The role of exercise intensity in the bone metabolic response to an acute bout of weight-bearing exercise

The paper we looked at for journal club was from the Journal of Applied Physiology and entitled ‘The role of exercise intensity in the bone metabolic response to an acute bout of weight-bearing exercise.’

What was the aim of the paper?

The researchers wanted to compare the effects of three different cardiovascular intensities on changes in bone turnover markers during and for 4 days after acute endurance running. They did not state a hypothesis, but previous research has shown that short-duration, high-impact exercise is beneficial for bone mass and endurance running exercise has been linked with detrimental effects on bone. The research was carried out by researchers in the UK.

What did the study involve?

The researchers recruited 10 men who performed at least one bout of endurance running a week. The study consisted of three counterbalanced 8-day experimental conditions, separated by at least a week. You can find out why the researchers carried out this design here. On days 1-3 of the trial, the participants did not carry out physical activity. Day 4 consisted of a single 60 minute bout of running at 55%,65% and 75% of VO2max, followed by 3 hours recovery. The researchers measured 60 seconds of expired air and RPE at 18,38 and 58 minute of exercise. They recorded heart rate continuously. Blood samples were taken at baseline, after 20,40 and 60 min of exercise and 0.5, 1,2 and 3 hours of recovery. The researchers measured the following markers: COOH-terminal telopeptide of procollagen type 1 (β-CTX), NH₂-terminal propeptides of procollagen type 1 (P1NP), osteocalin (OC), bone-alkaline phosphatase (ALP), osteoprotegerin (OPG), parathyroid hormone (PTH), albumin-adjusted calcium (ACa), phosphate (PO4) and cortisol. The participants consumed a standardised meal in the lab 3 hours after exercise, and around 4.5 and 7 hours after exercise. On days 5-8 the participants did not carry out physical activity but followed a prescribed diet and went to the lab for analysis.

What were the main results?

The researchers found that β-CTX concentrations were higher in the first hour following exercise at 75% VO2max, compared to 55% and 65%. P1NP increased significantly during exercise only and ALP concentrations increased significantly at 3 and 4 days after exercise, but neither were effected by exercise intensity. PTH and cortisol increased signficiantly with exercise at 75% only. OPG, ACa and PO4 increased signficiantly with exercise but were not effected by exercise intensity.

What can we take from it?

Although I found this paper quite challenging to read, I really enjoyed it and learnt a lot from it. The researchers recruited the participants well because they ensured that they did not have a bone fracture in the previous 12 months, no injury and did not have a condition or take any medication known to affect bone metabolism. They also took into account fasting vitamin D concentration, which is important because low serum vitamin D levels have been associated with low bone density.

I really liked the use of figures and tables in the paper. I think it was good to have a figure outlining the overall study design, which helped with the understanding of the protocol. The table with the subject characteristics was helpful and there was a good use of charts to graphically display the results.

A small point of interest in the study design is that the researchers relied on the participants correctly adhering to the diet that the researchers prescribed. There could have been variation in the timing of the consumption of food, however the researchers ensured that the participants verbally confirmed their adherence.

Overall, I think this was a good study with an appropriate design for the question being asked. Craig Sale, who is one of the authors of the paper, said in conclusion: “There is only a small and transient influence of intensity on the bone metabolic response to one hour of exercise. Most likely, from this and subsequent studies from our group, the effect of exercise duration on bone is more critical.” Further research in this area is being carried out by Craig and his research partners.

Craig has answered some questions about his research that were tweeted by followers on Twitter. It is a great opportunity to find out more about the research and I would highly recommend having a read through his answers.

Q&As with Craig Sale, Reader in Applied Physiology at Nottingham Trent University

For journal club we looked at the following paper:

Scott JP, Sale C, Greeves JP, et al. The role of exercise intensity in the bone metabolic response to an acute bout of weight-bearing exercise. J Appl Physiol 2011; 110 (2): 423-32

I was lucky enough to ask one of the authors, Craig Sale, some questions about his research. Craig is a Reader in Applied Physiology at Nottingham Trent University and one of his research areas of interest is bone health and metabolism.Thanks to those of you who tweeted your questions. I hope you find this interesting and it gives you a better insight into Craig’s research.

Q: Why did you decide to carry out this research?

A: To answer a possible question over the influences on the bone metabolic response to exercise that had arisen from two previous studies from our group.  Two obvious potential influences were exercise duration and intensity and we wanted to examine the latter here.

Q: How did you recruit the participants?

A: Primarily through posters, email and word of mouth.

Q: Why did you choose the study design of three 8-day experimental conditions, separated by a minimum of 1 week?

A: Some of the bone markers are sensitive to exercise and feeding, so for this reason we wanted a control period prior to the exercise trial itself, hence the 3 d lead in period.  In order to get the most relevant information on the bone metabolic response to exercise, the days following the exercise are most critical.  In this study we chose to examine a 4 d follow-up period for this reason, which was based upon our previous study.  You need at least a 1 week separation between trials as bone markers can remain elevated over this period.

Q: How did you randomise the participants into the experimental conditions?

A: There were 6 possible combinations of 3 conditions so the first 6 subjects were randomly assigned to one of the 6 combinations then the 6 combinations were ‘re-set’ and the last 4 subjects were assigned on this basis.

Q: How did you ensure that the participants adhered to the diet that you prescribed?

A: This study was conducted with free-living participants and so it is hard to directly confirm adherence to the dietary control.  However, subjects verbally confirmed their adherence to the dietary control on each occasion.

Q: Your findings showed that daily calcium intake of the participants exceeded the recommended daily intake of 700mg a day and there was large variation in calcium intake, do you think this could have affected your results?

A: The habitual dietary intake of the subjects was variable, but this was reduced somewhat by the dietary control imposed during the experimental trials.  Some variability would have remained but the effects of this on the bone metabolic responses to exercise is not likely to be large in this within subjects design.

Q: What populations do you intend your findings to be beneficial for?

A: These findings, particularly when combined with the evidence from other well-controlled exercise studies, would be of benefit for any athletic individual.

Q: How much, in your opinion, are the changes in bone turnover due to mechanical muscle tension placed on the bone?

A: As far as a specific quantification is concerned, this is obviously impossible to answer directly.  However, it is clear that muscle contractions, occurring concomitant with the direct impact loading of the bone, produced during exercise can generate further osteogenic loading. As such, the effects of muscle contraction in this sense are likely to contribute significantly to changes in bone with exercise and particularly training.

Q: What would you like people to take away from your research and how do you hope to develop your findings?

A: That there is only a small and transient influence of intensity on the bone metabolic response to one hour of exercise.  Most likely, from this and subsequent studies from our group, the effect of exercise duration on bone is more critical.  This makes sense from a mechanical loading perspective and it would be of interest to examine the influence of changes in “mechanical” intensity rather than cardiovascular exercise intensity. We now have several studies continuing on the effects of exercise and diet/nutrition on bone metabolism as well as studies examining the influence of exercise training on bone structure and geometry.

Thank you to Craig for taking the time to answer the questions about his research and sharing his knowledge.