Cryotherapy is used for a wide range of benefits, including, but not limited to, decreased nerve conduction velocity,1 cellular metabolism,2 localized blood flow,3 pain,4,5 and attenuated secondary hypoxia and edema.6 A common practice of cryotherapy for active individuals is the use of “to-go” ice bags; unfortunately, limited research exists on its practice and is isolated to the triceps surae.7,8 There are several anatomical areas where ice is applied in this manner to treat pain over an injured or sore area, such as the hamstrings. However, cryotherapy in this manner is applied to various anatomical locations to treat pain or soreness in the clinical environment. “To-go” ice bags commonly consist of plastic bags filled with ice and secured to the body with plastic or elastic wrap, allowing the patient to leave the treatment area; patients are provided with appropriate instruction and education on the proper use of the “to-go” ice bag.
“To-go” ice bags require the use of external compression to hold the ice bag in place. Clear or plastic wraps are commonly used because they can be discarded with the ice bag at the end of the treatment. Depending on the type of external compression wrap, surface and intramuscular temperatures decrease more than when ice is applied without compression.9 For elastic wraps, this seems to be due to an increased insulation effect, whereas clear wraps provide better contact with the skin compared to no wrap.9
Because walking activates the musculature of the lower extremities,10 there is concern that cooling may not be as effective because muscular contractions increase muscle temperature11 and blood flow.12 However, exercise prior to ice bag application has shown that cryotherapy treatment/application increased the rate of muscular cooling to pre-exercise temperature in the quadriceps. This may be explained by the large temperature gradient between the skin and ice bag.11 However, when ice was applied to the triceps surae during walking, no change in intramuscular temperature was observed during and immediately after walking for 30 minutes.7 Repeated muscle contractions that occur with activity (eg, walking) increase the metabolic demand within the muscles, causing heat production. This may limit the effect of cooling on muscle.
Guzzo et al.8 expanded on the previously mentioned study to examine the length of time needed to cool the gastrocnemius by 6°C after walking 15 or 30 minutes compared to a group that did not walk. An additional 18 to 19 minutes was needed to cool the gastrocnemius after walking stopped, resulting in a total treatment time of more than 30 minutes for both walking groups. The group that did not walk cooled by 6°C within 25 minutes, indicating that if a to-go ice bag is used for the calf, icing should continue for 20 minutes once walking ceases.8
The triceps surae is only one of many lower extremity muscles active during walking.10 Due to variations in the size of the muscles and activation patterns of the involved muscles, the results of the previous studies should not be generalized to all of the involved musculature, including the hamstrings. The hamstrings tend to be less active during walking compared to the triceps surae,10 indicating cooling while walking may be similar to cooling at rest.13
Cooling to a specific intramuscular temperature change, such as 6°C8 or 7°C,14 is arbitrary without support that this temperature change limits secondary hypoxia following acute injury, although it mimics a “typical” treatment.14 However, temperature changes by these degrees have been associated with decreased nerve conduction velocity and the perception of pain at the skin, which is cooler than muscle temperature following ice application.15 There are several factors that need to be considered when cooling muscles, such as subcutaneous tissue thickness, sex, and activity level of the person.16 Using an ice bag “to-go” may further complicate cooling at the intramuscular level because the heat produced from contracted muscles may slow the rate of cooling.7,8,11 It is unknown whether the moment when the patient feels numb (no longer feel ice bag on skin, or after the burning and tingling sensation goes away) corresponds to decreased intramuscular temperature of a “typical” ice bag treatment.
Cold application leads to a progression of sensations involving intense cold, aching pain, pins and needles/warmth, and numbness.17 Numbness equates to analgesia, but it is unknown whether cooling to the point of numbness during walking results in similar intramuscular temperature changes at rest. Therefore, the purpose of this study was to determine hamstring muscle temperature at the point of numbness while walking with a “to-go” ice bag and resting with an ice bag wrapped on the posterior thigh. Due to increased heat production from walking, we hypothesized that walking with a “to-go” ice bag would result in delayed numbness and a smaller change in intramuscular temperature compared to resting with an ice bag on the hamstring.
This was a single-blinded, crossover study design. The independent variable was condition (walking and resting). The dependent variables were intramuscular temperature, skin sensation, and time to reach numbness.
Fourteen healthy volunteers (7 males and 7 females) participated in this study (age: 21.0 ± 1.3 years; height: 171.2 ± 8.5 cm; mass: 74.0 ± 14.6 kg; medial hamstring muscle belly subcutaneous thickness: 7.08 ± 2.70 mm). All participants signed informed consent and the study was approved by the institutional review board of Illinois State University (IRB# 2011-0324). Exclusion criteria included a lower extremity injury in the past 6 weeks, lower extremity surgery, allergies to iodine, latex, or ice, altered skin sensation, or a known bleeding condition. There were no adverse events recorded and no participants dropped from the study.
For sanitation, the thermocouples were submerged in Cidex Plus (Johnson and Johnson, Irvine, CA) for 24 hours after each use and wiped off with a sterile saline wipe prior to being inserted into the participant. Prior to the start of the study, the thermocouples were inserted into a known temperature water bath to measure accuracy.
To measure intramuscular temperature and time to numbness, the Physitemp Thermes USB (Physitemp Instruments, Inc., Clifton, NJ) was used. An IT-21 thermocouple (Physitemp Instrument, Inc.) was inserted into the medial hamstring, using a posterior approach, at a depth 1 cm past subcutaneous tissue without the use of local anesthetic. A 16 gauge × 45 mm intravenous catheter needle (BD Insyte Autoguard, Franklin Lakes, NJ) was used to insert the thermocouple. The catheter was marked to the appropriate depth and inserted into the muscle. The needle was retracted, leaving the catheter behind. The thermocouple was threaded through the needle until muscular resistance was felt. The catheter was then backed out of the muscle, leaving the thermocouple in place. The thermocouple was secured with transpore tape (3M Corporation, St. Paul, MN) (Figure 1). When the thermocouple was removed after the intervention, measurements were taken to confirm the thermocouple did not move.
Application of Semmes-Weinstein Monofilamants (Fabrication Enterprises, Inc., White Plains, NY) to the skin. Pressure was applied until a crescent shape appeared in the monofilament.
Subcutaneous tissue depth was recorded using a diagnostic ultrasound machine (Terason t3000 M-series, Teratech, Burlington, MA). The linear array was 5 to 12 MHz. Measurement of subcutaneous tissue by ultrasound has been shown to be equal, and sometimes superior, to skinfold calipers.18
Skin sensation was measured using Semmes-Weinstein Monofilaments (SWM) (Fabrication Enterprises Inc., White Plains, NY). The SWM were applied perpendicularly to the skin until a crescent moon shape was formed (Figure 1).19 There are 20 SWM varying in diameter size. The SWM score is unit-less, but is associated with grams of force applied.20,21 They have also been shown to be reliable in measuring the threshold of sensation in the hand,22,23 as well as sensitive over dermatomes C4 (shoulder), T1 (medial upper arm), T8 (abdomen), and L4 (medial lower leg).24 A larger number indicates less sensation, indicating more force was needed to feel pressure on the skin.
Participants reported to the athletic training laboratory for testing on two separate occasions, separated by 1 week. Height and weight were recorded. The medial hamstring of the randomized leg (7 right and 7 left) was marked on the skin halfway between the ischial tuberosity and medial joint line while the participant was lying prone. The leg tested in session 1 was the same leg used in session 2. Ultrasound gel was applied to the area and the probe was placed linearly along the medial hamstring. Subcutaneous tissue thickness was measured as any biological tissue superior to the first fascial border of muscle (Figure 2). This thickness was used for proper thermocouple insertion.
Measurement of subcutaneous tissue was calculated from the top of the first fascial border of muscle to the top of the subcutaneous tissue.
Skin sensation of pressure was also recorded while the participant was prone. The investigator began with the smallest SWM and asked the participant, “Do you feel anything touching your skin?” The participant replied either “yes” or “no.” If the participant said “no,” then the investigator increased the SWM size by two and repeated. Once the participant reported feeling the SWM, the size of the SWM was decreased by one and applied to the skin. If the participant could feel the SWM touching the skin, that diameter was reported. If the participant did not feel it, the previous diameter was recorded.
Next, the thermocouple, using sterile technique, was inserted into the medial hamstring at its thickest girth. Povidone-iodine (10%) was applied to the area for 30 seconds in a circular motion using a swabstick (Dynarex, Orangeburg, NY). The catheter was marked 1 cm + subcutaneous tissue thickness to ensure proper depth on insertion into the muscle belly. The thermocouple was threaded through and measured on removal to make sure proper depth was maintained. A 10-minute period of rest followed to stabilize the intramuscular temperature within less than 0.05°C. The thermocouple remained in the hamstring for the duration of the study, including walking. Temperature was recorded every second.
The participants were randomly assigned as to which condition they would receive first (9 walking and 5 resting). The investigator taking the baseline sensation measurements left the room to be blinded to the condition. A 1-kg ice bag, with air removed and tied at ice level, was wrapped onto the hamstring using clear wrap. The same investigator applied the wrap to ensure consistency of pressure. For the walking condition, the participants walked on a treadmill at 4.8 kph, which is the same speed as a previous study.7 The investigator recorded the start time of walking from the thermocouple software, which began 15 seconds after the ice was applied. The participants walked until they reported numbness. Cues to help the participants know when they may be at the point of numbness included: “point after pins and needles” and “no longer feel the ice bag on the back of your leg.” At that point, the treatment duration was measured, the ice bag was removed, and the participants were positioned prone on a treatment table. The blinded investigator came back in the room to measure skin sensation. After this was recorded, the thermocouple was removed.
For the resting condition, the same procedures were followed as in the walking condition, except that after the ice bag was applied, the participants rested in a prone position until they reported numbness. At least 1 week later, the participants returned to the laboratory for the opposite condition.
Three paired-samples t tests were used to determine the effect of cooling while walking or resting had on intramuscular temperature, skin sensation, and time to numbness. For example, post-walking intramuscular temperature was compared to intramuscular temperature after resting. Alpha was set a priori at .05. Cohen's d effect sizes were calculated to interpret clinically meaningfulness of significant results. We performed a sample size estimation using minutes to cool to 6°C as the dependent measurement from a previous study.8 Seventeen total participants were needed to find significant differences with power set at 80% and alpha set at .05.
There were no differences between conditions in intramuscular temperature and skin sensation following the intervention (P > .315) (Figures 3–4), indicating the report of numbness occurred similarly between conditions. However, time to reach numbness was significantly less for the walking condition (10.2 ± 2.6 minutes) compared to the resting condition (15.8 ± 4.3 minutes) (P < .001), with a strong effect size of 2.15 (range: 1.22 to 3.08).
Skin sensation between conditions at baseline and point of numbness.
Intramuscular temperature between conditions at baseline and point of numbness.
While walking with an ice bag, participants felt numb approximately 5 minutes faster than when they rested quietly. For both conditions, skin sensation and intramuscular temperature decreased by the same magnitude rate. Effect sizes were strong, indicating the changes are more than likely clinically meaningful. Even though there was a 5-minute difference between conditions on reporting numbness, the levels of numbness and change in hamstring muscle temperature were the same for both conditions. The clear wrap applied to hold the ice in place was administered by the same person to allow for the same pressure to be applied consistently, indicating that walking caused the changes to be seen.
For clinicians who use “to-go” ice bags on the hamstrings, it seems walking does not hinder time to numbness, but actually increases how quickly the person feels numb. It also decreases intramuscular temperature the same as if the active individual were to use ice while lying on a treatment table. Based on our results, it appears that skin sensation at the point of numbness is related to a muscle temperature change of approximately 7°C. When time constraints are of concern, using a “to-go” ice bag will decrease the time needed to achieve numbness. If the active individual removes the ice once he or she feels numb, the desired effect of decreased pain would still be achieved. A reduction in pain may help the active individual perform activities of daily living easier, sleep better, or decrease the need for pain-relieving medications. However, if there was an acute injury to the hamstring, it is unknown whether reductions in metabolic demand would take place to decrease secondary cell death at this temperature,25 but pain could still be reduced.
It is important that the treatment goal for ice application be considered when choosing which form of cryotherapy to use. For example, if an active individual is sore following a “heavy” lift, repeated throwing, or unaccustomed activity, cooling to numbness will decrease pain sensation in the short term. Although skin temperature was not recorded in this study, analgesia is achieved when skin temperature is below 12°C, which can happen within 5 to 15 minutes of crushed ice application.26 If there is actual damage to the muscle, resulting in edema, swelling, and ruptured blood vessels, it is unknown what optimal intramuscular temperature is needed to effect these consequences from injury, although it seems to be between 5°C and 15°C.26 However, it is unlikely that muscle temperature during cooling will reach this point due to cold-induced vasodilation17 and previous research not reporting muscle temperature below 20°C.26 The current study also revealed the inability of the treatment to achieve this change in intramuscular temperature.
A possible explanation of why we found deep tissue cooling in the hamstring and others did not in the triceps surae7,8 may be due to the amount of muscle activation in each muscle during walking. The mean electromyography amplitude of the medial hamstring during walking is 16% ± 8% of a maximal isometric contraction, whereas that of the gastrocnemius and soleus are 41% ± 25% and 41% ± 14%, respectively.10 We can suspect that blood flow and heat production of the hamstring would increase during walking, but ice wrapped with clear wrap provides constant compression and some insulation for thermal exchange.7,9 As the skin cools, the thermal gradient between the skin and hamstring increases, resulting in cooling of the muscle. Because the hamstring muscle is not as active during walking,10 heat production would be less in that muscle group compared to the lower leg. This would result in the hamstring muscle producing less heat and not increasing muscular temperature, allowing for cooling to take place, beyond resting levels.
It should be noted that we did not assess temperature after the ice bag was removed. It is possible that differences during the rewarming phase may exist between the walking and resting condition. However, the intent of the study was to determine intramuscular temperature at the point of numbness. Future research should focus on the lasting effect of the treatment. Another limitation may be that a true control group was not used, but each participant served as his or her own control for the resting with ice condition.
Implications for Clinical Practice
“To-go” ice bags for the hamstring muscle group seem to be effective at causing numbness and decreasing hamstring muscular temperature. Clinicians can recommend the removal of a “to-go” ice bag from the hamstring when the active individual feels numb in the area and that intramuscular temperature will also be decreased. This may save the active individual and clinician time needed for additional cooling interventions. For other areas of the body, clinicians need to consider the amount of muscle activation of the area to be treated with a “to-go” ice bag. Increased muscular contractions may slow the rate of the cooling. The “dosage” or prescription for “to-go” ice bags should not be treated the same across muscle groups.
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- Guzzo SJ, Yeargin SW, Carr JS, Demchak TJ, Edwards JE, Cheatham S. The effects of walking on gastrocnemius cooling during an ice bag treatment. Intern J Athl Ther Train. 2014;19:34–40. doi:10.1123/ijatt.2014-0006 [CrossRef]
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