached to a cable robe fixed behind them above the head. The length of the cable was adjusted so that subjects reached a 90° angle with their elbow.
To measure the force of the calf raise, subjects sat on a calf-raise machine with their upper thighs placed under a fixed leg pad just above the knees.
A linkage construction was used to produce isometric working conditions. This consisted of steel chains linked by carbines and rigidly coupled with a force sensor system (System DigiMax, Fa. mechaTronic GmbH, Hamm, Germany). The signal from the calibrated force sensor was sampled at 1000 Hz. To remove high-frequency fluctuations from the force signal, data were filtered using a second-order Butterworth filter. Based on visual inspection of the filtering results, the cutoff frequency was set at 6 Hz, which allows adequate quantification of the maximum force value. For each exercise, the peak force produced was measured twice. The highest contraction force obtained from the recorded test trials was taken as the MVC force. All tested exercises revealed high correlations between the two MVC measurements at each measurement time point (all
Prior to the intervention, all subjects performed a 4-week standardized training program twice a week under the individual supervision of one of the investigators. Subjects trained four sets of each of the two exercises performing dynamic executions using a “15RM” load. The 15RM load was determined at the first training session, using a multiple repetition protocol (Fleck and Kraemer,
The experimental training sessions (12 in total) consisted either of MVC or IMC training three times each week for 4 weeks. It should be noted that all exercises during experimental intervention and testing were isometric. MVC training included four series of two maximal 5-s isometric contractions with a 10-s rest between contractions and a 90-s rest between series. IMC training was arranged in the same temporal pattern. Durations for active and mental trials were standardized regardless of differences to durations in dynamical task settings. All subjects completed their IMC sessions individually under the supervision of one of the investigators. They were instructed to imagine maximal contraction efforts as vividly as possible, using kinesthetic imagery (“you should imagine the sensation associated with a contraction effort, but your muscles must stay relaxed”). A preference for kinesthetic versus visual imagery is found in almost all instructions for IMC procedures. This approach is underpinned by studies showing that motor imagery is more effective when associated with “motor” tasks than visual imagery (Féry,
After each session, subjects were asked to rate the vividness of their kinesthetic images on a scale from 1 (
Maximum voluntary contraction gains of the individual exercises did not differ, either at Posttest 1,
A 2 × 2 analysis of variance (ANOVA) with the between factor IMC groups and with repeated measures for the comparison of individual differences between trained and not trained exercises was used to test for training effects, that means for different alterations from Pretest to Posttest 1 and from Pretest to Posttest 2, respectively. To test for group effects between the two posttests, that is, to determine whether MVC gains in the IMC groups differed from Posttest 1 to Posttest 2, a 2 × 2 ANOVA with the between factor IMC groups and with repeated measures for differences between Posttest 1 and Posttest 2 was computed.
The significance level for statistical analyses was set at
Irrespective of their later treatment, all subjects first completed a standardized (physical) training program. Training with submaximal loads in these sessions led to considerable strength gains as reflected in increasingly heavier training weights,
The experimental intervention started with 12 subjects within each of the four groups. A total of 43 subjects completed the pre
To test whether the training effects were influenced by the initial strength (training level at the start of the IMC/MVC sessions), correlations between the absolute MVC values at Pretest and percentage strength gains at Posttest 1 were computed for each of the four exercises. There are consistently negative correlations between the Pretest MVC values and percentage strength gains. This means that the strength gains were as higher the lower initial strength level was. However, only for the calf raise a moderate and significant correlation with
Expressed in SE (Figures
In competitive sports, particularly sports with weight categories, training often aims to increase maximal voluntary strength without increasing body mass. As a consequence, heavy resistance “neural” sessions and also plyometric exercises are typically applied with the aim of optimizing the neural activation of the muscles. This leads to some increase even in the maximal force per cross-sectional area of the muscle, and it has been linked to a centrally represented learning effect observed not only in highly trained athletes but also in recreationally trained and untrained persons (Saltin and Gollnick,
We preferred isometric strength training in this study for two reasons: first, maximal contractions produces strong increases in the maximal voluntary neural contraction of the muscles, whereas muscular hypertrophy of trained muscles remain insignificant. Second, relevant training variables can be determined and controlled very well in isometric strength training conditions, yielding high internal validity of the study. Due to this reason, and taking into account test specificity effects, isometric strength testing was assessed to evaluate IMC strength gains.
A comparison of the groups immediately after the training intervention (Posttest 1) underlines the expectation that considerable maximum strength gains can be achieved with a combination of “high-intensity” mental and physical strength training units. Different rates (75, 50, 25%) of IMC lead to nearly the same improvements as physical strength training alone. If neuromuscular training units are indicated, physical training units can be replaced by mental training units without any significant performance reduction. This holds for short-term effects as revealed by findings on the posttest immediately after the IMC training sessions. The results underline the concept of a functional equivalence between motor imagery and motor performance (Lotze and Halsband,
Since a cessation of heavy resistance training has been shown to enhance strength during the very first week after detraining (Schlumberger and Schmidtbleicher,
The IMC effect sizes in our study are smaller compared with other IMC studies (see Table
Finally, we shall discuss two outcomes related to the effect sizes of our IMC procedures that have both methodological and practical implications. First, as noted above, there is a considerable variability in strength gains within the groups. In the present study, we examined four sport-related strength exercises that differed in terms of extremity and complexity. Although strength gains of the individual exercises following the IMC/MVC interventions did not differ in our study, it cannot be ruled out, that IMC training effects are influenced by specific task demands. Lebon et al. (
However, IMC effects may also be influenced by individual differences in vividness of imagery. A supplementary data analysis revealed a moderate positive correlation between subjects’ subjective reports of vividness of motor imagery and strength gains. It has to be considered that imagery abilities do not differ greatly between participants. Therefore, correlations seem to underestimate the underlying connection between both variables. A median split of participants into good and excellent imagers revealed that excellent imagers showed better overall improvements of strength in IMC conditions. To our knowledge, this is the first time that a relationship between imagery ability and strength gains following IMC has been found. A relation between imagery abilities and motor learning processes in a more general sense have been stated for theoretical reasons (Munroe et al.,
Second, compared with other IMC studies, the total duration of the imagined muscle contractions is rather small (see Table
Naturally, strength training cannot be carried out without highly intensive real effort. However, high-intensity isometric strength training sessions can be partly replaced by IMC training sessions without any considerable reduction in strength gains. This holds at least for athletes at the recreational level. Moreover, we have shown that IMC training is also effective after a prior 4-week physical strength training phase. Our findings support the assumption that IMC training is an adequate supplementary method for improving muscle strength. This type of training may be advantageous for situations aiming to optimize the neural activation of the exercised muscles. In order to test the generalizability of the IMC effect future studies need to address training modalities that more closely represent actual training practice (e.g., dynamic strength training) and hence have a greater transfer to athletic or other skilled movement.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This research was supported by a grant from the Federal Institute of Sport Sciences, Germany (VF 07/05/11/2005).
The authors declare that all experiments performed in the study do comply with the current laws of Germany.