Chapman AR et al.  Neuromuscular Control and Exercise-Related Leg Pain in Triathletes.  Medicine and Science in Sports and Exercise. Vol. 24, No. 2; 2010; pp. 233-243.

Triathletes commonly experience musculoskeletal injury, estimated to occur in one-third of recreational triathletes and in up to 90% of highly-trained triathletes.  Of all injuries that disrupt training for triathletes, the most common is exercise-associated leg pain, which could include stress fractures, shin splints, tendon strain or compartment syndrome.  Because of the high frequency of injury, identification of athletes who are at high risk for injury is important.  Proper identification of risks also allows a more specific rehabilitation program to be formed for injured athletes.

It is known from previous studies that some triathletes who transition directly from biking to running exhibit abnormal muscle use patterns in the legs (muscle recruitment).  This does not appear to be related to muscle fatigue as much as it is simply a carry-over effect of “bike-like” muscle firing patterns into the run.  This activity-specific muscle firing is termed neuromuscular control.  This study looked at triathletes with and without exercise-associated leg pain, and tried to determine if athletes with pain had a higher rate of abnormal muscle recruitment in the legs than control subjects without leg pain.  For this study, 10 highly-trained triathletes (five male, five female) with a history of leg pain (but no leg pain currently) were matched to 24 triathletes at similar levels of training (weekly volume, intensity, number of training sessions, years of previous training and competition history) who had no history of exercise-related leg pain.  Triathletes who competed in other sports involving the legs were excluded from the study to avoid any confounding effect of these activities on leg muscle recruitment.

The exercise protocol in this study involved a baseline 10-minute run, followed by 60 minutes of recovery.  The athlete was then exercised with 20 minutes of cycling on their own bike using clipless pedals, followed immediately by a 30-minute transition run on a zero-grade treadmill.  Running pace at both runs was self-selected by the athlete at the speed where 30 minutes of constant running could occur without fatigue or decline in function.  Cycle intensity was maintained at a subjective level of between “somewhat hard” and “hard.”  Cycling cadence was altered during the course of the ride, with gearing adjusted to keep exertion at a constant level.  All athletes cycled in the upright position and all participants exercised on a bike set up for draft-legal Olympic distance competition.   Muscle electrical activity in the legs was measured with surface electrodes throughout exercise, and three-dimensional motion analysis of the limbs during exercise was also obtained.

When data was analyzed, 10/34 (29.4%) triathletes exhibited different muscle firing patterns in the legs between their baseline run and their post-cycle run, involving 23/170 total muscles tested (13.5%).  5/10 athletes with a prior history of leg pain (50%) showed altered muscle firing, compared with only 20.8% of those with no prior history of leg pain (5/24).  This means that triathletes with a history of leg pain are 2.4 times more likely to have altered neuromuscular control when coming off the bike than athletes without a history of pain.

This was a retrospective study, meaning that athletes with known histories of leg pain were compared to athletes without any similar history after the pain had already occurred.  Studies of this type allow for the identification of association, but not necessarily cause.  Thus, we aren’t really sure whether abnormal muscle firing causes increased stress through the leg with running and leads to pain, or if painful conditions develop that lead to abnormal muscle firing.  Because this study showed normal baseline muscle function with running before cycling, and because these athletes were pain-free with submaximal running at the time of the study, it is reasonable to infer that in this case, the cycling causes abnormal muscle firing for running, which then leads to injury.  To confirm this however, prospective studies would be necessary.  A prospective study follows a group of similar subjects over time, waits for the condition to occur (in this case, leg pain) and then measures the two groups to see if there have been measurable changes from baseline in the injured group.  Prospective studies take longer to do and are more difficult to control.  In all, this was a well-designed retrospective study.

So what is the practical importance of this study?  Simply, both training and rehabilitation strategies should take this phenomenon into account in order to try to minimize the development of problems.  With respect to training, since abnormal leg muscle recruitment during a run that immediately follows a bike ride does not seem to be a function of fatigue as much as muscle memory that carries over from the bike, training strategies that focus only on developing resistance to muscle fatigue miss the mark. Training interventions such as plyometric training, as well as ensuring that runs do not always follow bouts of cycling make sense.  In physical therapy, patients with leg pain should be assumed to have persistence of this altered muscle firing, which is typical after an injury.  It makes sense to start a patient’s rehab session with a bike ride in order to elicit abnormal muscle recruitment which can then be properly targeted on the rehab floor.

If you are an endurance athlete with leg pain, give us a call at (515)221-1102.  Chances are, we can help reduce your risk for ongoing injury.