Results/Discussion
The average heart rate (HR) and stride rate (SR) for each level per condition are presented in Table 1. As expected, the HR increased along with SR during DWR as well as TMR. Also, the stride rate for a given HR was lower for either of the DWR conditions compared to TMR. The stride rates during TMR are typical stride rates for submaximal running speeds.
The changes in anglular position during a stride for the ankle, knee and hip angles are illustrated in figures 2-4. In each figure, 0% represents heel contact, while toe off is indicated on each curve. The changes in ankle, knee and hip angles during a stride appear to follow similar patterns during DWR C1 and TMR. The main difference being that the ankle remains in a plantar flexed position during DWR C1 swing, while during TMR swing, the ankle undergoes dorsiflexion.
Knee flexion did not occur during the support phase of DWR C1 or C2. In comparison, during TMR, knee flexion occurs between heel contact to about mid-support. The difference between modes of exercise was expected because there was no ground contact during DWR.
Maximum knee flexion was greater than 180 degrees during the support phase of DWR C1, indicating hyperextension of the knee. The time of occurrence of maximum knee flexion was during early support. This observation seems to indicate that a high torque about the knee is present during support phase, indicating that the posterior region of the knee would be stressed to a greater magnitude compared to TMR. The reason for the high torque during early support is due to the position of the leg relative to the bouyant force. During the start of the support phase, the hip is near maximum flexion (of a stride) and the knee is fully extended. Considering the length tension relationship of muscle, the hamstring is not able to generate a high force at this point of the stride (i.e. it is fully stretched). Therefore, the dominant force would be the bouyant force, which is directed superiorily. This force, combined with the force of hip extension, along with the decreased ability of the hamstring to contract forcibly to stablize the knee joint results in hyper extension of the knee. This observation may indicate that during the cross country style of DWR, individuals should concentrate on not "over striding."
The hip angle appears to change similarly during DWR C1 and TMR, with less range of motion during DWR C1. A greater range of motion of the hip was observed during DWR C2 compared to TMR. It is understandable that the hip follows similar patterns during each of the activities. The hip functions to swing the lower extremity forwards and backwards.
To explore the temporal relationship between the ankle, knee and hip angles during each mode of exercise, a correlation matrix was generated between the 3 pairs of variables for each level per condition ( (see table 2.)
During treadmill running (TMR), the positive correlation coefficient between the knee and hip during each condition indicates that the knee and hip were generally extending and flexing simultaneously. A correlation coefficient of 1 would have indicated that the knee and hip were in phase. The correlation was not expected to be high because during the later portion of swing (i.e. just before heel contact), knee extension occurs simultaneously with hip flexion.
During DWR C2, the knee and hip were strongly linked temporally, regardless of the level of intensity, as indicated by the high correlation (r=0.97) for each level of intensity. This relationship is also indicated by examining time of occurrence of the maximum knee and hip flexion during DWR C2. Maximum knee and hip flexion occurred at 7.2% and 4.5%, respectively, of total stride time. The maximum knee and hip extensions occurred at 46.2% and 52.7% of stride. This result was expected due to the "piston" like action of the legs.
During DWR C1, the relationship between the ankle and hip was out of phase when compared to TMR. During DWR C1, maximum dorsiflexion occurred at 45.7% of total stride time. In contrast, during TMR, maximum dorsiflexion occurred at 91.1% of stride (just prior to heel strike). Likewise, maximum plantar flexion occurred at 92% and 38.7% of stride during DWR C1 and TMR, respectively. These differences are most likely due to hydrostatic forces present during DWR. As noted previously, the ankle remains in a maximum plantar flexed position during the majority of DWR C1 swing phase. During TMR, the ankle dorsi flexes during the swing phase up until heel contact. This observation indicates that there were high forces present about the ankle during the swing phase of DWR C1.
To further explore the temporal relationship between the knee and hip angles during DWR and TMR, angle-angle diagrams were generated for data sets of equivalent stride rates. Figures 5 through 7 illustrate the temporal relationship between the knee vs hip angle during DWR C1, DWR C2, and TMR, respectively. These levels of intensity were chosen because the stride rates were similar across conditions (DWR C1: 73 st/min; DWR C2: 76.1 st/min; TMR: 78.4 st/min).
Additionally, during the "swing" phase of a stride, the relationship between knee and hip was similar during DWR C1 and TMR. However, during the "stance" phase of gait, the relationship between knee and hip is different between DWR C1 and TMR due to the lack of need to counteract gravity during DWR.
The knee-ankle relationship during the different modes of exercise are illustrated in figures 8-10. The angle-angle relationship is clearly different during either of the DWR conditions and TMR. Also, the knee-ankle relationship is quite different between the two conditions of DWR, as indicated by the direction of the plot. During the stance phase during DWR C1, the ankle is dorsi flexing while the knee flexion is occurring. In contrast, during DWR C2 stance, the ankle is undergoing plantar flexion while the knee is extending. During TMR stance, the ankle undergoes dorsi flexion simultaneously with knee flexion until about mid stance. Then, the knee starts to extend while the ankle undergoes plantar flexion.
Interestingly, when considering the knee-ankle relationship of DWR C1 during levels 1 and 2, the relationship is quite similar to that of TMR. Figures 11 and 12 illustrate the knee-ankle relationship during levels 1 and 2 during DWR C1. Compared to TMR, the relationship between the ankle and knee angles during DWR C1 is quite similar. The main difference, again, is the lack of need for knee flexion during DWR stance.
Kinematic Conclusions
The running action during the cross-country style (C1) of DWR appears to be different compared to TMR only because of the lack of ground contact during the "support" phase of DWR. Also, there appears to be high forces about the ankle during the swing portion of DWR C1 due to the hydrostatic force of water. During DWR C1, the knee may have a tendency to hyper extend due to the torque about the knee during the early portion of the support phase. During this portion of the stride, because the hip is flexed and the knee is extended, the hamstring is not able to generate enough force to stablize the knee. The result is hyper extension of the knee. During DWR C2, there does not appear to the be the risk of hyperextension of the knee because the hip and knee flex and extend simultaneously throughout the entire stride.
Proceed to next section- EMG Results
