The hamstring muscles – the biceps femoris, semitendinosus, and semimembranosus – are very susceptible to injury in sports that involve running and sprinting (1). The contributing factors for hamstring injury are believed to include lack of hamstring strength, inadequate warm-up procedures prior to strenuous exercise, and plain-old-fashioned muscle fatigue. In addition, a lack of flexibility in the hamstrings is thought to spike the risk of hamstring injury (2). Indeed, research has suggested that athletes who have a history of hamstring injury have significantly less hamstring flexibility compared to uninjured fellow athletes, and are also very prone to the recurrence of hamstring troubles (3). Thus, there has been considerable interest in the development of routines that upgrade hamstring flexibility in athletes.
Many researchers have suggested that stretching regimens represent the ideal method for improving hamstring flexibility. While this approach certainly seems to be reasonable, the research concerning the effect of stretching on hamstring elasticity has been equivocal. For example, one investigation determined that 30 seconds of static stretching significantly improved hamstring flexibility (4), while another study found that a combination of static and proprioceptive-neuromuscular-facilitation stretching improved torque production by the hamstrings but failed to enhance hamstring flexibility at all (5).
In the training room, athletes and athletic trainers often use the application of moist heat to boost hamstring flexibility, and athletes report ‘good success’ with this technique. Unfortunately, there has been little controlled scientific research in this area, so the actual ability of moist heat to unkink tight hamstrings has been poorly understood.
To correct that deficiency, exercise scientists at the University of Texas at Austin and at the University of Louisville in Kentucky recently completed a study with 30 collegiate football players (6). In this investigation, the athletes were involved with football-conditioning sessions and resistance training three times per week. The age of the athletes ranged from 18 to 22, average percent body fat was 8.9%, and mean body weight was 100 kilograms. None of the participants in the study had a hamstring injury or a previous history of hamstring troubles.
At the beginning of the research, 15 subjects were randomly assigned to either a moist-heat or stretching treatment. The moist-heat therapy involved placing a hot (160-degree Fahrenheit), moist pack in a terry-cloth cover on each athlete’s hamstring muscle group for 20 minutes continuously as the athlete lay horizontally in a face-down position. The stretching treatment involved the completion of three sets of a static hamstring stretch for 30 seconds per set. The hamstring stretch was a classic one: athletes simply sat on a treatment table with their legs stretched in front of them and then engaged in an anterior pelvic tilt which produced considerable tension in the hamstring muscle group. This tension was then held for 30 seconds to complete a set.
One week later, the athletes convened in the laboratory again, but this time the moist-heat athletes carried out the static stretching, while the static stretchers tried moist heat. On both occasions, passive hamstring flexibility was measured after the two interventions (moist heat or static stretching) with the use of a goniometer. The subjects also completed a 15-item questionnaire which evaluated their opinions related to the effectiveness of various training and injury-prevention practices. Within this questionnaire were two items which measured the athletes’ attitudes toward stretching and moist-heat application as therapies for tightness in the hamstrings.
As it turned out, the 20-minute application of moist heat was associated with significantly greater hamstring flexibility, compared with three 30-second sets of static stretching. This was in spite of the fact that the athletes – based on their questionnaire responses – expected the stretching to be far more beneficial than the moist-heat application. In other words, there was an attitudinal bias toward stretching, but moist-heat application nonetheless induced greater hamstring flexibility. This of course strengthens the contention that moist heat is actually better than static stretching for upping hamstring extensibility.
Although this Texas-Louisville study makes moist-heat application ‘look good’ with regard to enhancing hamstring looseness, the results should be interpreted with caution. For one thing, hamstring flexibility was measured while the athletes were lying flat on examining tables, with hip flexion enforced by actions of the experimenters, not the athletes; thus, it is not certain that the upgraded hamstring flexibility associated with moist-heat application would also be present during actual, sport-specific movements, when the athletes were moving around the football field using dynamic, weight-bearing postures. Changes in static flexibility do not automatically transfer over to dynamic flexibility.
In addition, no warm-up was performed by the athletes prior to either the moist-heat application or the static stretching, even though warm-up can itself enhance flexibility and is a normal part of pre-workout and pre-competition preparations. Had a proper warm-up been performed, it is possible that moist heat might have held no advantages over static stretching.
So should high jumpers apply moist heat to their hamstrings between leaps over the bar? Should sprinters put moist heat on their hams just before their competitions and throughout the time periods associated with the running of ‘heats’ in a major competition? Should soccer players place moist heat on their legs during breaks from action or during time on the bench? Would these actions limit hamstring injuries and perhaps enhance performance?
The answer is not entirely clear, especially since what athletes really want is dynamic flexibility (proper extensibility of their muscles and connective tissues during movement), not static flexibility. Research concerning the effects of moist-heat application on dynamic flexibility has yet to be conducted. In the meantime (as you wait for such research to be carried out), you can conduct one of our practical exercises for enhancing dynamic hamstring flexibility – bicycle leg swings. If you carry out these swings three times a week, we guarantee that your hamstrings will be stronger and more flexible as you participate in your sporting activity. Here’s what to do:
Bicycle Leg Swings: 2 sets of 50 reps for each leg. Stand with your weight fully supported on your left leg (initially, you may place your right hand on a wall or other support to maintain balance). Begin by flexing your right hip and raising your right knee up to waist height (your right thigh should be parallel with the ground); as you do this, your right knee should be flexed to 90 degrees or more. Once your thigh is parallel to the ground, begin to extend your right knee (swing the lower part of your right leg forward, unflexing the knee) until your knee is nearly fully extended (your leg is nearly straight), with your right thigh still parallel to the ground. As your right knee nears full extension, allow your right thigh to drop downwards and backwards until the entire thigh and leg are extended behind your body (as if you were following through on a running stride). Your right knee should be near full extension (your leg should be basically straight) until it reaches the peak of the backswing. As your right hip nears full extension (as you approach the end of the backswing), raise your right heel by bending your right knee; your heel should move within close proximity of your buttocks as you do this. As this happens, move your right knee forward until it returns to the appropriate position in front of your body, with your right thigh parallel to the ground.
Repeat this entire sequence of actions in a smooth manner such that the hip and leg move through a continuous arc without stopping or pausing. Once you are able to coordinate the movement, strive to perform the swings at a cadence of about 12 swings every 10 seconds or so (slightly faster than one swing per second). After you become skilled, coordinated, and fatigue-resistant while carrying out this exercise, progress to performing the exercise with a stretchy, exercise cord of intermediate strength. Attach one end of this elastic cord around your ‘swing’ ankle and the other end to a fixed object at about knee height. Adjust the distance from the fixed object (and thus the tension on the exercise cord) so that the cord accelerates your leg ahead during the forward-swing portion of the bicycling action. This will force your hamstrings to upgrade their strength and flexibility even further.