Effect of heat and cold on tendon flexibility and force to flex the human knee
Med Sci Monit. 2013; 19: 661–667.
Published online 2013 August 12. doi: 10.12659/MSM.889145
It is commonly believed in medicine that using heat will increase the distensability and flexibility of soft tissue. If true, increased flexibility would be a positive factor to reduce injuries in sports. However, cold should have the opposite effect and is often used to treat sports injuries. This study was accomplished to quantify the effect of heat and cold on the force needed to flex the knee and laxness of the anterior and posterior cruciate ligaments.
The present study examined 20 male and female subjects to determine if heat would increase extensibility of the anterior and posterior cruciate ligaments of the knee and reduce the force needed to flex the knee. Cold exposure was examined to see if it would have the opposite effect. There were 4 experiments in the series: The first was a room temperature series; the second was a series where cold was applied with an ice pack for 20 minutes; in the third, hydrocollator heat packs were applied for 20 minutes; and in the fourth, ThermaCare heat wraps were applied for 4 hours on the quadriceps and knee. Tendon extensibility was measured with a KT2000. The force for flexing the knee was measured by passive movement being applied (CPM) to the knee through 30° and the force required to move the leg was measured.
The results show that the anterior and posterior cruciate ligament flexibility increased and the force needed to move the knee decreased with heat by about 25% compared to cold application.
Heat is beneficial in increasing muscle and ligament flexibility and may help reduce athletic injuries, but cold treatment may have the opposite effect.
Keywords: heat, tissue elasticity, sex differences, muscle, tendon
Athletic injuries are common in the adult population [1,2]. It is generally assumed that a warm up, by increasing tissue temperature, will increase tissue distensability and reduce the incidence of injury [3,4]. These warm-ups involve many modalities and can include stretching, heat, changes in tissue temperature by exercise, and even mental conditioning [3,4]. However, it is not clear which factor or factors actually reduce injury in the athlete. This is especially true in female athletes, whose biomechanics and hormonal influences make them very susceptible to anterior cruciate ligament tears and ankle injuries [5–8].
Heat is very commonly used before exercise . One advantage is that by increasing tissue metabolism the muscle is prepared for the metabolic challenge of exercise [9,10]. Heat also increases glycogen resynthesis and muscle recovery . Heat is also cited as increasing flexibility and thus reducing the chance of injury, and reducing energy cost of muscle contraction by reducing internal friction [11,12]. Internal energy costs are reduced by warming muscle, but many of these studies have been done in animals and very few in humans. There is only limited evidence that heat helps tissue stretching in humans. Further, when heat is used, there is no previous evidence of the effects of heat on the tendon or muscle itself, commonly referred to as the series and parallel elastic components in muscle .
In rat tendons, applying force to collagenous tissues causes stretching, which is greatly aided by heat . The response of stretch receptors in the cat are also increased by heat . The mechanism of the changes in the series’ elastic component is partly due to unwinding of the actin filaments in muscle due to stretch and partly due to collagen relaxation [11,12]. There is also rotation of the actin filaments with heat . While there is some evidence that heat increases muscle flexibility in humans, there has been no study of where the benefit is (parallel or series elastic components, or how long the benefit lasts) [16–19]. This was the purpose of the present investigation. We examined 2 heat modalities: a slow heat modality and a fast heat modality. We also examined cold packs to see what effect they might have on tissue elasticity, because cold is commonly used with injuries during sporting events.