The aim of this research was to evaluate marathon performance and asses the influence of this long-term running endurance exercise on the changes of muscle stiffness in middle-aged marathon runners. The hypothesis regarding muscle stiffness was not supported, as the current investigation revealed significantly lower levels of stiffness post-marathon for the calf muscles in the left leg (p = 0.016). No significant changes were noted with regard to muscle stiffness at the post-marathon assessment in the other two tested muscle groups (quadriceps, in both the left and right lower limbs, and calf in the right limb). Generally, this was very surprising.
The explanation of this phenomenon is likely to be difficult because none of the previously described studies have documented the impact of a prolonged running effort commonly defined as a marathon on muscle stiffness. Additionally, this requires the consideration of indirect analyses of other variables which effect the marathon effort. This approach is also considered difficult because many of the factors to be analysed were not included in this experiment. The reason for this is that many of these variables are difficult to measure without interfering with the running autonomy. However, it has been well described how long-distance running directly impacts running economy (RE) and muscle damage 11. It seems reasonable to combine all these factors, due to the non-exclusive relationships, to optimally assess the marathon effort and its direct impact on changes in the runner’s body after such a long effort. Knowing this may help runners not only to improve their marathon performance, but also to develop an appropriate training programme, which can optimally prepare them to run 42.195 m, regardless of their level: championship, intermediate, or recreational. An important element of such an analysis is the division into sex, but particularly into age categories, with a special emphasis on 50 + 37.
Essentially, marathon performance depends on the running economy (RE) in all the word weariness. RE is an ‘aerobic demand’ to maintain a proper pace: speed over distance. It is defined as the stationary oxygen uptake (VO2) associated with this speed 18,19. Comparing our participants with younger marathon runners aged 43.9 ± 8.3, the values of HRmax were lower by an average of 9.2 (bpm) and 4.29 (ml/min/kg) 38. Larger differences occur in comparison with the group of recreational runners (63 ± 32 km/week) aged 34 ± 8 years. The differences in Hrmax and VO2max are 14.5 (bpm) and VO2max by 18.69 (ml/min/kg), respectively, in favour of the younger runners 39. Similar relationships can be seen in the case of VO2 at the aerobic threshold (VT1) and anaerobic threshold (VT2). Younger runners are characterised by higher VO2 at these thresholds (3.67 for VT1 and 12.86 for VT2) 39. Despite the lower values of these indicators, the marathon runners studied in Wrocław consumed more oxygen in relation to their abilities than did younger recreational athletes. The percentage of participants of the Wrocław Marathon on VT1 achieved 76.23% VO2max, while the competitors studied by Lanferdini et al.39 only 59.49% VO2max. Comparing the capabilities at the VT2 threshold, the results were 91.3 and 84.65% HRmax, respectively. Despite this, our marathon runners showed a strong relationship between Vo2 max and speed on each subsequent 5-km section (in the range from 0.000034 to 0.000239). This confirmed previous research, which found that there is a strong relationship between VO2 max and the level of effort in a marathon run.
It is also obvious that the RE must be associated with the marathon runner’s individual running technique, and that this, in turn, depends on the resistance of the runner’s body to fatigue and falling running speed. In our experiment, marathon runners began to experience a drop in running speed after 12 km of a race. From that moment on, a continuous, slow decrease in speed began, which amounted to approx. 5.6% at the finish line. This did not confirm the reports of Hettinga et al. 40 that during the late stages of the marathon (the last 10–15 km) a considerable deceleration usually occurs. This affects even world-class runners and is recognised by runners as ‘hitting the wall’ 41. This is probably due to the fact that our runners are classified as slow, recreational runners and they were over 50 years of age, so their marathon effort can be defined as prolonged (4.30,05 h ± 35.12 min.) but of low intensity.
On the other hand, world-class marathon runners have developed training strategies to manage or prevent fatigue and sharp drops in running speed 42. The studies by Buckalew et al. 43 and Chan-Roper et al. 44 regarding the effects of fatigue on running technique showed that technique changed by decreases in step length rather than step frequency. These changes were directly responsible for the decreased speed. Marathon runners are predominantly rear-foot strikers, which is true for both world-class 45 and recreational long-distance runners 46. This can be applied to our marathon runners with an indication of the activity of the left leg, and with particular emphasis on the triceps muscle of the calf. They noticed a few disadvantages in this matter. A significant potential biomechanical limitation of landing with a rear-foot strike pattern is that the foot lands in front of the whole body’s centre of mass. This increases the braking force and directly impacts the speed, mainly reducing it by the resulting weaker take-off. This negatively influences the step length by shortening it. The second disadvantage of the running technique when fatigue appears is that landing almost the whole foot on the ground during the early stance and then continuing during the main amortisation phase significantly increases contact time. In turn, the high centre of mass is achieved through knee flexion. The greater the knee flexion, the longer the foot–ground contact time and the higher the reduction in speed. Additionally, according to Derrick et al.47, the presence of fatigue may decrease the utilisation of the stretch shortening mechanism, especially in the hip and knee joints. This causes the knee flexors and extensors to tire more quickly, which results in reduced leg stiffness. Despite this assumption, the relationship between running speed on each 5-km stretch (increasing fatigue with each km) and muscle stiffness was not confirmed with no change in muscle stiffness. However, a much greater correlation was found for the triceps muscle of the calf (mean significance level: p = 0.354612). This can be confirmed by the fact that this muscle has a greater functional impact in the running step technique. In turn, it weakens the ground reaction forces, thus significantly extending the contact time 48. All these elements mean that there is a significant reduction in speed, and the runners thus achieve poor results. In addition, these undesirable factors should be eliminated in training in order to achieve optimal results in the marathon in relation to motor preparation. At the same time, these parameters, which should not weaken the running technique, had a positive effect on muscle stiffness. This did not change after the marathon effort compared to the measurements before the race.
Here another problem arises that was not discussed earlier. It should be assumed that muscle stiffness is partly related to delayed-onset muscle soreness (DOMS). Delayed-onset muscle soreness appears when humans engage in exercise to which they are unaccustomed, or is prolonged in time. It is likely that after a long-term running effort – training for and participating in a run, e.g. a half marathon or marathon – runners show significant muscle damage and soreness. Muscle stiffness is associated with delayed muscle soreness. Muscle stiffness is defined as the change in strength divided by the corresponding change in muscle length. This occurs when the change in muscle length is caused by an external factor, e.g. additional external resistance such as uphill running, or by a change in the external load on the muscle – a sustained effort. In other words, the term ‘stiffness’ describes the resistance of a muscle to a change in length. Therefore, when the muscle is working (eccentric/concentric contraction, e.g. running step) using its entire length, it is able to generate maximum force, and this should automatically increase its stiffness. On the other hand, when the muscle is working, especially in eccentric contraction, with a significantly shorter length (large flexion of the lower limb in the knee joint), its length does not change significantly, which reduces the generation of force and thus reduces the level of its stiffness. This probably happens in marathon runners.
Following this statement, the question arises of whether, with no changes in muscle stiffness after the marathon, there were changes in DOMS, which are mainly manifested by increased muscle soreness and whether they are two independent or mutually exclusive work-related activities. Changes in the mechanical properties of the muscles observed after prolonged physical activity may be associated with increased joint stiffness. In terms of performance, the increased stiffness was found to be associated with increased speed, increased jump velocity, jump height, and running economy (measured by oxygen consumption)49. According to Beck et al.37, followed by Kerdok et al.50, a critical determinant of running economy is the spring-like storage and return of elastic energy from the leg during a stance.
Here we have to distinguish between two elements – muscle stiffness and joint stiffness – which is often equated with leg spring stiffness. The latter measures the stiffness of the muscle and tendon, but regarding how well a runner is able to recoil the elastic energy generated during ground contact in each stride. Therefore, increases in joint stiffness, mainly by eccentric contraction movement, shorten ground contact50,51 will generate more elastic energy. In turn, this indicates an improvement in running economy over time and an increase of delayed-onset muscle soreness.
According to Beck et al.37, it can be concluded that the assessment of older runners may be indirectly based on leg stiffness, through reduced tendon stiffness52,53, lower active peak ground vertical reaction forces (GRF)51, and greater flexion at the knee joint at landing54,55. This suggests that leg spring stiffness decreases with age37. Did this occur in our marathon runners? Although DOMS was not the subject of the study and we do not have relevant data, it should be assumed that the DOMS was at the tolerance level of the runner’s body immediately after the end of the run. The question therefore is how would the DOMS and muscles stiffness behave if measurements were taken several times, e.g. 12 h, 24 h, or 48 h after the marathon? These unknowns were partially answered by Chleboun et al.56, who stated that muscle swelling, as one of the factors of DOMS, does not necessarily account for the sudden increase in post-exercise stiffness, whatever the determinant of the subsequent muscle stiffness may be.
This is one of the limitations of this study: the absence of subsequent measurements of muscle stiffness, e.g. 12 h or 24 h after completing the marathon. This was not due to the technical feasibility of the measurements, but to the personal reasons of the competitors. Such measurements would have also allowed us to observe the changes in DOMS in relation to the delayed changes. Another limitation is the lack of a running technique evaluation on video recording e.g. 15 km or 40 km into the race. This would have allowed us to correctly describe the marathoners’ running technique and juxtapose it with VO2 in order to evaluate their running economy.