Evidence for the Late Swing Phase

There is something about the biomechanics of running at high speed that increases the risk of a hamstring injury. This is something of a review, focusing on the late swing phase of the gait cycle with the intent that if we understand what is going on, we can structure our training programs better.

Better training = injury prevention/ reducing the risk of injury = better performance.

In keeping with the principles of training (specificity, progressive overload), programs would ideally incorporate aspects (e.g. postures, muscle lengths, contraction type) that are most similar to those conditions associated with injury. Only then would athletes optimize the gains in functional strength and minimize the risk of future injury.

THE STANCE PHASE:

Some researchers, however, propose that the hamstrings are most likely to be injured during the stance phase of the gait. This is mostly due to the the external loading on the limb that comes as a result of ground contact. Lateral and medial hamstrings and gluteus maximus showed similar activities with peak levels of EMG during foot-strike (Jonhagen S, et al., 1996).

Yu B, et al. (2008) included the following in their abstract: "The potential for hamstring muscle strain injury exists during the late stance phase as well as during the late swing phases of overground sprinting."

CAVEAT

Notwithstanding, a landmark article describing the work by Lieber and Friden (1993), though largely uncited, should be considered. It was entitled Muscle damage is not a function of muscle force but active muscle strain. (Journal of Applied Physiology. 1993;74:520–6.) They interpreted their findings to signify that "it is not high force per se that causes muscle damage after eccentric contraction but the magnitude of the active strain (i.e., strain during active lengthening)."

THE LATE SWING PHASE: FORM AND FUNCTION

Conceptually, hamstring lengths are primarily a function of the angles of flexion found at the hip and knees during running. For example, the hamstrings are lengthening during late swing when the hip is flexing and the knee is extending, as both of these motions contribute to hamstring stretch.

•  However, the hip starts to extend prior to foot contact and continues to extend throughout stance (47); 
• the knee flexes until mid-stance and then extends until toe-off. Therefore, if hamstring stretch is to occur during the second half of stance, the hamstring lengthening due to knee extension would have to exceed the hamstring shortening due to hip extension. 
• Further because the hamstring moment arms at the hip are greater than at the knee (13, 47), the knee extension velocity would have to exceed the hip extension velocity. 
• However, sagittal hip and knee angular velocities during stance tend to be of comparable magnitude (27), making it unlikely that substantial hamstring stretch is occurring during stance.

THE LATE SWING: STUDY EVIDENCE

Heiderscheit BC, et al. (2005) identified, based upon the earliest indications in marker trajectories, a 130 ms period during the late swing phase as the period of injury. The biceps femoris (BF) reached a peak musculotendon length up to 12% beyond the length seen in an upright posture and exceeded the normalized peak length of the medial hamstrings. "This case provides quantitative data suggesting that the biceps femoris muscle is susceptible to an lengthening contraction injury during the late swing phase of the running gait cycle."

In an interesting 2009 study, Schache A, et al. collected 10 normal sprinting trials for kinematic and GRF analysis, and compared them with one after a hamstring strain. "For the pre-injury trials, the right leg compared to the left displayed greater knee extension and hamstring muscle-tendon unit length during terminal swing, an increased vertical ground reaction force peak and loading rate, and an increased peak hip extensor torque and peak hip power generation during initial stance."

"For the injury trial, significant biomechanical reactions were evident in response to the right hamstring strain, most notably for the right leg during the proceeding swing phase after the onset of the injury. The earliest kinematic deviations in response to the injury were displayed by the trunk and pelvis during right mid-stance. Taking into account neuromuscular latencies and electromechanical delays, the stimulus for the injury must have occurred prior to right foot-strike during the swing phase of the sprinting cycle. It is concluded that hamstring strains during sprinting most likely occur during terminal swing as a consequence of an eccentric contraction." (Biomechanical response to hamstring muscle strain injury.)

Using 3-D videographic and EMG data from the maximal overground (as opposed to treadmill) sprinting efforts of 20 male runners, soccer and lacrosse players, Yu B, et al (2008) interpreted their findings as such; "The potential for hamstring muscle strain injury exists during the late stance phase as well as during the late swing phases of overground sprinting." (Hamstring muscle kinematics and activation during overground sprinting.)

OTHER POINTS TO CONSIDER

• Repetition: The biarticular hamstrings undergo maximal stretching during the late swing phase of running gait and also present with a repetitive active stretch-shortening contraction. "When active stretch-shortening cycles are imposed on a musculotendon unit, the degree of strain taken up by the fibers can change with time, making the muscle potentially more susceptible to injury after repeated loading cycles" (Butterfield and Herzog, 2005). Basically, the more strides you take, the greater the likelihood for injury. (Ever notice how nobody pulls up straight out of the blocks?)
• Negative work: When running at high speeds, the amount of negative/concentric work increases significantly and thus, the hamstrings may be susceptible to a late swing injury as a result of repetitive strides of high speed running. (Heiderscheit BC, et al., 2005 and Schache A, et al., 2009)
• Biceps Femoris Observations: BF loading increases with speed during swing but not stance. (Chumanov et. al., presently reviewed paper.)

It is worth noting that this information derives from studies and observations when the hamstrings are involved in forward sprinting at a fairly constant speed as in track sprinters. The hamstring loads that occur during sudden directional turns and accelerations as in soccer are vastly different, and could certainly factor into injury risk.

Hope Solo

Defining the musculotendon demands during sprinting, with specific emphasis on which time injury is most likely to occur offer direction as to the type of resistance training may be useful for injury risk management. The examples above suggest that the late swing is the time the hamstrings are most susceptible to injury. with this evidence, the most effective resistance training would be when the hip is in flexion and the knee in extension, as in the swing phase. For example, deadlifts, good mornings, back extensions.

Pauline Nordin

This data also provides evidence for the use of eccentric contractions under inertial load in programming. This approach has already been shown to be efficacious as a prevention and rehabilitative tool. See my article, "The Best Exercises Soccer Players Are Not Doing" , for an eccentric focused training program, including progressions from rehab to sport-specific training.

REFERENCES

All references in this article, unless otherwise stated refer to

Chumanov E. S, Heiderscheit B. C, and Thelen D. G (2011). Hamstring Musculotendon Dynamics during Stance and Swing Phases of High Speed Running. Med Sci Sports Exerc. 43(3): 525–532.