ENERGY SYSTEM DEVELOPMENT AND LOAD MANAGEMENT THROUGH THE REHABILITATION AND RETURN TO PLAY PROCESS

Sports injuries are common and necessitate a structured return-to-sport (RTP) process to restore performance and reduce re-injury risk. The RTP continuum spans return to participation, return to sport, and return to performance, requiring coordinated decision-making among physicians, rehabilitation professionals, strength and conditioning coaches, sport coaches, and the athlete. While time-based standards aid communication, task-based, performance-driven criteria better reflect readiness. Traditional RTP algorithms (e.g., ROM, strength, hop tests) often overlook the amount and nature of training performed, leaving uncertainty about whether athletes have accumulated sufficient fitness and chronic training load to tolerate high-level competition. Incorporating training load assessment into RTP was highlighted in the 2016 Bern Consensus. Frameworks such as StARRT (assessing health risk, activity risk, and risk tolerance) can structure decisions; the authors propose embedding workload assessment within each of these domains to guide progression from medical clearance to return to performance. Strength and conditioning, including energy systems development, should be integrated throughout rehabilitation to prevent detraining and rebuild capacities. Energy system training depends on manipulation of mode, intensity, duration, and recovery to elicit specific adaptations. Effective monitoring of training ensures appropriate progression, avoids under/overexposure, and supports clear communication across the RTP team. This commentary outlines energy system physiology, practical training methods, and low-cost monitoring strategies to quantify and direct RTP.

• Energy system contributions and team-sport demands:

  • Three interacting metabolic pathways supply ATP: phosphagen, glycolytic, and oxidative; their relative contributions depend on intensity and duration.
  • Team sports rely on repeated sprint ability (RSA): intermittent maximal/near-maximal efforts interspersed with low-intensity activity. Fatigue in RSA is influenced by work-to-rest, recovery type, O2 kinetics, and metabolite accumulation.
  • High-intensity interval training (HIIT) improves VO2max, VO2 kinetics, mitochondrial biogenesis, and performance.
  • Although aerobic contribution to a single sprint can be ~≤10%, during repeated sprints it can rise to ~49%, underscoring the need for aerobic development to support recovery between efforts.

• Practical classification for short-duration repeated efforts (Chamari & Padulo):

  • Explosive Efforts: up to 6 s, maximal intensity; example 3–5 s work with 60–120 s passive rest.
  • High-Intensity Efforts: 6 s to 1 min, maximal; example 30 s work : 30 s rest (recovery RPE <2).
  • Endurance Efforts: >1 min at high intensity; example 3 min work : 3 min rest (work RPE 8–9; recovery RPE <2).
  • Longer-duration endurance work remains important: Extensive Zone 1 continuous (20–60 min), and Intensive Zone 2–3 intervals (e.g., 6–8 min or 4–6 min bouts with low Zone 1 recovery).

• Programming parameters and progression (Table 4 guidance):

  • Explosive: maximal <6 s efforts; 30–120 s passive recovery; 2–6 series; 2–3x/week over 2–3 weeks.
  • High-Intensity: 15–30 s maximal efforts; 30–120 s recovery at RPE <2; 4–10 series; 2–3x/week over 2–3 weeks.
  • Endurance Effort (short-duration repeated): 2–3 min at RPE 8–9; 2–3 min recovery at RPE <2; 6–10 series; 2–3x/week over 2–3 weeks.
  • Long-duration Extensive: Zone 1, 20–60 min, 3–5x/week for 2 weeks to 3 months.
  • Long-duration Intensive: Zone 2–3 intervals with 2–4 min low-intensity recoveries; 3–6 series; 2–3x/week for ≥2 weeks.
  • Modality progression across RTP: begin with non-injured-limb/low-impact options (e.g., bike, row), then linear running, change-of-direction, and finally open-skill sport-specific drills; progress intensity, work:rest, and specificity to match sport demands.

• Intensity prescription and affordable monitoring:

  • Use RPE-based zones when HR/GPS/power tech is unavailable: Binary (low vs high) or 3-zone model mapped to CR-10 RPE (Zone 1 ≤5; Zone 2 5–6; Zone 3 7–10).
  • Session RPE (sRPE) quantifies internal load as CR-10 RPE × session duration (min). Collect ~10–30 min post-session; minimal differences reported between 10 and 30 min.
  • sRPE is valid across aerobic, intermittent team-sport, and resistance training.
  • Practitioners can also prescribe interval work/recovery by RPE targets (e.g., work RPE 8–9; recovery RPE 2).

• Modeling training load and injury risk:

  • Acute:Chronic Workload Ratio (ACWR) = current week’s load / 4-week average. Values >1 indicate the current week exceeds the athlete’s recent chronic exposure.
  • In elite rugby league, ACWR >2.11 associated with 3.4× greater injury risk; flagging thresholds at ~1.0 and ~2.0 helps identify load spikes and plan deloads.
  • Visualization of acute load, chronic load, and ACWR supports RTP decision-making, helping ensure sufficient chronic loading before progressing to full sport.

• Integration with RTP frameworks:

  • Embed workload assessment (external and internal) within all three StARRT domains (health risk, activity risk, risk tolerance) to guide progression from return to participation to return to performance and align interprofessional communication.

This commentary addresses the gap between medical clearance and competitive readiness by integrating energy system development with quantifiable load management during RTP. By classifying and programming energy systems training aligned to sport- and position-specific demands (e.g., repeated sprint profiles), practitioners can restore both neuromuscular and metabolic capacities. Using affordable tools such as sRPE to quantify internal load and simple session logging to capture external load enables consistent tracking without costly technology. Modeling the training process via the acute:chronic workload ratio provides a pragmatic indicator of the athlete’s tolerance to current loads relative to their recent training history, highlighting potential spikes that elevate injury risk and guiding adjustments in volume or intensity. Embedding workload assessment into the StARRT framework facilitates interdisciplinary decision-making, ensuring athletes accumulate adequate chronic exposure before advancing along the RTP continuum. Overall, the proposed approach enhances communication, individualizes conditioning progression, reduces the likelihood of underprepared return, and supports a smoother transition to return to performance.

Return to play after injury requires coordinated, criterion-based progression that includes restoration of energy system capacities and quantification of training load. Incorporating workload assessment within a structured framework (e.g., StARRT) and programming energy system development across modalities and intensities allows clinicians to address readiness and tolerance to increasing sport demands. Low-cost monitoring with sRPE and modeling with the acute:chronic workload ratio help identify excessive loading and guide periodization so athletes accumulate sufficient chronic load before returning to high-intensity practice and competition. Approaching RTP in this manner ensures that capacity and workload risks are explicitly discussed and managed, supporting a safer and more effective return to performance. Future work should refine sport- and position-specific load targets, validate thresholds across populations, and explore integration of objective technologies with practical monitoring in rehabilitation settings.

• Clinical commentary (Level of Evidence 5): no experimental intervention or controlled outcomes; recommendations synthesize existing literature and practical experience.
• Generalizability: Acute:chronic workload ratio thresholds (e.g., >2.11 linked to higher injury risk) are derived from specific cohorts (e.g., rugby) and may not directly translate to all sports, levels, or injury types.
• Measurement constraints: Objective technologies (HR, GPS, power) can be costly and require expertise; heart rate is less practical for short-interval work due to kinetics. sRPE, while practical, is subjective and can be influenced by psychological and contextual factors.
• Terminology and prescription models (binary/3-zone, RPE mappings) are approximations and may vary across individuals; no single model fits all athletes or stages of rehabilitation.