When in the gait cycle do hamstrings strain?

[injuryupdate]

I keep reading that the likely time of hamstring strain is during late swing in an eccentric movement, as all 'experts' think that the hamstring must strain during an eccentric phase and therefore the late swing is when it would go.

I disagree. I think there are two important factors causing a hamstring to strain: FORCE (something pulling the hamstring to make it stretch) and STRETCH (the position the sarcomeres are in at the time), and of the two, force is one that actually tears the bloody thing (stretch just makes it susceptible to tearing).

What is the force acting against the hamstring in the late swing phase? As far as I can see there is nothing. The quadriceps is not contracting and even gravity and air resistance are going to help the hamstring not put force against it.

When the leg hits the ground (particularly if an overstride has occurred) then ground reaction forces may cause the hamstring to tear.

I don't think the muscle knows if it is in swing phase or ground phase or whether it is meant to tear or not. I just think it can either withstand the force stretching it (in which case it contracts) or it can't (in which case it tears). There just has to be a force. So I think the hamstring tears when the leg is on the ground.

The one concession I will make is that the error in the gait cycle that predisposes to a tear probably happens in the swing phase. That is, the hamstring might fatigue and not catch and therefore an overstride can occur. The die might be cast in late swing phase. But contrary to expert opinion, I think the strain occurs upon ground contact.

[Whoskins]

For a good overview on the topic you can try this link, however the topic along with why hamstring injuries have such a propensity to occur is a very complicated one that needs far more scientific coverage:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15922230&query_hl=11.

Firstly, it should be noted that it has not been scientifically proven that injury occurs at any particular stage of the gait cycle. It is more the case of authors presenting a hypothesis and their reasons behind the belief. A well designed research project is required to document at what stage of gait injury occurs. I personally believe injury can probably occur at both stages of the gait cycle. Often clinicians are after a magic bullet approach to the management of hamstring injuries when one doesn’t exist. There is still a lot we don’t know about hamstring injuries and a lot of quality research that needs to be done before injury rates significantly decrease.

With the exception of the short head of biceps, the hamstrings are biarticular. Because of the biarticular nature, the work of hamstrings is related to the range of movement in the hip and knee joints. When a muscle is monoarticular, there is a simple relationship between the amount of force that the working muscle exerts and its effect upon the joint. If there is enough force to overcome the resistance, the muscle will work concentrically, if there is insufficient force, the muscle will work eccentrically. For biarticular muscles there is no such linear relationship between muscle length and position of the joints. Another anatomical consideration that needs to be considered is the presence of extensive passive structures with the anatomy of hamstrings. This makes the muscle group suited to working from a prestretched position.

At the terminal stage of the swing phase of gait, quadriceps is contracting to extend the knee, but by far the most powerful movement is hip flexion (which includes rectus femoris of the quadriceps group). The power generated by the hip flexors increases significantly as speed of gait increases (Novacheck 1995, Instr Course Lect) and thus hamstrings needs to act as a break to counteract this force. In fact the main muscle group that appears to increase the speed of gait is that of the hip flexors (Mann et al 1986, AJSM). It should be also noted that the multijoint muscles are also preferentially recruited in open chain situations such as the swing phase of gait, which increases the force requirement of hamstrings. Moreover as a result of the series elastic component of the stretch shortening cycle you do not have to have muscle contraction to get injury. This is in fact one of the main issues with eccentric loading. There are a large number of studies supporting an eccentric mechanism of injury (refer to Brockett et al)

However, during the support phase of running, knee extension should occur simultaneously with backward flexion in the hip, thus hamstrings should act in a transducer of force capacity and transfer energy efficiently between body segments. Technqiue errors, such as overstriding may alter this function.

Some other thoughts. Running is regulated by way of the central nervous system, requiring afferent information carried from the motor apparatus to the CNS (kinesthesia). By means of muscle spindles and sensors in the tendon and joint, information about changes in muscle length, tension in tendons, positions of joints can be transmitted. Efferent impulses pass from the CNS to muscles and motor units. The force of a muscle can be regulated in muscles by contracting few or many of these motor units. Small motor units are the first to be recruited when force must slowly be built up in a muscle. Larger motor units are only switched on when much more force is needed. This serves to adjust precisely the amount of force generated. The locomotor and sensory systems are strongly interwoven. Being able to use the hamstrings accurately for running, especially at speed, will require sophisticated instruction about how the body moves. This may be why it has been suggested that learning how to control the hamstrings well during gait is more important during training that trying to develop force. Any slight alteration in the neurological control of hamstring contraction, either afferent or efferent will potentially predispose injury. Alterations in the recruitment of motor units may result in insufficient force generated by the hamstrings, favouring an eccentric mechanism of injury, or may even result in excessive force production and cause subsequent injury.

[hhh]

Does anyone else have a thesis to submit?

[jisles]

Decceleration forces (to prevent overstriding and knee hyperextension)are an important reason when in the accelerative phase and hip flexion is nearing maximal. Ground forces are important when hip flexion is less, but lumbar flexion has been maximised. At all the other times, there should be no reason for excessive forces being aplied, unless the hamstrings (biceps in particular) is or has recently been injured.

So MY ancillary yet unrelated question is - has anyone heard or come accross any postitve results from dietary supplements or medications in healing grade 1-2 strains? (COX II inhibitor anti-inflammatories and other NSAIDs have been done to death -so anyone with anything new or cutting edge???)

[hhh]

I think cybex testing is still considered cutting edge. The only data on NSAIDs suggest they may help improve recovey to competition (although this is statistically non-significant) but they also cause a much higher rate of reinjury. Other studies have found in terms of pain, they are no better than placebo for minor injuries and worse than placebo for severe pain. Bottom line is elite players pop these fellows like lollies. No other drug or medication has been studied. Given the dodginess in pharmaceutical funded research perhaps its best it stayed this way.

QUOTE=jisles]Decceleration forces (to prevent overstriding and knee hyperextension)are an important reason when in the accelerative phase and hip flexion is nearing maximal. Ground forces are important when hip flexion is less, but lumbar flexion has been maximised. At all the other times, there should be no reason for excessive forces being aplied, unless the hamstrings (biceps in particular) is or has recently been injured.

So MY ancillary yet unrelated question is - has anyone heard or come accross any postitve results from dietary supplements or medications in healing grade 1-2 strains? (COX II inhibitor anti-inflammatories and other NSAIDs have been done to death -so anyone with anything new or cutting edge???)[/QUOTE]

[injuryupdate]

My gut feel on NSAIDs (based on experience and understanding of the basic science but not RCTs) is that they are:
- a good option for 'minor' muscle strains that are unlikely to recur
- possible a dodgy option for structurally significant muscle strains that have a high risk of recurrence.

[hhh]

And what is your gut feel on cybex testing

[sydunisportsmed]

In terms of determining risk of recurrence on return to play, the Cybex scores almost certainly have a correlation (i.e. the worse your strength is the more likely it is you will recur). Whether it is going to change your decision for a particular player with a match this weekend, I don't know.

The two problems with Cybex testing are that on the eccentric phase of the machine, you actually can tear a previously uninjured muscle, and that if you decide that a player MUST get an H:Q ratio of 58% or whatever to be allowed to return (which some papers suggest) then you would be keeping some players out forever.

Just as there are players who win match of the match and can't remember the game (and who would never have passed a Cogsport test) there are also some players who would be running around with an H:Q ratio of less than 50 percent who get away with it.

[Whoskins]

A very interesting and original study.

Heiderscheit BC, Hoerth DM, Chumanov ES, Swanson SC, Thelen BJ, Thelen DG. Identifying the time of occurrence of a hamstring strain injury during treadmill running: A case study. Clin Biomech (Bristol, Avon). 2005 Aug 30; [Epub ahead of print]

BACKGROUND: While hamstring strain injuries are common during sprinting, the mechanisms of injury are not well understood. In this study, we analyzed the running kinematics of an athlete obtained at the time of an acute hamstring strain injury. The purpose was to identify the period of the gait cycle during which the hamstring was likely injured, as well as to characterize the biomechanical conditions associated with the injury. METHODS: A male professional skier injured his right biceps femoris long head while running at 5.36m/s on a treadmill with a 15% incline. Whole body kinematics were recorded at the time of injury. A linear periodic prediction model was used to determine when individual marker trajectories deviated from a cyclic periodic pattern, indicating the mechanical response to injury. A three-dimensional musculoskeletal model was used to compute joint angles and hamstring musculotendon lengths during the injurious running trial. These data were used with estimates of neuromuscular latencies and electromechanical delays to identify the most likely time period of injury. FINDINGS: Based upon the earliest indications in marker trajectories, a 130ms period during the late swing phase of the gait cycle was identified as the period of injury. During this period, the biceps femoris reached a peak musculotendon length that was estimated to be 12% beyond the length seen in an upright posture and exceeded the normalized peak length of the medial hamstrings. INTERPRETATION: 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.

With regards to cybex testing, after the Bennell et al 1998 study isn't it time to throw the baby out with the bathwater?

[sydunisportsmed]

Great case study, we'll put an article up on this site shortly regarding it. On initial read I agree that they have nailed the stride when it occurred but doesn't seem that they have proven that the injury occurred in late swing as opposed to early contact phase.

With respect to Cybex testing, a problem with the dual phase machines is that they can actually cause injuries themselves.

[hhh]

Need to consider that it is only a case report, the guy was running up hill and not very fast. A good case series would be to wire up guys like Kozshitski, Buckley, O'Loughlin, Xavier Clarke, David Peachey etc etc and make them run maximum 50m sprints repeated until they pop a hammy and then analyze the data.

[Rod Whiteley]

I'd side with late swing as being the principal villain on the basis of:
1. Highest EMG at this point in the gait cycle
2. Hamstrings contracting eccentrically, and
3. Theoretically hamstrings acting at the greatest length in the gait cycle.

I would caveat all of this though with a glance toward the following article which has made me think that muscle function is nowhere near as balck and white as I had thought:
Kawakami, Y. and T. Fukunaga (2006). "New insights into in vivo human skeletal muscle function." Exerc Sport Sci Rev 34(1): 16-21.
Approaches to understanding human muscle function through measuring in vivo muscle fiber behavior in humans are described briefly. Experimental results show the discrepancy between joint actions and muscle fiber behavior in many occasions, which complicates prediction of muscle mechanics from sole observations of joint actions. This is the result of muscle-tendon interaction during contraction, which is efficiently used during human movements.
Well worth a read