Discussion
In individuals with chronic LBP, pain intensity decreased following 6 months of general strength and conditioning but did not reach the minimum clinically meaningful difference of 20 points.20 As hypothesised, pain intensity was negatively associated with time under tension and participant reported RPE and positively associated with training session duration. Additionally, these associations were found when measuring the training parameters over the prior week (for time under tension) and session (for duration and RPE). Pain intensity was not associated with the frequency of training sessions or session-RPE at any time point (sessional, weekly or 2 weekly).
The current study showed that the exercise training variables of time under tension, RPE and training session duration were associated with pain intensity across a 6-month periodised programme of general strength and conditioning. A negative association observed with time under tension indicates that greater weekly volume (measured via time under tension) corresponded to lower pain intensity. For reference, every 60 s increase in weekly time under tension was associated with a lower pain intensity of 1.74 points. Similar results were seen in a study of 352 female office workers with musculoskeletal pain (various non-specific conditions), where total training volume was correlated with pain intensity following 16 weeks of resistance training.28 This suggests that for an exercise programme to be beneficial, a sufficient stimulus must be achieved to facilitate a physiological response to training. Likewise, the negative association between RPE and pain intensity found in the current study suggests that higher participant-reported intensities led to greater reductions in pain intensity. This finding is strengthened by the programme design, which was periodised with heavy and light blocks and progressed in a time-contingent manner irrespective of pain intensity. In other words, training intensity was not increased in response to decreases in pain intensity, which could lead to spurious findings. Similar benefits for high-intensity training have been found in individuals with chronic LBP following a 12-week programme of general strength and conditioning.29 Those completing the programme at a high intensity (100% VO2max and 80% 1RM) reported greater reductions in disability compared with those training at moderate intensity (60% VO2max and 60% 1RM), without any between-group differences in adverse events or adherence. Additionally, a meta-analysis of exercise training studies in individuals with chronic musculoskeletal conditions showed that protocols where participants ‘pushed into pain’ (up to 50 mm on a 100-point VAS) reported greater short-term improvements in pain intensity compared with protocols completed within a pain-free range.30 Combined, these results suggest a higher training intensity and volume may be beneficial for maximising outcomes in individuals with chronic LBP, even when participants report pain during exercise. However, it is unclear where the ceiling of this effect lies for maximising benefit while minimising the risk of adverse events. Future research should seek to identify an upper limit to exercise training volume and intensity that is safe and effective which may be best identified using non-linear modelling.
Despite correlations with other training variables, the current analysis showed that the number of weekly sessions attended over 6 months (training frequency; range: 0–2/week) was not related to improvements in pain intensity. Similar results were seen in the aforementioned study of female office workers with musculoskeletal pain,28 where training attendance (range: 0–3/week) was not correlated with reductions in pain intensity. It is unclear from the current results if the lack of relationship observed between pain intensity and training frequency would differ if participants had access to more than two training sessions per week. For example, when completing stabilisation exercises, 3–5 sessions per week has the greatest impact on improving LBP intensity.13 However, a direct translation between stabilisation exercise and general strength and conditioning is difficult due to the different mechanisms of action (eg, changes to motor control vs strength and capacity). Therefore, future research should explore the dose–response relationship between pain intensity and weekly attendance during a general strength and condition programme in which participants are randomised to low, medium and high training frequencies.
Importantly, the associations between pain intensity and training parameters were observed on a weekly (for time under tension) and sessional (for duration and RPE) basis. While monitoring training parameters over a 2-weekly basis did not provide any additional insight. To our knowledge, this is the first study to explore different time periods over which to measure pain intensity and training parameters. Analysing the response to training parameters measured over sessional, weekly and 2-weekly intervals can help to differentiate between acute and chronic responses to training load and intensity. The current findings suggest that short-term responses following a session or week of exercise training are more indicative of positive outcomes than intermediate responses (ie, 2 weekly). Additionally, measuring session duration alongside volume and training intensity on a short-term basis may prove important in reducing the risk of pain ‘flare-ups’ due to excessive exposure.
Clinical implications
When designing a general strength and conditioning programme for individuals with chronic LBP, training intensity and volume may be important variables to consider, with increased training intensity and volume associated with improvements in pain intensity. Therefore, ensuring the prescribed training load is adequate within each session to provide sufficient stimulus appears necessary to maximise reductions in pain intensity. Additionally, training session duration should be monitored alongside pain intensity to avoid periods of excessive load leading to poorer outcomes. When considering a programme consisting of up to two sessions per week, regular training frequency may not be the most important variable for reducing pain intensity. This information may be beneficial for individuals with limited resources (eg, time and money) to complete more than one training session per week. Additionally, individuals should not be discouraged if they miss a week of exercise training, as they can still achieve positive results from a general strength and conditioning programme.
Strengths and limitations
The current study provides valuable information to guide the delivery of exercise training interventions in individuals with chronic LBP. We used a single-item validated outcome measure to track pain intensity (VAS) every 2 weeks throughout the programme, which allowed us to explore how the accumulation of volume, duration and intensity impacted outcomes across a 2-week period. Additionally, allowing participants to self-select training frequency in the second half of the programme provides a pragmatic foundation to explore how training frequency interacts with recovery. However, the current study is not without limitations. First, pain intensity was the only outcome measure tracked on a 2-weekly basis. It is unclear how training variables relate to other outcomes that may be important in individuals with chronic LBP, such as disability, kinesiophobia, strength and muscular endurance. Training variables such as training frequency may be associated with these additional outcomes and retain importance in programme design. Second, while the single-item pain intensity VAS is a simple measure to administer and track, it does not distinguish between the type of pain experienced (eg, joint pain, neuropathic pain, general soreness). Hence, we could not determine whether pain intensity observed during some 2-week periods was due to an inability of neuromuscular tissue to cope with the imposed loads, an acute response due to delayed onset muscle soreness, a chronic overtraining response or other natural variations in pain. Future studies may benefit from measuring pain intensity more frequently around training sessions (eg, pre-exercise and postexercise session and 24 hours post) in addition to using a specific outcome measure to account for delayed onset muscle soreness. Third, additional training measures (eg, volume-load, weekly physical activity, metabolic equivalent) may have provided further information to identify which specific mode, volume or intensity variables most closely underpin the changes in pain intensity observed. Fourth, training data from the original randomised controlled trial pertaining to the motor control and manual therapy group were not available for analysis due to differences in recording. Fifth, these results are limited to adults aged 25–45 years with non-specific chronic LBP weighing less than 120 kg. Caution should be taken if generalising these findings to the broader public. Finally, although planned, most participants failed to return home-based exercise diaries at 3 and 6 months. Future studies should consider alternative methods of collecting home-based training compliance, such as via digital record keeping or wearable technology.