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The measurement of shock waves following heel strike while running
THE MEASUREMENT OF SHOCK WAVES FOLLOWING HEEL STRIKE WHILE RUNNING Jos A. DICKINSON.
STEPHEN
D.
COOK
and TED M. LEINHARDT
Tulx~r L’nivrrsiiy School of Medicine. Department ol Orthopardic Surgery. Neu
Orleans.
LA
1430 Tulane
Avenue.
701 I.‘_. U.S.A.
Abstract-A non-invasive method for demonstrating the shock wave which propagates through the skeletal system following heel strike is described. This wave was not seen in force plate studies where adequate shock absorption was provided by running shoes. In the present study six subjects ran across a force plate without shoes before and after they were fatigued on a treadmill to demonstrate possible changes in the heel strike transient. Most of the parameters measured were not altered by fatigue. and a relationship between the shock waveand height. but not the weight ofthe runner wasdemonstrated.The diffrrent mechanisms leading to this phenomenon. and its implication in the areas of osteoarthritic degeneration and running mechanics dr: discussed
al. (1978) and Voloshin and Wosk (1980). These waves travel up the axial skeleton and are attenuated along the way. Muscle stiffness seems to predominate as the main shock absorbing mechanism w-ith bony deformation, joint motion and articular cartilage compression also being important. A great deal of work on the transmission and attenuation of these incoming forces has been described by Wosk and Voloshin (1981) and others. The transmission of these shock waves through the skeletal system may pIa!- a role in various acute and chronic injury mechanisms. Recently there has been increasing interest in the effects of shock on the musculoskeletal system. Radin et al. (1982) demonstrated changes in the bone and the hexosamine content of cartilage indicative of osteoarthritis (Barland et al., 1966 Ballet, 1967) in the weight bearing joints of sheep who were walked for long periods of time over a concrete floor. Another group of sheep was walked over a floor covered with wood chips for the same length of time and showed no osteoarthritic changes. These results seem lo support the suggestion of Radin et al. (1975) that osteoarthritis results mainly from poorly handled mechanical loading, rather than from a disease process. Studies by Voloshin and Wosk (1982) have postulated that low back pain may be caused by reduced shock absorbing capacity in the musculoskeletal system in conjunction with impulse loading. The majority of the human and animal studies described in the literature deal with the forces transmitted while walking. In force plate studies on running subjects. peak forces of 2.5-3 times body weight are generated (Cavanagh and Lafortune, 1980). This is in sharp contrast to the forces of slightly over one body weight seen in studies on walking individuals. The 2-3 fold increase in forces seen while running in association with thi: experimental results indicative of possible degenerative processes occurring with normal walking forces clearly demonstrates the need for techniques to quantify these shock waves so methods for their attenuation can be attempted. et
INTRODUCIION
The sedentary lifestyle produced by twentieth century technology in conjunction with the realization of the beneficial effects of exercise upon health has resulted in the ‘fitness movement’ which has emerged throughout the world. This movement has produced a new type of athlete who is less conditioned, less knowledgeable about training techniques and more prone to injury than his counterpart in organized athletics. The large number of people participating in recreational athletics points to the need for studies on the acute and long term mechanisms of injury suffered by this group. This year approximately 40 million Americans will participate in some form of running. 50-70 y0 of these individuals will suffer some type of running related injury (Gudas, 1980). Running 20 miles a week over the next 30 yr will send approximately 50 million shock waves through the body of each runner. These statistics demonstrate the tremendous potential for degenerative changes which may not manifest themselves for a number of years. In this study a method for measuring the shock waves occurring on heel strike using a force platform is described. The presence of a shock wave traveling in bone on heel strike has been. reported by several authors. Light er al. (1980) described a skeletal transient using force plate and accelerometer techniques on a subject walking barefoot and in different types of footwear. They demonstrated that a skeletal transient was present (although somewhat attenuated) with shock absorbing footwear through an accelerometer mounted on the tibia. The force plate could not resolve this transient through shock absorbing footwear, but did demonstrate an initial heel strike spike in barefoot walking. Bone vibrations from 25 to 100 Hz at heel strike have been measured by Munro et al. (1975), Paul
415
416
JON
A. DICKII‘;SON. STEPHEEI D. Coax and
MATERIAIS
The data were collected on a Kistler multicomponent measuring platform type 9261A with a natural frequency of greater than 200 Hz. The force plate was mounted to a concrete floor, and situated in the middle of a 12.5 m raised wooden runway. Two photo cells were mounted 3.05 m apart with the force plate situated in the middle for velocity determination. An Apple computer system equipped with an A/D converter with a sampling rate of 1800 Hz was used in order to resolve the initial shock wave at heel strike. The computer began sampling when the first photo-cell was triggered so initial data points would not be missed due to electrical delay. The data obtained were stored on magnetic discs for later analysis. Ail plots and graphs were obtained using a Tektronic 4662 Interactive Digital Plotter. Concern that a sampling rate of 1800 Hz could lead to the evaluation of noise on heel strike prompted several preliminary experiments. Hard objects were bounced off of the force plate with increasing force and the plate was also struck with objects which remained in contact with its surface. These experiments demonstrated that the plate along with its electronic recording system has no tendency to resonate or produce electronic noise with impacts similar to those produced by the runners in this experiment.
METHOD
Six runners (all male) were used in this study. They ran an average of 38.4 km per week (S.D. 10.72) and their mean age was 26.3 yr (S.D. 1.75). All of the subjects wore their normal running shoes and were allowed to practice before the experiment so that they consistently struck the force plate with the left foot and felt that they had not altered their normal running gait in doing so. This was accomplished by having the subject start the same distance away from the plate, and having him lead with the same leg and take the same number of steps to reach the plate each time. This method minimized any changes in the data that could be due to differences in stride length, or speed. After each runner felt proficient at attaining consistency between runs he was instructed to run across the plate three times with shoes, followed by three runs without shoes. The subjects were also allowed to practice their runs bare-footed before being recorded. Emphasis as before was placed on leading with the same foot, taking the same number of strides and landing in the middle of the force plate using their usual heel strike technique. The distance the runners covered before striking the plate (6.25 m) is short compared to other studies in the literature. The shortened runway probably caused alterations in each runner’s natural running style, however, it was felt that it also resulted in greater consistency between runs. The fact that each runner
TED
M. LEINHARDT
had to use the same technique (same starting position and number of strides) to strike the middle of the force plate between treadmill runs minimized any change in running styles due to fatigue or the treadmill itself. This allowed the visualization of the influences of fatigue on the heel strike transient while minimizing changes in velocity and running style which could have greatly obscured the results. The absence of force plate studies in the literature demonstrating barefoot running necessitated that the runners be initially recorded in shoes. This demonstrated that our force plate and recording equipment produced similar results when compared to other studies in the literature (Cavanagh and Lafortune, 1980; Bates ef al., 1983) where ground reaction forces were recorded for runners wearing protective footwear. Next each runner ran for a total of 45 min on a treadmill in order to study the effects of muscle fatigue on the shock wave. The subjects were fatigued wearing their own running shoes on a Walton model G99-33 ‘stride setter’ motorized treadmill set to cover 7.42 km in 45 min. The treadmill was stopped at intervals of 15, 30 and 45 min and each subject was recorded running across the plate without shoes three times during each interval. Although studies (Bates et al., 1983) have reported the need for a minimum of eight trials to obtain stable values in force plate studies, a maximum of three trials during each interval was used due to the limitations in the recording equipment. Since the measurement of fatigue-induced changes was the main objective, it was felt that the 30 s processing time needed between runs allowed for only three trials before the effects of the treadmill-induced fatigue began to wear off. The mean value of the three trial runs was then calculated. A forced, pre-determined treadmill speed was used for fatiguing the runners in order to assure that they were all exposed to identical time and distance parameters. The design of the treadmill was also a reason for concern. The model used resulted in a discrepancy between the mean over ground speed of the runners (4.61 ms-‘) and the treadmill (2.75 ms-‘). The inability ofour treadmill to run at the higher overground speed seen in the runners may have led to changes in the running style of each subject. On comparison, the pre- and post-fatigue parameters of velocity, maximum vertical force, total foot contact time and the shape of the curve in the Fz axis were remarkably similar. Thus the treadmill design was probably not affecting (to a measurable extent) the parameters being measured. Any treadmill, irrespective of design may lead to changes in running styles simply due to the feeling one experiences after stepping off of a moving belt and attempting to run on a stationary platform. The velocity of each runner during each run over the plate was also recorded. However, there was no attempt to have all of the subjects run at the same speed. The runners were encouraged to approximate their natural pace, in hope of providing more accurate data on fatigue-induced changes in running.
The measurement of shock waves
RESULTS
Typical curves of the Fz (vertical force) axis are shown in Fig. 1 of the same subject running barefoot and in shoes. Cavanagh and Lafortune (1980) in a paper describing ground reaction forces in runners demonstrated similar results. The heelstrikers in his experiment were monitored between 3.6 and 4.1 m s- ’ in shoes with the first peak of 220% bw (body weight)
25s
7
A
Fig. 1. (a) The vertical force observed in a subject running barefoot across a force plate. (b) The same subject running in a pair of jogging shoes. (c)Figures la and lb are superimposed to demonstrate the differences seen while running barefoot and in shoes. Each graph represents a single trial. Additional trials of the same subject produced curves showing little variability with respect to shape, and the other parameters measured. Note the absence of the heel strike spike when the subject is wearing shoes,
417
at 23 ms and a maximum vertical force of 280 7; bw at 83 ms. In this experiment with the subjects wearing shoes the first peak averaged 140 y0 bw at 22.6 ms and the maximum vertical peak was 264% bw at 102 ms. Cavanagh concluded that the first peak correlates with the vertical impact or collision force and the maximum vertical peak correlates with forefoot loading. It was also concluded that the frough area between the two peaks occurs as the center of pressure moves across the longitudinal arch of the foot. This inflection caused by movement from heel to forefoot is seen at approximately 40 ms and is present in both curves in Fig. 1. The force plate was unable to record an initial shock wave at heel strike while the subject was wearing shoes. This was due to the damping effect of the substances used to construct the running shoe heel. The six subjects showed little change in the velocity (Table l), averaging 4.61 ms-’ (SD. 0.79) barefoot before the treadmill and 4.36 ms-’ (S.D. 0.66) after 45 min of treadmill running. No change was demonstrated in total foot contact time, with the runners averaging 207.2 ms (S.D. 23.2) in the unfatigued state and 212.3 ms (SD. 20.6) after 45 min. The amount of time needed to reach maximum vertical force and its magnitude also showed little change in the pre- and post-fatigue states with an average of 96.1 ms (S.D. 10.5)and 253 % bw (SD. 41) before fatigue and 94.3 ms (S.D. 11.0) and 259% bw (SD. 30) after 45 min of treadmill running. When the length of time that it took to reach the heel strike spike was measured, it was found that each individual consistently reached the maximum at the same time throughout the experiment, although these maximums varied from individual to individual. As a group the runners averaged 4.8 ms (S.D. 1.6) unfatigued and 4.9 ms (S.D. 1.0) after 45 min of treadmill fatigue. In an attempt to determine the etiology of the heel strike spike with regard to the physical characteristics of each runner, their height and weight were measured to see if they had any relationship to the magnitude of the spike or its peak time. Regressions of these relationships were then plotted (Figs 2a and b). There was a positive relationship (correlation coefficient 0.857) between the height of the subject and the time it took to reach the maximum value, whereas no relation could be demonstrated between the spike magnitude and the weight of the individual (correlation coefficient -0.076). Over the course of the treadmill-induced fatigue an increase in the magnitude of the heel strike spike was demonstrated. The initial spike averaged 186 % bw in the unfatigued state for all runners and increased to 203 y0 bw when the measurements taken at 15, 30 and 45 min were averaged (Table 1). It was difficult to determine whether these observed differences were accompanied by noticeable changes in the type of footplant and running style, or the result of more subtle changes. An oscillatory type of shockwave was also observed when heel strike was forceful. In Fig. 3, a plot of a typical forceful heel strike is shown. This type of curve
418
JON A. DICKINSON, STEPHEN D COOK and TED M. LEINHARDT
can be generated by instructing the runner to accentuate the heelstrike portion of his support phase. In this figure the peak, trough and second peak occur at 4, 14 and 22 ms respectively. Measuring the time for one oscillation (defined as the length of time between the two peaks) results in a natural frequency of 56 Hz which is well within the physiological limits of bone vibration of 25-100 Hz (Munro et al., 1975). In Fig. 4 one of the authors (Dickinson) who did not participate in any of the experimentation, but who is a regular runner and had become quite adept at running over the force plate demonstrated the midfoot striking technique which eliminates heel strike altogether. A typical midfoot strike with running shoes (Fig. 4a) produced a curve similar to those reported by Cavanagh and
Lafortune (1980). Figure 4b was recorded using the same midfoot striking technique without shoes. The subject was able to produce this pattern consistently (note the absence of any type of initial spike while running barefooted using a midfoot striking technique). DISCUSSION
The twice body weight forces seen during the heel strike spike might be expected to result in injury. Nevertheless, forces generated later in the gait cycle are larger, and when the compressive forces due to muscle contraction are taken into account, forces greater than twice the initial heel strike spike are seen. The maxi-
Table 1. The mean values of all six runners with respect to six different parameters measured during foot contact are shown. The number of trials averaged were: unfatigued with shoes (I 8). unfatigued barefoot (I 8) I5 min of fatigue (lb), 30 min of fatigue (I 7).and 45 min of fatigue (16). The number of trials averaged during fatigue are less due to failure of the recording equipment. The parameters measured are compared in the presence and absence of footwear, and after varying amounts of fatigue
Velocity of run (m s-‘)
Time of heelstrike peak (ms)
Total foot contact time (ms)
Magnitude of heelstrike peak ( O.bw)
Time of maximum vertical force (ms)
Magnitude of maximum vertical force ( :jo bw)
Mean
SD.
Mean
S.D.
Mean
S.D.
Mean
SD.
18.5
22.6
13.0
139
67
102.5
10.8
264
37
207.2
23.2
4.8
I.6
I86
55
96. I
10.5
253
41
0.59
215.1
23.0
4.8
1.7
205
43
97.4
11.3
256
28
4.46
0.65
211.5
23.7
4.6
1.3
211
50
92.3
13.1
255
27
4.36
0.66
212.3
20.6
4.9
1.0
194
47
94.2
11.0
259
30
Mean
SD.
Mean
SD.
Unfatigued with shoes
4.61
0.85
218.8
Unfatigued without shoes
4.61
0.79
15 min treadmill fatigue
4.43
30 min treadmill fatigue 45 min treadmill fatigue
6-
A
0
HEIGHT
OF RUNNER
Fig. 2. (a) The positive correlation between the time of the heel strike spike peak and the height of the runner. Each of the points plotted represent the mean for each subject when the data taken at 0, 15.30 and 45 min were averaged. The regression equation of the line is y = -29.90+0.192(x) and its correlation coefficient is 0.857.
419
The measurement of shock waves
250-
w ::
0
200-
%
k
0
f E iz
0 0
100 70
I
I 72
I 74
’
I 76
’
I 76
’
I 60
I 62
WEIGHT OF RUNNER
Fig. 2. (b) Shows the lack of correlation (correlation coefficient -0.076) when comparing each runner’s weight to the magnitude of the heel strike spike. The regression equation of this line is y = 194.6 -0.652(.x).
250, 200-
s s k! e
lsa-
100t 50-
O’,,r,,,‘,,,,,,,,,,,,,,,,,, ki
&
160
lb
2se
250
Fig. 3. This figure demonstrates the oscillations seen on a forceful heel strike of a single trial while running without shoes. The peak-trough-peak pattern occurring at 4, 14 and 22 ms respectively is suggestiveof an oscillatory motion in the axial skeleton occurring after impact.
mum moments at the hip, knee and ankle occur at 20, 40 and 60% of the stance phase, respectively. These moments are due to both the contraction of the extensor muscles and ground reaction forces (Winter, 1983). Approximately 75% of running injuries (tendonitis, shin splints, stress fractures, plantar fascitis and chondromalacia) seem to be due to the high forces
occurring at push off, when the forces across the knee and ankle are maximal (Winter, 1983). This evidence supports the contention that while all running injuries may not be due solely to high muscle forces during the gait cycle, the injuries listed above probably are not caused at impact. The most common type of training error results in
Jos A. DICKNON.
420
STEPHEN D. COOK and TED M. LEINHARDI
Z88
se-
Fig. 4. (a) This figure shows the force plate curve of one of the authors running with a midfoot striking technique wearing shoes.(b) Shows the same subject running across the plate without shoes using the same technique. Note the absenceofany initial skeletal transient when a midfoot strike running technique is used.
what is known as the ‘overuse injury’. Excessive mileage, rapid changes in the runner’s training routine, interval training, running on uneven or sloped surfaces and many other variables contribute to this type of injury. The structural factors which play a role in the pathogenesis of injuries complicate this picture even more. It is difficult to tell whether an injury resulting from an increase in mileage is due only to this increase, or to an asymptomatic biomechanical abnormality unmasked by the change in training techniques. The injury can also be due to both training error and structural abnormality. Similarly, exaggerated psychological stresses placed on the runner may also trigger breakdown by increasing the body’s susceptibility to the aforementioned mechanisms. Running surveys by James ef a\. (1978) and Gudas (1980) indicated that acute knee injury was the most common problem that runners experience, with ankle, foot and pelvis injuries all following in decreasing frequency. Stress fractures, while occurring with the least frequency, are perhaps the most debilitating. The majority of running injuries are due to either training errors or a structural problem in the runner himself (Sheehan, 1977; Newell and Bramwell, 1984; Clancy, 1980; Subotnick, 1977; Johnson, 1983). Impact loading
on the axial skeleton has not been implicated in any of the aforementioned running injuries. This phenomenon, however, still exists and these forces must be absorbed by the system, with muscle, bone, cartilage and joint movement providing the majority of the shock absorption as previously discussed. A survey of recent literature suggests that the impact absorbed on heel-strike may play an important role in various chronic injury mechanisms. Radin et 01. (1982) demonstrated osteoarthritic changes in the cartilage and surrounding bone of weight-bearing joints subjected to impact loading. Voloshin and Wosk (1981) showed a correlation between the decreased shock absorbing capacity of the skeletal system and low back pain. They then performed an experiment in which the clinical symptoms of lower back pain disappeared in 78% of the patients following administration of artificial shock absorbers inserted into the shoes of their subjects. Symptoms of low back pain and the possibility of degenerative changes resulting in osteoarthritis warrants further study of this phenomenon in runners. The data, assuming that the time of the heel strike spike maximum is related lo the vibrational frequency of theaxial skeleton,show no change in the vibrational frequency before or after fatigue. These data are in agreement with Paul et al. (1978). He showed that removal of various soft tissue components from the leg of a rabbit had no effect on the resonance frequencies of the bone, and very little effect on the magnitude of the transmitted forces upon impact loading when compared to an intact leg with or without muscle tone. When questioned, the subjects in our study all felt that the treadmill-induced fatigue was comparable to what they experience while running on their own. Although no change in the vibrational frequency on heel strike was demonstrated, it was shown that the magnitude of heel strike increased with fatigue. Its magnitude before fatigue averaged 186 % bw and increased to 203 % 6~. when the measurements taken at 15, 30 and 45 min were averaged. Whether the fatigue-induced changes in the ground reaction forces could be influenced by the musculotendinous unit was not addressed in this study. If this is the case, however, the change in heel strike magnitude seen with fatigue could be due to several factors including a decreased attenuational capacity of fatigued muscle, altered gait due to fatigue, or perhaps altered proprioception and pain sensation resulting from the release of the body’s own endorphins. Another factor in this increased heel strike could have been the speed or design of our treadmill, although studies by Elliott and Blanksby (1976) demonstrated no significant difference in the stride length, rate, and support or non-support time of subjects running on a treadmill between 3.33 and 4.78 ms-‘. An interesting phenomenon which was observed and not expected was the absence of correlation between the runner’s weight and the magnitude of the heel strike spike. It was expected that the heavier
421
The measurement of shock waves individuals would deliver the greatest amount of force on heel strike, however, this pattern was not seen. This observation suggests that each runner probably has his own characteristic style of running independent of weight. This variability in the magnitude of heel strike may put a runner at an increased risk for injury by overloading his shock absorbing mechanism. A study comparing the magnitude of a weight-adjusted heel strike spike vs the type and frequency of injuries suffered by runners might shed some light on this interesting phenomenon and document those who are more prone to injury. The positive correlation between the time that the heel spike reached its maximum and height of the individual also lends support to thecontention that the spike observed on heel strike is not artifactual, but a reliable measurement which can be used to study human biomechanics. If the axial skeleton behaves as a unit, then the greater its length, the lower its frequency of vibration and the greater the amount of time needed to reach the heel strike peak. The absence of this spike in forefoot running may indicate the need for reassessment by the running community as to whether or not heel striking is the correct way to run. Cavanagh and Lafortune (1980) showed that the mediolateral component of force for midfoot strikers was 3 times greater than that seen in heel strikers (35 % of bw vs I2 % bw). He also demonstrated that the anteroposterior force component for heel strikers showed a single gradually rising peak, whereas the forefoot strikers demonstrated a double peaked braking phase. This increased instability of the ankle joint in mid foot strikers is likely to lead to different patterns of injury seen in runners. It has also been shown (Winter, 1983) that in heel strikers the ankle functions principally as an energy generator and the knee as an absorber. Winter (1983) demonstrated that the ankle, in terms of generating energy does three times the work of the knee, whereas the knee absorbs 3.5 times more energy than it generates. By using the midfoot striking technique the runner forces the ankle to function differently, possibly increasing its role as an energy absorber, while maintaining its role as an energy generator. A survey comparingmidfoot and heel strikers with respect to the type of injuries which they sustain is presently underway. This study may clear up some of these questions and help to determine which pattern of running is less hazardous, keeping both acute and long term injury mechanisms in mind. The method outlined in this paper, which allows for the measurement of a heel strike transient in runners without protective footwear, has possible applications in several areas of biomechanical research. As previously noted this technique may demonstrate those runners who, because of a disproportionately forceful heel strike transient, may be placed at an increased risk for long term injury. Concerning the design of running shoes, the best overall protection may be served by two different types of material making up the sole. The heel should be made of a substance which provides the
maximum shock absorbing capacity, thus damping the vibration evident in the axial skeleton. The forefoot should be constructed so as to provide maximum cushioning capacity due to the forces of 2.5-3 times body weight seen in forefoot loading. With refinements in this technique it may be possible to demonstrate changes in the heel strike pattern secondary to bone and joint diseases in barefoot walking subjects. Future research needs to be directed at the demonstrated association between the osteoarthritic changes resulting from repetitive impulse loading in experimental animals, and the similar type of impulse loading seen in runners who practice their sport for prolonged periods of time. Shock absorbing footwear prevents the detection of the initial skeletal transient by force plate techniques. This has led to a de-emphasis of this phenomenon in force plate studies, even though Light et al. (1980) detected this transient on heel strike in a walking individual with protective footwear, where forces are considerably less. More work is needed on the possible adverse effects of this phenomenon, and running in general, so better counseling can be given by professionals interested in the field.
REFERENCES
Barland, P.. Janis, R. and Sandson, J. (1966) Immunofluorescent studies of human articular cartilage. Ann. rheum. Dis. 24, 156-164. Bates, B. T., Osternig, L. R., Sawhill, J. A. and James, S. L. (1983) An assessment of subject variability, subject shoe interaction, and the evaluation of running shoes using ground reaction force data. J. Biomechanics 16, 181-191. Bollet, A. J. (1967) Connective tissue polysaccharide metabolism and the pathogenesis of osteoarthritis. Ada. inf. Med. 13, 33-60. Cavanagh, P. R. and Lafortune, M. A. (1980) Ground reaction forces in distance running. J. Biomechanics 13, 397-406. Clancy, W. (1980) Runners’ injuries introduction, Am. J. Sports Med. 12, 137-138. Elliott, B. C. and Blanksby, B. A. (1976) A cinematographic analysis of overground and treadmill running by males and females. Med. Sci. Sports 8, 84-87. Gudas, C. J. (1980) Patterns of lower-extremity injury in 224 runners. Compr. 7Xerap. 6, 50-S9. James, S. L., Bates, B. T. and Osternig. L. R. (1978) Injuries to runners. Am. J. Sports Med. 6, 4CL-50. Johnson, R. (1983) Common running injuries of the leg and foot. Minn. Med. 66, 441-444. Light, L. H., McLellan, G. E. and Klenerman, L. (1980) Skeletal transients on heel strike in normal walking with different footwear. J. Biomechanics 13. 477-480. Mm-no, M. B., Abernethy, P. J., Paul, I. L., Rose R. M., Simon, S. R., Pratt, G. and Radin, E. L. (1975) Peak dynamic force in human gait and its attenuation by the soft-tissues. Whop. Res. Sot. 2 1, 65. Newell, S. and Bremwell, S. (1984)Overuse injuries to the knee in runners. Phys. Sporrsmed. 12, 81-92. Paul, I. L., Munro, M. B.,Abernethy, P. J.,Simon, S. R., Radin, E. L. and Rose, R. M. (1978) Musculo-skeletal shock absorption: relative contribution of bone and soft tissues at various frequencies. J. Biomechanics 11, 237-239. Radin, E. L., Paul, 1. L. and Rose, R. M. (1975) Mechanical factors in the aetiology of osteoarthrosis. Ann. rheum Dis. 34, 132-133. Radin, E. L.. Orr, R. B., Kelman, J. L., Paul, I. L. and Rose, R.
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JON A. DICKINSON.STEPHEND. COOK and TED M. LEINHARDT
M. (1982) Ekt of prolonged walking on concrete on the knees of sheep. J. Biomechanics IS, 487-492. Sheehan, G. (1977) An overview of overuse syndromes in distance runners. Ann. N. Y. Acad Sci. 301, 877-880. Subotnick, S. (1977) A biomechanical approach to running injuries. Ann. N. Y. Acad. Sci. 301, 888-899. Voloshin, A. and Wosk, J. (1980) Shock absorbing capacity of the human knee (in uirro properties). Proceedings of the Special Conference of the Canadian Society/or on ‘Human locomotion I’, London, Ontario,
Biomechanics
pp. 104-105.
Voloshin, A. and Wosk, J. (1981) Influence of artificial shock absorbers on human gait. C/in Orrhop. 160.52-56. Voloshin, A. and Wosk, J. (1982) An in cic;o study of low back pain and shock absorption in the human locomotor system. J. Biomechanics IS, 21-27. Winter, D. (1983) Moments of force and mechanical power in jogging. 1. Biomechanics 16, 91-97. Wosk, J. and Voloshin. A. (1981) Wave attenuation in skeletons of young healthy persons. J. Biomechanics 14, 261-267.
STEPHEN
D.
COOK
and TED M. LEINHARDT
Tulx~r L’nivrrsiiy School of Medicine. Department ol Orthopardic Surgery. Neu
Orleans.
LA
1430 Tulane
Avenue.
701 I.‘_. U.S.A.
Abstract-A non-invasive method for demonstrating the shock wave which propagates through the skeletal system following heel strike is described. This wave was not seen in force plate studies where adequate shock absorption was provided by running shoes. In the present study six subjects ran across a force plate without shoes before and after they were fatigued on a treadmill to demonstrate possible changes in the heel strike transient. Most of the parameters measured were not altered by fatigue. and a relationship between the shock waveand height. but not the weight ofthe runner wasdemonstrated.The diffrrent mechanisms leading to this phenomenon. and its implication in the areas of osteoarthritic degeneration and running mechanics dr: discussed
al. (1978) and Voloshin and Wosk (1980). These waves travel up the axial skeleton and are attenuated along the way. Muscle stiffness seems to predominate as the main shock absorbing mechanism w-ith bony deformation, joint motion and articular cartilage compression also being important. A great deal of work on the transmission and attenuation of these incoming forces has been described by Wosk and Voloshin (1981) and others. The transmission of these shock waves through the skeletal system may pIa!- a role in various acute and chronic injury mechanisms. Recently there has been increasing interest in the effects of shock on the musculoskeletal system. Radin et al. (1982) demonstrated changes in the bone and the hexosamine content of cartilage indicative of osteoarthritis (Barland et al., 1966 Ballet, 1967) in the weight bearing joints of sheep who were walked for long periods of time over a concrete floor. Another group of sheep was walked over a floor covered with wood chips for the same length of time and showed no osteoarthritic changes. These results seem lo support the suggestion of Radin et al. (1975) that osteoarthritis results mainly from poorly handled mechanical loading, rather than from a disease process. Studies by Voloshin and Wosk (1982) have postulated that low back pain may be caused by reduced shock absorbing capacity in the musculoskeletal system in conjunction with impulse loading. The majority of the human and animal studies described in the literature deal with the forces transmitted while walking. In force plate studies on running subjects. peak forces of 2.5-3 times body weight are generated (Cavanagh and Lafortune, 1980). This is in sharp contrast to the forces of slightly over one body weight seen in studies on walking individuals. The 2-3 fold increase in forces seen while running in association with thi: experimental results indicative of possible degenerative processes occurring with normal walking forces clearly demonstrates the need for techniques to quantify these shock waves so methods for their attenuation can be attempted. et
INTRODUCIION
The sedentary lifestyle produced by twentieth century technology in conjunction with the realization of the beneficial effects of exercise upon health has resulted in the ‘fitness movement’ which has emerged throughout the world. This movement has produced a new type of athlete who is less conditioned, less knowledgeable about training techniques and more prone to injury than his counterpart in organized athletics. The large number of people participating in recreational athletics points to the need for studies on the acute and long term mechanisms of injury suffered by this group. This year approximately 40 million Americans will participate in some form of running. 50-70 y0 of these individuals will suffer some type of running related injury (Gudas, 1980). Running 20 miles a week over the next 30 yr will send approximately 50 million shock waves through the body of each runner. These statistics demonstrate the tremendous potential for degenerative changes which may not manifest themselves for a number of years. In this study a method for measuring the shock waves occurring on heel strike using a force platform is described. The presence of a shock wave traveling in bone on heel strike has been. reported by several authors. Light er al. (1980) described a skeletal transient using force plate and accelerometer techniques on a subject walking barefoot and in different types of footwear. They demonstrated that a skeletal transient was present (although somewhat attenuated) with shock absorbing footwear through an accelerometer mounted on the tibia. The force plate could not resolve this transient through shock absorbing footwear, but did demonstrate an initial heel strike spike in barefoot walking. Bone vibrations from 25 to 100 Hz at heel strike have been measured by Munro et al. (1975), Paul
415
416
JON
A. DICKII‘;SON. STEPHEEI D. Coax and
MATERIAIS
The data were collected on a Kistler multicomponent measuring platform type 9261A with a natural frequency of greater than 200 Hz. The force plate was mounted to a concrete floor, and situated in the middle of a 12.5 m raised wooden runway. Two photo cells were mounted 3.05 m apart with the force plate situated in the middle for velocity determination. An Apple computer system equipped with an A/D converter with a sampling rate of 1800 Hz was used in order to resolve the initial shock wave at heel strike. The computer began sampling when the first photo-cell was triggered so initial data points would not be missed due to electrical delay. The data obtained were stored on magnetic discs for later analysis. Ail plots and graphs were obtained using a Tektronic 4662 Interactive Digital Plotter. Concern that a sampling rate of 1800 Hz could lead to the evaluation of noise on heel strike prompted several preliminary experiments. Hard objects were bounced off of the force plate with increasing force and the plate was also struck with objects which remained in contact with its surface. These experiments demonstrated that the plate along with its electronic recording system has no tendency to resonate or produce electronic noise with impacts similar to those produced by the runners in this experiment.
METHOD
Six runners (all male) were used in this study. They ran an average of 38.4 km per week (S.D. 10.72) and their mean age was 26.3 yr (S.D. 1.75). All of the subjects wore their normal running shoes and were allowed to practice before the experiment so that they consistently struck the force plate with the left foot and felt that they had not altered their normal running gait in doing so. This was accomplished by having the subject start the same distance away from the plate, and having him lead with the same leg and take the same number of steps to reach the plate each time. This method minimized any changes in the data that could be due to differences in stride length, or speed. After each runner felt proficient at attaining consistency between runs he was instructed to run across the plate three times with shoes, followed by three runs without shoes. The subjects were also allowed to practice their runs bare-footed before being recorded. Emphasis as before was placed on leading with the same foot, taking the same number of strides and landing in the middle of the force plate using their usual heel strike technique. The distance the runners covered before striking the plate (6.25 m) is short compared to other studies in the literature. The shortened runway probably caused alterations in each runner’s natural running style, however, it was felt that it also resulted in greater consistency between runs. The fact that each runner
TED
M. LEINHARDT
had to use the same technique (same starting position and number of strides) to strike the middle of the force plate between treadmill runs minimized any change in running styles due to fatigue or the treadmill itself. This allowed the visualization of the influences of fatigue on the heel strike transient while minimizing changes in velocity and running style which could have greatly obscured the results. The absence of force plate studies in the literature demonstrating barefoot running necessitated that the runners be initially recorded in shoes. This demonstrated that our force plate and recording equipment produced similar results when compared to other studies in the literature (Cavanagh and Lafortune, 1980; Bates ef al., 1983) where ground reaction forces were recorded for runners wearing protective footwear. Next each runner ran for a total of 45 min on a treadmill in order to study the effects of muscle fatigue on the shock wave. The subjects were fatigued wearing their own running shoes on a Walton model G99-33 ‘stride setter’ motorized treadmill set to cover 7.42 km in 45 min. The treadmill was stopped at intervals of 15, 30 and 45 min and each subject was recorded running across the plate without shoes three times during each interval. Although studies (Bates et al., 1983) have reported the need for a minimum of eight trials to obtain stable values in force plate studies, a maximum of three trials during each interval was used due to the limitations in the recording equipment. Since the measurement of fatigue-induced changes was the main objective, it was felt that the 30 s processing time needed between runs allowed for only three trials before the effects of the treadmill-induced fatigue began to wear off. The mean value of the three trial runs was then calculated. A forced, pre-determined treadmill speed was used for fatiguing the runners in order to assure that they were all exposed to identical time and distance parameters. The design of the treadmill was also a reason for concern. The model used resulted in a discrepancy between the mean over ground speed of the runners (4.61 ms-‘) and the treadmill (2.75 ms-‘). The inability ofour treadmill to run at the higher overground speed seen in the runners may have led to changes in the running style of each subject. On comparison, the pre- and post-fatigue parameters of velocity, maximum vertical force, total foot contact time and the shape of the curve in the Fz axis were remarkably similar. Thus the treadmill design was probably not affecting (to a measurable extent) the parameters being measured. Any treadmill, irrespective of design may lead to changes in running styles simply due to the feeling one experiences after stepping off of a moving belt and attempting to run on a stationary platform. The velocity of each runner during each run over the plate was also recorded. However, there was no attempt to have all of the subjects run at the same speed. The runners were encouraged to approximate their natural pace, in hope of providing more accurate data on fatigue-induced changes in running.
The measurement of shock waves
RESULTS
Typical curves of the Fz (vertical force) axis are shown in Fig. 1 of the same subject running barefoot and in shoes. Cavanagh and Lafortune (1980) in a paper describing ground reaction forces in runners demonstrated similar results. The heelstrikers in his experiment were monitored between 3.6 and 4.1 m s- ’ in shoes with the first peak of 220% bw (body weight)
25s
7
A
Fig. 1. (a) The vertical force observed in a subject running barefoot across a force plate. (b) The same subject running in a pair of jogging shoes. (c)Figures la and lb are superimposed to demonstrate the differences seen while running barefoot and in shoes. Each graph represents a single trial. Additional trials of the same subject produced curves showing little variability with respect to shape, and the other parameters measured. Note the absence of the heel strike spike when the subject is wearing shoes,
417
at 23 ms and a maximum vertical force of 280 7; bw at 83 ms. In this experiment with the subjects wearing shoes the first peak averaged 140 y0 bw at 22.6 ms and the maximum vertical peak was 264% bw at 102 ms. Cavanagh concluded that the first peak correlates with the vertical impact or collision force and the maximum vertical peak correlates with forefoot loading. It was also concluded that the frough area between the two peaks occurs as the center of pressure moves across the longitudinal arch of the foot. This inflection caused by movement from heel to forefoot is seen at approximately 40 ms and is present in both curves in Fig. 1. The force plate was unable to record an initial shock wave at heel strike while the subject was wearing shoes. This was due to the damping effect of the substances used to construct the running shoe heel. The six subjects showed little change in the velocity (Table l), averaging 4.61 ms-’ (SD. 0.79) barefoot before the treadmill and 4.36 ms-’ (S.D. 0.66) after 45 min of treadmill running. No change was demonstrated in total foot contact time, with the runners averaging 207.2 ms (S.D. 23.2) in the unfatigued state and 212.3 ms (SD. 20.6) after 45 min. The amount of time needed to reach maximum vertical force and its magnitude also showed little change in the pre- and post-fatigue states with an average of 96.1 ms (S.D. 10.5)and 253 % bw (SD. 41) before fatigue and 94.3 ms (S.D. 11.0) and 259% bw (SD. 30) after 45 min of treadmill running. When the length of time that it took to reach the heel strike spike was measured, it was found that each individual consistently reached the maximum at the same time throughout the experiment, although these maximums varied from individual to individual. As a group the runners averaged 4.8 ms (S.D. 1.6) unfatigued and 4.9 ms (S.D. 1.0) after 45 min of treadmill fatigue. In an attempt to determine the etiology of the heel strike spike with regard to the physical characteristics of each runner, their height and weight were measured to see if they had any relationship to the magnitude of the spike or its peak time. Regressions of these relationships were then plotted (Figs 2a and b). There was a positive relationship (correlation coefficient 0.857) between the height of the subject and the time it took to reach the maximum value, whereas no relation could be demonstrated between the spike magnitude and the weight of the individual (correlation coefficient -0.076). Over the course of the treadmill-induced fatigue an increase in the magnitude of the heel strike spike was demonstrated. The initial spike averaged 186 % bw in the unfatigued state for all runners and increased to 203 y0 bw when the measurements taken at 15, 30 and 45 min were averaged (Table 1). It was difficult to determine whether these observed differences were accompanied by noticeable changes in the type of footplant and running style, or the result of more subtle changes. An oscillatory type of shockwave was also observed when heel strike was forceful. In Fig. 3, a plot of a typical forceful heel strike is shown. This type of curve
418
JON A. DICKINSON, STEPHEN D COOK and TED M. LEINHARDT
can be generated by instructing the runner to accentuate the heelstrike portion of his support phase. In this figure the peak, trough and second peak occur at 4, 14 and 22 ms respectively. Measuring the time for one oscillation (defined as the length of time between the two peaks) results in a natural frequency of 56 Hz which is well within the physiological limits of bone vibration of 25-100 Hz (Munro et al., 1975). In Fig. 4 one of the authors (Dickinson) who did not participate in any of the experimentation, but who is a regular runner and had become quite adept at running over the force plate demonstrated the midfoot striking technique which eliminates heel strike altogether. A typical midfoot strike with running shoes (Fig. 4a) produced a curve similar to those reported by Cavanagh and
Lafortune (1980). Figure 4b was recorded using the same midfoot striking technique without shoes. The subject was able to produce this pattern consistently (note the absence of any type of initial spike while running barefooted using a midfoot striking technique). DISCUSSION
The twice body weight forces seen during the heel strike spike might be expected to result in injury. Nevertheless, forces generated later in the gait cycle are larger, and when the compressive forces due to muscle contraction are taken into account, forces greater than twice the initial heel strike spike are seen. The maxi-
Table 1. The mean values of all six runners with respect to six different parameters measured during foot contact are shown. The number of trials averaged were: unfatigued with shoes (I 8). unfatigued barefoot (I 8) I5 min of fatigue (lb), 30 min of fatigue (I 7).and 45 min of fatigue (16). The number of trials averaged during fatigue are less due to failure of the recording equipment. The parameters measured are compared in the presence and absence of footwear, and after varying amounts of fatigue
Velocity of run (m s-‘)
Time of heelstrike peak (ms)
Total foot contact time (ms)
Magnitude of heelstrike peak ( O.bw)
Time of maximum vertical force (ms)
Magnitude of maximum vertical force ( :jo bw)
Mean
SD.
Mean
S.D.
Mean
S.D.
Mean
SD.
18.5
22.6
13.0
139
67
102.5
10.8
264
37
207.2
23.2
4.8
I.6
I86
55
96. I
10.5
253
41
0.59
215.1
23.0
4.8
1.7
205
43
97.4
11.3
256
28
4.46
0.65
211.5
23.7
4.6
1.3
211
50
92.3
13.1
255
27
4.36
0.66
212.3
20.6
4.9
1.0
194
47
94.2
11.0
259
30
Mean
SD.
Mean
SD.
Unfatigued with shoes
4.61
0.85
218.8
Unfatigued without shoes
4.61
0.79
15 min treadmill fatigue
4.43
30 min treadmill fatigue 45 min treadmill fatigue
6-
A
0
HEIGHT
OF RUNNER
Fig. 2. (a) The positive correlation between the time of the heel strike spike peak and the height of the runner. Each of the points plotted represent the mean for each subject when the data taken at 0, 15.30 and 45 min were averaged. The regression equation of the line is y = -29.90+0.192(x) and its correlation coefficient is 0.857.
419
The measurement of shock waves
250-
w ::
0
200-
%
k
0
f E iz
0 0
100 70
I
I 72
I 74
’
I 76
’
I 76
’
I 60
I 62
WEIGHT OF RUNNER
Fig. 2. (b) Shows the lack of correlation (correlation coefficient -0.076) when comparing each runner’s weight to the magnitude of the heel strike spike. The regression equation of this line is y = 194.6 -0.652(.x).
250, 200-
s s k! e
lsa-
100t 50-
O’,,r,,,‘,,,,,,,,,,,,,,,,,, ki
&
160
lb
2se
250
Fig. 3. This figure demonstrates the oscillations seen on a forceful heel strike of a single trial while running without shoes. The peak-trough-peak pattern occurring at 4, 14 and 22 ms respectively is suggestiveof an oscillatory motion in the axial skeleton occurring after impact.
mum moments at the hip, knee and ankle occur at 20, 40 and 60% of the stance phase, respectively. These moments are due to both the contraction of the extensor muscles and ground reaction forces (Winter, 1983). Approximately 75% of running injuries (tendonitis, shin splints, stress fractures, plantar fascitis and chondromalacia) seem to be due to the high forces
occurring at push off, when the forces across the knee and ankle are maximal (Winter, 1983). This evidence supports the contention that while all running injuries may not be due solely to high muscle forces during the gait cycle, the injuries listed above probably are not caused at impact. The most common type of training error results in
Jos A. DICKNON.
420
STEPHEN D. COOK and TED M. LEINHARDI
Z88
se-
Fig. 4. (a) This figure shows the force plate curve of one of the authors running with a midfoot striking technique wearing shoes.(b) Shows the same subject running across the plate without shoes using the same technique. Note the absenceofany initial skeletal transient when a midfoot strike running technique is used.
what is known as the ‘overuse injury’. Excessive mileage, rapid changes in the runner’s training routine, interval training, running on uneven or sloped surfaces and many other variables contribute to this type of injury. The structural factors which play a role in the pathogenesis of injuries complicate this picture even more. It is difficult to tell whether an injury resulting from an increase in mileage is due only to this increase, or to an asymptomatic biomechanical abnormality unmasked by the change in training techniques. The injury can also be due to both training error and structural abnormality. Similarly, exaggerated psychological stresses placed on the runner may also trigger breakdown by increasing the body’s susceptibility to the aforementioned mechanisms. Running surveys by James ef a\. (1978) and Gudas (1980) indicated that acute knee injury was the most common problem that runners experience, with ankle, foot and pelvis injuries all following in decreasing frequency. Stress fractures, while occurring with the least frequency, are perhaps the most debilitating. The majority of running injuries are due to either training errors or a structural problem in the runner himself (Sheehan, 1977; Newell and Bramwell, 1984; Clancy, 1980; Subotnick, 1977; Johnson, 1983). Impact loading
on the axial skeleton has not been implicated in any of the aforementioned running injuries. This phenomenon, however, still exists and these forces must be absorbed by the system, with muscle, bone, cartilage and joint movement providing the majority of the shock absorption as previously discussed. A survey of recent literature suggests that the impact absorbed on heel-strike may play an important role in various chronic injury mechanisms. Radin et 01. (1982) demonstrated osteoarthritic changes in the cartilage and surrounding bone of weight-bearing joints subjected to impact loading. Voloshin and Wosk (1981) showed a correlation between the decreased shock absorbing capacity of the skeletal system and low back pain. They then performed an experiment in which the clinical symptoms of lower back pain disappeared in 78% of the patients following administration of artificial shock absorbers inserted into the shoes of their subjects. Symptoms of low back pain and the possibility of degenerative changes resulting in osteoarthritis warrants further study of this phenomenon in runners. The data, assuming that the time of the heel strike spike maximum is related lo the vibrational frequency of theaxial skeleton,show no change in the vibrational frequency before or after fatigue. These data are in agreement with Paul et al. (1978). He showed that removal of various soft tissue components from the leg of a rabbit had no effect on the resonance frequencies of the bone, and very little effect on the magnitude of the transmitted forces upon impact loading when compared to an intact leg with or without muscle tone. When questioned, the subjects in our study all felt that the treadmill-induced fatigue was comparable to what they experience while running on their own. Although no change in the vibrational frequency on heel strike was demonstrated, it was shown that the magnitude of heel strike increased with fatigue. Its magnitude before fatigue averaged 186 % bw and increased to 203 % 6~. when the measurements taken at 15, 30 and 45 min were averaged. Whether the fatigue-induced changes in the ground reaction forces could be influenced by the musculotendinous unit was not addressed in this study. If this is the case, however, the change in heel strike magnitude seen with fatigue could be due to several factors including a decreased attenuational capacity of fatigued muscle, altered gait due to fatigue, or perhaps altered proprioception and pain sensation resulting from the release of the body’s own endorphins. Another factor in this increased heel strike could have been the speed or design of our treadmill, although studies by Elliott and Blanksby (1976) demonstrated no significant difference in the stride length, rate, and support or non-support time of subjects running on a treadmill between 3.33 and 4.78 ms-‘. An interesting phenomenon which was observed and not expected was the absence of correlation between the runner’s weight and the magnitude of the heel strike spike. It was expected that the heavier
421
The measurement of shock waves individuals would deliver the greatest amount of force on heel strike, however, this pattern was not seen. This observation suggests that each runner probably has his own characteristic style of running independent of weight. This variability in the magnitude of heel strike may put a runner at an increased risk for injury by overloading his shock absorbing mechanism. A study comparing the magnitude of a weight-adjusted heel strike spike vs the type and frequency of injuries suffered by runners might shed some light on this interesting phenomenon and document those who are more prone to injury. The positive correlation between the time that the heel spike reached its maximum and height of the individual also lends support to thecontention that the spike observed on heel strike is not artifactual, but a reliable measurement which can be used to study human biomechanics. If the axial skeleton behaves as a unit, then the greater its length, the lower its frequency of vibration and the greater the amount of time needed to reach the heel strike peak. The absence of this spike in forefoot running may indicate the need for reassessment by the running community as to whether or not heel striking is the correct way to run. Cavanagh and Lafortune (1980) showed that the mediolateral component of force for midfoot strikers was 3 times greater than that seen in heel strikers (35 % of bw vs I2 % bw). He also demonstrated that the anteroposterior force component for heel strikers showed a single gradually rising peak, whereas the forefoot strikers demonstrated a double peaked braking phase. This increased instability of the ankle joint in mid foot strikers is likely to lead to different patterns of injury seen in runners. It has also been shown (Winter, 1983) that in heel strikers the ankle functions principally as an energy generator and the knee as an absorber. Winter (1983) demonstrated that the ankle, in terms of generating energy does three times the work of the knee, whereas the knee absorbs 3.5 times more energy than it generates. By using the midfoot striking technique the runner forces the ankle to function differently, possibly increasing its role as an energy absorber, while maintaining its role as an energy generator. A survey comparingmidfoot and heel strikers with respect to the type of injuries which they sustain is presently underway. This study may clear up some of these questions and help to determine which pattern of running is less hazardous, keeping both acute and long term injury mechanisms in mind. The method outlined in this paper, which allows for the measurement of a heel strike transient in runners without protective footwear, has possible applications in several areas of biomechanical research. As previously noted this technique may demonstrate those runners who, because of a disproportionately forceful heel strike transient, may be placed at an increased risk for long term injury. Concerning the design of running shoes, the best overall protection may be served by two different types of material making up the sole. The heel should be made of a substance which provides the
maximum shock absorbing capacity, thus damping the vibration evident in the axial skeleton. The forefoot should be constructed so as to provide maximum cushioning capacity due to the forces of 2.5-3 times body weight seen in forefoot loading. With refinements in this technique it may be possible to demonstrate changes in the heel strike pattern secondary to bone and joint diseases in barefoot walking subjects. Future research needs to be directed at the demonstrated association between the osteoarthritic changes resulting from repetitive impulse loading in experimental animals, and the similar type of impulse loading seen in runners who practice their sport for prolonged periods of time. Shock absorbing footwear prevents the detection of the initial skeletal transient by force plate techniques. This has led to a de-emphasis of this phenomenon in force plate studies, even though Light et al. (1980) detected this transient on heel strike in a walking individual with protective footwear, where forces are considerably less. More work is needed on the possible adverse effects of this phenomenon, and running in general, so better counseling can be given by professionals interested in the field.
REFERENCES
Barland, P.. Janis, R. and Sandson, J. (1966) Immunofluorescent studies of human articular cartilage. Ann. rheum. Dis. 24, 156-164. Bates, B. T., Osternig, L. R., Sawhill, J. A. and James, S. L. (1983) An assessment of subject variability, subject shoe interaction, and the evaluation of running shoes using ground reaction force data. J. Biomechanics 16, 181-191. Bollet, A. J. (1967) Connective tissue polysaccharide metabolism and the pathogenesis of osteoarthritis. Ada. inf. Med. 13, 33-60. Cavanagh, P. R. and Lafortune, M. A. (1980) Ground reaction forces in distance running. J. Biomechanics 13, 397-406. Clancy, W. (1980) Runners’ injuries introduction, Am. J. Sports Med. 12, 137-138. Elliott, B. C. and Blanksby, B. A. (1976) A cinematographic analysis of overground and treadmill running by males and females. Med. Sci. Sports 8, 84-87. Gudas, C. J. (1980) Patterns of lower-extremity injury in 224 runners. Compr. 7Xerap. 6, 50-S9. James, S. L., Bates, B. T. and Osternig. L. R. (1978) Injuries to runners. Am. J. Sports Med. 6, 4CL-50. Johnson, R. (1983) Common running injuries of the leg and foot. Minn. Med. 66, 441-444. Light, L. H., McLellan, G. E. and Klenerman, L. (1980) Skeletal transients on heel strike in normal walking with different footwear. J. Biomechanics 13. 477-480. Mm-no, M. B., Abernethy, P. J., Paul, I. L., Rose R. M., Simon, S. R., Pratt, G. and Radin, E. L. (1975) Peak dynamic force in human gait and its attenuation by the soft-tissues. Whop. Res. Sot. 2 1, 65. Newell, S. and Bremwell, S. (1984)Overuse injuries to the knee in runners. Phys. Sporrsmed. 12, 81-92. Paul, I. L., Munro, M. B.,Abernethy, P. J.,Simon, S. R., Radin, E. L. and Rose, R. M. (1978) Musculo-skeletal shock absorption: relative contribution of bone and soft tissues at various frequencies. J. Biomechanics 11, 237-239. Radin, E. L., Paul, 1. L. and Rose, R. M. (1975) Mechanical factors in the aetiology of osteoarthrosis. Ann. rheum Dis. 34, 132-133. Radin, E. L.. Orr, R. B., Kelman, J. L., Paul, I. L. and Rose, R.
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JON A. DICKINSON.STEPHEND. COOK and TED M. LEINHARDT
M. (1982) Ekt of prolonged walking on concrete on the knees of sheep. J. Biomechanics IS, 487-492. Sheehan, G. (1977) An overview of overuse syndromes in distance runners. Ann. N. Y. Acad Sci. 301, 877-880. Subotnick, S. (1977) A biomechanical approach to running injuries. Ann. N. Y. Acad. Sci. 301, 888-899. Voloshin, A. and Wosk, J. (1980) Shock absorbing capacity of the human knee (in uirro properties). Proceedings of the Special Conference of the Canadian Society/or on ‘Human locomotion I’, London, Ontario,
Biomechanics
pp. 104-105.
Voloshin, A. and Wosk, J. (1981) Influence of artificial shock absorbers on human gait. C/in Orrhop. 160.52-56. Voloshin, A. and Wosk, J. (1982) An in cic;o study of low back pain and shock absorption in the human locomotor system. J. Biomechanics IS, 21-27. Winter, D. (1983) Moments of force and mechanical power in jogging. 1. Biomechanics 16, 91-97. Wosk, J. and Voloshin. A. (1981) Wave attenuation in skeletons of young healthy persons. J. Biomechanics 14, 261-267.