Hi Clive, you are asking should you do base (aka HVLI aka endurance) training on its own or should you combine HVLI with HIIT? or are you asking should you do cycling endurance AND strength training…sorry the terminology is confusing.
I will answer the first question. Should you do just base training alone? OK this is very clear. Unless you are a raw beginner, you should always combine base with HIIT (which means polarized or pyramidal TID). The debate is more what you should do with threshold and sweetspot…maybe some but not instead of all HIIT. You might be aware I reviewed the evidence here: https://www.youtube.com/watch?v=jFXnjPvyO1M&feature=&t=15m30s but I will take a few minutes to look at the evidence again. Anyone who states you can be base on its own, or threshold alone or sweetspot alone needs to cite their evidence.
Seiler et al. (2013 Scand. J. Med. Sci. Sports 23, 74–83) analyzed the performance adaptations of different types of high intensity interval training in 35 recreational cyclists were randomized to four training groups. base (aka HVLIT) group trained 4–6 sessions/wk and the three HIT groups trained two sessions 4 × 4 min (94% HRmax), 4 × 8 min (90% HRmax), or 4×16 min (88% HRmax) plus 2–3 sessions HVLIT. All three interval training groups tended to improve in physiological capacity after the training period, while the HVLIT group remained relatively unchanged.
Stöggl and Sperlich (2014 Front. Physiol. 5:33. doi: 10.3389) compared HVLIT (83/16/1%) vs. THR (46/54/0%) vs. HIT (43/0/57%) vs. polarized TID (68/6/26%) in 48 well-trained runners, cyclists, triathletes and cross-country skiers. While all four groups increased time to exhaustion, the polarized TID increased VO2peak (+11.7%), time to exhaustion (+17.4%), and peak performance (+5.1%) to the greatest extent. Exclusive emphasis of THR or HVLIT did not lead to further improvements in endurance performance.
Stöggl and Björklund (Front Physiol. 2017 Aug 2;8:562) randomly 36 athletes to assigned to one of three groups (HIIT; polarized training POL; HVLIT applying no HIIT). Only the HIIT group achieved improvements in peak power/velocity and peak lactate and acute HRR was improved in HIIT (11.2%, P = 0.002) and POL (7.9%, P = 0.023) with no change in the HVLIT . They conclude that HVLIT had no effect on any performance or HRR outcomes.
So the conclusion is that all three good quality well powered RCTs found base training alone to be not optimal compared to other options. Hope that helps!
regards alex from FFT