Adaptation and injury: why timing is everything

“The runner has three great challenges, and the greatest of these is the mile. The others are the dash and the marathon. Taken together they comprise all of exercise physiology. They correspond to the three major sources of muscular energy, and they call on man in his various guises as body, mind, and spirit.”

Sheehan understood that a runner’s performance was contingent upon melding the power of the dash with the persistence of the marathon. Most runners however have tipped their training balance towards the latter side of the equation. And perhaps, as a result, injury and performance have been potentially compromised.

New Zealand coach Arthur Lydiard (1917-2004) is credited with developing one of the first periodised training programs for runners, and its design has had a lasting influence (Ed – see also John Shepherd’s article elsewhere in this issue). Lydiard’s cornerstone was an extensive block of marathon training, which included 100 miles of aerobic running per week with “as many easy running miles as possible.” In total, his middle and distance runners of the 1960’s were logging between an astonishing 160-200 miles per week!^{Arthur Lydiard’s Runner’s Bible. Copyright 1970 United States Track and Field Federation} Following these months of mileage, Lydiard advocated shorter cycles of hill bounding, anaerobic intervals, topped off with ‘skill’ training, and race-specific paced efforts (see figure 1).

Base building science

Physiologists have found that aerobic capacity (V02 max) improves sharply when running up to about 50 miles per week but plateaus shortly thereafter. Training 60-90 minutes per day appears to be the saturation point at which measurable improvements in aerobic metabolism occur (but note there might be further adaptations in other areas with somewhat higher mileages such as running economy). These adaptations, including increased capillary and mitochondrial density, are largely realised within only six to nine training weeks.

If you are new to running, you can expect to see improvements in your V02 max and performance by increasing your mileage to around 35-40 miles per week (mpw)^{Running Research News 1986; 2(6):1-2, 5-6}. Evidence for this can be seen in a study of first time marathoners. It found that higher-volume runners, averaging 50mpw, finished no faster than runners averaging 40mpw. The average finishing times for both groups were identical at 4hrs:17mins and 4hrs:51mins for males and females, respectively^{Running Research News 1995; 11(2):1, 5-6}. However, a study of more experienced marathoners reported that mileage may matter as an increase in training mileage (from 47 to 56 mpw) WAS associated with faster finishes (from 3hrs:20 to 3hrs:10)^{Aust J Sports Med 1977; 9:58-61}.

Intensity matters too. A Finnish study investigated the responses to two different base training intensities^{Finnish Sports Exer Med 1984; 3:104-112}. A lower intensity group (70-75% of maximum heart rate) failed to improve V02 max at all while a higher intensity group (82-88%) made significant improvement. Meanwhile, a McMaster University study found that subjects performing four sets of 6 X 30-second sprint intervals with four minutes of low intensity recovery were able to improve time trial performance, mitochondrial enzyme activity, and glycogen storage to a greater degree than a control group that trained continuously at 65% V02 max for 90-120 minutes^{J Phys 2006; 575(3):901-911}.

Injury and adaptation

In his book ‘Running Science’, running guru Dr. Owen Anderson states: “No evidence exists to suggest that higher–quality training heightens the risk of injury during base periods. Since the total time spent training (e.g. volume) is usually the best predictor of running injury, a traditional endurance oriented base, in which distance is steadily ramped up, might be much harder on muscles and tendons than a high intensity base.” But why should this be the case?

Studies reveal as many as 75% of runners will experience an annual injury. Among the numerous risk factors include being a novice runner, high training loads and poor running form. Why is it such a risk factor to be a new runner? One reason is that the body’s structural elements, like bone and tendon, are MUCH slower to adapt to training loading than the aerobic systems. Human bone for example requires four months to adapt to higher running speeds by becoming stronger and denser, and may actually lose some strength initially before ultimately adapting. In other words, while your aerobic fitness improves rapidly, your structural integrity lags well behind when it comes to adaptation.

A rule of thumb is that your structural integrity adapts in a manner that is ‘just strong enough’ to prevent injury for your habitual speed and form. As such, training with extensive mileage below race pace will NOT improve your race pace strength or neuromuscular control. In fact, it may even reduce it; studies of bone density reveal that it actually declines when running volume exceeds about 55 miles per week (see figure 2).

Graph shows the bone mineral density (BMD) of different regions of the lower leg versus running mileage. Notice that very high volume runners (running 60-75 miles per week) had lower BMD than high-volume runners (running 40-55 miles per week), and values that were no better than completely inactive individuals (controls).
*J Appl Phys 1992; 73(3):1165-1170

Minimum effective strain

The primary signals that govern bone and connective tissue strength are force and frequency. Bone studies reveal that very few repetitions of loading are required to create an adaptive response; provided THE MAGNITUDE OF FORCE EXCEEDS A CERTAIN THRESHOLD STRAIN. In biomechanics, this is known as the ‘minimal effective strain’ (MES).

In order to improve structural strength, a level of force that supersedes the system’s present ceiling for adaption must be applied. Infinite numbers of repetitions below the MES have NO strengthening effect. Studies of both humans and animals reveal that as few as 10 repetitions of loading above the MES can induce significant improvements in structural strength. This is a really important point because it suggests that a relatively small number running strides performed above habitual running speeds are more ‘bone strengthening’ than infinite numbers of strides at slower paces!

However, it also seems that the minimal effective strain is better if applied quite frequently. To illustrate this point, a 2008 rat study evaluated the change in lower leg bone strength as a consequence of different weekly jump training frequencies (see table 1).

Table 1

Number of weekly training sessions	Total number of jumps/week	% improvement in bone strength
Control (0)	0	0
1	10	7.5%
3	30	12.6%
5	50	12.0%
7	70	19.8%

The results revealed that training only once per week had no significant effect on bone strength. Training 3-5 times per week resulted in a 12% strength gain and daily training resulted in a further 40% improvement^{J Bone Min Metab 2008; 25(5):456-60.}. The takeaway point here is that a little bit of regular loading at the right intensity goes a long way in improving strength and resilience. Here, there are similarities with aerobic fitness because studies of V02 development have also concluded that frequency is a primary determinant of metabolic adaptation. For example, one study found that runners who trained four times per week improved their fitness to a significantly greater degree than those that ran the same total miles but in two or three sessions per week^{Arch Phys Med Rehab 1975; 56(4):141-145}.

Form, function and structure

There are numerous studies correlating running form (technique) to both injury and performance. One of the strongest correlates with injury is impact or ‘ground reaction force’. Higher levels of landing impact are positively correlated with more severe running injuries^{Br J Sports Med 2016 Jul; 50(14):887-92}. Landing impact is governed by a number of factors including bodyweight and foot strike pattern (see figure 3).

% stance refers to time from when foot first touches ground (0%) to when it leaves the ground (100%). Steeper curves indicate greater impact rates. Averaged throughout the whole stance, a midfoot strike results in the lowest total impact.

On an intuitive level, most can appreciate the relationship between impact and injury. What is less frequently discussed is that landing impact distinguishes runners of different performance levels. Simply stated, for a given running speed, better runners create less braking impact and as a result run faster with greater ease. The features in form which are consistently correlated with better function and less structural damage include:

• Landing closer to the general center of mass.
• Mid foot landing.
• A more vertical lower leg (shin) at landing
• Minimal vertical oscillation.
• Greater body extension at terminal stance.
• A cadence of around 180 steps per minute.
• Increased knee flexion during swing phase

Summary

Let’s summarise and see what the science tells us about adaptation and injury risk. Firstly, aerobic metabolism can be developed quickly with moderate running mileage relative to what has been traditionally recommended. As part of that, exercise intensity appears to be a critical determinant in the development of your aerobic fitness; interval training appears to be significantly more economic than high-volume, lower-intensity approaches.

There’s another downside to high volume training, low-intensity training because it may lead to adverse structural consequences, including loss of bone mass and muscle strength/power. It turns out that your structural adaptation to training (bones, tendons and ligaments) requires months or years to be fully realised – ie it is a much slower process than gaining endurance fitness see figure 4).

*from Counsilman 1991

Key points to remember and apply

Structural adaptation is also contingent upon force, frequency, and form. Therefore, in order to reduce your injury risk and maximise performance, all runners should remember/think about the following points:

• Metabolic adaptations, including the traditional grail aerobic capacity, are quick to develop and require only weeks of training. Strength of connective tissues, including bone and tendon, require at least four months of training for adaptation to be realised.
• Running economy and structural strength are ‘pace specific’. Running extensive mileage at eight minutes per mile pace will not necessarily make running a 10K race at seven minutes per mile feel any easier, or improve your race pace durability.
• If you are new to running, have a history of injury, or are building a new base, FOCUS ON FORM. Most runners will benefit from landing closer to their general center of mass and increasing cadence. Video analysis of your technique and use of a cadence counter can be invaluable training aids.
• All running performances can be reduced to the simple formula of stride length times stride rate. In order to improve your performance, one or both of these factors must be increased. Running FORM is the root of perFORMance and paramount in reducing injury. Compare your present running form to elite runners such as Haile Gabreselassie and Tirunesh Dibaba in order to identify relevant differences. You can find links to these videos here and
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• Practice your running frequently in short doses. As physical therapist and strength coach Gray Cook recommends, “First move well, then move often.” Fitness runners or triathletes should reap tremendous return on frequency investment by running four times per week.
• If you are a performance runner logging more than 60 miles per week and not improving, consider reducing your volume and increasing intensity. High yield strategies include training at VV02 max pace and/or race-specific paced training are both proven ways to boost fitness and economy.
• All runners should advance their training in a manner that is consistent with maintaining both form and function. Significant deterioration in running technique and consequently speed is an absolute contraindication to proceed in both training and racing conditions.
• Consider using an interval approach to running development. There is no metabolic or structural advantage gained by running continuously at a consistent speed. Conversely, a strong scientific argument can be made in favor of training with intervals at variable intensities.
• Finally, here are two tips tips for race injury avoidance:
  
  • Perform race-specific pace running (very short intervals count!) at least three times per week for four months.
  • Maintain your running form for the race’s entire duration.