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Thanks for sharing your research. It definitely promotes healthy discussion. Please can you clarify your definition of Cross Training? (alternative endurance training or strength training) I agree that Cross training may not improve Vo2 max but there are credible studies which show that running economy is improved through maximal (Helgerud/Storen) and explosive strength training (Paavolainen). With regard to injury prevention, I definitely do not agree that it "can only prevent running injuries when it replaces running". To give you just one example - the elite triathlon squad in Leeds (Multiple Olympic/World/European and Commonwealth medallists) all have individually created strength programmes designed to develop or maintain balance, stability and specific joint strength to complement their weekly run programme. Often this work is referred to as "prehabilitation training" by their physios because they do it to prevent injury. If they didnt do these exercises then they would only be prescribed as a way of recovery. I think I'd prefer to do the work first and avoid injury rather than get injured (and frustrated) and have to do the work as part of my recovery.
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Your discussion centred on three aspects, the first being a clear definition of cross training, the second on the role of maximal and explosive training and the third a challenge to the use of cross-training in injury prevention.
Let’s start with the easy one first, cross-training would constitute a different activity for different sports. In essence it would be any activity that departs from your core sport activity. As an example, for a MdS ultra-endurance multi-day athlete, running, walking, interval training etc would all constitute core activities, but cycling or swimming would not, so cycling or swimming would constitute cross-training. This would be different for a triathlon athlete who would have three core activities, nl, running, cycling and swimming, for this athlete cross-training would be yoga, weights etc. This brings us to a somewhat grey area at times, such as does static cycling in the gym (which don’t require core balance) constitute a core activity or cross-training for a tri-athlete, the same goes for using the elliptical machine, treadmill etc. My personal view is that the closer a training activity is to the race condition the closer it is to being a core activity and the further away from the race condition it moves the closer it moves to becoming a cross-train activity. Per definition, I would consider running on the treadmill in the heat chamber a core-activity, although it would be better to run on the train in 40degC as part of preparation, this may not be practical for everyone, so the treadmill and heat chamber combination would constitute an acceptable core activity (not ideal, but acceptable). I hope this makes sense, I imagine we could setup a spreadsheet that classes the different activities for different sports in terms of core or cross-train activities.
Next, let’s look at the prevention of sport injuries through prehabilitation training. Primarily, the studies conducted around this have centred on pre-surgery exercises to aid in a faster recovery after surgery. Evidence that this is transferable from a pre/post-surgery environment to the sport injury prevention environment is not that clear. What is clear is that, the fitter a person is when he/she sustains an injury or undergoes surgery the faster their recovery should be (Jack, West, Grocott 2011, et al). There seems to be a much stronger case for total rest as a mechanism for injury prevention opposed to cross-training. The high-level of injuries associated with cross-training could possibly have nothing to do with the cross-training activity, but rather, that cross-training is used by athletes as a form of ‘rest’ (just a personal thought here – no study to support this view yet!). Cross-training and the use of cross-training as a successful injury prevention strategy has little proof as the 10% rule athletes adhere to religiously. A study by Andersen and Williams suggest that a number of environmental conditions can cause psychological stress which can lead to physical injuries. These are New Environments (such as moving from one city to another), Competition (the event itself), Personality Factors (how the athlete responds to competition), and Resources (financial and social support). Dr. Jack Raglin, a kinesiologist at Indiana University, states that training too hard for too long will eventually result in a drop in performance which is what he calls, ‘staleness syndrome’. The only way to ‘reset’ is to rest, for several weeks, and possibly even months. According to Raglin, athletes who experience ‘staleness syndrome’ are likely to experience other symptoms like poor sleep, loss of appetite, upper respiratory infections and muscle fatigue (which inevitably leads to more serious injuries). He states that severe mood disturbances are common in this group. Some experts recommend periodization, a regimen that builds in recovery time and is intended to prevent the symptoms of overtraining. In both these studies, injuries are not prevented by incorporating non-core activities such as cross-training, but rather by responding to your body by providing sufficient rest. I would, therefore, venture to define rest as the meta cross-training activity for all sports, and if rest ‘cross-training’ is employed then cross-training is the right thing to do.
Your final point, that of running economy, is a little more complex. The concept of running economy is not as well defined as one would hope. The first observation of most of the studies in support of maximal training such as those done by Helgerud and Storen, you mentioned, is the sample size. Their entire conclusion is based upon the results obtained from 17 runners divided into two groups of 8 and 9 respectively. It is also worthwhile noting that the 17 runners were all highly experienced, high-performance, athletes who participated over an 8-week period. The evidence from this study is that by using ‘weight’ based exercises the intervention group were able to take on average 72 seconds longer before reaching exhaustion then the control group both running at a 1.5% incline on a treadmill. This is significant for sprinters and middle distance athletes running on a track, but no such study has been replicated for the cross-country / trail runners, running at higher inclines, over more severe terrain and for prolonged periods of time in highly extreme environments. The same advantage can be achieved by the MdS runner who move his/her training outdoors and into a +35degC environment compared to a runner training on a treadmill in a gym. A more meaningful study would have had three groups, an intervention group, a track control group and a cross country control group. Their conclusion may have relevance to short and middle distances, however, according to Midgley, “the critically important factor in the enhancement of running economy (for the distance runner) may therefore be the cumulative distance the runner has covered over the years of training” (870). The researchers noted that interval training, at between 93% and 106% of VO2max, has been shown to improve running economy significantly. This would seem to indicate that the same improvement result can be achieved without the runner having to do training outside of the primary activity, and without having to take the risk associated with cross-training. Your second source, the study by Leena Paavolainen, et al, claimed that explosive strength training (plyometric training) has performance benefits for 5km running time. Her sample consisted of 22 elite cross-country runners of which 4 (18%) had to withdraw during the 9-week test period due to injury or sickness (see my argument linked to lack of sufficient rest earlier). Again her test lacked ultra-distance, ‘real-world’ application for an MdS runner who won’t be running 5km’s, and definitely not on a track. However, even if the conclusion and result of the study is transferrable to multi-day, self-sufficiency races, the 18% injury / sickness rate is much higher than the average reported in other studies and one has to question the soundness of employing plyometric training. Dr. Ross Tucker from the University of Cape Town and Sports Science Institute of South Africa warns that “the risk of injury is high, and so this should neither be tried out by novice runners, or done too often”.
The emerging picture is that (a) core activities (those directly related to your sport) is what makes you really good at what you will be doing. For the MdS runner, that is running, running long distances, over uneven / rough terrain, with a back-pack in very high heat. The more of these a runner can incorporate the better. (b) Rest should be exactly that, rest, if rest is replaced by a non-core activity, then the non-core activity seems to add to the already high load with a high risk of injury, so the possible benefit seems to be negated, and (c) there seems to be enough other core-type activities that can be incorporated in training (such as interval training, or hill work, for the runner) to make cross-training unnecessary.