Data exclusion and measurement quality
One subject of the controls had to be excluded from the passive measurement due to the poor quality of the ultrasound video. In this low-quality ultrasound video, the MTJ was not identifiable with the necessary precision.
The mean ICCs of the interrater test of the ultrasound video analysis were 0.96, 0.98, 0.95, 0.96, and 0.95 for muscle thickness, pennation angle, and fascicle length during the RoM measurement, and MTJ displacement during PRT measurement, respectively. Values above 0.90 are classified as high (Vincent & Weir,
2012).
The mean values of the Pearson correlation coefficients at the linear regression were 0.92, 0.96, and 0.97, with all P < 0.05, for passive tendon stiffness, muscle stiffness, and muscle–tendon stiffness, respectively.
Discussion and conclusions
This pilot study compared the muscle and tendon tissue properties of competitive soccer goalkeepers, midfielders, and less-active controls. No significant differences were found in the functional parameters of RoM, PRT, and muscle–tendon stiffness, and several structural parameters, including muscle and tendon stiffness, muscle thickness, fascicle length, and pennation angle. However, the MVC torque was significantly higher in the goalkeeper and midfielder groups compared to the control group. Though, since this is a pilot study with a small sample size further studies are necessary to strengthen the results of the present study.
Higher MVC torque values in athletes (sprinters and endurance athletes) compared to non-active persons have also been reported by Arampatzis et al. (
2007). In their study, it was found that the sprinters also had significantly higher MVC torque values (154.6 Nm) than the endurance athletes (126.3 Nm). Stenroth et al. (
2015) found a tendency for higher MVC torque in older sprinters (153 Nm) compared to older endurance athletes (age of both groups around 74 years; 116 Nm). In the present study, both the goalkeepers (156.6 Nm) and the midfielders (160.6 Nm) had similar torque values to the sprinters in the study of Arampatzis et al. (
2007). One could therefore assume that training for midfielders and goalkeepers leads to a similar development of strength in the plantar flexor muscles. In our first hypothesis, we expected higher MVC torque values in the more sprinter-like goalkeeper group compared to the more endurance-like midfielder group. Surprisingly, this was not confirmed by our results. One could therefore assume that training for midfielders and goalkeepers leads to similar development of strength in the plantar flexor muscles. However, the higher volumes of activity lead to higher maximum isometric torque values. This was confirmed by the significant positive correlation (
r = 0.58;
P = 0.01) between the training volume and MVC torque values in our data. Similar to the present study, Faria et al. (
2013) found no differences in MVC torque between several playing positions (defenders, midfielders, wingers, forwards, non-players) in their soccer study. Due to the results of the study of Faria et al. (
2013) and our study, one could deduce that the specific training loads of the playing positions in soccer seem to affect the adaptation of the MTU in a similar way.
Regarding further parameters of the MTU, Arampatzis et al. (
2007), in a study of active track and field athletes, but not Stenroth et al. (
2015), in a study of older athletes, found stiffer tendons in the sprinter group than in the endurance and non-active groups. Due to the explosive characteristic of goalkeeper movements (similar to the sprinters in the study of Arampatzis et al. (
2007)), we expected that goalkeepers would have stiffer tendons and/or muscles than midfielders and the controls. However, we did not find any difference in passive tendon stiffness, muscle stiffness, muscle thickness, pennation angle, or fascicle length between the three groups (goalkeepers, midfielders, controls). This is in contrast to some results from the literature which showed that training and other chronic processes led to specific adaptations of the MTU. Several studies found structural adaptations due to endurance training habits in the muscle cross sectional area (Stenroth et al.,
2015), fascicle length (Oda et al., 2015), tendon length (Sano et al.,
2013), and tendon stiffness (Oda et al.,
2013; Kunimasa et al.,
2015). Moreover, Reeves, Narici, and Maganaris (
2003) determined that repeated strength training increased patella tendon stiffness. It was also found that repeated isometric strength training increased MVC torque and muscle volume in a study by Kubo et al. (
2009). Furthermore, Foure, Nordez, and Cornu (
2012) found higher muscle stiffness and a tendency for increased muscle–tendon stiffness (
P = 0.09) following a plyometric training program over 14 weeks. Csapo, Maganaris, Seynnes, and Narici (
2010) showed higher Achilles tendon cross-sectional area and stiffness and shorter fascicle lengths following the chronic use of high heels. Soccer training, especially in the preparation phase, also consists of strength training and plyometric training. With regard to the findings in the literature, one could therefore assume that these training regimes lead to higher tendon and muscle stiffness. In general, training regimen of goalkeepers and midfielders are different. Therefore, one could assume that these groups will have different muscle and tendon function and structure.
The lack of significant differences in structural parameters between the goalkeepers and the midfielders but also between the athletes groups and the non-active controls could possibly be explained by the small sample size of the pilot study. Therefore, we determined the effect size and statistical power of all the parameters between the three groups (see Table
2). While effect sizes were quite high between athletes and controls (mean 0.77, range 0.02–1.33) they were rather small between goalkeepers and midfielders (mean 0.19, range 0.00–0.45). Statistical power was rather low (<0.8) in all our results, indicating that possible differences between the groups might be overlooked. At one hand, it appears that the amount of subjects was too small to detect significant differences between athletes and controls. An a priori power analysis for two independent means (d = 0.8; power = 0.8; α = 0.05) has shown that 21 subjects for each group would have been appropriate. On the other hand small effect sizes indicate that there is no physiological meaningful difference between the athletes group. This is underlined by a respective power analysis with the observed difference between the athlete groups (d = 0.19, power = 0.8, α = 0.05) which produced a necessary sample size of 344 for each group which cannot be realized. Hence, we are confident about the main conclusion (no differences between the athlete groups) of the present study.
Table 2
Differences, effect size (ES), and power of any parameter of the goalkeepers and midfielders, of the goalkeepers and controls, and of the midfielders and controls
A | Range of motion (°) | −1.8 | 0.24 |
0.11
| 3 | 0.39 |
0.17
| 4.8 | 0.75 |
0.37
|
Muscle thickness (cm) | 0 | 0.00 |
0.05
| 0.2 | 0.81 |
0.41
| 0.2 | 0.85 |
0.44
|
Fascicle length (cm) | 0.2 | 0.34 |
0.15
| 0.3 | 0.47 |
0.21
| 0.1 | 0.12 |
0.08
|
Pennation angle (°) | −0.7 | 0.24 |
0.11
| 2.5 | 0.87 |
0.46
| 3.2 | 1.29 |
0.74
|
B | Passive resistive torque (Nm) | 2.5 | 0.16 |
0.09
| 15.3 | 0.87 |
0.46
| 12.8 | 1.02 |
0.56
|
Passive tendon stiffness (N/mm) | 3 | 0.45 |
0.20
| −0.2 | 0.02 |
0.05
| −3.2 | 0.49 |
0.22
|
Muscle stiffness (N/mm) | 0.9 | 0.22 |
0.10
| 4.2 | 0.92 |
0.49
| 3.3 | 0.98 |
0.5
|
Muscle-tendon stiffness (Nm/°) | 0 | 0.00 |
0.05
| 0.4 | 0.88 |
0.46
| 0.4 | 1.09 |
0.61
|
C | MVC torque (Nm) | −4 | 0.13 |
0.08
| 52.7 | 1.21 |
0.69
| 56.7 | 1.33 |
0.76
|
Concerning the lack in significant differences (except MVC torque) between the athletes and the non-active controls, one possible explanation could be in the higher (but not significantly so) RoMs in the goalkeepers (25.1°) and midfielders (26.9°) compared to the controls (22.1°). It was reported in some studies (Mahieu et al.,
2007; Konrad et al.,
2015) that alongside with increased RoM tendon stiffness decreased following a stretching regime. It is possible that the frequent stretching by the goalkeepers and midfielders led to a similar stiffness to the controls. Furthermore, some of the ‘non-active’ persons actually undertook up to 4 h of activity a week, which might have already led to adaptations of the MTU.
There are some limitations to this study. First, the sample size of the groups was small, and this may have influenced the results. However, power analysis (results not presented) has shown that the main outcome of the study would have been the same with a larger number of participants. Thus, we believe that this pilot study could provide important information for future studies on this topic. Second, the persons taking the measurements were not all blind to the intervention. Therefore, a bias in the results cannot be completely excluded, although the interrater reliability was excellent (mean ICC: 0.95–0.98). Third, the method of measuring the moment arm of the ankle joint in vivo was quite simple. However, the values obtained in this study were very similar to others obtained using magnetic resonance imaging data (Rugg, Gregor, Mandelbaum, & Chiu,
1990) or ultrasound (Lee & Piazza,
2009). Fourth, according to Kubo et al. (
2002) and Mahieu et al. (
2007,
2009), the GM muscle force was estimated assuming that it comprises 18% of the whole plantar flexors muscle group. This might be a rough estimation in some subjects. Fifth, although we monitored the measurements carefully, we cannot rule out small changes in the axis of the ankle joint in relation to the dynamometer axis due to heel displacement. This would have led to differences between the measured ankle joint angles/moments and real ankle joint angles/moments during measurements (Arampatzis et al.,
2005a). An implication of this limitation would be erroneous assessments of the MTJ displacement (Arampatzis et al.,
2005b) during the passive measurements and hence to incorrect estimations of muscle and passive tendon stiffness. However, there is no indication that errors due to both limitations four and five are systematically different in the different subject groups and, therefore, would not significantly affect the general outcome of the study.
In conclusion, this pilot study found no difference in the functional and structural parameters of the MTU between soccer goalkeepers and midfielders. However, there was a significant difference in MVC torque between the athlete groups (goalkeepers and midfielders) and the less-active persons. Further studies should explore possible changes of the MTU over an entire soccer season. Moreover, other types of sports (handball, basketball, volleyball, tennis, etc.) with different requirements should also be investigated in this regard.