5.1 Kinematic Adaptations to the Knee, During Uphill Running

CHAPTER 5

Discussion

The principal aim of this study was to identify the kinematic and postural adaptations associated with distance running on uphill inclines of 3 and 6, and on a horizontal surface. Spatio-temporal variables were also investigated.

5.1 Kinematic Adaptations to the Knee, during Uphill Running

It was hypothesised that as the gradient increased during uphill running the amount of knee flexion would increase, throughout the stride. The significant results gained from this investigation support this hypothesis. The maximum values for the knee angle show that there was a significant difference between the level running condition and the two inclined running conditions (Table 3). The maximum value results for the knee angle show that the knee was more extended during level running (169.63), compared to the inclined conditions (163.47 and 163.53).

Comparison of these results to other studies is difficult, as all the studies have used different speeds. Williams (1985) stated that differences in speed affect the majority of biomechanical parameters measured during running. Despite this, studies by Paradisis and Cooke (2001) and Paradisis et al. (1999) obtained results, which support the trends found in the current study. Both studies found that during uphill sprinting the knee was more flexed throughout the stride, in comparison to horizontal sprinting.

The motion of the leg in running can be divided into two distinct phases; there is a period of non-contact (swing phase) and a support (contact) phase where one leg is in contact with the ground (Durward et al., 1999). During the time of contact with the ground, the leg supports the body and projects it forwards. After thrusting the body forward, the leg moves from behind the body to a position in front of the body during the swing phase (Dillman, 1975). The graph (Fig. 3) shows that during the stride there are two peaks of knee flexion. At heel strike the extended leg begins to flex. The first peak of knee flexion is half way through the contact phase, and after this the leg begins to extend again until take off. After take off the knee flexes rapidly. The greatest amount of knee flexion is observed, half way through the swing phase. Each phase of the running stride will be compared independently.

5.1.1 Kinematic Adaptations to the Knee, at Heel Strike

At heel strike significant differences were identified between all three conditions. It was found that as the gradient increased the amount of knee flexion at heel strike increased (Table 1, Fig. 2a), and this contributed to a reduced stride length during uphill running. During level running the mean knee angle at heel strike was 165.24. This decreased to 159.76 during the incline condition of 3, decreasing further to 151.40 as the gradient increased to 6. In support of this finding Paradisis and Cooke (2001) and Paradisis et al. (1999) also found that at heel strike, as the gradient increased the amount of knee flexion increased.

During the running cycle, the greatest stress applied to the body undoubtedly occurs during the contact phase (Williams, 1985). Therefore, in level running, as the foot contacts with the ground the joints of the leg begin to flex (Fig. 3) to absorb the landing of the body (Dillman, 1975). During uphill running the study found that knee flexion increased at heel strike, compared to level running. Consequently, it could be assumed that this finding was due to the body needing to absorb more force whilst running uphill. This explanation however, is scrutinised by Hamill et al. (1984) who actually found there were significant decreases in leg shock with increasing gradient.

An alternative reason could be that during uphill running the centre of mass needs to remain in front of the driving leg (Leroux et al., 2000) to ensure that the energy is used in a positive direction. If the knee is more flexed during uphill running at heel strike, then the whole posture of the body is able to remain further forward. Subsequently the centre of mass will remain in front of the driving leg, even though the incline is steeper. This would make the running cycle more efficient. During distance running it is essential that the body works efficiently as the athlete is trying to move the body quickly but with respect to a rate that will enable the runner to have enough energy to complete the race (Dillman, 1975).

It is also thought that increased knee flexion occurs to enable the powerful hip extensor muscles, such as the gluteus maximus, to be involved in the action (Stone and Stone, 2000). The hip extensor muscles are important during uphill running, as they are a strong group of muscles (Stone and Stone, 2000), which support the body during the contact phase and can thrust the body off the surface at take-off more strongly. This is particularly important during uphill running as there is an increased force of gravity acting on the body, thus, more force is needed to aid the runner up the hill.

5.1.2 Kinematic Adaptations to the Knee, at Take-Off

Once the body has absorbed the landing, the supporting leg begins to extend (Fig. 3) to drive the body forwards (Hay, 1993). Dillman (1975) reported that better runners during level running have greater knee flexion during the support phase and are therefore, able to extend the knee joint in a wider range prior to take off. This would enable the take-off and drive forward to be more powerful. The finding of this study could be equated to the results found in the present investigation, during uphill running.

Although the knee in the current study had increased flexion at immediate heel strike during incline running at 6, it didn’t continue to flex quite so much during the support phase. The results at take-off (Table 2, Fig. 2b) showed that during the inclined conditions of 3 and 6 the leg was slightly more extended (161.81 and 161.91) compared to the level running condition (160.14). As the leg was slightly more extended at take-off in the uphill running conditions, this would enable the take-off and drive forward to be more powerful (Dillman, 1975).

5.1.3 Kinematic Adaptations to the Knee, during the Swing Phase

Figure 3 showed that most knee flexion occurred during the swing phase. In addition, the graph showed that the knee was most flexed during the inclined condition of 6. The least flexion occurred during the swing phase, whilst running on level ground.

During the initial swing phase the knee is rapidly flexed behind the thigh. Researchers have remarked that this rapid knee flexion occurs to reduce the rotational resistance and moment of inertia of the leg about the hip joint (Durward et al., 1999). Consequently, the thigh can be brought through more quickly, enabling the leg to travel further producing a greater range of motion. This means the knee can be raised higher more efficiently (Dillman, 1975). Increased efficiency is important as distance running places a premium on the minimisation of metabolic energy output (Dillman, 1975).

When the athlete’s thigh reached an almost horizontal position, the lower leg swung forward about an axis through the knee and begins to extend (Fig. 3), before contact was made with the ground again.

5.1.4 Potential for Injury during Uphill Running

A study by Paradisis and Cooke (1998) found that training on combined uphill-downhill surfaces was beneficial to the leg flexor muscles (hamstring group). The results indicated that the hamstrings were able to exert higher forces at a greater rate, after just 8 weeks of training on combined uphill-downhill surfaces. The present study found that increased knee flexion was an important adaptation to uphill running, suggesting that it is this frequent knee flexion which strengthens the hamstring muscles. This would be beneficial to runners as a common cause of injury during running is caused by a muscle imbalance (Worrell, 1994), between the quadriceps and hamstrings. As it has been found that training on combined uphill-downhill surfaces strengthens the hamstrings (Paradisis and Cooke, 1998), this would reduce muscle imbalance and therefore, minimising the risk of injury.

Although it has been suggested that uphill running might be of benefit, by reducing potential injuries, adversely there are other injuries that can be caused by too much uphill running. Synovitiis of the knee is a painful condition, in which the synovial membrane of the knee becomes inflamed (Garrick and Webb, 1990). This condition is an overuse injury, caused by instability in the knee (Garrick and Webb, 1990). As the gradient is altered in uphill running this worsens the instability of the knee. Although there are no known kinematic changes that could be made during the running cycle to rectify this, it should be noted that training should be built up gradually. This would allow the supporting structures in the knee to strengthen, therefore, improving the stability of the knee.

5.2 Kinematic Adaptations to the Hip, during Uphill Running

It was hypothesised that as the gradient increased, hip flexion would increase. The results supported this hypothesis, indicating that throughout the stride, as the gradient increased the hip became more flexed (Fig. 4). Previous studies by Paradisis and Cooke (2001), Paradisis et al. (1999) and Swanson and Caldwell (1999) support the results found in this study. All the studies found that as the gradient increased, the amount of hip flexion throughout the stride increased. However, the results gained in these studies were not significant. Each phase of the running stride will be discussed individually.

5.2.1 Kinematic Adaptations to the Hip, at Heel Strike

The results from this study show that the hip begins to extend, prior to heel strike. The hip does not flex at heel strike in response to impact with the ground, but continues to extend during the shock absorption phase of support (Fig. 4, Fig. 2a). The results also exemplify that there was a significant difference between the three conditions (Table 1). During the level running condition the hip angle was 34.79. As the gradient increased to 3 the hip angle value at heel strike increased to 39.62, this increased further to 48.22 at the inclined condition of 6. Due to the angle convention used, the results show that the greater the hip angle value, the greater the amount of hip flexion. The results therefore, show that as the gradient increased the hip became more flexed.

Paradisis et al. (1999) also found that at heel strike, as the gradient increases the hip becomes increasingly flexed. It is thought that this occurs simply because during the swing phase the hip angle is much more flexed in the inclined condition.

The hip extends rapidly in the latter part of the support phase (Fig. 4). This action of the hip drives the body over the support leg (Durward et al., 1999), during the contact phase.

5.2.2 Kinematic Adaptations to the Hip, at Take-Off

The runner’s aim during the take-off phase is to thrust downwards and backwards against the surface. This drive is brought about by the forceful extension of the hip (Hay, 1993). Data analysis of this investigation showed that, the hip was fully extended at take-off for all three conditions (Fig. 4, Fig. 2b). During level running at take off the hip value was –4.19, which changed to -2.37 during the inclined condition of 3. At the 6 gradient the hip angle was 1.16. This shows that the hip was slightly less extended during the incline conditions, although this difference was not significant. The full extension of the hip helps to drive the body over the supporting leg (Hay, 1993).

The hip extensor muscles are a strong group of muscles, which are important during uphill running (Stone and Stone, 2000) helping to support the body during the contact phase and thrust the body off the surface at take-off very strongly. During uphill running the knee has increased flexion and this action allows more hip extensors such as the gluteus maximus, to be involved in the action (Stone and Stone, 2000). This is beneficial during uphill running, as increased work and force is necessary to counter the elevation change (Martin and Coe, 1991).

5.2.3 Kinematic Adaptations to the Hip, during the Swing Phase

During the swing phase it can be seen that the hip was at its most flexed (Fig. 4). Due to the angle convention used, the maximum hip angles (Table 3) found correspond to the time when the hip was most flexed. The results from this show that the hip was most flexed approximately half way through the swing phase. It is thought that maximum knee flexion occurs during the swing phase, so that the hip would be able to go through a greater range of motion between the swing phase and take-off. This would mean that the angular acceleration at take-off would be greater, thus, enabling the take-off and drive forward would be more powerful (Hay, 1993).

The maximum values of the hip angle (Table 3) also showed a significant difference between all three conditions. During the horizontal running condition the maximum hip angle value was 47.02. This increased to 54.08 at the 3 incline, and then to 63.13 as the gradient increased to 6. Therefore, out of the three conditions, greatest hip flexion was observed at 6. Figure 4 also shows that the hip is much more flexed in the inclined conditions.

Evidence in the literature supports the findings of this study. Leroux et al. (2000) found that a key mechanism in their study, when adapting to uphill walking, was to lift up the swinging leg by performing a simultaneous increase in the hip and knee flexion of that limb. Martin and Coe (1991) stated that a quick and powerful knee lift by action of the hip flexors is an important element in hill running. These findings suggest that increased hip flexion during the swing phase is an advantageous characteristic, during uphill running. It is thought that increased hip flexion during the swing phase allows for a greater range of movement in the hip before take-off. Therefore, if the knee is raised higher during inclined conditions then the angular acceleration at take-off would be greater, enabling the take-off and drive forward to be more powerful. This is important during uphill running, as more power is needed to counteract the effects due to gravity (Martin and Coe, 1991).

5.3 Kinematic Adaptations to the Trunk, during Uphill Running

It was hypothesised that during uphill running the forward lean of the trunk would increase as the gradient increased. The results obtained support this hypothesis. A significant difference for the maximum trunk angle values (Table 3.) was found between the level running condition and the incline running condition at 6. The maximum value for the trunk angle decreased from level running (78.02) to the incline condition at 3 (76.44), decreasing again as the gradient increased to 6 (73.08). The trend showed that as the gradient increased the trunk angle decreased and therefore, the forward lean of the trunk increased (Fig. 2). The time that the maximum trunk angle occurred varied between participants, although it tended to occur around heel strike. The results gained for the trunk angle at heel strike and take off also showed the same trends (Fig. 5), however, the differences between the conditions at these points in the stride were not significant.

Comparison of results is difficult, as all the studies referenced investigating trunk angle with increasing gradient, have used very different speeds. Williams (1985) stated that the differences in speed affect the majority of biomechanical parameters measured during locomotion. Despite this, all the studies found similar trends to the present study. Two previous studies (Leroux et al., 2000; Vogt and Banzer, 1999) investigated the postural strategies to adapt to uphill treadmill inclination during walking. Both studies found there was an increased forward lean of the trunk in the direction of progression, with increasing gradient.

Studies by Paradisis and Cooke (2001) and Paradisis et al. (1999) also found that increasing the gradient of the incline had the same effect on trunk lean, even though these studies investigated sprinting. Paradisis and Cooke (2001) found that uphill sprint running at a gradient of 3 produced significant reductions in the trunk angle of 8.7% at heel strike, and a reduction of 9.8% at take-off. These results are comparable to the results obtained in the present study. Only one study using a distance running speed has looked at the kinematic variable of trunk lean. In agreement with the present study, Klein et al. (1997) found that during the inclined condition of 5%, greater trunk lean was displayed although this was not significant.

Despite the variation in speeds investigated, all the studies showed that the main postural requirement when adapting to inclined walking or running was to change the trunk orientation relative to the earth’s sagittal plane. The studies indicated as the gradient increases the forward lean of the trunk increases, to overcome the effects of gravity.