Stephen ). Kinzey, Mitchell L. Cordova^ Kevin J. Gallen, Jason C. ... ML, Gallen K(. Smith |C, Moore IB, The effects of tryotherapy on ground-react ion forces.
Original Research Reports The Effects of Cryotherapy on Ground-Reaction Forces Produced During a Functional Task Stephen ). Kinzey, Mitchell L. Cordova^ Kevin J. Gallen, Jason C. Smith, and Justin B. Moore Objective: To determine whether a standard 20-min ice-bath (1 CC) immersion of the leg alters vertical ground-reaction-force components during a 1 -legged vertical jump. Design: A 1 x 5 factorial repeated-measures model was used. 5ert/ng: The Applied Biomechanics Laboratory at The University of Mississippi. Participants: Fifteen healthy and physically active subjects (age = 22.3 ± 2.1 years, height = 177.3 ± 12.2 cm, mass = 76.3 ± 19.1 kg) participated. Intervention: Subjects performed 25 one-legged vertical jumps with their preferred extremity before (5 jumps) and after (20 jumps) a 20-min cold whirlpool to the leg. The 25 jumps were reduced into 5 sets of average trials. Main Outcome Measures: Normalized peak and average vertical ground-reaction forces, as well as vertical impulse obtained using an instrumented force platform. Results: Immediately after cryotherapy (sets 2 and 3), vertical impulse decreased (P = .01); peak vertical ground-reaction force increased (set 2) but then decreased toward baseline measures (P= .02). Average vertical ground-reaction force remained unchanged (P> .05), Condusions: Jhe authors advocate waiting approximately 15 min before engaging in activities that require the production of weight-bearing explosive strength or power. Key Words:force platform, functional strength, lower extremity rehabilitation, closed kinetic chain Kinzey 5|, Cordova ML, Gallen K(. Smith |C, Moore IB, The effects of tryotherapy on ground-react ion forces produced during a functional task. / Sport Rehabit. 2OOO;9:3-l 4. © 2000 Human Kinetics Publishers, Inc.
Cryotherapy is a common treatment technique used by allied-health professionals in the care of acute and chronic injuries. This use is based on the physiological responses to a decrease in tissue temperature. Specifically, various physiological responses to cryotherapy include decreased tissue S. J. Kinzey and J. C. Smith are with the Applied Biomechanics and Motor Performance Laboratory, Department of Exerdse Science and Leisure N4anagement, at The University of Mississippi, University 38677, M. L. Cordova is with the Sports Injury Research Laboratory, Athletic Training Department, at Indiana State University, Terre Haute 47809. K. J, Gallen is with the Physician Assistant Program at Philadelphia College of Osteopathic Medicine, Philadelphia, PA 19131, J, B, Moore is with the Department of Kinesiology and Health Education at the University of Texas, Austin 78712.
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temperature, metabolism, inflammation, circulation, pain, and muscle spasm and increased tissue stiffness.' The decreases in pain and muscle spasm are attributed to alterations in the neurological pathways.^ As tissue temperature decreases, there is a linear decrease in sensory-nerve conduction velocity. When sensory-nerve temperatures fall below^ 100", cryotherapy causes a total block of conduction.^ Similarly, cryotherapy causes a decrease in motor-nerve conduction velocity. These changes have been attributed to an increase in the nerves' threshold for stimulation.^'' The effects of cryotherapy on subcutaneous tissue largely depend on the depth of the tissue. Deeper tissues take longer to cool than more superficial subcutaneous tissue; however, subcutaneous tissues begin to warm almost immediately after cryotherapy treatment, whereas deeper tissues continue to cool after removing the cooling agent." With respect to muscle strength, cryotherapy has been shown to reduce isolated joint isometric and isokinetic force production."'" Also, cryotherapy has been found to cause a 27% reduction in ground-reaction force measured during a 2-legged vertical jump." These changes in strength have been attributed to a decrease in adenosine 5'-triphosphate (ATP) hydrolysis, which causes a reduction of the maximal velocity of muscular contraction. The 2-legged vertical jump has been classified as an index of one's ability to perform daily functions.'' As a result, researchers have attempted to investigate the effects of cryotherapy on functional activities that relate more to the specific demands placed on athletes. Generally, it has been demonstrated that performance during tests evaluating functional agility remains unaffected after therapeutic applications of cryotherapy to the lower extremity. ' ^' ^ Although the effects of cryotherapy on motor performance (agility, speed) have been studied, its effects on force production during a functional task remain unknown. Lower extremity functional strength has been defined as lower extremity force production during a movement specific to sport.'' The 1-legged jump has been reported to require nearly the same amount of stability as that encountered during physically demanding activities.'^ Because many sports involve jumping movements or similar activities dependent on the generation of lower extremity power,'*" the vertical jump appears to provide a useful means of estimating lower extremity functional strength.'^ Quantifying the vertical ground-reaction forces duringa 1-legged vertical jump allows for lower extremity force assessment during sportspecific activity. The peak vertical ground-reaction force generated during the 1-legged jump is directly related to the vertical projection of the body's center of mass and is the result of lower extremity muscular effort performed in a coordinated manner.'^'" Although the effects of cryotherapy on functional agility and speed have been studied,"'^ there are few data that describe cryotherapy's effects on lower extremity force production during a functional task. This information is crucial, because diminished
Cryotherapy and Ground-Reaction Forces 5
functional strength can contribute to unsafe or inefficient movement. Thus, the purpose of this study was to determine whether cryotherapy affects characteristics of the vertical ground-reaction force produced during a 1legged vertical jump.
Methods Experimental Design and Setting A quasi-experimental, multivariate, repeated-measures design was used in this study. The single within-subjects factor was set having 5 levels: set 1 (5 jumps before cryotherapy), set 2 (5 jumps immediately postcryotherapy), set 3 (5 jumps after set 2), set 4 (5 jumps after set 3), and set 5 (5 jumps after set 4). The 3 dependent variables measured were peak vertical groundreaction force (PVGRF), average vertical ground-reaction force (AVGRF), and vertical impulse (VT). All data collection took place in the Applied Biomechanics Laboratory at The University of Mississippi.
Participants Fifteen physically active participants, 8 women and 7 men, (age = 22.3 ± 2.1 years, height = 1773 ± 12.2 cm, mass = 76.3 ± 19.1 kg) participated in this study. Before participation, the purpose of the study was explained and each participant signed an informed consent form approved by The University of Mississippi's Institutional Review Board. All participants were without cold allergies and were free from lower extremity injuries within the past 6 months. Furthermore, participants did not suffer from any known vestibular, circulatory, vision, or orthopedic disorders that might have affected their ability to successfully perform the required tasks in this study. No control group was employed in this study because all participants performed a set of jumps prior to the cryotherapy treatment and served as their own controls against the experimental conditions. All participants demonstrated qualitative consistency in their ability to perform a 1-legged vertical jump and were introduced to this particular form of cryotherapy for 10 min during the orientation session; no other measurement of previous exposure to cryotherapy was recorded.
Instrumentation The ground-reaction force data were collected using a strain-gauge force platform {Model OR6-5, AMTI, Watertown, Mass) mounted to the laboratory Boor. Output voltages from the force platform were amplified by a signal condiHoner (Model SGA-6, AMTI), with the gain set at 2000 and the frequency response set at 10.5 Hz. These signals were sampled (1000 Hz) and low-pass filtered at 6 Hz (fourth-order Butterworth digital filter) using
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DataPac III Version 1.59 software (Run Technologies, Laguna Hills, Calif). Jump initiation was defined as the point at which the countermovement began (drop from baseline value), and jump completion was defined as the point at which the participant left the force plate (drop from peak amplitude). Each of the ground-reaction force variables (PVGRF, AVGRF, and VI) was normalized to body weight and measured for each jump.
Experimental Procedures The testing process required participants to attend both an orientation session and a testing session. The orientation session allowed the participants to become familiar with the equipment and protocol used in the study. The testing took place within 3 days of the orientation session and in the same location as the orientation. Participants performed the 1-legged vertical jump using their preferred leg. During orientation, they performed no more than 10 and no less than five 1-legged vertical jumps at 1-min intervals on the force plate. Force-plate data were collected and visually examined for evidence of consistency across a minimum of 5 vertical jumps. This consistency was interpreted as a participant's ability to perform the task without a learning effect during the subsequent testing procedure. The criterion for consistency was defined as a participant's ability to demonstrate similar ground-reaction force production. After demonstrating jump consistency, participants placed their preferred leg in a 10°C ice bath for 10 min. This 10-min cryotherapy treatment was used to introduce the participants to the discomfort associated with this form of cryotherapy and to determine whether participants wished to continue with the testing process. When the 10-min introduction to cryotherapy was complete, participants performed 2 trials of the 1-legged vertical jump on the forceplate. They were asked to perform the jumps according to earlier instructions, but no additional instructions were given, and the participants were able to begin the jumps freely. At the end of the orientation session, a mutually convenient testing time was scheduled. During the actual testing session the participants were given specific directions as to when the jumps were to be performed. On command, participants began the jump sequence by placing their preferred foot in the middle of the force plate and the other foot directly next to it. They then flexed the contralateral knee to 90° to prevent it from contacting the force platform during the jump. An initial countermovement, which was characterized by dorsiflexion, knee flexion, and hip flexion, was allowed during the jump; however, participants crossed their arms over the chest to eliminate arm movement. Restricting arm swing was important because arm swing has been shown to increase peak ground-reaction force during vertical jumps.''^ After completing the entire jump sequence, which included reading a set of instructions (approximately 9 seconds) and the actual jump (average jump duration: 812 ± 52.2 milliseconds), each participant sat in a chair placed
Cryotherapy and Ground-Reaction Forces 7
next to the force platform and remained seated for 50 seconds. After the 50second rest period, the participant was instructed to stand in front of the force platform, at which time the jump sequence began again. The jump process w^as repeated until 5 jumps had been recorded. After the pretreatment fifth jump, the participants immersed their preferred leg (inferior pole of patella to distal end of foot) in a 10°C (mean temperature = 9.8°C ± 0.3°C) whirlpool, without the circulation jet running, for 20 min. To avoid a thermal barrier, the participants were instructed to continuously move the leg during the 20-min session. When the treatment terminated the participants were instructed to move immediately to the force platform. The total time between removing the leg from the whirlpool and the first posttreatment jump was approximately 2 min (M = 2.15 ± 0.23 min). Participants then performed the same jump sequence as previously mentioned until a total of 20 jumps had been recorded over a 20-min time span. During the testing procedure, participants were consistently reminded to perform a maximal jump. All oral commands given during the trials were read from a standard jump-protocol script in order to avoid jump sequence variability among participants. Participants performed all testing without footwear. AH dependent measures were recorded and processed during each of the 25 one-legged vertical jumps. The values for each dependent variable associated with each jump were averaged across sets of 5. Thus, 1 pretreatment and 4 posttreatment values for each dependent variable were used for statistical analyses.
Statistical Analysis A 1-way repeated-measures multivariate analysis of variance was used to examine the effects of a 10°C whirlpool treatment to the leg and foot on the linear combination of PVGRF, AVGRF, and VI. After the sigruficant multivariate test, univariate F tests were used to assess the effects of cryotherapy on each dependent variable. Specific differences between sets were located using the least significant difference post hoc procedure. The level of significance for all tests was established at P < .05.
Results The means and standard deviations for each dependent variable across each set are displayed in Table 1. When all of the dependent measures were considered linearly, the characteristics of the vertical ground-reaction force curve were altered by cryotherapy (Roy's largest root[4,46] = .38; P = .001; rf = .28; 1 - ^ = .96). Specifically, cryotherapy did not affect normalized AVGRF (f^^ = 2.18; P = .08; r}~ = .14; 1 - j3 = .61; Figure 1). Normalized VI was lower in sets 2 and 3 than in sets 4 and 5 {F^^ = 3.73; P = .01; rf = .21; 1 - ^ = .86; Figure 2). Normalized PVGRF was larger in set 2 than in both sets 4 and 5 (F,^ = 3.13; P = .02; rf = .18; 1 - /3 = .78; Figure 3).
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Table 1 Descriptive Statistics {M ± SD) for Normalized Dependent Measures Before and After Cryotherapy* Seti
Set 2
Set 3
Set 4
Set 5
NAVGRF(%BW)
0.86+0.06
0.85 ±0.05 0.84 ±0.05
0.83 ± 0.05 0.84 ±0.05
NPVGRF(%BW)
1.25±0.16
1.25±0.17 1.21 ±0.16
7.22±0.16
1.21 ±0.16
NVI(%BWxs)
4.14±1.17 3.89±1.18 4.04±1.45 4.24±1.50
4.41±1.46
*NAVGRF indicates normalized average vertical ground-reaction force; BW, body weight; NPVCRF, normalized peak vertical ground-reaction force; and NVI, normalized vertical impulse.
Cryotherapy Treatment Figure 1 Normalized average ground-reaction forces. No differences were found across sets ( P > .05). Values are expressed as percent body weight ± standard deviation.
Discussion This investigation evaluated the effects of cryotherapy on various characteristics of the vertical ground-reaction force produced during a functional movem£;nt. The vertical ground-reaction force produced results from segmental rotations from the hip, knee, and ankle in propelling the body's center of mass vertically in a rectilinear path.^'' Previous research has predominantly focused on evaluating cryotherapy effects on isolated joint strength,-' ^' but not strength produced during a functional task. Some studies have shown no changes in muscle strength after cooling,^''^^'^ but most of the available
Cryotherapy and Ground-Reaction Forces 9
4.5 4 3.5 CO
3
3
O
t:
1
>
0.5 0 Cryotherapy Treatment
Figure 2 Normalized vertical impulse ± standard deviation. Values are expressed as percent body weight x seconds. *lndicates the difference between set 2 and sets 4 and 5. **lndicales the difference between set 3 and sets 4 and 5.
1.41.2-
I '' ^ : 'i'0.8o
o
^ 0.6-1 (Q
a. 0.4 0.20-
Cryotherapy Treatment Figure 3 Normalized peak vertical ground-reaction force. Values are expressed in percent body weight ± standard deviation. *lndicates higher peak vertical ground-reaction forces in set 2 than in both sets 4 and 5.
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Studies revealed diminished force output with cooling.^^''-^^-^ Although these inquiries are important in establishing the effects of cooling on individual muscle groups, they did not consider how these effects translate into altering the force-producing capabilities of muscles during sport-specific movement patterns. With the respect to the force-time curve, vertical impulse represents the area under the curve. Essentially, vertical impulse represents the product of force and time during the propulsion phase of the jump. Because vertical impulse is a function of force and time, a change in this variable would represent a change in the amount of force produced or manipulation in the time that the force is generated. Vertical impulse can be characterized as an accelerating force with which a change in the body's momentum occurs.^" In this study, it was found that cooling significantly decreased vertical impulse; however, no differences were found when pretreatment jumps (set 1) were compared with those performed in set 2. Previous researchers^ reported that cold whirlpool treatments caused a continual decrease in intramuscular temperature over a 30-min posttreatment duration. Therefore, a likely explanation for the lack of differences between sets 1 and 2 is that the subcutaneous temperature was not low enough. Our results indicated that set 2 vertical impulse values were lower than those in sets 4 and 5. Furthermore, it was found that 5 min after cooling (set 3 jumps), vertical impulse was still diminished compared with that in sets 4 and 5. Based on these data, it appears that cold-water immersion of the leg negatively affects lower extremity force production up to 10 min after treatment. The decrease in vertical impulse can be partly explained by the depressive effects of cooling on motor-nerve conduction velocity.' A decrease in motor-nerve conduction velocity acts to decrease the neural drive and inhibit the muscle groups responsible for the movement. It has been shown that the ankle joint contributes approximately 36% of the total work produced during a vertical jump.^" Because of the large contribution of the ankle plantar flexor moment to vertical jumping, it is not surprising that cooUng the leg (ankle and foot) would yield this result. The decrease in vertical impulse after cold-water immersion is sirrular to the results reported by other researchers."•^'^ Ferretti et ar' found that maximum instantaneous muscle power produced during a 2-legged vertical jump decreased by 27% after cold-water immersion, and Gallant et aF" reported a 7.2% decrease in vertical jump force after a 20-min ice application to the quadriceps. The fact that cooling diminishes vertical impulse during a functional task might have large clinical implications. First and foremost, the reduction in the accelerating force produced during a functional weightbearing movement can certainly manifest in decreased performance. Vertical jump performance, and other activities requiring explosive pushlike movements, can suffer if the participant fails to maximize leg-extension acceleration when creating the impulse during the jump.'" However, based on our data it appears that this deficit is short-lived and lasts no longer than 10 min. Second, if one considers the return to practice after treatment
Cryotherapy and Ground-Reaction Forces
11
of a lower extremity injury {ankle sprain), could this predispose the athlete to re-injury and put him or her at future risk? It is our recommendation that patients returning to practice or competition after cold-water immersion wait approximately 15 min before engaging in an activity that specifically requires the affected extremity to develop maximal or explosive force in a coordinated functional manner. The average vertical ground-reaction force represents the arithmetic mean of the amount of force produced during the propulsion phase of the jump. Unlike vertical impulse, calculating the average force does not include considering the time in which the force is generated. In this study we foimd that cryotherapy did not alter this characteristic of the force-time curve. This finding is important because it suggests that the amount of force generated throughout the functional task remains unchanged. In other words, the amount of force required in performing the movement does not become compromised. This can be viewed as a positive result from a pure strength perspective; however, when considering force production during a sport-specific movement, one must consider the time in which the force is produced (impulse). Peak vertical ground-reaction force represents the greatest amount of force obtained throughout the propulsion phase of the vertical jump. This force is created, in part, by the net muscle moments produced by the hip and knee extensors and the ankle plantar flexors. In this study, it was found that with treatment immediately postcryotherapy (set 2), peak force increased over that in sets 4 and 5. These results are similar to those of previously reported studies^'-^' that found increases in peak torque after cold-water immersion. Because the lower leg was the object of cooling, the triceps surae complex was the muscle group affected that contributes to vertical jump force. Within this complex, the gastrocnemius muscle contains predominantly fast-twitch muscle fibers^^ that rapidly produce the force necessary to accelerate the body during the vertical jump. The increase in maximum tension or force might best be explained by variations in cooling slow-twitch or fast-twitch muscle fibers; it has been found that maximum tension in fast-twitch muscle increased with cooling." Other data have been reported that indicate that ankle plantar flexion peak eccentric torque remained unaffected after a 30-min ice-bath immersion of the leg." Despite the discrepancies in the literature, this study evaluated peak force produced in a weight-bearing environment, and not in an isolated joint open chain environment. Perhaps another explanation for the increased peak force is the increased activation (recruitment) from the thigh musculature in performing the jumps. If in fact cryotherapy diminishes a muscle's ability to produce peak force, it might be that other muscles involved in performing the vertical jump compensate for the decreased response from the ankle plantar flexors. Furthermore, exploration using electromyography might provide greater insight into the effects of cryotherapy on neuromuscular function during a weight-bearing, explosive movement. Increased peak vertical
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ground-reaction force after cooling can be viewed as a positive result from this study. However, caution should be exercised in interpreting this result. We must emphasize that the peak force produced during the jump represents only 1 point along the force-time curve—further investigation is warranted on this possible cause-and-effect relationship. In conclusion, the results of this investigation indicate that immediately after a 20-min cold-water immersion of the leg, there was a decrease in vertical impulse produced during a 1-legged vertical jump. This diminished response appeared to last no longer than 10 min. Average force produced during the vertical jump remained unaffected by cooling, whereas peak force increased immediately after cooling. After reviewing our data, we advocate waiting approximately 15 min before engaging in activities that require weight-bearing, explosive strength or power. More research is needed to imderstand the effects of cryotherapy on lower extremity force production during functional movements. I
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Acknowledgments The authors gratefully acknowledge the technical assistance they received fi-om Richard Lambert and the editorial comments made by Paul Devita, Stanley P. Brown, and Judith L. Cole.
